IV. Receiver Features and Their Use
Contents
1. Passing Alarm Figure IV 5 illustrates the passing some times termed the arrival off course alarm As the name implies this alarm warns the mariner that a waypoint has been passed technically that Receiver Features and Their Use Iv 25 the vessel has passed a line perpendicular to the intended track at the waypoint without trigger ing the arrival alarm Anchor Watch Figure IV 6 shows an anchor watch alarm which might be thought of as the mirror image of the arrival alarm The mariner defines a waypoint where the anchor is dropped and an alarmcircle sufficient to accommodate the swing circle of the vessel Directions for how to do this vary by make and model for some models the Swing circle is preset in other models it is adjustable The alarm will sound whenever the vessel penetrates the shaded area in other words whenever the anchor drags and the vessel drifts outside of a user defined swing circle The low power consumption of the loran ensures that the ship s battery won t be run down excessively if the generator is not running or not available and the loran is left on overnight so as to use the anchor watch Overall an anchor watch is a desirable fea ture But it is important to have a realistic appreciation of the limitations of this feature First an anchor watch probably won t be of much help in a very crowded anchorage where the swing circles of other vessels are just boat lengths away T2. ADJUSTABLE XTE MARGIN ALARM SOUNDS IN THIS REGION check the appropriate charts to determine if a course alteration can be made safely If an autopilot is coupled to the loran receiver the autopilot will maintain a correct course to the next waypoint To simplify navigation many receivers en able an adjustable XTE alarm to be set so as to warn the user when a pre defined XTE tolerance isexceeded Figure IV 2 shows this graphically As in the first illustration the vessel is assumed to be in transit between two waypoints The XTE alarm is an audible alarm that can be set to warn the mariner of any excursions outside of a lane of adjustable width between the waypoints In Figure IV 2 for example the XTE alarm would sound whenever the vessel strays into the shaded area An XTE alarm would typically be set for voyage legs where navigational hazards e g shoals rocks heavily traveled shipping lanes fish trap areas lie to one side or the other of the intended track The XTE alarm should be set so as to enable the vessel to return to course in ample time to avoid the navigational hazard IV 20 Loran C User Handbook Name Arrival Passing Off Course Cross Track Error amount Boundary Border Anchor Watch Therefore the navigator should allow an ad equate margin of safety to ensure safe passage This safety margin should reflect among other things an allowance
3. 8 3 Based upon sample of 11 makes and models 2 0 3 3 Depends upon whether receiving or only in standby mode Radar 3 0 18 0 Based upon sample of 62 makes and models Power required a function of maximum range of the radar Depth Sounder 0 3 5 0 Depends upon whether indicator only or video or paper chart type ae Navigation 3 Based upon three 12 watt lights this number could be larger Lights 1 4 Bilge Pump Unlikely to run continuously Cabin Lights pea Varies with number and wattage Single Side 1 Receive Not needed within VHF range of shore Band Radio 12 Xmit NOTE The above table is furnished for illustrative purposes only Consult the owner s manual for each piece of equipment for details Based upon sample of 8 makes and models information changes With aircraft lorans a cartridge is shipped periodically to update the original factory supplied data Magnetic Variation Most Loran C receivers are equipped with a chip that provides magnetic variation data throughout the areas of the world covered by the loran chains that can be used by the receiver On some models this data base includes average annual changes in variation Once the user enters the date and year the receiver can compute the variation at any relevant location In practice therefore the user can do all navigation with reference to either true or magnetic north De viation is not accounted for on any production loran as of this writin
4. s manual Generations of Loran C Receivers Although the earliest Loran Creceivers were a substantial improvement over Loran A receiv ers these early so called first generation Lo ran C receivers were extremely primitive by today s standards The first Loran C models were difficult to operate similar to the Loran A sets and provided only TD information Later so called second generation models offered little more than a display of measured TDs from two secondaries in a Triad Users would need to specify the GRI and secondaries to be tracked and take the measured TDs convert these to latitude and longitude if desired and otherwise do the time honored navigator s days work of plotting determining course to steer estimated time enroute ETE estimated time of arrival ETA and the many other tasks common to the practice of navigation Commercial videotapes are available to supplement the owner s manual for many makes and models Receiver Features and Their Use IV 3 The advent of microchips and miniaturization of computers over the years since the Loran C sys tem has been in place has created a revolution in the design and features of the modern receiver Loran C receivers now select chains and secondaries auto matically convert TDs to latitude longitude warn of any lack of system integrity do many of the typical calculations made by navi gators and talk to other on board electr
5. be unreli able On this model wait is displayed if there is a low SNR cycle error blink code etc Other models are capable of displaying much more detailed information In general itis desirable to have more detailed information because the user can often intervene e g by switching secondaries to remediate the problem Receiver Features and Their Use IV 15 With one exception the definitions of the alarms and status indicators in Table IV 3 are clear and do not need elaboration It is appropri ate however to say a few words more about SNR indications on Loran C receivers SNRs are very important to the user Low SNRs warn the user of possible acquisition or tracking difficulties the need to switch secondaries and or that on board electrical interference prob lems exist High SNRs are generally desir able but abnormally high SNR values in what would otherwise be fringe areas could warn of skywave contamination see Chapter II For these and other reasons SNR values are important to the mariner Some Loran C receivers display SNR infor mation only in qualitative terms e g by letter codes e g A excellent B very good etc or word descriptors such as very low or very high Other sets display a two or three digit numerical code e g ranging from 00 to 99 where 00 is worst and 99 is best Pay particular attention to the text in the owner s manual to interp
6. but also for designing a load shedding strategy in the event of alternator failure A heavy duty marine battery of 100 ampere hour rating for example could supply current to service the ship s electronics load at the rate of 100 amperes for one hour 50 amperes for two hours 25 amperes for three hours etc without being recharged In the event of alternator failure all nonessential electrical equipment would be shut off to conserve the battery Mariners are left to decide exactly what is nonessential depend ing upon the circumstances of the voyage As Table I V 2 indicates the current drain for most loransis sufficiently small that the loran receiver would probably not have to be shut down in the event of alternator failure Even this small draw could be reduced by the simple expedient of shutting off the display lights and using a small flashlight for illumination if necessary Automatic Alarms and or Status Indicators Most Loran C receivers have the capability to display a variety of automatic alarms and or status and warning indicators Table IV 3 pro vides a sample of these alarms and status and warning indicators for marine lorans These alarms and the names and display codes vary from receiver to receiver so the owner s manual should be consulted for details For example one receiver model combines all of these alarms into one warning flag wait to indicate that positions displayed by the loran may
