Home

USER GUIDE FOR EGNOS APPLICATION DEVELOPERS

1. s MR H EM M A o 6 How to use EGNOS messages In theory EGNOS transmits data only for IGP marked in black or blue in the figure below In practical terms only IGP marked in blue are monitored on a regular way FIGURE 18 EGNOS IGP Masks 6 2 3 1 Message type 18 IGP mask Again with the aim of optimising message size the mask principle is applied once again to associate ionospheric corrections with the IGPs to which they relate Each message contains the mask for one band A bit positioned at 1 means that the information is provided for the corresponding IGP 6 2 3 2 Message type 26 ionospheric corrections Type 26 messages provide for the IGPs present in the mask data for computing the ionos pheric corrections or Grid lonospheric Vertical Delay GIVD and a parameter for estimating the accuracy of corrections called a GIVE indicator GIVEi This information can be provided for a maximum of 15 IGPs per message As the ionospheric bands can contain up to 201 IGPs the IGPs present in the mask are grouped into blocks of 15 IGPs Thus block 0 contains data for the first 15 IGPs activated in the mask and so on The o GIVE values are obtained through correspondence with the GIVE indicators transmitted in the message 46 6 How to use EGNOS messages GIVEi 2272 m IGP Status 0 0 0084 Use 1 0 0333 Use 2 0 0749 Us
2. GNOS EGNOS EGNOS EGNOS EGNOS EGNOS EGNOS p EGNOS EGNOS ISA EGNOS EGNOS APPLI EGNOS EGNOS 0 EGNOS EGNOS re A EGNOS EGNOS ee EGNOS EGNOS EGNOS EGNOS SE EGNOS EGNOS TE GNOS EGNOS EGNOS2 EGNOS EGNOS a NOS FONOS S EGNOS EGNOS USER GUID 15 GNOS EC E ER de cnes CENTRE NATIONAL D TUDES SPATIALES esa Preface After more than 10 years of development and qualification efforts bringing together actors from across the European continent including project teams industrial teams and operators for the first time Europe has at its disposal a GNSS infrastructure which delivers on a permanent basis and according to international civil aviation standards a satellite navigation service covering much of the continent For all space sector participants it is extremely gratifying to see a programme having origina ted in research and development leaving the space agency sphere and entering into the world of services addressed at a vast community of users After telecommunications meteorology oceanography search and rescue and Earth observation we are now seeing the emergence of a new area of operational space activity which clearly illustrates the unique and essential contribution space can make to citizens The success of a development activity is a crucial step on the way to the success of a pro
3. 24 E 2 Advantages of EGNOS Access to the EGNOS signal is like the civil GPS signal free of charge Most commercially available receivers for professionals and the general public use the EGNOS signal is also possible to access EGNOS messages by means of other distribution channels available on the internet such as SISNeT and EDAS These channels are presented in detail in section 4 2 3 SERVICES TERMINOLOGY In the official literature the services provided by EGNOS are often grouped together using the following terms e Open Service OS refers to the use of accuracy improvement Safety of Life SoL refers to use of the integrity function This service was originally intended for the world of safety critical transport aviation shipping railways etc but is also suited to other applications such as those requiring legal guarantees Commercial Data Distribution Service CDDS refers to the use of additional data by certain professional users This service is not provided by the EGNOS signal broadcast by the geos tationary satellites but by the EDAS system see section 4 2 2 4 EGNOS PERFORMANCE LEVELS 2 4 1 Accuracy One of the main advantages of EGNOS 1 the improved accuracy in relation to a position solely calculated using GPS by the broadcasting of differential corrections to GPS orbits GPS clocks and the ionosphere The horizontal accuracy provided is of the order of 1 to 3 metres the vertical a
4. 9 3 3260 10 5 1968 11 20 7870 12 230 9661 13 2078 695 14 Not Monitored 13 N A Do not Use DU 48 gt al 6 How to use EGNOS messages 6 2 6 Message type 24 a special case Message type 24 contains two types of correction fast and long term as well as the associated integrity parameters UDREi Message type 24 can be broadcast if the number of satellites in the last mask is less than 6 The first part of the message will contain the fast corrections and the UDREis while the second will contain the long term corrections 6 3 USING INTEGRITY INFORMATION 6 3 1 Generation of alerts and protection levels Satellite Alarms EGNOS transmits for each GPS satellite being monitored an integrity signal with three values showing whether the status of the satellite is in keeping with use for a safety of life application OK ananomaly has been detected with the satellite Do not Use DU the data on the satellite are insufficient to monitor it Not Monitored NM The system has 6 seconds in which to inform the user of any integrity fault that is no more than 6 seconds may elapse between the moment when the problem impacts the user and the moment when the alert is available to the user The alert is repeated in the signal for 4 conse cutive seconds in order to counteract any message loss Anomaly information Do not Use and Not Monitored is transmitted wit
5. A3 2 A3 2 System architecture The GPS system 1 based on three segments the space segment consisting of a constellation of satellites that emit the navigation signals the ground segment which monitors and controls the space segment In particular it provides the satellites with their orbital parameters for redistribution to the users the user segment consisting of all the GPS receivers which calculate their position velocity and time PVT using the signals received Space segment The space segment of the GPS system is specified as nominally consisting of 24 satellites distributed evenly across 6 circular orbital planes at an altitude of 20 184 km spaced at 60 intervals and with an inclination of 55 to the equatorial plane Additional positions have been allocated for when the number of satellites in the constellation exceeds 24 FIGURE 35 GPS Constellation source http pnt gov In practice the number of operational GPS satellites is higher 31 satellites at the end of 2011 Each satellite s orbital period lasts around 12 hours The configuration of the constellation ensures that at any one time there are at least 6 satellites visible and the service 15 available at any point on the globe with nevertheless a few availability limitations at higher latitudes The GPS satellites carry several highly accurate atomic clocks up to 4 each to time the precise moment at which the satellite transmits 5 data
6. Ban 1 Why do we need EGNOS 1 4 EGNOS The European Geostationary Overlay Service EGNOS complements the American GPS system which is made up of a number of navigation payloads aboard satellites in geostationary orbit a ground based network comprising a series of positioning stations and several control centres all of which are interconnected EGNOS while dependent on GPS is able to offer services today that are close to those that in future will be offered by Galileo byimproving GPS positioning accuracy by providing the user with information on GPS reliability by sending integrity messages giving confidence thresholds and alarms in the event of anomalies by emitting a signal synchronised with Coordinated Universal Time UTC Three principal players have been behind the development of EGNOS the European Union represented by the European Commission the European Space Agency ESA and Eurocontrol European Organisation for the Safety of Air Navigation The European Space Agency acted as system prime during the development validation and initial exploitation phase until March 2009 Eurocontrol established the requirements called for by system users among the civil aviation community The European Union contributes towards codifying the requirements of all its users and validating the system It also takes care of the establishment of EGNOS by taking all the necessary measures notably the leasing of the payloads
7. FIGURE 12 Message validity principle 38 5 messages Refresh and validity periods For each message type transmitted there is thus a maximum refresh period which must be taken into account by the system in the transmitted signal A validity period is also defined it must be applied by the user and can depend on the application These intervals and periods are given in the table below Validity period Types Data contained En Route Precision Terminal NPA Approach 0 Don t use for safety applications 6 60 60 1 PRN mask 120 mes 600 600 2to6 24 UDREI 6 18 12 2to5 24 Fast Corrections Variable ote 1 Variable e1 Variable oe 24 25 Long Term Corrections 120 360 240 9 GEO Navigation Data 120 360 240 7 Fast Correction Degradation 120 360 240 10 Degradation Parameters 120 360 240 18 lonospheric Grid Mask 300 nete 1200 1200 26 lonospheric Corrections 300 600 600 12 UTC Timing Data 300 86400 86400 17 Almanac Data 300 None None 27 Service Level 300 if used 86400 86400 Note 1 The value depends on the degradation factor for the fast corrections for further infor mation refer to section 2 1 1 4 9 of MOPS DR2 Note 2 When the masks are modified see sections 6 1 and 6 2 message type 1 or 18 must be repeated several times before the new mask can be used This ensures that all users have received the new mask before it is a
8. SEPTENTRIO SEPTENTRIO SEPTENTRIO SEPTENTRIO SEPTENTRIO SEPTENTRIO SEPTENTRIO SEPTENTRIO SEPTENTRIO SEPTENTRIO SOKKIA SPIRIT DSP SPIRIT DSP D 4 i 8 General Information Caption m Available NS Not stated NA Not Applicabie Applications 2IWto33W 3 5W to 4W AW to SW NS NS NS NS 235x154x71 235x154x71 235x154x71 147x13x45 147x13x45 147x113x45 147x113x45 60x100x9 1 46x71x13 46x71x13 46x71x13 46x71x13 60x100x13 85x125x13 185x160x71 61x100x13 5 130x185x46 60x90 77x120 NS 60x90 60x90 130x185x46 130 185 46 60x90 160 100 13 160 100 13 160 100 13 285140237 285140237 285x140x37 235x140x37 235x140x37 47 5x70 147x100x40 30x40x6 30x40x6 NS 112 168x100x15 160x100x14 2 157x48x170 197x90x46 NS NS Interfaces SEE nm cp o ap n m n General Information Applications 21W 1 4W t01 5W 24W to 2 3 NS NS NS NS 72 6x62 51x9 159x173x113 159x173x113 NS
9. Similarly if broadcasting of an alert cannot wait until the next type 2 3 4 or 5 message is broa dcast a message type 6 will be broadcast immediately A message type 6 contains integrity information on all the mask s satellites the maximum number of satellites in the PRN mask is 51 Such messages also contain Issue Of Data Fast Correction IODF data which associate UDREi values with the corrections contained in the type 2 to 5 and 24 messages type 6 messages are not directly linked to the mask DIRECTION OF DATA FLOW FROM SATELLITE MOST SIGNIFICANT BIT MSB TRANSMITTED FIRST 250 BITS 1 00 22 IODF amp 2 BITS EACH PARITY 22 20 51 BIT WDRER un on 6 BIT MESSAGE TYPE IDENTIFIER 6 8 BIT PREAMBLE 24 BITS TOTAL IN 3 CONTIGUOUS BLOCKS FIGURE 21 format 0 50 E 6 4 USING TIME DATA The EGNOS system transmits via message type 12 the parameters for synchronising EGNOS Network Time ENT obtained during computation of the user s position with Coordinated Universal Time UTC MT12 is updated a maximum of every 300 seconds Note Although all the parameters needed to calculate UTC are broadcast in the message type 12 few receivers compute and automatically generate UTC from ENT For details on the calcu lations to be done refer to Annex 8 6 5 GEO RANGING Messages Type 9 and 17 aim at providing information about GEO satellites na
10. trique est quantifi e par une valeur sans unit appel e Dilution of Precision DOP On distingue plusieurs types de DOP Geometric Dilution Of Precision PDOP Position Dilution of Precision 3 D HDOP Horizontal Dilution of Precision Latitude Longitude VDOP Vertical Dilution of Precision Altitude TDOP Time Dilution of Precision Temps 91 92 The lower the value of the DOP the greater the accuracy of the point If we assume a measurement error common to all the satellites we will have an XEP positioning error equal to X P Hou V UERE Geodetic models The coordinates longitude latitude altitude of a point are relative to a given geodetic model Thus the GPS system uses the WGS84 system developed by the US DoD This system models the Earth with an ellipsoid whose centre 15 close to the centre of the Earth s masses whose 2 axis is close to the centre of the Earth s axis of rotation and whose OXY plane is that of the equator The WGS84 system has the following characteristics The model is accurate to within one metre The associated ellipsoid is the IAG GRS80 The associated projection is the UTM Depending on the applications it may be necessary to convert to other geodetic models In France for example the legal reference system is the R seau G od sique Francais 1993 RGF93 with a flat representation using the Lambert 93 pro
11. 12 2 Time distribution 12 3 Using SISNeT 12 4 Using the integrity service 1 2 3 4 5 6 7 8 9 ACRONYMS REFERENCES GPS ELEVATION OF A GEOSTATIONARY SATELLITE EGNOS AND NMEA CALCULATING IONOSPHERIC CORRECTIONS CALCULATING THE HORIZONTAL PORTECTION LEVEL HPL SYNCHRONISATION WITH UTC STANFORD DIAGRAM 83 87 88 94 96 98 101 105 107 Acknowledgments The following people participated in preparing this document Messrs A ALLIEN and TAILLANDIER from Mrs CAPO and PRISELOW from the Centre National d Etudes Spatiales CNES Multimedia Design Workshop Messrs J LEGENNE J MARECHAL and M JEANNOT of CNES with the support of other CNES experts whom we would like to thank for their help The autors wish to thank you in advance for any comments you may wish to provide on this guide subsequent to having read and used it or any suggestions you may have on how it could be improved Please send all such comments to guide_egnos cnes fr
12. 3 Synchronisation with UTC 244 EGNOS Reference Frame 25 Coverage EGNOS ARCHITECTURE 3 1 Step 1 Collecting measurements and data from the GPS satellites 3 2 Step 2 Calculating differential corrections and estimating residual errors 3 3 Step 3 Transmitting the EGNOS messages to users via the geostationary satellites OTHER WAYS OF ACCESSING EGNOS 4 1 SISNET Signal In Space through the interNET 42 EDAS EGNOS Data Access System EGNOS MESSAGES Size and bit rate Message types Structure of message types Message validity period Type 0 and type 0 2 messages 5 5 1 What purpose do they serve 5 5 2 What impact does this have on my receiver HOW USE MESSAGES 6 1 Applying the PRN mask 6 2 Using differential corrections 6 2 1 General information on differential corrections 6 2 2 Issue of data IOD 6 2 3 lonospheric corrections 6 2 4 Long term corrections 6 2 5 Fast corrections 6 2 6 Message type 24 a special case Using integrity information 6 3 1 Generation of alerts and protection levels 6 3 2 Message type 6 a special case 6 4 Using time data 65 GEO Ranging COMPARISON OF GPS AND EGNOS PERFORMANCE 7 1 Accuracy 7 2 Integrity 7 3 Availability LIMITATIONS FINDING OUT THE LATEST EGNOS STATUS 9 1 Programme status 9 2 Current Status of GEO satellites 9 3 Useful tools UPGRADES CHOOSING A RECEIVER EXAMPLES OF PRACTICAL APPLICATIONS 12 1 Precision farming
13. 5 1 of DO229D 102 M 0 ANNEX 7 Calculating the Horizontal Protection Level HPE ure is the variance of ionospheric correction errors as defined in Annex 6 For more details refer to Appendix A sections A 4 4 10 and A 4 5 2 of DO229D d represents the square of tropospheric correction residual error the latter being defined tropo oe 1 1 Where Tropospheric vertical error Orve 0 12 meters A simplified computation of m El can be got through below equation for satellite elevation angles E above 4 1 001 7 0 002001 sin EI m El For more clarification refer to section A 4 2 4 of DO229D 07 air is provided thanks to the expression 0 0 noise O imuttipath O i divg With 00 10 03 50 36 with elevation angle of satellite in degrees 0 36 si Niveau signal min applying a linear 0 15 si Niveau signal max 2 2 2 1 2 i O divg i 1 lt interpolation between these two values elevation min value 5 and max value 90 Definition of SUDRE SUDRE parameter is a multiplying factor of Supre applied when inside or outside defined areas 5 at the maximum all these parameters being provided through MT27 or MT28 In EGNOS MT27 1 used in a basic way the defined area being the ECAC one with an SUDRE maximum outside this area ANNEX 8 Synchronisation wi
