Development of a Cheap, GPS-based, Radio
Contents
1. DC DC voltage converter Electronics board showing left to right and top to bottom radio Although the initial design took a simplistic approach that doesn t use any method of advanced power reduction it is a workable design thanks to the relatively low power nature of the individual components as shown in Table II Future versions are planned that will allow for turning off and or putting to sleep devices when they aren t needed which would achieve roughly a 2 3 reduction in power Even as built the 2 2 Ah 12 V lead acid batteries used for the development will operate the electronics for about six hours Table II POWER CONSUMPTION OF THE DRIFTER FOR EACH OF THE TWO OPERATING STATES Device Power Consumption Watts Transmit Receive Sleep Micro controller 0 9 0 9 0 10 GPS 1 1 1 1 0 00 Radio 0 7 0 3 0 13 Fully running Power 2 30 16 7 duty cycle 0 68 power Other battery chemistries such as NiCd Alkaline and Li ion offer performance improvements in the available power Proc of the IEEE OES Seventh Working Conference on Current Measurement Technology battery volume and weight A rechargeable 15 V 15 Ah Li ion pack would occupy roughly the same space weigh 1 4 less and cost an order of magnitude more 223 but would extend the operating time to about 64 hours running continuously or on the order of nine days with power management strategies Regardless of the battery type the drifter housing is designed to a2. The drifter is designed to operate in a variety of different modes ranging from self contained to remote tracking This variety results from the different uses planned by CBI Fig 5 Initial testing of the prototype drifter For this version 1 6 cm 4 in PVC pipe was used Since then everything has been shrunk to fit into a 1 2 cm 3 in PVC pipe with a longer and narrower antenna A Self contained mode The simplest mode is the self contained un tracked mode In this mode the drifters are deployed where needed and the drift logs are retrieved after recovery A major drawback with this mode is the required visual tracking of the individual drifters that are deployed The use of this mode of operation is planned for spot checking areas of interest Proc of the IEEE OES Seventh Working Conference on Current Measurement Technology B Boat based drifter tracking Over the past year CBI has been developing and operating a spill of opportunity vessel 4 This vessel is equipped with an ADCP dual frequency fathometer and a towed Acrobat sensor package CTD fluorometer particle size analyzer etc In this mode a group of drifters can be deployed tracked from the boat in real time and treated as part of the sensor suite C Fixed based drifter tracking mode The third mode of operation planned for the drifters is in support of the fixed monitoring platforms and HF Radar that CBI operates The platforms are e
3. Fig 2 Active GPS Antenna mounted on the top of the test drifter 67 For initial development the NMEA message GGA Global Positioning System Fixed Data is used and only the UTC time Latitude and Longitude are stored Although it is generally preferred to store the entire message it was parsed in order to reduce the storage The GPS offers both a low power sleep mode and a shut off mode but neither of these features was utilized for the initial design Another feature being explored is using the differential GPS DGPS option built into the receiver by re broadcasting a differential signal to the drifter over the telemetry link which would improve accuracy and precision of the computed water speed C Radio The radio link used to communicate with the drifters is based on a 100 mW 900 MHz frequency hopping spread spectrum OEM transceiver manufactured by MaxStream 3 It is a physically small and low power device with many features The 1200 bps radio to radio version was used for its increased range In addition to real time tracking a radio provides the capability to download data and program the drifter without disassembling it In fact the data can be retrieved without interrupting the drifter s ongoing operation The radio s range is constrained by the physical limitation of the need to have a rather low profile drifter housing This limits the drifter s antenna height Since the drifter operates over water the ra
4. Proc of the IEEE OES Seventh Working Conference on Current Measurement Technology Development of a Cheap GPS Based Radio Tracked Surface Drifter for Closed Shallow Water Bays John C Perez James Bonner F J Kelly Chris Fuller perez cbi tamucc edu bonner cbi tamucc edu fkelly cbi tamucc edu cfuller cbi tamucc edu Conrad Blucher Institute for Surveying and Science Texas A amp M University Corpus Christi 6300 Ocean Drive Corpus Christi TX 78412 361 825 5891 Abstract This paper describes the development of an inexpensive 500 GPS based radio tracked surface drifter system designed to operate within a closed shallow water bay Texas bays and estuaries are generally shallow weather event driven complex non linear systems In spite of the amount of activity that occurs within the bays very little real time information is collected The Conrad Blucher Institute for Surveying and Science CBI at Texas A amp M University Corpus Christi is working toward better understanding of these bays by operating a pair of SeaSonde HF Radar units within Corpus Christi Bay This experience has shown the need for a specialized drifter that can be used in conjunction with the HF Radar operation and other bay research activities I INTRODUCTION The main design criteria used in the development of this shallow water surface drifter are that it be inexpensive operate in a closed shallow bay environment be self logging and have the ab