7. details IV 28 Loran C User Handbook the average component of the vessel s SOG along the direct course to the destination The VTD will equal the SOG provided that the vessel s COG is exactly equal to the bearing to the next waypoint If not the VTD is equal to the SOG multiplied by the cosine of the angle between the COG and the bearing to the waypoint Figure IV 8 illustrates the definitions of SOG VMG and VTD The scale has been exagger ated for clarity In this instance a vessel has drifted off course to the south right of the track between two waypoints The operator is at tempting to correct for this deviation but the correction is insufficient because the SOG vec tor is not directly aligned with the bearing to the next waypoint The VTD is the projection of the SOG vector on an axis directly oriented with the bearing to the next waypoint The VMG is the projection of the SOG vector on an axis parallel to the original track In general both VMG and VTD are less than or equal to SOG The relation between VTD and VMG depends upon the geometry Time Information Many Loran C receivers have a built in calendar and time display Additionally most lorans can be used to calculate either or both of the estimated time enroute ETE also called TTG or the estimated time of arrival ETA at the next waypoint These quantities can be found in one of the pages of the navigational display The TTG or ETE display will ch
8. for the accuracy of the loran system the reaction time of adistracted helms man and the speed and reaction capability of the vessel Although vessel are generally thought of as being comparatively slow these can still cover a surprising distance in a short span of time A sport fisherman on plane at 30 knots for example will cover more than 1 500 ft in 30 seconds Incidentally it is noted above that the dis tance to go DTG to the next waypoint is the great circle distance between the vessel or aircraft s present position and the waypoint In circumstances where the vessel s intended course differs from this great circle e g because the vessel is following a meandering river this DTG could be a significant understatement of the TABLE IV 5 ADJUSTABLE ALARMS OR TYPICAL LORAN C SETS Alarm to indicate that vessel has penetrated to within an adjustable radius of the next waypoint Alarm to indicate that vessel has passed a waypoint and is enroute to the next waypoint in a route sequence Sometimes termed arrival off course alarm or perpendicular crossing alarm Alarm to indicate that vessel has cross track error larger than preset Alarm to indicate that vessel has penetrated a defined exclusion zone parallel to the track between two waypoints Alarm to indicate that vessel has departed from within a predefined swing circle about waypoint actual distance remaining In turn other
9. more below while other manufacturers use terms such as velocity made good VMG or speed of advance SOA to describe the same term As a second example one manufacturer calcu lates the time to go TTG to the next waypoint as the distance to the waypoint distance to go DTG divided by the speed over the ground another as the DTG divided by the componentof the ground speed in the direction of the waypoint The control buttons of different receivers have different names or alphanumeric designa tors and operating procedures likewise differ The only way to master a particular make and model of Loran C receiver is to read the accompanying manual and call the manufac turer or dealer if you have any questions The old saw when all else fails read the direc tions is simply not good advice when it comes to today s sophisticated marine electronics or avionics There is no substitute for careful study of the owner s manual Owner s manuals differ significantly in the amount of detail presented and in their clarity and accuracy The prospective purchaser is well advised to read this document and include the quality of the manual among the attributes to be considered in the purchase decision The old saw when all else fails read the directions issimply not good advice when it comes to today s sophisticated marine electronics There is no substitute for care ful study of the owner
10. navigationally relevant information based upon this quantity such as the time to go would also be in error To minimize this error the navigator should within the memory limitations of the receiver enter as many waypoints as necessary to represent the vessel s meandering course to destination Failing this the navigator should recognize that the distance to go may understate the actual miles over the route to be followed Other Alarms The XTE sometimes called off course alarm is only one of several adjustable alarms that can be set by the user to assist in navigation Table IV 5 provides a list of several other alarms commonly incorporated into Loran C receivers These are next discussed Although these are described as audible alarms in the manufacturer s literature the sound of the alarm may not carry very far particularly in a noisy Receiver Features and Their Use IV 21 FIGURE IV 3 ARRIVAL ALARMS INFORM THE USER WHEN THE VESSEL PENETRATES AN ADJUSTABLE RING SURROUNDING A WAYPOINT ADJUSTABLE RADIUS WAYPOINT ALARM SOUNDS WHEN VESSEL PENETRATES SHADED CIRCLE environment and some manufacturers provide for an external connection to a loud alarm Although alarms can be used to great advan tage these should be used judiciously Many types of modern marine electronics are fitted with alarms those described below for the loran depth alarms on the sonar intrusion alarms on radar sets etc Th
11. requirements for Loran C receivers are typically quite modest As Table V 1 shows Loran C receivers do not draw much current e g 0 15 to 1 75 amperes among a sample of 20 receivers at least in comparison to many other types of marine electronics found on board a recreational vessel Thus to operate a Loran C receiver for a 24 hour period would require from 3 6 to 42 ampere hours Power requirements for Loran C receiv ers are typically quite modest compared to other marine electronics This enables Loran C receivers to be operated almost continuously on sailboats power vessels or aircraft experiencing alternator fail oe he D e These memory batteries need to be replaced periodically to ensure that memory contents are not lost A battery lifetime of from 3 to 7 years is typical for lithium batteries used for this application IV 14 Loran C User Handbook TABLE IV 3 AUTOMATIC ALARMS STATUS AND WARNING INDICATORS Brief Description Alphanumeric display to warn that accuracy of displayed position may be poor Ambiguity Alarm to note that ambiguous position information is being received probably as result of operating in area of baseline extension No Solution Receiver unable to compute latitude longitude from available information may be provided in lieu of ambiguity alarm Warns that SNR of master or secondaries is poor Cycle Select Warns that receiver may not be tracking correct cycle in Loran C puls
12. second ary pairs in the manner noted above Prospective purchasers are cautioned toread the user s manual carefully on this point _ Usersshould consult tl als to determine exactly be received by the set of tracking all extant C USSR system while others are ited The introduction of the recent SOCUS and equate reception on older rod Loran C User Handbook IV 6 Illustration of a Loran C receiver integrated into a plotter Other display sizes include signal status course devia tion indicator and route information Photograph courtesy of Raytheon Marine Company Displays Nearly all modern marine loran receivers use a liquid crystal display LCD which is energy efficient and easy to read in daylight as well as darkness Dimmer switches are handy to control nighttime cockpit or nav station light levels These displays indicate position infor mation TDs or latitude longitude as well as many other ancillary quantities stations in use signal characteristics navigational information etc The size of the display screen varies among models Some models feature paged displays in which different information is displayed on different pages of the display By pressing a mode or other key the user can p