14. Airborne Equipment version D describes the implementation of SBAS services for receivers in civil aviation use An annex to DO229D contains the specifications for the SBAS signal and message This document is available for a fee and can be obtained from the RTCA website at http www rtca org The RTCA provides regular updates to these standards 1 Why do we need EGNOS The various SBAS systems WAAS EGNOS MSAS GAGAN were developed in accordance with this common standard and are therefore all compatible in other words do not interfere with each other and interoperable a user with a standard receiver can benefit from the same level of service and performance whether located in the EGNOS or WAAS coverage area Applications of SBAS Outside the civil aviation sphere SBAS systems are used in all fields where accuracy and inte grity are of foremost importance In particular SBAS is indispensable for all applications where people s lives are at stake or for which some form of legal guarantee is required SBAS makes it possible for example to improve and extend the scope of applications for GPS in areas such as precision farming the guidance of agricultural machinery on road vehicle fleet management oil exploration for the positioning of platforms out at sea or scientific applications such as geodesy etc The following figure illustrates the coverage areas of the various SBAS systems FIGURE 3 Coverage areas of SBAS systems
15. HPL The HPL parameter is defined as the radius of a circle located in the horizontal plane i e tangential to the WGS84 ellipsoid whose centre is the actual position of the antenna and which thus describes the zone guaranteed to contain the horizontal position calculated The HPL enables a limit to be estimated for the position errors It is calculated by the receiver or equipment using information transmitted by the EGNOS system the receiver s own parameters and geometric factors The following parameters transmitted by EGNOS are needed to establish the protection levels UDRE User Differential Ranging Error which characterises the estimated residual errors in the orbit clock corrections for each satellite GIVE Grid lonospheric Vertical Error which describes the potential error level in the ionos pheric corrections For this the following EGNOS messages must be retrieved Message type 1 to obtain the PRN mask Message types 2 5 6 24 for orbit and ephemeris errors UDRE Message types 18 and 26 for ionospheric error GIVE HPL Ky npa ed naor In this equation K npa is set to 6 18 for NPA phases and so for aeronautics domain correspon ding to a probability of non integrity of 1 10 hr This parameter may however be modified for other applications for example terrestrial ones so enabling to have reduced HPL values for less important non integrity probabilities Values for K can therefore be
16. The GPS satellites transmit on two frequencies known as L1 1575 42 MHz and L2 1227 6 MHz The standard positioning service is currently broadcast exclusively on L1 Satellites from the IIR M launched from 2003 onwards and 11 blocks first launch performed in 2010 also broadcast a civil signal on the L2C frequency whereas L5 is broadcast by 11 satellites Ground segment The GPS satellites are permanently controlled by a network of five control stations with the Master Control Station being located in Colorado Springs The ground segment has several roles e To recalibrate the satellites atomic clocks To generate the data that enable the user to calculate a position satellite ephemeris data clock corrections To load the previous data onto the satellites for distribution to users To control and command the satellites 89 90 User segment This segment consists of the GPS receivers It is important to bear in mind that a GPS receiver only monitors signals sent by the satellites and does not establish any contact with them Therefore a GPS receiver cannot be used by a third party to find out a user s position without his knowledge The table below lists the main error types that a user typically comes across Error type Orbit and synchronisation 1m Tropospheric error 0 25 lonospheric error 2m Receiver noise 0 5 Multipath 0 2 UERE 1 0 2 31 m HDOP function o
17. been operational since May 2006 For further information go to http www egnos pro esa int sisnet 36 E 4 Other ways of accessing EGNOS 4 2 EDAS EGNOS DATA ACCESS SYSTEM European Commission has put in place a system to make EGNOS data available EDAS This system allows to have access to data issued from EGNOS infrastructure Main types of available data are the following e GPS GLONASS and EGNOS raw data collected by RIMS stations network EGNOS augmentation messages as received by a user via EGNOS geostationary satellites Coordinates of RIMS antenna phase centre Details of this information and means to access it are described on GSA website on page http www gsa europa eu go egnos edas 5 1 SIZE AND RATE The EGNOS system transmits its messages over band L1 1575 42 MHz at a rate of 250 bits per second It uses the same modulation as GPS but at a transmission rate five times higher The size of every message transmitted is 250 bits which enables one message to be trans mitted per second 5 2 MESSAGE TYPES Several message types can be transmitted by the system the various message types currently standardised are listed below FIGURE 10 List of EGNOS messages 5 3 STRUCTURE OF MESSAGE TYPES All EGNOS message types can be broken down into the following structure The first 8 bits of each 250 bit message correspond to part of the preamble The preamble is a unique 2
18. gramme For it to become a total success efforts must now focus on ensuring that users all application areas can obtain easy access to services making those services straightforward to use and of course on guaranteeing quality of service over time This guide 15 designed to acquaint the user with the system and to provide the essential tech nical information that users and application developers require if they are to make the best possible use of EGNOS CNES the European Space Agency and the European Commission are proud to have contri buted to the development of the EGNOS system and thank all participants in this effort both public and industrial for their contributions to and support for the programme They also wish every success to the EGNOS operational exploitation phase and hope that this guide will allow users from all walks of life to make use of EGNOS in a great many application areas Mana Yannick d Escatha Matthias Ruete Jean Jacques Dordain President Director General Director General Centre National d Etudes Spatiales DG Energy and Transport European Space Agency European Commission DISCLAIMER This guide is designed to be used by developers of applications for the European satellite navigation system EGNOS Under no circumstances must it be taken to be a manual certi fied by the designers and developers of the EGNOS system or by any legal and regulatory authorities The information it contains
19. in 1994 The system called Galileo will offer a range of services which will be compatible and directly interoperable with the GPS open service The first experimental satellites GIOVE A and GIOVE B have been launched in 2005 and 2008 followed in October 2011 by the first two satellites of the IOV phase Finally China with Beidou has begun implementing a regional satellite navigation system having launched the system s first satellites in 2000 Extension of that regional system to form a global system named Compass is ongoing since 2010 By allowing anyone with a GPS receiver to determine their position to within a few metres their speed to within a few cm s and the time to within a few hundredths of a microsecond around the clock and across the entire globe GPS has revolutionised the world of navigation and has opened the way for new applications based on navigation positioning and time determination Today the use of satellite navigation systems grouped under the term GNSS for Global Navi gation Satellite System has become essential to a multitude of applications whether they be strategic professional or simply leisure oriented In 2011 the GPS system is the only fully operational global satellite navigation system 1 2 GPS HOW IT WORKS ITS PERFORMANCE AND LIMITATIONS This section gives some information on how the GPS system works as well as on its mance and limitations Refer to Annex 3 for more information
20. longitude latitude and alti tude in accordance with WGS84 the World Geodetic System 1984 reference system Sources of error and how they affect positioning Various errors interfere with pseudorange measurements It is not possible to know exactly what these errors are but their distributions can be characterised statistically lt so happens that errors that adversely affect GPS system accuracy follow distributions that closely mirror Gaussian distributions One characteristic of these distributions is that 95 of the population is situated in the band 20 20 where o represents the typical deviation of the distribution around the mean In practice therefore the errors E affecting pseudorange measurements are often expressed as 2 o which means that the probability of the real error being less than E is 95 Given that the notion of error is directly linked to that of accuracy and that each of the error components contributes to the calculation of the position or time the positioning accuracy is also expressed at 95 For a description of the different error types see Annex 2 which provides the principal GPS errors Using the various error components one can determine a UERE User Equivalent Range Error which provides the accuracy of the pseudorange measurement between the user and each satellite Bo 1 Why do we need EGNOS 1 2 2 Performance The performance of a satellite navigation system is expressed according t
21. only general and marginal account of local errors arising from local multipath errors prevalent in this type of environment The integrity concept cannot therefore be used as is in areas where these are prevalent urban areas forest cover etc Action is being taken in various forums to look into solutions that would resolve this difficulty for example further processing at receiver level Sensitivity to ionospheric effects The EGNOS system has been designed to operate in single frequency mode this can give rise to degraded service availability in the event of very strong ionospheric turbulence Sensitivity to jamming As GPS and EGNOS signals are received on the ground at very low power levels they are relatively susceptible to jamming deliberate or otherwise 9 FINDING OUT THE LATEST EGNOS STATUS 9 1 PROGRAMME STATUS Detailed information on the EGNOS programme and its current status are available on the websites of the European Commission GSA ESA and ESSP the EGNOS operator http ec europa eu enterprise policies satnav egnos index_en htm http www gsa europa eu go home egnos http www esa int esaNA egnos html Site of the firm ESSP SAS France http www essp sas eu 9 2 CURRENT STATUS OF GEO SATELLITES Information on the status and performance of the EGNOS system and in particular on the availability of the geostationary satellites is supplied in real time on ESSP User Support and ESA websites http egn
22. parameters telling it to use EGNOS if necessary exclude the satellite used for EGNOS testing if necessary force the use of EGNOS despite the broadcasting of an 2 type message If the receiver uses the NMEA protocol to send data Annex 5 provides details on how to detect that EGNOS 1 being used 12 2 TIME DISTRIBUTION Context of the application Time distribution is a system in which a master clock is responsible for synchronising one or more slave receiver clocks In this application the master clock synchronises itself using the UTC time provided by EGNOS and redistributes this time to the slave clocks Advantages of EGNOS Here EGNOS Network Time ENT is synchronised with the UTC time issued by the Paris Observatory UTC OP in order to synchronise the master clock 76 12 Examples of practical applications Architecture Application engine GPS EGNOS receiver FIGURE 31 Architecture of a Time Distribution system The master clock consists of a GPS EGNOS receiver linked to an application engine a micro controller and an integrated or offset antenna The antenna must have a clear line of sight in order to pick up the GPS and EGNOS signals The receiver transmits its data to the microcon troller via a serial link Functions used When the receiver uses the GPS and EGNOS data to calculate the PVT the time calculated 1 ENT To obtain UTC time a correction model is used applying data from th
23. receiver 1 Why do we need EGNOS PSEUDO RANGE CORRECTIONS GPS DATA LINK 27 a SP gt CORRECTIONS PROCESSOR MOBILE STAT REFERENCE STATION Figure 1 How DGPS works Depending on the applications the reference stations can be independent or networked They can also be either fixed or movable 1 3 1 2 GBAS Ground Based Augmentation System GBAS is a local augmentation system to GNSS standardised by ICAO International Civil Aviation Organization for precision approach and landing operations with a high level of integrity Its principle is similar to that ofs DGPS GBAS is made up of a ground subsystem comprising two to four GNSS reference receivers and an airborne subsystem Using data from reference receivers the ground based subsystem calculates corrections to the pseudoranges for all visible satellites The ground subsystem also monitors the quality of the information transmitted to the airborne subsystem by performing a large number of tests on the differential corrections and pseudoranges These corrections are transmitted to the aircraft using the VDB VHF Data Broadcast system A GBAS system provides its services to all aircraft present in its coverage area of up to 20 Nautical Miles at the minimum GBAS 1 designed to respond to the problems posed by the most demanding of operations all weather precision approach The civil aviation community is currently working towards standard
24. shall be no substitute for official EGNOS linked documents and shall be considered information provided as is with no guarantee of any kind explicit or implicit notably in respect of its accuracy reliability exhaustiveness appropriateness for and adapta tion to a specific use or the needs expressed by the users of this guide It implies no obligation on the part of the European Commission CNES and ESA In no sense does the EGNOS guide exempt users from their obligation to exercise prudence in its application and in the interpretation they make of it in the performance of their activi ties whatever those may be Consequently its use and implementation is under the exclu sive responsibility of its users while the European Commission CNES and ESA can under no circumstances be held liable for damages of any kind whether caused by the implementation use or interpretation of the information contained in the EGNOS guide Furthermore the European Commission CNES and ESA may make any changes to this guide they deem to be useful notably following any future evolutions of the EGNOS system 0 INTRODUCTION EGNOS the European Geostationary Navigation Overlay Service is designed to complement the American GPS system It comprises a number of navigation payloads on board satellites in geostationary orbit and a ground based network consisting of a series of monitoring stations and several control centres The EGNOS system has been op
25. signal using space based augmentation notably from the 80s onwards at the instigation of CNES and the DGAC Civil Aviation Authority in France These were the beginnings of EGNOS notably with the CE GPS European Complement to GPS experiments But it was really from October 1994 when the US government offered civil aviation the possibi lity of using GPS free of charge the Russians did the same with GLONASS in June 1996 that large scale work got under way It was then that ICAO International Civil Aviation Organization began studies on complemen tary systems to compensate for certain disadvantages of GNSS in terms of accuracy essentially in the vertical a phenomenon which at that time was made worse by the deliberate degradation applied to GPS until 2000 integrity continuity of service and availability Indeed neither GPS nor GLONASS meet ICAO operational requirements in respect of the most critical phases in aircraft flight in particular landing This work gave rise to the SBAS Satellite Based Augmen tation System concept and the beginnings of the process of standardisation carried out by ICAO T 1 Why do we need EGNOS The SBAS concept is based on the transmission of differential corrections and integrity messages for navigation satellites which are within sight of a network of reference stations deployed across an entire continent A key characteristic of SBAS is that the data link frequency band and signal modulati