5. ccomplish recharging without opening it A Microprocessor The design is based around the RCM2300 microprocessor core module from Rabbit Semiconductor This microprocessor was chosen because of is low cost ease of use low power and because it has enough built in resources to accomplish the needed tasks The core module version of this chip allows for rapid development by providing all the needed support circuits in an easy to use package Using the core modules doesn t prevent future versions of the drifter from eliminating the modules and directly incorporating the needed support chips for improved capabilities such as more data storage The core module features a 22 1 MHz clock 256K Flash amp 128K SRAM memory Real time clock 29 I O lines and 4 serial ports The module is relativity small 41x29x12 mm and has minimal power consumption while supporting a real time operating system that uses the C programming language 1 The controlling software can be downloaded into the flash memory while the SRAM is used for data storage B GPS The GPS used is an OEM module that is manufactured by Laipac and is based on the SiRF star I LXTM chip set It is a 12 channel receiver with a position accuracy of 25 m CEP without SA 2 The drifter utilizes the active antenna option of the GPS as shown in Fig 2 The GPS outputs variety of NMEA Nation Marine Electronics Association messages that are parsed for the needed information
6. ent around existing data collecting platforms The existing data loggers can be set up so that when a platform undergoes regularly scheduled service the drifters can be deployed and recovered with the data automatically retrieved and transmitted back to CBI i e independently of the field personnel s tasks See Fig 6 69 27 48 097 22 W 097 20 w Fig 6 Typical drifter drift track within Corpus Christi Bay the drifter sampled its location every 10 minutes V SUMMARY The initial development of the drifter accomplished the goal of a simple to use inexpensive 500 shallow bay surface drifter Based on what was learned several changes are planned that will increase the drifter s usefulness but not the cost such as the ability to power down or put to sleep the drifter between samples which will significantly increase the battery life Changing the operating frequency of the radio which will improve the range that the drifters can be tracked is another improvement planned Software enhancements such as error detection and packet retransmission will allow for more a more robust data link ACKNOWLEDGEMENTS This work was supported by the Texas General Land Office under the Interagency Cooperation Contracts 00 43R and 02 115R REFERENCES Rabbit Semiconductor Davis CA online Available http www rabbitsemiconductor com GPS Engine Board TF Series Laipac Technology Inc Ontari
7. gned to accomplish each task Four tasks were defined that needed to be addressed by the drifter software they are communication with the radio communication with the GPS timekeeping and a coordinating task to interconnect the other tasks into a functioning product The process that communicates with the radio accepts data coming from the radio and queues the characters until a full line of data has been received as signaled by a carriage return This process allows the rest of the software to only deal with full commands and not worry about each byte of information as it comes in This method also allows for future enhancements such as implementing checksum bytes for error detection and retransmission of lost data The task of communication with the GPS includes parsing the NMEA messages For now only the GPS time Latitude and Longitude are parsed from the NMEA GGA message Although the internal clock of the microprocessor is used for operating the drifter the GPS time is used to coordinate the data retrieved from several drifters Although the GPS also reports the NMEA VTG course over ground and ground speed which contains land speed information this information is not being used While the VTG message would provide an instantaneous speed and bearing a more accurate measurement can be obtain by calculating the speed of the drifter from the distance traveled and the time difference The expected errors using the worst case va