13. track all usable secondaries in a chain Although two loran LOPs are sufficient to determine a fix reception of additional second It is convenient to distinguish between the receiver s front end or ability to acquire and lock on to signals and the software or computer programs that are used to process and interpret these signals Sensitivity dynamic range and minimum SNRs necessary for acquisition and lock are largely but not entirely determined by the front end Navigation features latitude longitude conversions and ease of use are more a function of the software Best is placed in quotation marks for the reasons discussed in Chapter III Receiver Features and Their Use IV 5 TABLE IV 1 GENERIC STAGES FOR SIGNAL RECEPTION Stage Description Search Status Search and Acquisition Looking for signals of selected GRI i e establishing the approx imate location in time of the master and ach of the selected secondaries with sufficient accuracy to permit subsequent settling and tracking Settling Detecting the front edge of Loran C pulse Selecting the correct cycle 3rd to be tracked Tracking Lock Tracking 3rd cycle i e main taining the synchronization of the receiver with the selected signals Lock on is the state of the re ceiver in which acquisition and settle have been completed and the receiver is tracking the selected signals SOURCE Report of the Special
14. CHAPTER IV Receiver Features and Their Use Introduction Earlier chapters have addressed the theory of the Loran C system This chapter narrows the focus to the shipboard component of the sys tem Loran C receivers Specifically this chap ter provides an overview of the key features and characteristics of Loran C receivers relevant to the navigator Readers wishing to learn about the operating instructions for a specific make and model of loran receiver are advised to consult the appropriate owner s manual for details As of this writing there are at least 25 brand names of marine Loran C units and several manufac turers of aircraft lorans and most manufactur ers produce several models so the user has a wide array of choices Available Loran C re ceivers differ substantially in design type of displays and operating instructions making it impractical to cover this information in requisite depth in a chapter of reasonable length More over any such discussion would rapidly become obsolete because new models are continually being introduced This chapter serves as a supplement to the owner s manual providing perspective ratio nale and theory to explain the features of mod ern Loran C receivers Prospective purchasers of a loran receiver may also find this chapter useful to identify the potentially desirable fea tures of loran sets Readers should be aware however that neither the Coast Guard nor any othe
15. Committee No 70 Minimum Performance Standards Marine Loran C Receiving Equipment Radio Technical Com mission for Marine Services RTCM Washing ton DC 1977 NOTES Different receiver manufacturers use slightly different terminology for these stages Some receivers display numeric codes to identify the various stages The time required to complete each stage depends upon the SNR of the signal at the receiver aries is desirable As noted the availability ofan additional secondary can be used to resolve ambiguous positions when operating near areas of baseline extension Moreover statistical tech niques can be used Kalman filtering to derive more precise position information if three or more LOPs can be measured The receiver s computer automatically determines the position which minimizes the weighted mean square error of all the LOPs At least one receiver manufacturer has designed a Loran C unit with the capability to use signals from two chains simultaneously a so called dual chain receiver Statistically optimal positions can be derived from LOPs from many station pairs from the two chains An advantage claimed for the dual chain receiver is that it can provide accurate positions at greater distances than single chain counter parts It is important to note however that simply because a Loran C receiver tracks all secondar ies does not mean that it is capable of using information from more than two master
16. a tion as TDs or as latitude longitude Aircraft lorans use latitude longitude exclusively TDs are what is measured by the receiver in all cases and these are converted to latitude longitude by mathematical algorithms using ASF informa tion e g the tables shown in Chapter II stored in computer memory Some manufacturers have gone to great lengths to ensure that ASFs are as accurate as possible Although the ASFs stored in the internal memory of the Loran C receiver are highly accurate for many makes and models this is not true uniformly Studies conducted at the Coast Guard Research and Development Center in Groton CT see Frazier 1988 indicated that the internal ASF tables in some models were very inaccurate for some locations Indeed for some makes and models the internal ASF cor rections resulted in greater latitude longitude errors than if no corrections were applied at all As of this writing there is no industry standard for coordinate conversion and each manufac turer uses a slightly different variant It is pos sible therefore for two Loran C receivers lo cated next to each other to register exactly the same TDs but slightly different positions in latitude and longitude terms Because of this lack of standardization and because ASFs are only approximate in any event use of TDs is preferred for most accurate navigation although accuracy differences may not be large for top of the line L
17. age through the available information Some lorans can inter face with voice synthesizers so that the user does not have to look at a display to acquire necessary information a mechanical voice continually broadcasts data from the receiver Some users find lorans equipped with voice synthesizers to be very convenient to others this feature is an irritating distraction Prospective receiver purchasers are well advised to pay particular attention to the type of display screen of the set This may seem an odd point to emphasize but it is absolutely true that the most sophisticated Loran C receiver in the world is of little use unless the navigator can quickly read and interpret the available informa tion As with many other features of the loran receiver the design of the display reflects nu merous compromises and tradeoffs A large screen for example is more costly consumes more power and may be incompatible with the overall size of the receiver The available infor mation from a Loran C receiver see below includes status indicators and warning informa tion identification of the GRI and secondaries in use SNRs of the master and secondaries chosen alarm settings position information in latitude longitude or TDs and navigational information e g waypoint descriptors bearing and dis tance to waypoint ETE ETA information cross track errors speeds and courses to cite just a few elements In total a modern Lora
18. an error in programming reliability errors etc These technological marvels can en courage laziness among the unwary It is impor tant to remember that the vessel operator or pilot in command has the ultimate responsibility for the safe passage of the vessel or aircraft The above caution is not intended to be a Luddite epistle Used properly and in conjunc tion with all available information and the ordinary practice of good seamanship this system offers tremendous capabilities es the navigator of the burde cally fixing the vessel by ila ble Means nor of the ordinary practice of goodseamanship ANONYMOUS QUOTED IN SCHLERETH 1982