26. subsequent messages are related The mask contains 51 bits An nth bit at 1 shows that the nth satellite is being monitored by EGNOS Bit mask Satellite PRN 1 37 GPS constellation 38 61 Glonass slot number plus 37 62 119 Future constellations 120 138 GEO SBAS PRN 139 210 Future constellations In the example below the PRN mask shows that EGNOS will supply in its subsequent messages corrections and integrity information for the GPS satellites whose PRN codes are 3 5 and 7 The first correction supplied by EGNOS will correspond to PRN3 the second to PRN5 and so on 41 42 6 How to use EGNOS messages Bit N 4 2 3 4 5 6 7 PRN 0 0 1 0 1 0 1 GPS GPS GPS 3 5 7 FIGURE 13 mask 6 2 USING DIFFERENTIAL CORRECTIONS 6 2 1 General information on differential corrections A short explanation is required here on what is done by EGNOS on the corrections and what needs to be processed at application and or receiver level Thus Inthe case of the ionospheric correction parameters the user must choose either to use the GPS s Klobuchar parameters or to apply the parameters from the ionospheric grid trans mitted by EGNOS which is far more accurate For the other parameters ephemeris corrections and or clock corrections Timing Group Delay TGD correction GPS corrections must be applied first and
27. system for an agricultural vehicle This system consists of an offset antenna placed on the vehicle s roof a GPS EGNOS receiver and a computer running the guidance application As an option the receiver can be connected to an odometer which improves guidance accuracy GPS EGNOS Antenna Receiver Guidance application Odometer optional FIGURE 30 Architecture of the guidance system for an agricultural vehicle Functions used All the differential corrections broadcast by EGNOS are used for this application see Section 6 2 i e fast corrections long term corrections e ionospheric corrections As this is not needed the receiver used does not process the integrity information Receiver constraints There are no particular constraints with the receiver Any EGNOS compatible model i e which can calculate all the differential corrections will be suitable The offset antenna on the vehicle s roof offers a better reception of the GPS and EGNOS signals un 12 Examples of practical applications Implementation details Using the EGNOS service is relatively simple with this kind of application The differential corrections broadcast by EGNOS are taken into account directly by the receiver The receiver is generally connected to the computer running the guidance application via a serial link When the guidance application starts up it is however necessary to send the receiver the configuration
28. then the EGNOS corrections 6 2 2 Issue of Data IOD IODs are attributes of masks and of current long term and fast corrections They are therefore set inside the concerned messages and enable the various data transmitted as well as the successive updates to be handled in a coherent manner e IODP Issue of Data PRN identifies the current PRN mask e IODFj IOD Fast Corrections identifies current fast corrections refers to the type of message 2 to 5 IODE IOD Ephemeris identifies current long term corrections ODI IOD lonosphere identifies current ionospheric corrections 6 How to use EGNOS messages 6 2 3 lonospheric corrections To estimate the onospheric error for each receiver satellite line of sight the receiver must identify the lonospheric Pierce Points IPPs Each IPP 1 defined as being the intersection between the atmospheric layer located at altitude of 350 km and the line originating at the receiver position and which is directed at the GPS satellite in question Direction Local tangent plane IGP lonospheric Grid Point Earth Ellipsoid Earth Centre lonosphere FIGURE 14 Principle of the Ionospheric Pierce Point IPP EGNOS transmits ionospheric corrections enabling the ionospheric error to be estimated for each IPP These ionospheric corrections are broadcast for each of the points on a virtual grid situated at an altitude of 350 km These points are c
29. 2010 04 03 23 59 59 GEO 126 SBAS msgs 86396 gt 5 00 500 460 420 380 340 300 250 220 180 140 100 LATITUDE o 3 Ww 9 8 p z gd 9 9 9 q q q 4 ON 6 o ON e d x R eos 9 amp 9 X 9 8 8 3 8 amp F 8 8 amp 8 8 S 9 z 9 2 9 9 Y 9 e 9 9 LONGITUDE ECLAYR v5 21 Produced by ESA FIGURE 23 Horizontal positioning performances obtained with PRN 126 Geo Test S V 7 Comparison of GPS EGNOS performance 7 2 INTEGRITY Despite 5 great accuracy the reliability of data supplied by the GPS system 1 not guaranteed notably in the event of a malfunction of an atomic clock onboard a satellite which may lead to very significant positioning errors see Annex 3 A3 3 Caution is therefore called for depen ding on the applications for which GPS 16 used This is where EGNOS input is key thanks to permanent monitoring of the GPS constellation it is able to assign a confidence level to the data transmitted to a user and detect GPS satellite faults What EGNOS does 15 transmit estimates of the confidence a user can have in the differential corrections These data are used by the GPS EGNOS receiver to work out the protection levels The following graph shows for a fixed receiver at a known position that the vertical protection level VPL shown in green protects the user properly by delimiting the actual vertical errors in blue The p
30. 297x689x306 183x89x190 297x69x306 240x120x50 240x120x50 100x80x17 100 106 7 12 7 100x60x11 6 100x84 9x11 6 261x140x55 10x11x12 16x12 2x2 13 17x22 4x2 13 19x19x2 54 19 19 2 54 19 19 2 54 26x26x6 19x19x2 54 1148x56x216 215x99x77 215x99x77 215x99x77 234x99x56 234x99x56 106x40x146 240x120x50 106x40x146 129x74x30 129x74x30 129x74x30 129x74x30 A76x100x50 240 120 50 240 120 50 Interfaces General Information Interfaces 135x85x240 135x85x240 190x112 190x112 i 5 m 190x100 m 6 5x8x1 2 17x22 4x2 4 17x22 4x2 4 17x22 4x24 17x22 4x2 4 17x22 4x2 4 10 1x9 7x2 5 5 10 1x9 7x2 5 10 1 9 7 2 5 16 12 2 2 4 5 El 16x12 2 2 4 4x4 0 85 5 8x8x0 85 3 4x4x0 85 5x6x1 1 8x8x0 85 8 0 85 Caption Available Not stated Not Applicable EXAMPLES OF PRACTICAL APPLICATIONS This section gives four practical examples of EGNOS applications The first application illustrates the advantages of using EGNOS for precision farming The second application explains how to use EGNOS to create a time distribution system The third application shows how EGNOS is used through the SISNeT service Lastly the fourth application illustrates the use of EGNOS s integrity mechanism
31. 4 bit word 01010011 10011010 11000110 spread over three successive messages which enables the initial part of the data to be synchronised during the acquisi tion phase E 5 EGNOS messages The next 6 bits identify the message type 0 to 63 The subsequent 212 bits correspond to the useful data contained in the message which are specific to the message type see section 6 The last 24 bits correspond to the parity bits which ensure that the data were not corrupted during transmission no bit error DIRECTION OF DATA FLOW FROM SATELLITE MOST SIGNIFICANT BIT MSB TRANSMITTED FIRST 250 BITS 1 SECOND 24 BITS L1 212 BIT DATA FIELD PARITY En 6 BIT MESSAGE TYPE IDENTIFIER 0 63 8 BIT PREAMBLE OF 24 BITS TOTAL IN 3 CONTIGUOUS BLOCKS FIGURE 11 Messages type structure 5 4 MESSAGE VALIDITY PERIOD The EGNOS system is designed to provide users with the most up to date integrity parameters and differential corrections However EGNOS allows for the possibility of a user not being able to receive all the messages due for example to an erroneous bit In such a case in order to guarantee system performance certain users need to apply degradation models to the information supplied for example aircraft in precision approach phase Validity period Validity period Validity period RE Re eee Refresh period Refresh period Refresh period
32. AVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS General Information Interfaces Applications 120x120x40 54 7x129 5 109x71x28 71 1x40 6x12 109 2x71 1x16 375x105x25 458x113 37 71 1x40 6x13 4 160x114x45 160x114x45 160x114x45 160x114x45 188 114 71 178x120x46 600x160x180 417 158 69 189x114x71 101x97x35 101x97x35 101x97x35 236x199 a 254x83 5x12 7 38 1x76 2x20 3 38 1x76 2x20 3 38 1x76 2x20 3 50 8 50 8 25 4 NS NS a NS 148x85x35 148x85x35 148x85x35 109 35 141 109x35x141 109x35x141 109 35 141 m 109 35 141 m 109x35x141 100x80 100x80 m 182x61x190 m 182x61x190 m 132x61x190 Caption Available NS Not stated Not Applicabie JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS JAVAD GNSS John Deere ESHE kv mouse inc Leica Geosystems AG Leica Geosystems AG Leica Geosystems AG Leica Geosystems AG Leica Geosystems AG Leica Geosystems AG Leica Geosystems AG Leica Geosystems AG L
33. DoD had noticed that by using measurements of the Doppler effect of signals emitted by Sputnik it was possible to plot one s position on Earth provided that one knew the satellite s orbital parameters Armed with this discovery in 1958 the US began its first satellite navigation programme named TRANSIT This system which went into operation in 1964 made use of the Doppler effect to establish a position to within an accuracy of 200 to 500 metres but it had a number of disad vantages with only 6 satellites positioning was not possible at any point on the globe 24 hours a day and in some cases it took up to 24 hours to establish a position To overcome these disadvantages the US military began thinking about how to create a more effective system that would make it possible to establish one s position speed as well as the time with great accuracy 24 hours a day at any point on the globe Its research gave rise to the current GPS system or to go by its full name NAVSTAR GPS short for NAVigation System with Time And Ranging Global Positioning System The first prototype GPS satellite was launched in 1978 and the system was declared operational in 1995 with 24 satellites in orbit GPS offered two services the first of which was called precise positioning service only accessible to the US armed forces and to their allies and the second which was called standard positioning service or open service with a degraded performance lev
34. Equipment IS GPS 200 R vision E 08 06 2010 Navstar GPS Space Segment Navigation User Interfaces EGNOS Service Definition Document Open Service Ref EGN SDD OS V1 1 EGNOS Safety of Life Service Definition Document Ref EGN SDD SoL V1 0 88 1 Overview Civil and military GPS When it was originally designed the GPS system was intended for military use However it soon became apparent that there could also be many advantages for the civil user community Today the system s ease of use low equipment costs and accuracy have led to the growth of a considerable market with several tens of millions of devices sold each year The Precise Positioning Service PPS is still reserved for authorised controlled users gene rally military whereas the Standard Positioning Service SPS is available to the international civil community Until 2000 the accuracy of the standard positioning service was deliberately degraded so as to induce 100 metres of error in the horizontal position 95 This degradation known as Selec tive Availability SA was deactivated in May 2000 Access to the services provided by GPS is free for all users They must however have the equipment needed to process the data distributed by the GPS system The cost of such equi pment varies from just a few euros to several thousand depending on the functionality and performance required and the content exploited for example mapping or traffic info
35. H NPA 3 717 for a non integrity probability of 10 3 034 for a non integrity probability of 10 etc 2 2 2 2 d Ce a i 2 4 di dia gt Seast i S E Su east i Sorti T i i l j 2 2 2 2 O O tO ugg tO iair i tropo 101 ANNEX 7 Calculating the Horizontal Protection Level HPL For a navigation solution computed from the least square method is defined by Seast l Seast2 Seast N Snorthl Snorth2 Snorth N 1 6 Sul 502 SUN 5 5 2 St N With cos El sin Az cos El cos Az sin El 1 i line of G and 1 4 gt 0 0 1 wa d w Jan 2 ve 07 represents the variance of fast and long term correction residual error It can be computed this way Cuvee SUDRE En Ee EV 241 if RSS unre 0 55 0 On Where lupre SUDRE Ex t Ene TEn 82 RSS og UMessageTypel0 RSSupre root sum square flag in Message Type 10 model parameter in Message Type 2 6 24 UDRE SUDRE factor for user location if defined in Message Type 27 ou 28 otherwise SUDRE equals 1 see specific point below Efe Ewe Eer degradation parameters for respectively fast correction data range rate correction data long term correction or GEO navigation message data and en route through applications For more clarification refer to Appendix A section A 4
36. Institute of the University of Bern in order to provide data in a single format that has been collected in proprietary formats by different brands of receiver This format is gene rally supported by professional receivers It is also used by IGS servers for supplying GNSS data In this format the GNSS data are provided as text files There are six distinct file types containing Observation data GPS navigation messages meteorological data GLONASS navigation messages navigation messages from the geostationary satellites information on receiver and satellite clocks A description of this format is available free of charge on the University of Bern server ftp ftp unibe ch aiub rinex rinex211 txt 64 E 11 Choosing a receiver NMEA 0183 The National Marine Electronics Association NMEA is an American organisation whose aim is to standardise the interfaces of the electronic equipment carried on ships The defined stan dards include NMEA 0183 which relates to the GPS receiver and which has been adopted by the majority of receiver manufacturers owing to its simplicity and flexibility The aim of this section is not to provide full details of the NMEA standard the complete standard which can be purchased for a fee is available at http www nmea org but to provide important information about SBASs and EGNOS in particular The NMEA 0183 standard specifies both the protocol and the physical link between the rec
37. NeT Typically this architecture can be provided by a smartphone with a Bluetooth connection to communicate with the receiver and an Internet connection over GPRS The correction module can be integrated directly in the smartphone Functions used For this application the EGNOS corrections broadcast via SISNet are used as well as the ephemeris data and parameters of the Klobuchar ionospheric correction model also broadcast SISNeT Receiver constraints For this application it is important for the GPS receiver not to use EGNOS in native mode as the corrections are applied outside the receiver The receiver must be able to send the time and position longitude latitude altitude as well as the list of satellites used to calculate the PVT solution Most commercially available GPS receivers can do this With regard to internet connectivity because the compressed SISNeT messages are compact the mean bandwidth needed is around 500 bits s Therefore an Internet connection via GSM or GPRS is amply sufficient 12 Examples of practical applications Implementation details This section details the implementation of the GPS SISNeT correction module A full descrip tion of the algorithms is given in the DO 229D document addition ESA has placed all the publications describing the use of SISNeT on its website at http www egnos pro esa int sisnet publications html The GPS SISNeT correction module must carry out
38. S located mainly inside and around its service area There are also a few RIMS in Canada French Guiana and South Africa to improve orbit deter mination performance 3 EGNOS architecture FIGURE 8 EGNOS SITES Courtesy ESSP in April 2011 3 2 STEP 2 CALCULATING DIFFERENTIAL CORRECT AND ESTIMATING RESIDUAL ERRORS Consolidation of data and calculation of corrections by means of CPFs and MCCs Data gathered by the RIMS are processed by the Central Processing Facilities CPFs which estimate the differential corrections and integrity information and generate the EGNOS messages 3 34 3 EGNOS architecture E RIMS A P E NLES Stations 2 Network of monitoring stations FIGURE 9 How CPFs work For redundancy and maintenance purposes there are five identical CPFs distributed over four sites known as Mission Control Centres MCCs Two of the CPFs are at Langen Germany one is at Torrej n Spain another at Swanwick United Kingdom and the fifth at Ciampino Italy 3 3 STEP 3 TRANSMITTING THE EGNOS MESSAGES TO USERS VIA THE GEOSTATIONARY SATELLITES Satellite uplinking NLES The Navigation Land Earth Stations NLES receive the EGNOS messages from the CPFs and transmit them to the geostationary satellites for broadcasting to users ensuring synchronisation with the GPS signals Two NLES one active and one providing hot redundancy are deployed for each geostationary satel