8. ility to operate in either autonomous or real time tracking modes A radio link also offers a means to download the data log and program the unit without opening it The drifter design was broken into two parts the hardware software characteristics including GPS radio battery software and the drifter housing Only the hardware software system Table I costing around 500 is discussed in detail below Table I COST ITEMIZATION OF DRIFTER S ELECTRICAL COMPONENTS Item Cost Micro Controller 42 00 Radio 201 00 GPS 93 00 Circuit board 91 00 Battery 21 00 Miscellaneous items 40 00 Total 488 00 II DRIFTER The electronics portion of the drifter fits on a 7 5x22 cm board as shown in Fig 1 This small size makes it capable of being used in a wide variety of drifter enclosures The board follows a modular design and consists of a main circuit 0 7803 7813 X 03 17 00 2003 IEEE 66 board that various off the shelf components can plug into All the components operate on 5V which is supplied by a DC DC converter built on the main board The DC DC converter is used instead of a linear regulator because it allows for a wider range of battery pack voltages yet offers an operating efficiency in the range of 80 90 The main board also features a 3 3V Li ion backup battery used to maintain the real time clock on the micro controller and to reduce the startup time of the GPS Fig 1 micro controller GPS backup battery
9. lues from the GPS are shown in Fig 3 for various time intervals Reported Error vs Water Current 50 40 30 20 10 ees en ae 0 Spates 5 10 15 Water Current cm 20 25 6 min 30 min 60 min Fig 3 The expected water current error for using a time interval of 6 30 and 60 minutes An independent task is used to keep track of the CPU time and to determine when to take a sample This task provides an easy method for changing the sampling rate and the time the radio will transmit The CPU clock was chosen to 30 68 determine when to sample because it is always available and doesn t rely on the operation of the GPS The final process acts as a master and coordinates the activities of the other processes It also determines when the drifter is operating in a data logging mode or in the user interface mode The user interface provides methods for changing the time on the RTC changing the sampling rate and downloading the data as well as other functions as shown in Fig 4 CBI Drifter 1 Help Menu 2 Display Time 3 Set Time 4 Read GPS 5 Download Data 6 Print Out Latest Data 7 Change Sampling Rate 8 Exit Menu Fig 4 User menu from the drift drifter showing the available options II DRIFTER OPERATION The electronics portion of the drifter along with the test housing is shown in Fig 5
10. nge is governed by the interaction of the line of sight Eq 1 with the interference of the first Fresnel zone Eq 2 d hios r 1 cos 2r Where hios Line of sight height in meters r Effective radius of the earth in meters d Distance between transmitter and receiver in meters h ndd Where n Fresnel zone number h Height of the Fresnel radius for zonen in meters A Wavelength of radio wave in meters d Distance from the transmitter to point in meters Initial calculations determined that for a 6 m high base antenna and a 0 5 m high drifter antenna the expected radio range is about 900 m This value corresponds to what was found by field testing the radio link Range can be improved by modifications to the system First a 2 4 GHz version of the radio can be used The increase in operating frequency will decrease the effects of the Fresnel zone interference which will increase the range Second the drifter antenna can be elevated to 1 m Lastly by switching to a 10 m high Proc of the IEEE OES Seventh Working Conference on Current Measurement Technology antenna located on a platform in Corpus Christi Bay the expected range should be about 7 km D Software The software used to control the drifter was written in C and utilizes the real time operating system pcos Using the ucos operating system allows for breaking the firmware into discrete tasks and then creating an independent process desi
11. o Canada user manual MaxStream Orem UT http www maxstream net T Ojo J S Bonner M Sterling F J Kelly C A Page J Perez C Fuller Adaptive sampling for coastal environmental monitoring using a geo referenced mobile platform 3 euroGOOS conference 2002 in press online Available
12. quipped with a sensor suite similar to that of the spill of opportunity boat but are used to record time series information about the bays In this mode the drifters can be deployed around the platform during routine service trips and tracked real time by the data loggers already present on the platforms IV DATA RETRIEVAL SYSTEMS Drifter data are retrieved via its radio The base station radio that communicates with the drifter has been implemented in two versions A Radio with Rabbit core A RCM2200 rabbit core is interfaced to the same radio model used in the drifter It is essentially the same as the RCM2300 that is used in the drifter but it includes an Ethernet port The Ethernet port allows the base station to FTP the drifter data directly to a fileserver for use and archiving It also allows the user the ability to Telnet into the base station in order to modify its operation such as how often the data are transferred to the fileserver This version of the base station is used for downloading the data from the drifters when they are operated in the self contained mode and during the boat based tracking deployments B Radio with Data logger This version of the base station is simply a radio connected to one of the serial ports of an existing data logger With this method the information reported by the drifter will be treated as another sensor attached to the data logger This method is planned for deploym
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