19. ange through out the voyage as the vessel nears the next waypoint If the vessel slows down the TTG or ETA will increase if it speeds up the TTG or ETA will decrease Incidentally receivers differ in how the TTG is calculated Most receivers calculate the Datacard being inserted into aviation laser receiver Datacards can be periodically replaced to ensure the database is current Photograph courtesy of II Morrow Inc Receiver Features and Their Use IV 29 TTGas the distance to go DTG divided by the vessel s SOG This calculation will be correct only if the vessel is headed directly toward the waypoint At least one model loran calculates the TTG as the DTG divided by the VMG arguably a more realistic estimate Routes The route capabilities of Loran C receivers are discussed in more detail in the next chapter However it should be noted here that most lorans have the capability to store a route as a sequence of waypoints Once a route is as sembled and entered intomemory the waypoints appear in sequence anew waypoint becomes the destination waypoint whenever the current des tination waypoint is passed With some models only one route can be stored with others several in some models even hundreds of routes can be stored The number of possible waypoints in each route may also be limited Voyage Planning Most receivers can be used in a mode that facilitates voyage planning For example when a sequence o
20. d models than their marine counterparts More over the annual cost of operation of the aviation is larger because the databases see below have to be periodically replaced to ensure that vital navigational information is kept current As of this writing only a few marine loran receivers have prestored data bases while this feature is common on aviation loran receivers Data available from an aircraft loran includes airport information location runway lengths radio frequencies for communications etc airspace information restricted areas terminal control areas etc airway information altitude information e g minimum safe altitudes mini mum enroute altitudes etc and a host of other data Greater computer power is required to access and rapidly process this information particularly when the aircraft s greater speed is considered For example it is no small task to find quickly and display the bearing and dis tance to the nearest airport information that could be critical in the event that a precautionary or emergency landing were necessary As a second example aviation receivers need to keep track dynamically of the aircraft s position in relation to restricted or special use airspace Of course these functions are being undertaken along with the usual signal processing and navi gation functions IV 30 Loran C User Handbook Some aviation lorans can make special pur pose calculations unique t
21. e Blink Warns whenever blink code is received Latitude longitude or TDs will blink on and off as well on some receivers Battery Feature on portable loran sets to warn that batteries used as power supply are low and need to be replaced May also present remaining battery life in hours Power Failure Warns that battery voltage shipboard batteries has dropped below usable voltage e g 10 volts and that restart procedures must be used Memory Warning on some lorans that internal battery is weak or has failed and memory Battery contents may be lost ASF Indicates that ASFs permanently stored in memory or manually entered are in use for latitude longitude conversion Indicates that course and bearing displays are reference to magnetic rather than true north Variation may be input manually or stored on memory chip Manual Offset Indicates that user supplied offsets ASFs are being applied for latitude longitude conversions ECD Warns that envelope to cycle difference is out of specification and that the receiver may be tracking the wrong signal Display may be integrated with cycle alarm Above list is illustrative not all receivers are able to display each warning or status message Names given in this list are illustrative Actual designations vary from receiver to receiver Power requirements for marine electronics are relevant not only for selecting the storage batteries and sizing the generator alternator
22. e IV 27 FIGURE IV 8 THREE SPEED VELOCITY TERMS THAT CAN BE DISPLAYED ON SOME LORAN C RECEIVERS ARE SOG VMG AND VTD ONLY THE COORDINATES DIFFER WAYPOINT USER S POSITION tween the average SOG and the VMG is equal to the cosine cos of the angle that the vessel s course makes with the intended track If for example the angle between the actual track and the direct course between the two waypoints were 50 degrees and the vessel s SOG were 6 knots the VMG would equal 6 knots times cos 50 or approximately 3 9 knots The sailor has a practical optimization problem to solve Generally speaking a sailing vessel is faster off the wind than when sailing close to the wind The specific relation between the wind direc tion and the sailboat is termed a polar diagram WAYPOINT and differs from vessel to vessel But sailing further off the wind increases the distance to be covered The optimal course is one that maxi mizes the VMG A Loran C receiver that has the capability to display VMG could be very handy in determining the optimal course to steer The mariner would make multiple minor adjust ments on course watching the loran closely and allowing for averaging lags finally setting on the course that maximizes the VMG Velocity Towards Destination VTD The velocity towards destination YTD is The situation is a little more complicated than this simplified discussion suggests See Alexander 1988 for more
23. e United States Canada and Mexico The severity of the effects of interfering signals is a function of many factors but interfering signals can reduce the SNR of the loran signal and degrade the accuracy of the position determined To avoid the degradation in SNR associated with these interfering sources loran sets are equipped with so called notch filters that can be used to attenuate or notch out the interfering signal Some receivers contain built in spectrum analyzers to display levels of interfering signals a useful feature when setting adjustable notch filters Some receivers are equipped with preset notch filters others with adjustable notch filters and yet others with so called Pac man or seek and destroy filters These latter filters automatically search for interfering signals near the loran band and dynamically notch out this interference Refer to the owner s manual for instructions on how to use the notch filters fora particular make and model of receiver It should be noted that the purpose of notch filters is to control the effects of interfering signals not any noise or interference associated with shipboard equipment Control of internal noise sources is addressed in more detail in Chapter VII Integration with Other Systems Loran C receivers can be integrated with This receiver can process Loran C or GPS inputs and couples not only to shipboard electronics but also pe
24. e assembled that will provide the navigator with a series of norms for comparison It is only by this method that the navigator has the information to determine that something is amiss Perhaps there are storms between the LORSTAs and the vessel perhaps a newly installed piece of shipboard equipment needs noise suppression or the receiver s ground has been impaired etc These phenomena or problems can only be detected by a systematic comparison of SNR values with historical norms which depend upon the make and model of receiver installation technique and vessel loca tion Navigation Features The above features of loran receivers would be in themselves more than satisfactory for navigational purposes However all modern lorans also incorporate a wide variety of naviga tional functions that taken together transform loran from simply an instrument to determine position such as a hand bearing compass or a sextant into a complete navigational system Navigational information and functions of mod ern Loran C receivers are next discussed The Loran C receiver knows the user s position in TD and or latitude longitude terms at any instant in time As well the receiver has a very precise clock Knowledge of position and time information enable the calculation of the user s speed course and other relevant information for navigation In principle SNR values could also be calculated from a two digit or