39. Use PRN3O 2010314 1 L utilisateur ne doit plus utiliser le les correcbons rapides sont toujours mises o 5 Fast Corrections EGNOS du PRN3O L utilisateur est prot g il recoit en quelques secondes Horloge 2 les corrections pour cette anomalie d horloge GPS stable Ss Panne d horloge du GPS 30 20h02m Corrections EGNOS stables Erreur imm diate sur la mesure 7 m tres minute Dur e totale 12minutes Erreur de mesure max 175 m tres 5 7 i 20 00 20 02 20 04 20 06 20 08 20 10 20 12 FIGURE 25 Fault detection 7 3 AVAILABILITY EGNOS availability is usually calculated in relation to the percentage of time when the protec tion levels HPL and VPL are below their threshold values set for a type of operation by the alarm limits i e HAL and VAL EGNOS is currently available over its service area for 9996 of the time for the civil aviation service APV1 HAL 40m VAL 50m HPL lt HAL and VPL lt VAL for Measured Availability 2010 04 03 00 00 00 2010 04 03 23 59 59 GEO 126 SAAS mags 66396 Y 28 82783823 555 LATITUDE 824 LONGITUDE FIGURE 26 APV 1 service availability provided by EGNOS on 6 February 2011 with the RIMS network then deployed 54 8 LIMITATIONS The implementation of EGNOS has brought users many advantages Its user interface complies with a standard common to all SBASs It should be no
40. alled lonospheric Grid Points IGPs The receiver knows the position of these particular points and the estimated delay for each of them and is thus able to estimate the ionospheric delay for each IPP and therefore each pseu dorange In order to do that the receiver must perform an interpolation between the values provided for the IGPs close to each IPP The receiver takes into account an obliquity factor angle at which the ionosphere is traversed Y grid point 2 grid point 3 2 FIGURE 15 IPP interpolation principle 43 6 How to use EGNOS messages The IGP grid consists of 11 bands numbered 0 to 10 Mercator projection Bands 0 to 8 are vertical and bands 9 and 10 are defined horizontally around the poles there being a total of 1808 IGPs The following figure shows bands 0 to 8 85 p N75 o u 5 22 D SEI BEES du eerie er ur E ps 55 0600000000000000000000000000000000000000000 65 e o o o En o 2 e e o o 8 S75 o ee W140 W100 W60 W20 E20 E60 E100 E140 FIGURE 16 IGP grid In each of the bands 0 to 8 the IGPs are numbered 1 to 201 as shown below FIGURE 17 IGP numbering principle In bands 9 North Pole and 10 South Pole the IGPs are numbered 1 to 192 from West to East and by increasing latitude
41. ation Identifier lonospheric Grid Point International GNSS Service Inertial Navigation System Issue Of Data Fast Ephemeris Issue Of Data Fast Correction Issue of Data lonosphere Issue Of Data lonospheric Pierce Point International Terrestrial Reference Frame International Telecommunications Union Jet Propulsion Laboratory 1 Acronyms MCC MOPS 229D MSAS MTO N NANU NATS NAV EP NAVSTAR GPS NLES NM NMEA 05 PDOP PPP 1PPS PPS PRN RAIM RDS RF RIMS RINEX RTCA RTCM RTK Monitoring and Control Center Minimum Operational Performance Standards for Global Positioning System Wide Area Augmentation System Airborne Equipment version D Multi functional Satellite Augmentation System Message Type 0 Notice Advisory to NAVSTAR Users National Air Traffic Services Navegac o A rea de Portuga NAVigation System with Time And Ranging Global Positioning System Navigation Land Earth Station Not Monitored National Marine Electronics Association Original Equipment Manufacturer Open Service Performances Assessment And Check out Facility Position Dilution of Precision Precise Point Positioning 1 Pulse Per Second Precise Positioning Service Pseudo Random Noise Position Velocity and Time Receiver Autonomous Integrity Monitoring Radio Data System Radio Fr quency Ranging and Integrity Monitoring Stations Receiver INdependant Exchan
42. ation Civile Differential GPS Department of Defence Dilution of Precision Do not Use European Commission European Civil Aviation Conference EGNOS Data Access System European Economic Interest Grouping European Geostationary Navigation Overlay Service Ente Nazionale di Assistenza al Volo EGNOS Network Time European Space Agency European Satellite Services Provider EGNOS Wide Area Network Fault Detection Fault Detection and Exclusion Framework Programme for research and technological development 84 1 Acronyms G GAGAN GBAS GDOP GIVD GIVE GIVE GLONASS GNSS GPGGA GPGLL GPGSA GPGSV GPRS GPS GSM H HAL HDOP HPL ID IGP IGS INS IODE IODFj IODP IPP ITRF ITU J JPL GPS And GEO Augmented Navigation Ground Based Augmentation System Geometric Dilution Of Precision Grid lonospheric Vertical Delay Grid lonospheric Vertical Error GIVE indicator Global naya Navigatsionnaya Sputnikovaya Sistema Global Navigation Satellite System Global positioning system fixed data Geographic position latitude longitude GNSS DOP and active satellites GNSS satellites in view General Packet Radio Service Global Positioning System Global System for Mobile Communications Horizontal Alert Limit Horizontal Dilution of Precision Horizontal Protection Level International Association of Geodesy Geodetic Reference System International Civil Aviation Organiz
43. by EGNOS Message type 0 content During these test phases the content of MTO is nevertheless identical to that of MT2 and can therefore be used in the same way i e fast corrections will be provided the only difference being the message identifier The message is therefore known as MT0 2 MTO may also be transmitted when a major problem occurs and the entire system becomes unavailable When that happens the MTO content is completely empty and EGNOS must not be used for any applications at all 5 5 2 What impact does this have on my receiver In the case of non safety of life applications and in order to enable the utilisation of the data transmitted by EGNOS most receivers can process the data contained in MT0 2 Where this is the case you must simply make sure that 2 processing is activated by default in the receiver or that it can be activated by the user see section 11 6 HOW TO USE EGNOS MESSAGES This section describes the main types of EGNOS message A more detailed description of the messages can be found in MOPS DR2 which 1 the official reference 6 1 APPLYING THE PRN MASK Each GPS satellite and each EGNOS satellite has a unique pseudo random noise PRN code which makes it identifiable by the user Message type 1 MT1 contains what is known as PRN mask data This mask enables the size of EGNOS messages to be optimised by showing to which satellites PRN the data contained in the other
44. ccuracy 2 to 4 metres 20 95 see diagram in section 7 1 In addition EGNOS was also designed to free users from the intentional degradation of the civil GPS signal by Selective Availability SA which has been deactivated since May 2000 2 Advantages of EGNOS 2 4 2 Integrity The GPS system s errors or malfunctions may depending on the satellite geometry have serious repercussions for user safety if not detected in time and restrict significantly the range of possible applications Another important EGNOS differentiator is the integrity it delivers Indeed in contrast to GPS for which no guarantee is given EGNOS broadcasts an integrity signal giving users the capacity to calculate a confidence interval alerting them when a GPS satellite malfunctions and is not be used for an application where safety is a factor The data produced and transmitted by EGNOS thus include estimates of GPS satellite orbit and clock errors and estimates of errors due to GPS signals crossing the ionosphere These parameters enable users to evaluate a limit from its position error Four parameters characterise integrity alarm limit protection level integrity risk Time To Alarm TTA If the positioning error exceeds the stated protection level an alarm must be transmitted to the user That alarm must be received by the user within the Time To Alarm limit The probability of an alarm not being transmitted to the user
45. ces Applications 1 2W to 1 6 m 1 3W to 1 7W m 0 854 58x56x11 1 9W to 24W 100x80x13 ASHTECH a NS 190x90x43 a ASHTECH a NS 146x64x29 HANDHELD ASHTECH NS 195x90x46 Bomer ASHTECH 215x200x76 ASHTECH 215x200x76 omer ASHTECH m 35W 190x58x160 Bomer ASHTECH m 190x58x160 HANDHELD ASHTECH 190x90x43 HANDHELD ASHTECH a 190x90x43 a LU ASHTECH 228x188x84 oom ASHTECH mn 0 0 wine mno 0 wom meme 000000 0 00 0 mm u M 2E LM 66 11 Choosing a receiver S 2 Em ES Hemisphere GPS om Hemisphere GPS m Hemisphere GPS om Hemisphere GPS Hemisphere GPS Hemisphere GPS o Hemisphere GPS Hemisphere GPS Hemisphere GPS eomer Hemisphere GPS Hemisphere GPS Hemisphere GPS Hemisphere GPS Hemisphere GPS eomer Hemisphere GPS Hemisphere GPS Hemisphere GPS HANDHELD Hemisphere GPS Hemisphere GPS IFEN GmbH IFEN GmbH Jackson Labs Technolog Jackson Labs Technolog Jackson Labs Technolog Jackson Labs Technolog Jackson Labs Technolog Jackson Labs Technolog Jackson Labs Technolog JAVAD GNSS JAVAD GNSS JAVAD GNSS J
46. ctions are broadcast by EGNOS to correct rapid variations in the ephemeris errors and clock errors of the GPS satellites These corrections are provided in type 2 to 5 messages Message type 2 contains the data for the first 13 satellites of the mask that have the same IODP Issue Of Data PRN value Message type 3 contains data on satellites 14 to 26 of the mask that have the same IODP value and so on If the number of satellites in the mask or in the remaining part of the mask is less than 6 type 2 to 5 messages can be replaced by a message type 24 The structure of type 2 to 5 messages 1 as follows DIRECTION OF DATA FLOW FROM SATELLITE MOST SIGNIFICANT BIT MSB TRANSMITTED FIRST E SS y IODP 2 BITS S REPEAT FOR 12 gt REPEAT FOR 12 MORE SATELLITES UDRE MORE SATELLITES PRG 212 BIT DATA FIELD l 12 BIT FAST CORRECTIONS gt 13 4 BIT UDREIs 24 BITS IODF 2 BITS 6 BIT MESSAGE TYPE IDENTIFIER 2 3 4 amp 5 8 BIT PREAMBLE OF 24 BITS TOTAL IN 3 CONTIGUOUS BLOCKS FIGURE 20 Format of MT2 to 5 fast corrections Type 2 3 4 and 5 messages also contain a parameter enabling the accuracy of corrections to be estimated known as UDRE or rather UDRE indicators UDREi UDREi CA re Status of satellite 0 0 0520 OK 1 0 0924 2 0 1444 8 0 2830 OK 4 0 4678 OK 5 0 8315 OK 6 1 2992 OK 7 1 8709 8 2 5465
47. duct s main circuit board It is an ideal solution for prototyping embedded applications The unit cost is relatively high between 10 and 100 depending on the model 62 11 Choosing a receiver 1111111111122412 FIGURE 28 OEM version of the receiver source Faxtrax OEM Original Equipment Manufacturer version consists of the bare receiver without casing then needs to be integrated in the casing that will house the application This is also a good solution for quickly producing prototypes with embedded solutions The price is in the same range as for the auxiliary card versions FIGURE 29 Stand alone receiver source Thales Stand alone consists of a complete receiver which comes in a number of different forms portable rackable etc Prices vary from a few tens of dollars to several thousand dollars for professional receivers What is meant by WAAS Capable and WAAS Enabled When selecting a receiver it is essential to check that it supports the information generated by EGNOS and to understand how this 16 taken into account In particular it is important to identify how the MTO message is interpreted what kind of corrections are used and above all in the event that some of the calculations relating to EGNOS corrections are performed outside the receiver to ascertain whether the EGNOS message 1 available as output from the receiver The possibility of excluding a satellite used for tests mu
48. e 2 0 1331 Use 4 02079 Use 5 0 2994 Use 6 0 4075 Use 0 5322 8 0 6735 9 0 8315 Use 10 1 1974 Use 11 1 8709 Use 12 3 3260 Use 13 20 7870 Use 14 187 0826 Use 15 Not Monitored Not Monitored On the basis of GIVD and oGIVE data provided for each GPS satellite in sight and by applying an obliquity factor calculated from the elevation of the corresponding satellite user s view the receiver obtains a slant range correction and a standard deviation value for the residual ionos pheric error written Ope Note An EGNOS receiver will usually automatically calculate ionospheric corrections for details of the calculations to be done refer to Annex 6 6 2 4 Long term corrections Long term corrections are broadcast by EGNOS to correct long term variations in the ephe meris errors orbit parameters and oz and clock errors Sa of the GPS satellites These corrections are provided in type 25 messages long term satellite error corrections DIRECTION OF DATA FLOW FROM SATELLITE MOST SIGNIFICANT BIT MSB TRANSMITTED FIRS 250 BITS 1 f V LociTy coot no BER IODP SECOND HALF OF MESSAGE o 10 ee OF DATA SEE 1 PRN MASK NUMBER 6 BIT MESSAGE TYPE IDENTIFIER 25 8 BIT PREAMBLE OF 24 BITS TOTAL IN 3 CONTIGUOUS BLOCKS FIGURE 19 Format of MT25 long term corrections 6 How to use EGNOS messages 6 2 5 Fast corrections Fast corre
49. e EGNOS type 12 message Receiver constraints In this application the receiver is not considered to accept the MT12 corrections sent by EGNOS in native mode but it can nevertheless calculate a PVT solution using GPS data and EGNOS s fast long term and ionospheric corrections MT2 5 6 18 24 and 26 The receiver must be able to supply the complete EGNOS message to the master clock application The UTC correc tion is performed in this application 12 Examples of practical applications Implementation details The microcontroller must initialise the receiver s parameters in accordance with the manufac turer documentation so that it can use EGNOS data ignores the type 0 message where necessary excludes if possible the use of the geostationary satellite used for EGNOS testing Artemis in 2008 issues the EGNOS message as well as the PVT solution Once this initialisation has been completed the microcontroller has to accept the messages from the receiver and wait for it to issue a PVT solution that takes EGNOS into account the protocols indicate whether the solution has been calculated with GPS alone or with an SBAS satellite and an EGNOS message When the microcontroller has received the PVT solution and the EGNOS message it must then extract the type 12 message Section 6 4 and correct the UTC time as described in Annex 8 The UTC time obtained can then be sent to the slave clocks correcting wh
50. eica Geosystems AG NAVIS AVIS NAVIS NAVIS NAVIS Navman Wireless OEM Navman Wireless OEM Navman Wireless OEM Navman Wireless OEM Navman Wireless OEM Navman Wireless OEM NAVSYNC NavSys Corporation NOVATEL NOVATEL NOVATEL NavCom Technology Inc NavCom Technology Inc NavCom Technology Inc General Information Applications lt 20W 0 3W to 0 9 1 24 1 20 mw 56 mW 80 mW 56 mW 125 mW 28 mW lt 69mW lt 145mW NS 132x651x190 182x61x190 182x61x190 182x61x190 182x61x190 182x61x190 182x61x190 182x61x190 182x61x190 182x61x190 100x80 100x80 100x80 100x80 55x40 55x40 57x66 57x66 178x96x178 NS NS 1524x1676x889 212x166x79 220x200x94 212x166x79 186x89 196x198 278x102x45 323x125x45 278x102x45 323x125x45 120 100 11 203x111 164x117x60 240x140x73 78 7x53 6 35x35x7 50x75x15 90X96X15 11 11 2 25 25 4 25 4 3 71 1x40 6x10 17 15 2 7 30x30 11 11 2 25 21x16 4x2 4 21x16 44x2 52 185x162x76 235 154 71 Interfaces NOVATEL NOVATEL NOVATEL NOVATEL NOVATEL NOVATEL NOVATEL NOVATEL NOVATEL NOVATEL NOVATEL NOVATEL NOVATEL NOVATEL NOVATEL SEPTENTRIO SEPTENTRIO SEPTENTRIO SEPTENTRIO SEPTENTRIO SEPTENTRIO SEPTENTRIO SEPTENTRIO SEPTENTRIO SEPTENTRIO
51. eiver and the host equipment The latest version of the standard is 3 01 which was published in January 2002 With regard to NMEA 0183 v3 01 the standard specifies the use of an RS232 type link a baud rate of 4800 8 bit no parity 1 stop bit 8N1 An addendum to this standard NMEA 0183 HS v1 01 specifies a rate of 38400 baud The data from the receiver are sent as data packets containing a maximum of 80 characters The receiver can send a maximum of 6 packets per second due to the transfer rate Data are encoded as directly readable ASCII characters These packets are referred to by the stan dard as sentences The NMEA protocol is bi directional Not only can data be received but it can also be sent to the receiver NMEA 0183 standardises a certain number of sentences all beginning with GP In addition some manufacturers add specific sentences to their products identified by PXXX where XXX is a manufacturer s code allocated by the NMEA association for example SRF for SirF SSN for Septentrio The list of codes is available free of charge from the NMEA website Most of the time manufacturers of GPS equipment do not implement all of the sentences Nevertheless the receivers transmit the six main sentences GGA GLL GSA GSV RMC and VTG 11 Choosing a receiver Message name Description Global positioning system fixed data This message gives lati GPGGA tude longitude altitude and time the HDOP and number