25. e sound of numerous alarms going off simultaneously may actually compli cate decision making in a hazardous situation So while it is nice to have the capability to set various alarms these should be used with some discretion Moreover the navigator should be fully familiar with the sound or tone patterns of the various alarms lest valuable time be wasted in identifying which alarm has tripped An external connection to a loud alarm could be particularly useful if an anchor alarm is being set While at anchor crew might not be near the helm and might even be sleeping IV 22 Loran C User Handbook FIGURE IV 4 THE BOUNDARY BORDER ALARM IS SIMILAR TO THE XTE ALARM EXCEPT FOR THE REGION WHERE THE ALARM SOUNDS ADJUSTABLE MARGIN Arrival Alarm An arrival alarm can be programmed to sound whenever the vessel passes within a user defined distance of the next waypoint in se quence Figure IV 3 illustrates the arrival alarm The arrival alarm will sound whenever the ves sel penetrates the shaded area The alarm can be turned off manually and on some models will automatically shut off whenever the vessel exits the shaded area in Figure IV 3 Arrival alarms are useful in circumstances of bad weather or otherwise restricted visibility to alert watch standers to be particularly vigilant in searching for an entrance buoy for example The arrival alarm may also signal the helm to reduce speed to avoid overrunning or
26. escribe these features Receivers differ widely i in the number and type of features offered The features dis cussed here providea useful sample for the prospective purchaser IV 4 Loran C User Handbook Basic Function Reception and Display of Position Information In broad terms the functions of the Loran C receiver are to acquire and lock on the appropri ate transmissions and at a minimum to display the TDs associated with the selected master secondary station pairs Additionally all receiv ers now being marketed have the capability to convert from TDs to latitude and longitude termed a coordinate conversion capability All modern receivers also have a navigation mode that enables the user to monitor the progress of a flight or voyage and make necessary correc tions to stay on course Receiver circuit designs are generally pro prietary and in any event beyond the scope of this handbook Nonetheless receivers do em ploy different hardware and software and dif fer substantially in their ability to acquire and process signals These differences can be impor tant to the mariner particularly a mariner who frequents fringe areas near the limits of the coverage area or areas where interference is high With some older receivers it is necessary to select the chain and the station pairs as part of the setup process Newer receivers incorporate automatic transmitter selection ATS or auto matic
27. f waypoints is assembled into a route the loran will display the bearing and distance from each waypoint to the next and on some models the entire route distance Interface With Electronic Charts As noted above many Loran C receivers either have built in plotters or can interface with electronic charts and or plotters In either case the vessel s actual ground track can be displayed and the waypoints along a route can be superimposed on the electronic chart of the area This feature is convenient for many rea sons But itis particularly convenient because it facilitates the detection of blunders in entering waypoint coordinates in the loran The actual waypoints are displayed on the electronic chart and it is easy to see if the waypoint is grossly in error Aviation Lorans In conceptual terms aviation lorans are very similar to marine lorans However there are also some important differences in features and method of operation See Connes 1990 for additional details on aircraft receivers Avia tion Loran C receivers are considerably more complex than marine counterparts This added complexity is found chiefly in the computa tional and data base functions of the aviation receiver Partially because of this additional complexity and partially because of the respec tive sizes of the aviation and marine markets aviation lorans are considerably more expensive by as muchas a factor of 10 for some makes an
28. g When directions are referenced to magnetic north the loran receiver displays a flag such as MAG or other abbreviation to indicate that directions are refer enced to magnetic rather than true north Power Requirements Nearly all Loran C receivers used by recre ational vessels or aircraft operate on DC power In most cases these receivers are designed to use on board power Consult the specifications for each receiver for the acceptable voltage range e g 10 15 volts or 7 40 volts However some receivers are portable and use self con tained batteries e g 6 AA cells Power batter ies should not be confused with those lithium batteries required to maintain the receiver s waypoint memory Receiver Features and Their Use IV 13 If the voltage drops outside of the acceptable range because the batteries are run down or as aresult of starting the engine the loran receiver may crash and have to begin the entire acqui sition to lock cycle anew This could cause a problem if for example the engine s were shut down to increase the likelihood of hearing a sound signal from a critical buoy in circum stances of restricted visibility If on restarting the engine s the loran were to crash the navi gator would lose critical navigational data at an inopportune time Where possible itis desirable to use a different battery to power marine elec tronics from that used to start the engine s Power
29. g inputs can ease the task of entering data Display screens differ in the viewing angle through which the numbers can be clearly read Some displays are quite difficult to read when not standing directly in front of the set Finally the display should be evaluated in terms of where the loran receiver will be mounted in the vessel Mounting directly in front of the helm station e g on a power vessel may not A paged two line display showing course and speed information Photograph courtesy of Micrologie Inc IV 8 Loran C User Handbook require a display as large as if the set were mounted at some distance from the navigator s eye as might be required on a sailboat Aircraft lorans are generally rack mounted on the instru ment panel Keypad The keypads used on loran receivers differ Some use membrane or flat keypads others use raised keys In general raised keys as on a computer keyboard have a better feel and are easier to use Membrane keypads are easier to make waterproof or water resistant however Some receivers emit a beep when a key is depressed and the information is entered This feature compensates to some degree for the lack of tactile sensation when using a membrane keyboard The size of the keys likewise differs among receivers Closely spaced keys invite entry er This Loran C receiver features an artificial horizon display that facilitates understand
30. ging times e g 30 seconds will track changes in the user s speed more readily but at the expense of stabil ity Some receivers have the capability of deter mining the average course and speed with respect to the ground since the last waypoint i e arguably the true CMG and SMG values Velocity Made Good VMG Illustrated in slightly exaggerated form in Figure IV 7 velocity made good VMG also called velocity along route VAR by atleast one WAYPOINT major manufacturer and speed ofadvance SOA by another is a term very familiar to sailors VMG represents the component of a vessel s speed over the ground in the direction of the waypoint In Figure IV 7 a sailing vessel is travelling from the waypoint to the west to the one to the east The bearing of this second waypoint is 090 degrees from the first How ever in this example because the wind is as sumed to be coming from the east the vessel s actual track must consist ofa series of tacks with the result that the actual path over the ground is a series of zig zags shown by the dotted line Obviously the distance along the dotted line between the two waypoints is larger than the great circle distance shown by the solid straight line As a consequence the overall VMG as measured along the solid line would be sub stantially smaller than the vessel s SOG In general it can be shown that the relation be Receiver Features and Their Us
31. he XTE optionally in nautical or statute miles Cross track error is illustrated in Figure IV 1 which shows the aircraft s intended track the solid line between two waypoints and the actual track denoted by the dashed line In this illustra tion the aircraft has drifted to the right south of course The bearing BRG sometimes called course to steer CTS would be the angle from the aircraft s present position and the DTG the distance great circle from the user s present position to the next waypoint in sequence CROSS TRACK ERROR XTE IS THE PERPENDICULAR DISTANCE FROM THE USER S POSITION TO THE INTENDED TRACK WAYPOINT CROSS TRACK ERROR XTE Knowledge of the XTE enables the user to alter the vessel s or aircraft s course to compen sate for the observed drift effects of maneuver ing to avoid traffic and or inattention at the helm Additionally many receivers display a course deviation indicator CDI often by an arrow that indicates the appropriate angular correction to return to course It is important to remember that the mere fact that the loran indicates the XTE does not imply that there is safe water or airspace between the vessel and the waypoint It is the navigator s responsibility to Receiver Features and Their Use IV 19 FIGURE IV 2 THE CROSS TRACK ERROR XTE ALARM SOUNDS WHENEVER THE PRESCRIBED XTE HAS BEEN EXCEEDED ALARM SOUNDS IN THIS REGION ITR es
32. he number of waypoints that can be stored in the receiver s memory varies by make and model but most receivers can store 100 or more waypoints Waypoints are stored as a waypoint number and set of coordinates Some receivers permit an alphanumeric waypoint designator e g home buoy 01 etc to be used In use waypoints are either places to be visited e g checkpoints along a route orplaces to be avoided e g shoals rocks or other ob structions to navigation Often a navigator will lay out a sequence of waypoints linked into an overall route for the voyage The Loran C receiver keeps track of the user s progress from waypoint to waypoint At all times the user can determine the distance to go DTG and bear ing BRG to the next waypoint in sequence an angular course direction to the next waypoint and the time to go TTG to reach the next waypoint These functions are discussed in more detail below Cross Track Error The cross track error often abbreviated XTE on loran displays is the perpendicular distance from the user s present position to the intended track between waypoints Nearly all Aircraft loran displaying bearing and distance to waypoint The second line shows the magnitude 0 35NM and direction of the XTE Photograph courtesy of IT Morrow Inc IvV 18 Loran C User Handbook FIGURE IV 1 WAYPOINT modern Loran Creceivers can display t
33. herepeatable accuracy ofa loran may not be sufficient for this purpose Second unless an external land alarm is fitted the noise of the anchor alarm may not be sufficient to wake crew sleeping some distance from the loran receiver Course and Speed Information As noted above position and time data in the loran receiver enable the computation of course and speed estimates In the case of loran all course and speed estimates are referenced to motion over the ground rather than motion relative to the water Thus for example the course and speed estimates are really course over the ground COG and speed over the ground SOG COG and SOG information are particularly useful to the navigator because these quantities reflect the combined effect of the vessel s motion through the water and the current set and drift When navigating to a destination the user simply alters the heading of the vessel to maintain a zero XTE or to maintain the COG equal to the intended track and the vessel will arrive at the chosen waypoint Navi gators should remember that the vessel s head ing per standard compass will generally differ from the COG because of compass deviation and the correction or crab angle necessary to compensate for current or winds aloft in the case of aircraft Reference to numerous owner s manuals indicates that there is little or no uniformity in the nomenclature employed by various manu facturers to desc
34. in the locations of the thousands of airports throughout the country The user can call up attributes of the airport e g the longest hard surfaced runway with only a few keystrokes and navigate to this airport In the event of an in flight emergency the loran receiver can display the distance and bearing of the nearest airfield having the requisite runway characteristics Likewise marine loran receiv ers with data bases contain the locations of buoys and other features of navigational interest One marine loran comes with a data base of approxi mately 8 000 lights and 6 000 buoys along the coast of the continental United States Great Lakes Hawaii and Alaska A data base can be very convenient but it is also necessary to have some means to update the data base as the locations of the entries or other IV 12 Loran C User Handbook TABLE IV 2 CURRENT DRAW FOR SEVERAL TYPES OF MARINE ELECTRONIC EQUIPMENT TURNING OFF THE LORAN C IS UNLIKELY TO FEATURE IN ANY LOAD SHEDDING STRATEGY Equipment Typical Type Current Draw Amperes RDF ADF 0 5 3 0 Older units likely to be less efficient ADFs aircraft type are at the upper end of this range Loran C 0 07 1 75 More complex Loran C including integrated loran GPS naviga tion systems draw more current VHF Radio 1 0 5 5 Varies depending upon whether transmitting or operating in receive only Video Charts 0 8 2 5 Based upon sample of 10 makes and models Video Plotter 1 1
35. ing of navigation information Photograph courtesy of I Morrow Inc rors particularly when on the bridge of a pitch ing or rolling vessel or in the cockpit of an aircraft encountering turbulence Nearly all receivers have a numeric keypad in addition to function keys On some models however it is necessary to push or buttons to increase or decrease an entry an inconvenience when entering waypoint coordi nates Some marine lorans have calculator style numeric keyboards i e with the keys 7 8 9 on the top row Other marine lorans have tele phone style numeric keyboards i e with the keys 1 2 3 on the top row There is no best choice for the keyboard type but a matter of individual preference Remote Readout Some receivers offer the option of a remote readout or display to be used instead of or in addition to the principal readout Receivers with this feature can be used to provide naviga tional information in two locations such as at the bridge and at a separate navigator s station Of course the same effect can be achieved by using two lorans one at each station but the remote Receiver Features and Their Use Iv 9 readout ensures that both stations display the same navigational information and at a cost somewhat less than that for a second loran Coordinate Conversion All modern marine Loran C receivers have the capability of displaying position inform