52. el accessible to all civil users without restriction Today in 2011 GPS is still operational with some 30 satellites currently in orbit A new gene ration of satellites is currently being developed Block III with a view to further improving the system s performance In the 1980s during the Cold War the Soviet military aware of the strategic importance of possessing a satellite navigation system came up with its own answer to the GPS system in the shape of GLONASS short for GLobal naya NAvigatsionnaya Sputnikovaya Sistema a system with similar objectives and performance to GPS offering a means of precisely determi ning ones position anywhere on the planet GLONASS was declared operational in 1996 But after some ups and downs due to technical problems and a lack of funding in the wake of the Cold War the GLONASS system went into stagnation and with no more than 6 operational satellites was unable to offer any real availability In 2002 the Russian Federation decided to relaunch the programme and is now studying a new generation of satellites with a view to having a fully operational system by 2015 In 2011 the GLONASS system is approaching its nominal configuration 1 Why do we need EGNOS Europe in turn aware of the strategic importance of satellite positioning systems for its economy and independence decided to develop a satellite navigation system of its own under civil control and began the first studies to that end
53. erational since 1 October 2009 for non sensitive uses that do not jeopardise human life The EGNOS Safety of Life SoL service which can guide aircraft on their approach flight path was opened on 2 March 2011 Most commercially available GPS receivers currently receive and use EGNOS signals thus permitting the implementation of a a great number of applications or various types of experiments The purpose of this guide is to provide practical information to EGNOS users SMEs scientific laboratories application developers etc who are not specialists in the use of the system It therefore is addressed primarily at those outside the aviation community which has been involved in the development of EGNOS from the outset and is familiar with its use It aims to answer questions such as can EGNOS enhance my application How in practice can use EGNOS signals and messages etc It then explains how you obtain the latest up to date information on EGNOS and on evolutions of the system and gives advice on how to choose a receiver that makes best use of EGNOS functionalities Finally some specific examples of applications are provided which serve to illustrate its use 1 WHY DO WE NEED EGNOS 1 1 SATELLITE NAVIGATION SYTEMS FROM TRANSIT TO GALILEO The USSR could never have imagined when launching the satellite Sputnik in 1957 that in doing so it would be giving the USA the idea for GPS But the US Department of Defense
54. ere necessary any delays due to calculation or data transmission These operations are repeated according to the refresh period for the PVT solution sent by the receiver In most cases this period is one second 12 3 USING SISNET Context of the application In certain constrained environments GPS and EGNOS signals may be difficult to acquire For example when a vehicle is driving along a road hemmed in by rows of tall buildings the vehi cle s onboard receiver may have difficulty picking up the satellite signals Positioning in these environments which are known as urban canyons is poor The use of SISNeT see Section 4 1 compensates for this EGNOS reception problem This kind of environment also generates a lot of multipath errors whose effects can only be dealt with using receiver level techniques RAIM to identify and exclude erroneous measurements and multipath rejection algorithms 78 E 12 Examples of practical applications Architecture The system consists of a GPS receiver that communicates with equipment running the EGNOS SISNeT correction software GPS SISNeT correction module as shown in the diagram below The equipment consists of an interface that is compatible with the GPS receiver an Internet connection and an interface for sending the corrected positions to the application SISNET Internet 5 GPS SISNET me GPS Receiver ne Application FIGURE 32 Architecture ofthe connection to SIS
55. erpolate the ionospheric correction and its variance This operation is carried out using information provided in the ionospheric mask and must be done while taking into account whether the IGP is moni tored not monitored or do not use If one of the IGPs is identified as not monitored inter polation is done within a triangle containing the IPP If two of the IGPs are not monitored the interpolation cell must be widened All the IGP selection rules are given in section A4 4 10 2 of Annex A to DO 229D DR2 All the rules for interpolation of the IPP s vertical ionospheric delay and its variance are given in section A4 4 10 3 of Annex A to DO 229D DR2 grid point 2 grid point 3 2 1 M App A2 A FIGURE 39 Principle of interpolation of the IPPs 6 Calculating ionospheric corrections Once the user has calculated the vertical error for the IPP he must then multiply this vertical error the Obliquity Factor F_ to obtain the ionospheric correction IC to add to the pseudorange measurement Brp App Fpp Delir dy in which m is defined as follows 1 2 R cos E Mills RS a ES o ye I then calculated as follows F o 2 O UIRE UIVE 5 5 2 2 5 5 o 40 60 Elevation FIGURE 40 Change in Obliquity Factor according to Elevation INNEX 7 Calculating the Horizontal Protection Level
56. es used by the receiver Note that the satellite number uses NMEA identifiers NMEA 105 The following correlation table must therefore be used Satellite ID ARTEMIS 124 37 INMARSAT 3 F2 120 33 INMARSAT 3 F5 126 39 In the example below the INMARSAT 3 F2 EGNOS PRN 120 satellite is being tracked Field Example Description Sentence ID GPGSA Mode 1 A A Auto 2D 3D M Forced 2D 3D Mode 1 3 1 fix 2 2D 3 3D Satellite used 1 01 Satellite used on channel 1 Satellite used 2 20 Satellite used on channel 2 Satellite used 3 19 Satellite used on channel 3 Satellite used 4 13 Satellite used on channel 4 Satellite used 5 33 Satellite used on channel 5 Satellite used 6 Satellite used on channel 6 Satellite used 7 Satellite used on channel 7 Satellite used 8 Satellite used on channel 8 Satellite used 9 Satellite used on channel 9 Satellite used 10 Satellite used on channel 10 Satellite used 11 Satellite used on channel 11 Satellite used 12 Satellite used on channel 12 PDOP 40 4 Position dilution of precision HDOP 24 4 Horizontal dilution of precision VDOP 32 2 Vertical dilution of precision Checksum Terminator CR LF 5 Caution this is not enough to deduce whether EGNOS is actually being used It may be that an EGNOS satellite is being tracked by the receiver i e the
57. f Life service which has been awarded by European Commission in March 2011 2 ADVANTAGES OF EGNOS N 2 1 ADVANTAGES OF EGNOS As stated above as it currently stands the EGNOS system enables users with an EGNOS compatible GPS receiver to improve positioning accuracy by a factor of two to three to have integrity data for validating the signals transmitted by GPS satellites they have confidence thresholds regarding the calculated position and are alerted in near real time less than 6 seconds of any data reliability shortcomings to benefit from accurate and reliable synchronisation with UTC to improve availability te to date the functionality of providing additional pseudorange measurements Geostationary satellite P d Ranging Differential Estimate of Integrity GPS type corrections residual Use Don t Use signals positioning error l 4 Accuracy Integrity Availability Continuity FIGURE 4 EGNOS functionality from geostationary satellites has not been activated To benefit from the advantages provided by EGNOS users need simply use an EGNOS compa tible GPS receiver Thanks to the broadcasting of signals that are compatible and interoperable with GPS signals with frequency and modulation identical to GPS these receivers differ very little from standard GPS receivers and do not require a communications connection to refe 2 2 rence stations HOW TO USE EGNOS
58. f the geometry of the 14 visible satellites me positioning precision error 2 54 m pee positioning precision error 5 08 m TABLE 2 GPS error assessment typical orders of magnitude Note Typical orders of magnitude are shown with actual results depending on the conditions encountered in particular status of GPS constellation place date and time of day elevation of satellites above the horizon possible masking of satellites by obstacles reflection of signals onto obstacles behaviour of the ionosphere and troposphere age of broadcast orbit and clock data etc The sum of these errors enables an estimator known as UERE User Equivalent Range Error to be determined which corresponds to the accuracy of the distance measurement between the user and each satellite Calculation of position the receiver Using pseudoranges the satellites orbital parameters and the error corrections the receiver can calculate a position to within ten metres expressed in longitude latitude and altitude in the World Geodetic System 1984 standard WGS84 The number and position of the satellites used by the receiver affect the accuracy of the tion The satellites must be geometrically well distributed in order to minimise position error as illustrated in the figure below Good geometric distribution Poor geometric distribution FIGURE 36 Impact of the geometric distribution of the satellites La r partition g om
59. ficantly increased accuracy and enables him to define a level of confidence corresponding to his position 12 Examples of practical applications SP tay Harbour wall Harbour wall Harbour wall FIGURE 33 Example of a port environment The level of confidence known as the HPL is a circle centred on the ship s current position that assesses the risk to the ship to be 10 7 for every 150 seconds that it remains inside the circle EGNOS has been designed to guarantee the position within a maximum radius of 40m horizon tally 99 of the time In the event of an anomaly in the GPS constellation EGNOS warns the navigator within 6 seconds that his position can no longer be guaranteed This system enables the navigator to enter the port in full safety without risking a collision with the harbour walls Advantages of EGNOS EGNOS helps to improve accuracy by correcting the measurements in the GPS signals and in particular by providing the integrity service Architecture GPS EGNOS Rawmeasirements Navigation Receiver computer FIGURE 34 Functional architecture of the terminal 80 12 Examples of practical applications Functions used All the messages distributed by EGNOS as described in section 6 3 are used in this application particularly the o parameters udre give Receiver constraints In this application it is considered that the receiver does not calculate the radius of protection by
60. for the geostationary satellites The industrial prime contractor role for EGNOS has been given to France s Thales Alenia Space EGNOS funding up until the exploitation phase was provided by ESA the context of its Artes 9 programme by the EU through its TEN T budgets and 5 and 6 R amp D Framework Programmes and by air navigation service providers AENA Spain DFS Germany DSNA France ENAV Italy NATS United Kingdom Skyguide Switzerland and NAV EP Portugal These service providers joined forces in 2001 to set up a European Economic Interest Grouping EEIG christened ESSP European Satellite Services Provider which allowed it to become EGNOS operator and to coordinate the activities of its members in providing operations and system maintenance tasks ESSP initially headquartered in Brussels transferred to Toulouse in 2008 in doing so becoming ESSP SAS a limited liability company under French law 22 1 Why do we need EGNOS In 2005 under contract from ESA ESSP started the initial operation phase of EGNOS with a view to its qualification In April 2009 system ownership was transferred to the European Commission now in charge of the contracts for the exploitation and maintenance of the system which is expected to have a nominal exploitation of at least 20 years In July 2010 ESSP went through a process of certification to become an Air Navigation Service Provider first step to declaration of Safety o
61. ge Radio Technical Commission for Aeronautics Radio Technical Commission for Maritime Services Real Time Kinematic 85 86 ANNEX 1 Acronyms 5 SBAS SISNeT SME SOL SPS T TCP IP TDOP TGD TTA TTL U UDRE UDREI UERE UTC UTM v VAL VDB VDOP VHF YPL WAAS WGS84 WiFi WN XAL XPL XEP Selective Availability Satellite Based Augmentation System Signal In Space through the interNET Small or Medium Enterprise Safety Of Life service s curit de la vie Standard Positioning Service Transmission Control Protocol Internet Protocol Time Dilution of Precision Time Group Delay Time To Alarm Transistor to Transistor Logic User Differential Range Error User Differential Range Error Indicator User Equivalent Range Error Universal Time Coordinated Universal Transverse Mercator Vertical Alarm Limit VHF Data Broadcast Vertical Dilution of Precision Very High Frequency Vertical Protection Level Wide Area Augmentation System World Geodetic System 1984 Wireless Fidelity Week Number Horizontal or Vertical Alarm Limit Horizontal or Vertical Protection Level Horizontal or Vertical Protection Level DR1 DR2 DR3 DR4 DR5 2 References GPS SPS Performance Standard 4th edition September 2008 RTCA MOPS DO 229D 12 13 2006 Minimum Operational Performance Standards for Global Positioning System Wide Area Augmentation System Airborne
62. h of the four satellites PR represents the pseudoranges measured for each of the four satellites b is the clock bias between the receiver and the satellites is the speed of light Xu Y et Z represent the coordinates to be calculated of the receiver s position 77779 Qp Q 1suuu 2 1 1 Why do we need EGNOS Satellite positions and clocks Each satellite transmits a constant stream of information in the form of a navigation message which can be used to precisely determine its position in space at a given time T This informa tion is known as almanac and ephemeris data Almanac data consist of parameters which allow a medium term estimate to be made of the position of all the satellites as a function of the time They are used during the acquisition phase to identify those satellites that are visible Ephemeris data consist of a set of parameters describing very accurately the orbit of a satel lite as a function of the time making it possible to calculate the satellite s position at a precise moment t to within about 1 metre The navigation message also includes data which can be used to correct certain errors such as clock corrections for the satellites How the receiver calculates its position Using pseudoranges the satellites orbital parameters and error correction a receiver can calculate a position to within several metres generally expressed in
63. hin UDRE parame ters values 14 and 15 see section 6 2 5 lonosphere alerts EGNOS also transmits for each IGP being monitored an integrity signal with three values and showing its status if an anomaly is detected or if it is not being monitored However the Do not Use alert is generated through the maximum value of the GIVD ionos pheric delay not by a particular GIVE value As with the satellite alerts the system has 6 seconds in which to inform the user of any integrity fault Again the alert is repeated 4 times To 6 How to use EGNOS messages Protection levels The parameters transmitted to estimate the accuracy of the corrections GIVE and GIVD enable the receiver to compute horizontal and vertical protection levels see section 2 4 2 Generally only receivers used for aviation purposes calculate and automatically generate protection levels However the entire set of parameters needed to calculate them is broadcast in particular through type 2 to 5 6 24 18 and 26 messages for details on the calculations to be done refer to Annex 7 6 3 2 Message type 6 a special case Type 6 messages are used in two instances e to refresh UDRE indicators UDREi to be able to broadcast satellite alerts very quickly if necessary DU It should be pointed out that although UDREi are contained in messages 2 to 5 with the fast differential corrections their validity period may require more frequent updating