36. initialization as it is sometimes called If the receiver has this capability all the user need do to initialize the set is to enter the user s latitude longitude and the set automatically se lects the best GRI and station pairs This feature is convenient but as noted in Chapter III it is sometimes necessary to override this automatic selection Once the GRI and the secondaries are se lected the receiver goes through a sequence of steps to search for acquire settle and lock on to the transmissions from the desired secondar ies Table IV 1 identifies the generic steps in signal acquisition and lock The time for receiv ers to complete these steps varies with the re ceiver and the SNR of the master and the second aries Typically this time varies from less than a minute for very strong signals to 15 minutes or more for signals with low SNRs Most receivers display an alphanumeric code to identify the stage of the setup procedure As noted in earlier chapters the requisite SNR for reception differs with the receiver The SNR required for acqui sition is generally greater than that for tracking This is why it sometimes occurs in an out and back trip that the loran receiver can continue to track a previously acquired signal in circum stances where the same receiver could not ac quire the signal Some receivers can acquire and track only a master and two secondaries while others can acquire and
37. n C re Some earlier marine lorans used light emitting diodes LED which draw more power and are more difficult to read in strong light conditions Aircraft lorans use LED displays more frequently Receiver Features and Their Use IV 7 ceiver may have the capability of displaying hundreds of pieces of potentially relevant data Practical constraints limit the overall size of display screens and size of lettering numbering so that it would be impossible let alone confus ing to present all this information on one dis play Segmenting the display into pages each with defined and logically grouped contents is a viable design alternative However paged dis plays do not solve all problems For example a user cannot simultaneously examine the con tents of more than one page so that not all information is rapidly accessible Large clear numerals or letters without distracting and hard to read letters made from numerals are easiest to read It is also helpful if the screen can display several items at once perhaps in different sizes so that the user does not continually have to switch pages to find related information Some information is more easily and rapidly understood in analog e g pointers arrows etc rather than digital form Arrows symbols or mini charts if well de signed and logically grouped can also enhance theinterpretability ofadisplay So called menu screens i e those with self promptin
38. o aviation g com putation of density altitude true airspeed etc others compute estimates of winds aloft the aviation equivalent of current sailing compu tations and yet others are integrated with fuel management systems so that the aviator can compute and update fuel reserves along with the other routine navigational bookkeeping There are circuitry differences between avia tion and marine lorans as well but these are less significant than the computer and software dif ferences Detailed specifications for aviation loran receivers are provided in Radio Technical Com mission for Aeronautics RTCA Document Number DO 194 as amended by the Federal Aviation Administration FAA Technical Stan dard Order TSO C60b This latter document contains minimum standards that must be satis fied if the loran is used for instrument flight Pitfalls The capabilities of the modern Loran C receiver are almost astounding It is literally possible with the right equipment to move the vessel from the dock enter in the required ws THE GREATEST HAZARD TO NAVIGATION IS A BORED NAVIGATOR waypoints and route sequence engage the auto pilot and do nothing thereafter until the vessel reaches the final waypoint However such a voyage could be very foolhardy Built into every electronic system is the possibility of error arising from the inherent limitations of the sys tem hum
39. onic systems such as radar electronic charts autopi lots and other marine electronics If the early receivers could be called radios in some sense the later receivers should really be termed navigation computers Additionally the price of full fea ture lorans has decreased substan tially over the years as manufac turers amortized research and de velopment expenditures captured economies of scale and responded to competitive pressures Receiv ers have therefore become much more affordable for owners of rec reational vessels and aircraft Example of a hand held loran powered by 6 AA batteries The antenna is telescoping and no external ground is required Considering the sophistication of most modern receivers these are remarkably user friendly 1 e easy to operate Nonetheless itrequires time and some diligence to master the use of a given receiver not unlike that required to use a computer Although some lorans are much easier to operate than others all require a modicum of user so phistication Features This section describes the relevant features of various Loran C receivers now available Photograph courtesy of Micrologic Inc commercially Notall receivers include all of the features discussed below but all of these fea tures can be found among commercial loran sets Some manufacturers use company unique or trade names different from those in this hand book to d
40. oran C re ceivers As noted above aircraft loran receivers provide latitude longitude information only The width of a typical airway highway in the sky is 4 nautical miles either side of a centerline so precise position information is less important in the enroute mode When making instrument approaches to airports standard ASFs appropri ate to each airport are entered into the loran from the published instrument approach procedure When prestored ASFs are being applied there is generally some indication on the display e g the code sequence ASF to indicate that this is the case Most lorans also enable the user to input a predefined ASF latitude longitude offset or bias as an alternative to the stored values Experienced navigators needing o Loran C accuracy are well Published waypo TDs as measured E Notch Filters Loran C signalreception can be impaired by interference from other signals broadcast on A few loran receivers being marketed as of this writing do not have internal ASF tables or the capability of manually entering these factors RTCM is presently working on such a standard Interested readers should contact RTCM for details and current information IV 10 Loran C User Handbook slightly different frequencies e g radio broad cast stations military radio transmitters and other navigation equipment Appendix A pro vides a list of known sources of interference in th
41. r agency of the U S government has the responsibility of performance evaluation or pub lication of comparative performance statistics of recreational vessel loran receivers although marine lorans used on certain types of vessels and aircraft lorans must meet technical perfor mance criteria There are private sector publi cations that do preser t technical data and frank evaluations of Loran C receivers It is also worth reminding the reader that receiver photo Some of these are undoubtedly made by the same manufacturers with only minor changes in the size or shape of housing or other cosmetic differences to distinguish private label IvV 1 IV 2 Loran C User Handbook graphs included in this handbook are for illustra tive purposes only and should not be construed as endorsements of any particular make or model As with this handbook generally the focus of this chapter is on marine users Where appro priate supplemental material relevant to other users is included Read Your Owner s Manual Although all Loran C receivers operate on the same general principles there are important differences among these receivers in features methods of operation and even in the definitions of terms used in the operating instructions For example one manufacturer uses the term ve locity along route VAR to describe the com ponent of the aircraft s or ship s speed over the ground in the direction of the course