64. hree space platforms carry no signal genera tors They are fitted with a transponder which does nothing more than relay the signal processed on the ground and sent into space As with GPS satellites each EGNOS satellite is allocated a unique PRN Pseudo Random Noise number which allows it to be identified by the user The NMEA standard used in output mode by most commercially available receivers allocates a unique identifier to each EGNOS satellite as described in the table below As a general rule 2 satellites out of the 3 available are used operationally for the broadcasting of the EGNOS message the 3rd being used for the purposes of maintenance testing and vali dation The table below describes the situation at the time of writing of this guide but that situa tion is subject to constant change The reader is therefore advised to go to the ESSP or ESA websites see section 09 for the list of links from which to obtain operational situation pertaining to the EGNOS satellites Satellite ARTEMIS INMARSAT AOR E INMARSAT IOR W FIGURE 6 EGNOS satellite coverage It should be noted that geostationary satellites due to being positioned in the equatorial plane are vertically above a user located at the Equator Therefore the further a user travels towards the poles towards high latitudes the more the satellite drops down towards the user s horizon When the satellite is too close to the horizon
65. ic Time TAI by an integral number of seconds 2 4 4 EGNOS Reference frame Though very close EGNOS corrections are not directly referenced to GPS terrestrial reference frame WGS84 but are periodically aligned on ITRF International Terrestrial Time Frame in order to provide a consistency of an order of a few centimetres Therefore this means that for most applications positions provided by an EGNOS receiver can be used in WGS84 frame including GPS cartography databases 2 5 COVERAGE Unlike the GPS system which uses dedicated satellites on medium orbits at roughly 20 000 km altitude in 6 different orbital planes the EGNOS system uses payloads on board 3 telecommu nications satellites placed in geostationary orbit at an altitude of 36 000 km Note geostationary orbits are geosynchronous orbits having a period of revolution identical to that of the Earth in the equatorial plane with the result that a satellite following that orbit always appears stationary relative to any point on the Earth s surface 28 2 Advantages of EGNOS As things stand in 2011 the EGNOS signal is broadcast by three geostationary satellites two Inmarsat and ESA s Artemis satellites positioned above Africa and East of the Atlantic These three satellites orbits are in the equatorial plane at three different longitudes with each able to broadcast EGNOS services across the whole ECAC area Unlike the GPS and GLONASS satellites these t
66. ising GBAS for category Il and precision approach which is likely to be operational as of 2015 2020 Ban 1 Why do we need EGNOS 1 3 1 3 Other ground based augmentation systems RTK Real Time Kinematic This technique is based on a principle similar to that of DGPS with a single reference station and a means of communication between the receiver and the station but in this case it is not corrections that are transmitted but raw data These raw data then enable specialised receivers to calculate the satellite to receiver transit time based on the phase of the wave received and not on the code sequence This method which requires more complex receivers makes it possible to achieve accuracy of roughly 3 to 5 cm conditional upon being within a distance of up to 100km from the reference station It also takes considerable time to initialise and requires dual frequency receivers A variant of this method known as interpolated RTK makes it possible to achieve even greater accuracy by using a denser network of reference stations in France for example the Teria Orpheon and Sat Info networks In this case the errors in the receiver measurements are interpolated with measurements carried out by the stations situated around the user PPP Precise Point Positioning The Precise Point Positioning method PPP is a different approach which makes use of undif ferentiated code and phase observations from a single or dual frequency rece
67. itis no longer usable As regards EGNOS beyond latitude 75 the service becomes barely usable Sometimes it is necessary to calculate the elevation of the EGNOS geostationary satellites relative to one s position to see whether they will be visible in the area intended for the use of the application Annex 4 details the method for calculating the elevation of the geostationary satellites relative to one s position 3 EGNOS ARCHITECTURE EGNOS service provision requires the following steps Step 1 Collection of measurements and data from the GPS satellites Step 2 Calculation of differential corrections estimation of residual errors and generation of EGNOS messages Step 3 Transmission of EGNOS messages to users via the geostationary satellites A data integrity verification process is conducted in parallel with these steps EGNOS like GPS consists of three segments a space segment which comprises the payloads of the three satellites a ground segment which is composed of the terrestrial infrastructure and a user segment made up of all the receivers EGNOS also includes a support segment consisting of the following two entities The Performance Assessment and Checkout Facility PACF which serves to coordinate operations and maintenance and monitors the functioning of the system The Application Specific Qualification Facility ASQF which provides applications support and the user interface The operati
68. itself this is done by the navigation computer In this case the receiver must be able to provide all the raw GPS data pseudoranges navigation messages and all the EGNOS messages Implementation details The receiver only provides the computer with the raw GPS and EGNOS data It is not designed to provide a position The stages involved in implementing this system are as follows Correct the pseudoranges for each GPS satellite using the EGNOS messages and exclude the pseudoranges for satellites declared by EGNOS as Do Not Use and Not Monitored Calculate a PVT solution using these corrected pseudoranges Calculate in parallel the HPL by following the stages described in Annex 7 Display for the user the position HPL and if necessary any integrity alarms 1 Acronyms AAIM ABAS APV ASCII ASQF CDDS Aircraft Autonomous Integrity Monitoring Aircraft Based Augmentation System APproach with Vertical guidance American Standard Code for Information Interchange Application Specific Qualification Facility Commercial Data Distribution Service CE GPS CNES CONUS CPF DAB DFS DGAC DGPS DoD DOP DU EC ECAC EDAS EEIG EGNOS ENAV ENT ESA ESSP EWAN FD FDE FP Compl ment Europ en du GPS European Complement to GPS Centre National d Etudes Spatiales Continental US Central Processing Facility Digital Audio Broadcast Deutsche Flugsicherung GmbH Direction G n rale de l Avi
69. iver This method is principally used in deferred time since it requires correction data to be received PPP uses these precise orbital data and clock corrections to calculate an extremely accurate absolute position static or kinematic to the decimetre or even centimetre in kinematic mode using precise IGS products available with 3 weeks delay Unlike with RTK common errors the effect of tides or ocean loading for example are not eliminated Obtaining a position that is both absolute that is not relative to a reference station and extremely accurate makes it possible to observe phenomena such as Earth tides or crustal deformation under the influence of ocean loading Some commercial service providers Omnistar Starfire Veripos etc referred as GSBAS Global Satellite Based Augmentation System offer commercial real time correction products broadcast via geostationary satellites carried out thanks to a global sensor stations network The claimed optimal precision is decimetric 1 3 2 Receiver level technologies RAIM RAIM Receiver Autonomous Integrity Monitoring is an algorithmic technology for improving integrity based on the use at receiver level of redundancy of the available GNSS pseudoranges allowing comparison between the positions established by different groups of four satellites within visual range 7p tuUq177 OcUm ge 1 Why do we need EGNOS RAIM can function in two way
70. jection Details on the different models as well as the conversion tools can be found on the website of the French National Geographical Institute IGN http www ign fr A3 3 Integrity and availability of the GPS system The command and control segment of the GPS system manages satellite unavailability periods Each one leads to a report known as the NANU Notice Advisory to NAVSTAR Users being published by the United States Coast Guard These reports are available at http www navcen uscg gov GPS nanu htm Over the last decade there have been about one or two of these satellite unavailability periods per year and per satellite Although limited these unavailability periods can place severe constraints on the system s use A number of unexpected malfunctions of the GPS system have been recorded including the following Problems with the satellite clocks as in July 2001 or January 2004 where the failure of the PRN23 clock resulted in a range error of 285 m before the satellite was identified as unhealthy by the system Signal modulation errors in 1994 when problems of signal distortion led to vertical errors of 2 to 8 metres Errors in transmitting the ionospheric model such as were observed from 28 May to 2 June 2002 Incorrect modelling of satellite orbits as observed from 12 to 22 March 1993 which caused distance errors of more than 40 metres Undeclared orbital manoeuvres in 1995 where during an i
71. lite making six NLES in total Distributing data to users the geostationary satellites EGNOS messages received by the three geostationary satellites are transmitted directly to users The message sequences differ between the three satellites 4 OTHER WAYS OF ACCESSING EGNOS 4 1 SISNET SIGNAL IN SPACE THROUGH THE INTERNET SISNeT is a service offered by the European Space Agency ESA available on the internet enabling EGNOS differential corrections and integrity information to be accessed in real time It is a free service but users need to register with the European Space Agency For details go to http www egnos pro esa int sisnet uas html The service is normally used via mobile internet connections GSM GPRS etc The messages sent are EGNOS messages The service makes it possible to Receive EGNOS messages even when the receiver does not have the EGNOS function However it must be able to use the data transmitted by SISNeT that is integrate the diffe rential corrections and integrity information Access EGNOS data in areas where the geostationary satellites are masked Important this system is available only if the receiver has internet access capability Regarding coverage all mobile telephony operators offer access to the internet in Europe The SISNeT service is therefore accessible almost everywhere in Europe Moreover there is an increasing number of wifi hotspots in towns and cities SISNeT has
72. n orange that enables the non integrity events to be identified If there is no failure in integrity the XPLs should be located on the left of this straight line XPL gt XPE In addition the availability is calculated using the number of samples of XPL that are less than XAL since if XPL gt XAL then no integrity guarantee can be established for the calculated position 70 Vertical Protection Level VPL in m 1 navigation 60 not available 1 1 50 10 20 30 40 50 60 FIGURE 41 Stanford Diagram If XPL XAL grey area The navigation integrity service is unavailable If XPL lt XAL and XPE XPL green area Nominal case of the integrity service XPL XPE XAL orange area Loss of system integrity no data integrity XPL XAL XPE red area Loss of navigation integrity for the user the system sends an alarm to the user in less than 6 seconds 107 Table of contents INTRODUCTION WHY DO WE NEED EGNOS 1 1 Satellite navigation sytems from Transit to Galileo 1 2 GPS How it works its Performance and Limitations 1 2 1 GPS works 1 2 2 Performance 1 2 3 Limitations Augmentation systems 1 3 1 Ground based augmentation systems 1 3 2 Receiver level technologies RAIM 1 3 3 Space based augmentation systems 14 EGNOS ADVANTAGES OF EGNOS 2 1 Advantages 22 How to use EGNOS 2 3 Services terminology 2 4 EGNOS performance levels 2 4 4 Accuracy 2 4 2 Integrity 2 4
73. o four criteria accuracy integrity continuity and availability Accuracy corresponds to the difference between the measured and the real position speed or time value Integrity refers to the confidence the user is able to have in the calculation of the position Integrity includes a system s capacity to provide confidence thresholds as well as alarms in the event that anomalies occur Continuity defines a system s ability to function without interruption throughout the operation the user wants to carry out for example landing a plane Continuity is the probability from the moment that the accuracy and integrity criteria are fulfilled at the beginning of an opera tion that they continue to be fulfilled throughout that operation s entire duration Availability is the percentage of time in which over a certain zone geographical area the accuracy integrity and continuity criteria are fulfilled Note the notion of integrity is also used in computing where it has a different sense in compu ting it is defined as the property of a piece of numerical data having undergone no alteration during its storage or transfer 1 2 3 Limitations 1 2 3 1 Accuracy The accuracy of the GPS system has improved continually over the last few years Nonethe less the accuracy which can be expected today remains in the order of several metres which can prove inadequate for certain applications Vertical positioning in particula
74. obability factor of the order of 10 7 The XPLs can be calculated from certain data supplied by the EGNOS system and the geometry of the GPS satellites used See Annex 7 EGNOS broadcasts parameters which enable users to assess the degree of confidence they can have in the differential corrections and to estimate a limit to their positioning error Say 2 Advantages of EGNOS 2 4 3 Synchronisation with UTC The EGNOS system uses a system time known as ENT EGNOS Network Time linked to UTC Coordinated Universal Time notably through the installation of an EGNOS ground station on the site of the Observatoire de Paris which itself provides UTC reference time for France All the differential corrections broadcast by EGNOS are referenced according to ENT Thus the time obtained by the user when he calculates his position using EGNOS data is also referenced in ENT not in GPS time In addition EGNOS also broadcasts a specific message containing several parameters allowing the receiver to estimate a UTC The user then has a precise reliable time directly synchronised with UTC Section 6 4 and Annex8 describe the way to link ENT time to UTC time The accuracy obtained relative to UTC 16 less than 50 nanoseconds Note UTC Coordinated Universal Time represents a time scale which serves as international reference time It is close to Universal Time UT directly linked to the Earth s rotation and differs from International Atom
75. of visible satellites GPGLL Geographic position latitude longitude This message gives the latitude longitude and time GNSS DOP and active satellites This message gives the list of GPGSA satellites used to calculate the PVT solution as well as information on the geometry of these satellites Dilution Of Precision GPGSV GNSS satellites in view This message gives the elevation azimuth and the signal to noise ratio of the satellites used by the receiver GPRMC Recommended minimum specific GNSS data This message gives the time longitude latitude speed and course GPVTG Course over ground and ground speed This message gives infor mation on the speed and course Annex 5 to this guide explains how to ensure EGNOS is using the NMEA protocol The following table provides a non exhaustive range of EGNOS compatible receivers as well as their characteristics Data from this table are manufacturer ones and have not been tested in the frame of this guide writing Most Mass Market as well as some professional receivers use EGNOS signals and messages but without processing integrity parameters For more details refer to datasheets or ask clarifications to manufacturers You can also consult the receiver list managed by GSA at the following address http egnos portal gsa europa eu developer platform developer toolkit receiver list 11 Choosing a receiver General Information Interfa