42. ret the SNR values provided by a particular make and model A measure of the signal to noise ratio fa vored by electrical engineers is the SNR in decibels abbreviated dB The SNR in dB is numerically equal to 20 log SNR Thus for example an SNR of 0 5 would be equal to 6 02 dB Equivalently the SNR corresponding to a particular dB reading is SNR 10 Table IV 4 shows the relationship between the SNR as a fraction and the equivalent in decibels As can be seen from inspection of this table the SNR in dB will change by approximately six units whenever the actual SNR is either doubled or halved Some Loran C receivers have the capability of displaying SNR values in dB This display is preferable because alone among the IV 16 Loran C User Handbook TABLE IV 4 RELATION BETWEEN SNR AND DECIBEL SNR MEASUREMENT Decibels dB various methods of indicating SNR the actual SNR value can be calculated from the dB fig ure SNR displays are useful for setting notch filters determining the sources and sig nificance of shipboard electrical interfer ence detecting skywave contamination and selecting secondaries for use Quanti tative displays showing the actual SNR as a ratio or in decibels are best Recording SNR Data in the Navigation Log Navigators should make it a practice to record the SNR for various stations as a memo item in the navigation log for each trip Over time a data base can b
43. ribe course and speed informa tion Moreover the apparent definitions of these terms are generally at variance with accepted navigational nomenclature For a summary of the traditional definitions of many course and speed terms please refer to Appendix C In what follows the course and speed features of a sample of modern lorans are summarized All modern lorans have the capability to display COG and SOG or some reasonable facsimile of these quantities According to defi nitions used by some manufacturers these are incorrectly termed course made good CMG and speed made good SMG respectively consult the owner s manual for your set Ac cording to traditional definitions the COG and SOG are instantaneous values In the case of loran receivers these quantities are in fact short term average values where the averaging period e g from seconds to minutes is adjustable by the user Because of this time averaging the IV 26 Loran C User Handbook FIGURE IV 7 AN ILLUSTRATION OF VELOCITY MADE GOOD COMPARED TO SOG VMG IS LIKELY TO BE MORE RELEVANT TO SAIL CRAFT THAN POWER VESSELS WAYPOINT values displayed by the receiver will lag the vessel s actual direction and speed e g the speed indication for a decelerating vessel will be overstated Long averaging times e g as many as 7 minutes for some models will tend to be quite stable and accurate provided the vessel does not alter speed Short avera
44. rsonal com puters Photograph courtesy of Trimble Navigation IV 11 Receiver Features and Their Use The Digital Selective Calling DSC radio pictured at left interfaces with a marine loran receiver so that the vessel s position can be transmitted This is particularly helpful in the event of distress Photograph courtesy of Ross Engineering other shipboard systems in two ways Some Loran C receivers are actually built into an other piece of electronic gear e g a depth sounder fish finder or plotter Some receivers are integrated with GPS offering additional flexibility and redundancy As well most re ceivers have output jacks with a standardized output three common protocols are the Na tional Marine Electronics Association NMEA 0180 0182 and 0183 formats that enables interconnection with plotters video charts au topilots and radars Some models can also be interconnected with a gyrocompass or fluxgate compass and speed log enabling the electronic determination of the set and drift of the current This interface capability can be a considerable help to the navigator Prospective purchasers should ensure that the correct output formats are available to tie into ancillary equipment Data Bases Some Loran C receivers typically those intended for use on aircraft but also some marine models incorporate a self contained data base On an aircraft loran for example this data base would conta
45. running into the waypoint Gf a physical object such as a buoy or a light structure Incidentally many lorans use differ ent tones or tone patterns for the different alarms One manufacturer for example uses the Morse code A for the arrival alarm Generally speaking an arrival alarm would be set only for those waypoints where some action is required by the operator or crew such as a course or speed change When traveling Receiver Features and Their Use IV 23 FIGURE IV 5 PASSING ALARMS INFORM THE USER THAT THE VESSEL AIRCRAFT HAS PASSED THE WAYPOINT BUT HAS NOT SATISFIED THE ARRIVAL CRITERION WAYPOINT ESS Ss ARRIVAL ALARM ZONE WAYPOINT towards waypoints where no operator action is required the alarm can be disabled This prac tice is desirable because it reinforces the idea that when an alarm sounds some action must be taken by the operator Alarms that sound rou tinely have a desensitizing effect the cry wolf syndrome which could mean that a genuinely significant alarm would be overlooked or that alarms will not be set in the first place Arrival alarms are particularly useful in cases where a waypoint must be reached exactly and in circumstances e g reduced visibility S PASSING ALARM ZONE with a high potential for distraction As with the XTE alarm the arrival alarm should be set at a sufficient distance to avoid overrunning the waypoint Upon hearing the arrival alarm
46. the vessel operator would normally slow down and carefully monitor the DTG and BRG indications to steer to the waypoint A prudent navigator should use all available means e g depth sounder radar to help locate the waypoint If the waypoint were an entrance buoy for ex ample and visibility were impaired e g by fog or darkness the operator might wish to initiate a systematic search pattern to ensure that the IV 24 Loran C User Handbook FIGURE IV 6 AN ANCHOR WATCH CAN BE SET TO ALERT THE MARINER THAT THE VESSEL HAS DRIFTED OUTSIDE A DEFINED SWING CIRCLE ALARM RADIUS EQUALS SWING CIRCLE buoy was located prior to proceeding to the next waypoint Boundary Border Alarm Figure IV 4 illustrates the border alarm It may be thought of as the mirror image of the XTE alarm warning the user that the vessel is about to penetrate a lane of defined width between two waypoints This could be used to warn the mariner that the vessel has entered a traffic separation lane As a second illustration this feature might be used by a commercial fisherman to avoid fishing in illegal fishing ALARM SOUNDS IN THIS REGION areas of defined dimension These illegal areas are separated from legal zones by an imaginary line between two points of latitude longitude or TDs Penalties for fishing within illegal areas can be very substantial so many commercial fishing vessels find these alarms particularly useful
47. three digit code However a survey of 20 owner s manuals for different sets has failed to find one example where this correspondence has been provided Waypoints As noted in earlier chapters all Loran C receivers in current production have the capabil ity of entering and storing waypoints These waypoints are simply sets of coordinates which describe a location of navigational interest Waypoints could include a dockside location where the vessel is berthed fixed and floating aids to navigation channel centerlines turnpoints productive fishing areas wrecks shoals etc Aviators would typically define different types of waypoints from those given above Possible waypoints relevant to aviation uses of loran could include airports locations of the initial approach fix locations of radionavigation aids airway intersections locations of pub lished holding fixes turnpoints and other rel evant information Note that aviation lorans equipped with a data base may have many of these locations preprogrammed in the loran A subscription service is available to update these locations Waypoints can generally be entered into and stored by the loran either by visiting the area and pressing the appropriate control button on the set or can be entered as coordinates typically as TDs latitude longitude or as Receiver Features and Their Use IV 17 distance and either true or magnetic bearing from another waypoint T
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