76. on GPS 1 2 1 How GPS works The basic principle underpinning satellite positioning is the use of distance measurements at a precise moment in time T between a receiver and several satellites whose exact positions in space are known Pseudoranges The satellites emit electromagnetic waves which are propagated through space at the speed of light It is then possible to calculate the distance separating the satellite from the receiver by determining the time a wave takes to travel from satellite to receiver using the following formula d c t where d is the distance c the speed of light and t the time it takes for the wave to travel from satellite to receiver Ban 1 Why do we need EGNOS To estimate the time that signals take to travel between a given satellite and the receiver the receiver compares a unique code linked to the satellite s navigation signal with a copy of the same code generated by the receiver itself Since the time interval between the codes corres ponds to the transit time this can then be used to calculate the distance or pseudorange The use of pseudo in this term is because this distance does not correspond to the geometric distance between satellite and receiver due to the bias between the time reference used by the GPS system and that used by the receiver as explained below With at least three distance measurements to three different satellites it is theoretically possible to determine
77. on are identical to those of GPS signals In addition the SBAS signal is broadcast by geostationary satellites able to cover vast areas with each error source being isolated Several countries and regions have implemented their own satellite based augmentation System For example the North American SBAS component WAAS Wide Area Augmenta tion System covers the continental United States CONUS Canada and Mexico The Euro peans for their part have EGNOS the European Geostationary Navigation Overlay Service which covers Europe s area while Japan is covered by MSAS Multi functional Satellite Augmentation System Now India and Russia have launched their own SBAS programme respectively named GAGAN GPS And GEO Augmented Navigation and SDCM System of Differentional Correction and Monitoring Note ECAC the European Civil Aviation Conference is an organisation of 44 Member States whose role is to promote intergovernmental cooperation on air transport matters in Europe FIGURE 2 ECAC Member States LL O 20 All of these systems are interoperable and adhere to the RTCA aviation standards MOPS DO229D while at the same time having their own unique characteristics RTCA is an orga nisation that issues standards for civil airborne equipment MOPS 229D Minimum Opera tional Performance Standards for Global Positioning System Wide Area Augmentation System
78. onal components are all interconnected via the EGNOS Wide Area Network EWAN and are designed to transmit data in near real time Processing NLES Stations EGNOS Signal GPS Signal GPS Signal RIMS e 1 Network of monitoring Central stations Facilities NLES Stations FIGURE 7 EGNOS Infrastructure 32 E 3 EGNOS architecture 3 1 STEP 1 COLLECTING MEASUREMENTS AND DATA FROM THE GPS RIMS network To ensure optimum continuous gathering and observing of measurements and data from the various visible GPS satellites and of ionospheric variations a network of observing stations called Ranging and Integrity Monitoring Stations RIMS was set up mainly in Europe The RIMS gather data and transmit them at a rate of 1 Hertz to the computation centres or Central Processing Facilities CPFs for exploitation There are three types of RIMS Type A RIMS supply raw measurements from visible EGNOS GPS satellites These data are used by the CPFs to calculate corrections and estimate confidence thresholds Type B RIMS also supply raw measurements from visible EGNOS GPS satellites These data are used by the CPFs to verify broadcast messages and guarantee EGNOS integrity e Type C RIMS are given over to the detection of specific faults known as evil waveforms a corrupted navigation signal waveform caused by an anomaly on board a GPS satellite The EGNOS system comprises about 40 RIM
79. onospheric storm a satellite swit ched to nuclear detection mode and drifted from its nominal orbit EGNOS can detect these malfunctions in real time and correct them with differential correc tions or Do not Use 93 4 Elevation of a geostationary satellite The following section gives details on how to calculate the elevation of a geostationary satellite according to the user s observation position Note All values must be expressed in radians The formula for converting an angle expressed degrees to radians is Angle radians Angle degrees x Pi 180 Notations and numerical values Radius of the Earth R 6 378 km Apogee of the geostationary orbit AG 35 786 km User s position latitude longitude is lat_user long_user The satellite s position is identified by its longitude long_sat An Eastern longitude 15 positive a Western longitude is negative 1 The user s coordinates are calculated in Greenwich reference time by Xuser R cos lat _ user cos long user Yuser R cos lat _ user sin long _ user Zuser sin lat user 2 The satellite s coordinates are calculated by Xsat R AG cos long sat Ysat R AG sin long _ sat Zsat 0 3 The distance separating the user from the satellite is calculated by D J Xsat Xusery Ysat Yusery Zsat Zusery 4 Elevation of a geostationary satellite 4 The user satellite unit vector is calcula
80. ors or breakdown can also have serious repercussions for user safety if not detected in time and have the effect of restricting the number of possible applications In parti cular they make the system unsuitable for critical applications such as civil aviation or those with regulatory or legal ramifications such as transaction timing automatic billing etc It is with a view to overcoming the limitations of GPS with respect to integrity therefore that augmentation systems have been developed 1 3 AUGMENTATION SYSTEMS The ease of use and round the clock availability at any point on the globe of GPS combined with its unrivalled intrinsic performance have led many users to want to use GPS for specific applications for which it was not initially designed Bo 1 Why do we need EGNOS Among such applications are those for which a high degree of integrity is required aircraft landing command and control systems for trains etc or those for which accuracy to within a metre or below is necessary geodesy ship docking etc To respond to such user demand it was necessary to implement systems to complement GPS which could compensate for certain inadequacies or improve its performance while at the same time continuing to benefit from the technological and operational advancement offered by GPS These complementary systems known as augmentation systems are either made up of ground based or space based infrastructures or otherwise
81. os user support essp sas eu http www egnos pro esa int IMAGEtech imagetech_realtime htm ESA Helpdesk for EGNOS egnos esa int 9 3 USEFUL TOOLS ESA has made a set of tools available to satellite navigation professionals at http www egnos pro esa int GSA provides some tools for application developers available on its website http egnos portal gsa europa eu developer platform developer toolkit CNES also has a dedicated server providing access to an archive of EGNOS messages transmitted by each of the geostationary satellites http sis perfandata cnes fr 10 UPGRADES Since March 2011 EGNOS system has been operational for aeronautical navigation thanks to opening of SoL Safety of Life mode With the aim ofimproving performances and notably availability on the coverage area the system undergoes periodic evolutions through installation of new RIMS or algorithm optimization Moreover EGNOS is fully interoperable with the GPS system which is currently being moder nised GPS will be transmitting new civil signals on the L5 frequency band and this will improve the system s performance In addition Galileo the European satellite navigation programme should be operational in 2014 for lOC Initial Operations Capability Phase including a constel lation of 18 satellites in total This will be followed by FOC Full Operations Capability phase which will see the full Galileo constellation 27 satellites 3 spares A number of
82. pplied In addition the EGNOS system also continuously monitors the correctness of the values broa dcast throughout their validity period The sequencing of the various broadcast message types takes account of constraints that are due to the validity periods and refresh periods of each message This sequencing is not predic table reaction of system algorithms to the internal and external environment and differs from one geostationary satellite to another 40 E 5 EGNOS messages Degradation models For some corrections the user should apply degradation models between two refreshes and during the validity period Degradation factors are provided by message type 10 for long term and ionospheric corrections and by message type 7 for fast corrections especially for UDRE degradation For further infor mation refer to section 4 5 of MOPS DR2 5 5 TYPE 0 AND TYPE 0 2 MESSAGES 5 5 1 What purpose do they serve Message type 0 is transmitted by EGNOS for as long as the signal is uncertified by Civil Aviation as this is the case with the test signal The broadcasting of this message therefore means that information provided by the system does not have to be used for safety of life appli cations for example civil aviation Since March 2011 EGNOS has been officially declared usable for Safety of Life Applications SoL Service MTO message has therefore been removed from operational messages trans mitted
83. r constitutes the main limitation in terms of accuracy The GPS system accuracy specifications provided by the US Department of Defense see Annex 2 DR1 are given in the following table divided into horizontal vertical and temporal positioning service 777 5v m 13 1 Why do we need EGNOS Real expected GPS Specifications performance Accuracy 5 17 metres 9596 Horizontal service 7 1 available 99 of the time or more lt 37 metres 95 Vertical 13 2 available 99 of the time or more p Temporal service Accuracy 5 40 ns 9596 12 ns See also information in the Annexes 1232 Integrity Currently the GPS system does not make it possible to guarantee the position for some deman ding applications such as airport approaches by aircrafts In particular The probability of loss of integrity of a GPS satellite is far greater than that which is required for the purposes of navigating an aircraft Inthe event of system breakdown or malfunction clock drift broadcasting of erroneous data etc pseudorange measurement can be biased by anything from a few metres to a few kilo metres Due to the system architecture and specifically the limited number of GPS ground stations these errors may impact the user for several hours 6 hours maximum GPS system err
84. receiver has allocated a reception channel to this satellite but it is not using its data As well as the verification described above the type of PVT solution calculated by the receiver must also be identified This involves analysing the content of the integrity field given in the RMC RMB VTG or GLL sentences This field must have the value D Differential if EGNOS data is being used The table below shows an example of the RMC sentence Field Example Description Sentence ID GPRMC UTC Time 092204 999 hhmmss sss Status A A Valid V Invalid Latitude 4250 5589 ddmm mmmm N S Indicator 5 North South Longitude 14718 5084 dddmm mmmm E W Indicator E E East W West Speed over ground 0 00 Knots Course over ground 0 00 Degrees UTC Date 211200 DDMMYY Magnetic variation Degrees Magnetic variation E East W West Integrity D eng a Estimated N not valable S Simulator Checksum 525 Terminator CR LF To sum up to find out whether the receiver is using EGNOS and the correction data one of the satellite identifiers must be present in a GSA message and the integrity field of one of the RMC RMB VTG or GLL sentences must have the value D 6 Calculating ionospheric corrections EGNOS transmits ionospheric corrections which enable the ionospheric error to be estimated for each IPP These ionospheric corrections are broadcast for each point on a vi
85. rtual grid located atan altitude of 350 km These points are known as lonospheric Grid Points IGPs Direction to SV Pierce Point pp App local tangent plane e IGP lonosph ric Grid Point Earth Ellipsoid Earth Centre lonosphere FIGURE 36 Principle of the IPP Ionospheric Pierce Point The following equations provide the latitude and longitude A of an IPP A Ppp Sin sin COSY t cos SINY cos 4 expressed in radians yal R where V us I corresponds to the angle in radians between the user position and the direction of the IPP taken back towards the Earth centre A is the satellite azimuth angle in relation to the user s position measured in relation to the direction of North E is the satellite elevation angle in relation to the user s position measured in relation to the local tangent plane R is an approximation of the Earth s radius 6 378 km h is the height of maximum electron density 350km 6 Calculating ionospheric corrections The longitude of the IPP 1 given by If c gt 70 and tan w cos gt 2 or if lt 70 and tan w cos A rr tan r 2 then siny pp sin A App A Arc sin otherwise Sin Wppsin A App Arc sin RDA COS Ppp After calculating the position of his IPP the user must select which IGPs to use to int
86. s 12 1 PRECISION FARMING Context of the application Precision farming is used to facilitate the cultivation of agricultural land thus enabling farmers to make substantial savings They are able to manage the yield from plots of land by taking into account its variability the crops needs in terms of fertiliser and seed dispersal management of input factors However the cost of the equipment needed for precision farming remains high which restricts its use to large farms This is mainly due to the fact that equipment manufacturers offer solutions using RTK or DGPS techniques Now EGNOS is able to give farmers on small and medium sized farms access to high performance equipment at low cost Advantages of EGNOS The two key parameters for precision farming are accuracy of positioning to help with guidance of agricultural vehicles accuracy of positioning from one pass to the next and from one year to the next for the agricultural vehicle revisit capability EGNOS appears particularly well suited to this type of application as it considerably enhances the positioning accuracy and revisit capability compared to the use of GPS alone In addition the services offered by EGNOS are affordable requiring only the use of a single frequency receiver which costs much less than RTK positioning systems 74 E 12 Examples of practical applications Architecture The diagram below shows the architecture of a simplified guidance
87. s The first consists in detecting a unhealthy faulty satellite Fault Detection FD while the second detects and then excludes the unhealthy satellite from the positioning calculation made by the receiver thus allowing the user to continue working Fault Detection and Exclusion FDE Certain RAIM algorithms can also use the speed and acceleration information provided by platform sensors for example accelerometers altimeters odometers speed sensors etc thereby improving their performance RAIM is generally used on commercial aircraft GPS receivers to provide autonomous monito ring of GPS signals A variant of the principle used for RAIM algorithms is AAIM Aircraft Autonomous Integrity Moni toring which is used in the aviation field but is equally applicable to other modes of transport AAIM uses data generated through coupling with an inertial navigation system INS and takes advantage of INS GPS complementarity Though INS provides short term error stability inertial drift increases over time while GNSS errors are limited temporally GNSS and INS are there fore highly complementary with GNSS serving to recalibrate INS which in turn detects short term anomalies with GNSS RAIM and AAIM are also known by the term ABAS Aircraft Based Augmentation System 1 3 3 Space based augmentation systems Well before the operational deployment of GPS research work was being conducted with the aim of improving the GPS
88. st also be considered 11 Choosing a receiver In fact although some manufacturers clearly specify that EGNOS is supported others indicate that their receivers are WAAS Capable WAAS Enabled with WAAS referring to both the north American SBAS system and the SBAS standard In practice WAAS Capable means that the receiver can use SBAS services but that this function needs to be activated once only or each time it starts up WAAS Enabled usually means that SBAS reception is activated by default by the receiver The best course of action is to ask the manufacturer for details on how EGNOS is implemented and or to request a sample from the reseller in order to conduct tests Interface types and protocols Several interface types are offered by receiver manufacturers Among the most common are asynchronous serial interfaces complying with TTL RS232 or Bluetooth formats Receivers specialising in time applications use TCP IP or 1PPS 1 Pulse Per Second interfaces With regard to communications protocols manufacturers generally use proprietary protocols which give access to almost all the data pseudoranges satellite navigation messages SBAS messages etc associated with a standardised protocol NMEA 0183 Some receivers also generate data in RINEX Receiver INdependent EXchange format RINEX RINEX 15 an exchange format that is independent of the receiver lt was developed by the Astronomical
89. studies are currently being conducted to investigate potential EGNOS upgrades particularly taking into consideration corrections and integrity of GPS signals broadcast on L5 signals transmitted by Galileo system Extending the coverage area to include countries on the edge of the European Union as well as to Africa and the Middle East is also being considered 11 CHOOSING A RECEIVER The choice of receiver depends on the targeted application the EGNOS functions that will be used and the integration constraints To begin with you should establish whether the receiver correctly supports EGNOS then select the interface type and lastly check that the protocols supported by the receiver allow retrieval of the data required for the targeted application Receiver types A number of different receiver types are available Chipset consists of one or two components that must be installed on a circuit board The routing of the RF part is sensitive This compact solution is also the least expensive 1 to 5 FIGURE 27 GPS Chipsets Source SiRF Hybrid component consists of a single component integrating the RF and signal processing parts to be installed on a circuit board Routing is easier than with chipsets The price is higher than for the chipset solution around 10 Auxiliary card piggyback all the receiver and peripheral components are integrated on a ready to use card which has to be connected to the final pro
90. ted by Ux Xsat Xuser D Uy Ysat Yuser D Uz Zsat Zuser D 5 The zenith vector is calculated by Zx cos lat user cos long user Zy cos lat user sin long user Zz sin lat user 6 The elevation of the satellite is deduced by Elev arcsin Ux Zx Uy Zy Uz Zz The figure below shows the change in elevation of the EGNOS satellites at zero longitude y Cor ding to observation fot an observation from zero longitudes PRN 120 AORE PRM 124 ARTEMIS PRN 126 POR 5 degree Satelite elevation deg ees BIN 8s es amp 95 9 8 89 3 Z4 7 e 3 2 5 s n a Observation knude degrees FIGURE 37 Change in the elevation of EGNOS satellites by latitude If the elevation of the satellite and the receiver s immediate environment are known the availa bility of the EGNOS satellites can therefore be predicted 96 5 The protocol offers no direct solution determining whether the receiver is using EGNOS Certain messages must first be interpreted in order to deduce this Caution only version 2 3 and later releases standardise the information needed to detect EGNOS Detection of the geostationary satellite used Detection is via the GSA sentence GPS DOP and active satellites which gives among other information the list of satellit
91. ted however that the ranging and GLONASS corrrection functionalities have not yet been implemented The main utilisation limits are as follows Utilisation in a constraining environment The EGNOS system was initially designed for use by aviation in the various flight phases and particularly the most critical This generally implies a clear environment in terms of satellite visi bility and a spectrum management policy meeting criteria Use of EGNOS requires at least one of its three geostationary satellites to be in view For terrestrial applications especially in an urban environment satellite visibility is often not as good as for aviation applications This leads to potential masking not only of several GPS satellites but also of the EGNOS geos tationary satellite providing the differential corrections and integrity message However this can be resolved by using the SISNeT service see section 4 1 In some cases accuracy of position computed by an EGNOS receiver can be degraded compared to the one obtained by a stand alone GPS receiver This is the case for example when the GPS receiver computes a position with more GPS satellites than a receiver using EGNOS some of them being able to be excluded in this last case if an insufficient number of RIMS are able to monitor them However in the big majority of cases EGNOS provides a better stability of position GPS alone Moreover the EGNOS error calculation model takes
92. th UTC EGNOS data enable to correlate EGNOS time ENT EGNOS Network Time with UTC This correlation of ENT is carried out from parameters provided by Message Type 12 Message Type 12 consists of the 8 bit preamble a 6 bit message type identifier 12 followed by 104 information bits for the UTC parameters Details of correlation parameters are provided in section A 4 4 15 of DO229D DR2 Parameters definition as well as algorithms to be used are defined in sections 20 3 3 5 1 6 and 20 3 3 5 2 4 of IS GPS 200 DR3 with the exception that the UTC parameters will correlate UTC time with EGNOS Network Time rather than with GPS time Between UTC and ENT there are three equations to be applied which depend on the rela tionship between the effective time and the user s ENT time NB the user must take into account the truncation of WN WN and WN to the eight least significant bits of the complete week number which contains a total of 10 bits Condition a of IS GPS 200D When the effective time as indicated by the WN and DN values is not in the past relative to the user s present time and the user s present time falls outside the time interval beginning six hours before the effective time and ending six hours after the effective time UTC is obtained by the following equation ture te modulo 86400 seconds UTC Where At rc At HA t 604800 WN WN seconds t EGNOS time estimated b
93. the following operations Open the connection to the receiver and configure it so that it does not use EGNOS or SBAS in general e Open a connection to the SISNeT server The server uses a standard TCP IP connection and the information exchanged is in plain text Once the receiver has a valid GPS position the module must carry out the following operations Decode the GPS position and the list of satellites used from the NMEA datastream Transform this position into pseudoranges for each satellite This entails the reverse process to the one used by the receiver to calculate a position This is possible because the SISNeT server makes available the satellite ephemeris data and Klobuchar parameters so as to ignore the rough ionospheric corrections produced by the GPS receiver Retrieve the EGNOS message decompress it and apply the corrections to each pseudorange Recalculate a positioning solution using the corrected pseudoranges and the ephemeris data Reformat the result in the NMEA standard and transfer it to the application 12 4 12 4 USING THE INTEGRITY SERVICE Context of the application Imagine that a ship wishes to enter a port in conditions of reduced visibility The navigator s problem is how to ascertain that his position is accurate and reliable The GPS system alone cannot ensure the reliability of his position Where there is no local DGPS system the combined use of GPS and EGNOS offers the navigator signi
94. the position of the receiver if and only if the receiver s clock is perfectly synchro nised with those on board the satellites Unfortunately though all the satellites may be equipped with perfectly synchronised atomic clocks the same is not true for receivers which for reasons of cost and compactness are equipped with internal clocks that are not synchronised with the satellite clocks and whose stability is very poor compared with those aboard the satellites The following table illustrates the performance in terms of stability of various clock or oscillator types Equivalent in terms of Temperature controlled quartz oscillator as used in GPS 10 milliseconds 3 000 km receivers Thermostatted quartz oscillator 0 1 millisecond 105 Several microseconds Ultra stable oscillator 1055 Atomic clock as used on GPS Galileo satellites Atomic clock from ACES PHARAO 22 Ten picoseconds 10 78 3 mm scientific project Ten nanoseconds 10 Since a 1 millisecond difference between a satellite clock and receiver clock can produce a 300 km positioning error this clock bias must be compensated for That is why distance measure ments are made to a fourth satellite in order to calculate the bias To sum up this method entails solving the system of four equations with four unknowns as follows PR A XA AY Zi Z 0 b where i 1104 Where Yi Zi represent the coordinates for the positions of eac
95. they implement specific techniques at receiver level 1 3 1 Ground based augmentation systems 1 3 1 1 DGPS Differential GPS DGPS or differential GPS is a real time positioning method which uses fixed reference stations to transmit information to users within the coverage area so as to enable a receiver to correct certain errors in relation to the satellites pseudoranges All error types can be handled except for local errors generated by the user receiver multipath errors inherent to the receiver envi ronment measurement noise The accuracy achieved depends directly on the distance between the reference station and the user and deteriorates sharply beyond 100 to 150 km Each station constantly calculates its GPS position and compares it with its real position deduc ting from that applicable correction parameters known as differential corrections which it trans mits by terrestrial radio to receivers located in the coverage area In addition to the ground based infrastructure use of this system requires users to be equipped with a data link system able to receive the messages emitted by the reference stations A transmission rate of 100 bits s and a refresh rate of 10s are usually sufficient for most applications In the case of DGPS for maritime applications the RTCM SC 104 standard is generally used to transmit the differential corrections that make it possible to obtain accuracy to within a metre using a single frequency
96. ticular status of GPS constellation place date and time of day elevation of satellites above the horizon possible masking of satellites by obstacles reflection of signals onto obstacles behaviour of the ionosphere and troposphere age of broadcast orbit and clock data etc Thanks to the improvements made an EGNOS receiver can provide accuracy in the order of 1 2 metres 20 that is two to three times more accurate than a standard GPS receiver Moreover EGNOS provides extremely good stability over time as shown in the following graph blue line GPS accuracy on the other hand can be very variable pink line even though its overall performance is satisfactory Using EGNOS makes it possible to overcome these occa sional positioning error variations 7 Comparison of GPS and EGNOS performance a SBAS GPS Solution Comparison from 12 03 2011 00 15 to 12 03 2011 23 59 5 T T T T Error m 0 5 10 15 20 GPST Longitude GPST Horizontal 10 T T T 0 2 JOP iaa po ah Ar ee firm pe E 10 mel LM 54 M d 20 1 1 1 1 0 5 10 15 20 GPST Vertical BRST 120 11MAR12 L3 Processed by NTM F Team CNES Toulouse PEGASUS 4 6 FIGURE 22 Improvement in GPS accuracy thanks to EGNOS Brest France The figure below displays horizontal positioning performances obtained with 126 Geo Test SV HPE Percentile 95 2010 04 03 00 00 00
97. urple line represents the number of satellites seen by the receiver and monitored by EGNOS Verticale Performances from 12 03 2011 00 00 to 12 03 2011 23 59 45 D gt o o LL LLDnDL 1 4 1 1 5 i A 4 A Yen 1 An 4 di al LA 11 1 V1 Lt MI ni 1 A ur run Y l Y 1 wp 8 gt VPE blue VPL DO229C green amp SV number PEGASUS 4 6 Te a o T 5 15 GPST TRO1 120 11MAR12 L3 Processed by NTM F Team CNES Toulouse FIGURE 24 Integrity performance Tromsoe Norway The following graph shows EGNOS capacity to detect GPS faults such as that which occurred in June 2006 in the active atomic clock on GPS satellite SVN30 This quickly led to errors of more than 1 6 km observed at the Grasse France site EGNOS detected this anomaly almost instantaneously and informed all its users via the navigation message 7 Comparison of GPS and EGNOS performance Panne d horloge du satellite GPS PRN3O le 30 juin 2006 Corrections EGNOS et Mesure de pseudo distance du PRN3D corng e des variations d orbite et du biais utilisateur PRN30 mesure de pseudo distan 20 4 EGNOS Fast corrections gt valeur 14 Don t Use SV 15 4 10 Alarme EGNOS Don t
98. vigation They indeed provide respectively ephemeris and almanac positions for these satellites MT 17 contains the almanac for up to three GEO satellites as well as Health and Status mainly required for acquisition purposes More information is available in MOPS Section A 4 4 12 provide GEO ephemeris needed for the use of the GEO as a ranging source In addition a URA User Range Accuracy as defined for GPS satellites is also provided Details can be obtained in MOPS Section A 4 4 11 Though data are actually included in these messages GEO ranging service is currently disabled on EGNOS Ranging Off 7 COMPARISON OF GPS AND PERFORMAN 7 1 ACCURACY Improvements brought by EGNOS to the various GPS error components and thus to final accuracy for the user are shown in the following table Error type GPS EGNOS Orbit and clock synchronisation 1m 0 5 Tropospheric error 0 25 0 25 lonosph ric error 2m 0 3m Receiver noise 0 5 0 5 Multipath 0 2 0 2 m UERE quadratic sum of errors 1 0 2 31 0 83 m HDOP function of geometry of visible satellites Tel 1 1 Horizontal itionin r rror ZN ZW Horizontal positioning accuracy error 2 95 5 08 1 84 m TABLE 1 Summary of GPS EGNOS errors typical orders of magnitude Note Typical orders of magnitude are shown with actual results depending on the conditions encountered in par
99. within the time limit must be lower than the integrity risk Users of a GNSS system wishing to obtain a certain degree of integrity must state their needs in line with these four parameters for a given application EGNOS is specified to deliver the following integrity performance levels Parameter Performance Level Integrity risk 2x10 per 150 seconds Time To Alarm 6s Vertical Alarm Limit 50m Horizontal Alarm Limit 40 m 26 E 2 Advantages of EGNOS The required 6s Time To Alarm between the point when the problem impacts the user and the moment when the alarm is available at the user end is both a major and very design critical component of the EGNOS system In practice since the actual position error is unknown to the user estimates of these errors called Protection Levels XPL X designating the horizontal H or vertical V component are compared to the alarm limits A civil aviation approach procedure corresponding to an alarm level XAL will be authorised only if the XPL protection level is less than XAL See also the presentation of the Stanford diagram in Annex 9 which can be used to measure integrity performance from a receiver in a known position Alarm volume defined by the flight procedure FIGURE 5 Integrity limits principle EGNOS has therefore been so designed to enable users to perform more critical operations and to provide them with XPL limiting the actual error with a pr
100. y the user in seconds expressed relative to the beginning end of the week WN current week number in sub frame 1 of the GPS navigation message Condition b of IS GPS 200D When the user s current time is included in the time window beginning six hours before the effective time and ending six hours after the effective time UTC 1 provided by the following equation W modulo 86400 At At seconds where W t At 2743200 modulo 86400 43200 seconds and the definition of At see a above applies throughout the entire transition period UTC Condition c of IS GPS 200D and DN under When the effective time of the presence of the leap second as indicated by the WN sp values is in the past relative to the user s current time the relationship given for t condition a remains valid but the value of At is substituted by that of At i e UTC tura tz Atyre modulo 86400 seconds UTC where At rc At gp A t 604800 WN WN seconds A 1 OS 9 Stanford Diagram One way of representing both EGNOS s availability and integrity on the same graph is to use a Stanford Diagram This kind of graph initially used to validate SBAS systems displays for a known position the protection limit VPL or HPL according to the errors observed vertical or horizontal The alarm limit values VAL or HAL are shown on the two axes as well as the straight line y x i

USER GUIDE FOR EGNOS APPLICATION DEVELOPERS

Contents

Download Pdf Manuals

Related Search

Related Contents