Communications in GPS

1. Communications in GPS • Tracey Willmott • City of Whittlesea • GPS Co-Ordinator • SatCon Services • Magellan/Ashtech dealer

2. Overview • Real time GPS • differential GPS • four commercial providers in Australia • other options • Real time GPS communications to GIS • NMEA

3. Differential GPS • This concept requires 2 GPS receivers working at the same time, where one is the base station, set up at a known position; and the other is the rover that moves around to the unknown positions. • Two methods of applying the differential correction:- • Post Processing - where the differential corrections, as acquired at the base station, are applied after the fact; • Real Time Differential - where the differential signals are transmitted directly (via radio signal) to the rover.

4. Differential GPS

5. Differential GPS • The differential correction components (X, Y, Z) are transmitted to the rover via radio wave • High frequency radio is usually used, because of the clarity of data, and because coverage is restricted to the local area • Rover GPS must have a radio receiver as well as GPS receiver. • Differential signals need to be received every few seconds to maintain DGPS status (position accuracy)

6. RTCM • Radio Technical Commission for Maritime Services Special Committee 104 (RTCM SC-104) • = the messages transmitted by a base station containing the differential corrections • RTCM is the broadcast standard for real time DGPS • Approximately 23 RTCM messages • DGPS uses Types 1 & 9, with possibilities of Types 2, 3, 5, 6, 7, 16 • RTK uses Types 1 & 9 as well as Types 18 & 19

7. Real Time Differential GPS • Five options for real time differential GPS in Australia • Marine Beacon (AMSA) • FM Radio (AUSNAV) • Omnistar (Fugro) • Landstar (Thales, formerly Racal) • Private radio • Achievable accuracies ranging from decimetre to 3-5 metres

8. 1. Marine Beacon • Operated and maintained by the Australian Maritime Safety Authority (AMSA) • Designed for marine safety/navigation • Corrections transmitted at low frequency for greater coverage (low frequency radio travels great distances over water) • (Cape Schank) signal can be received as far inland as Ballarat, Shepparton, Bairnsdale

9. 1. Marine Beacon • www.amsa.gov.au/ns/dgps

10. 1. Marine Beacon • Benefits • free-to-air • very good coverage up east coast of Australia • reliable service because navigation safety relies upon it • Disadvantages • accuracy can be reduced further away from base station • does not work inland of coastal areas

11. 2. FM Radio • Subscription service operated privately through Ausnav Services (formerly govt operated through AUSLIG - sold in 1997) • Differential correction signal piggy backed on ABC FM (Triple JJJ) radio signal • Base stations established at or close to ABC transmitting towers • Coverage does not always include areas where Triple JJJ can be received

12. Differential GPS

13. 2. FM Radio • Benefits • good coverage and accuracy in populated populated states • small unobtrusive equipment • Disadvantages • $1200 subscription cost per year • does not work outside nominated areas • difficulty in black “dropout” areas

14. 3. Omnistar • Satellite based world-wide DGPS service offered internationally through Fugro (world’s largest survey company) • Signal is transmitted from base stations are various control points around the world to Optus satellites • DGPS signal is received from more than one satellite to offer a Wide Area solution in most locations on the earth’s surface

15. 3. Omnistar Coverage  Receiver 

16. 3. Omnistar • Benefits • good coverage all over Australia and overseas • good accuracy in Australia through 12 base stations • Disadvantages • expensive

17. 4. Landstar • Satellite based world-wide DGPS service offered internationally through Thales (a French company who have recently purchased Ashtech/Magellan) • Signal is transmitted from base stations are various control points around the world to Optus satellites • DGPS signal is received from more than one satellite to offer a Wide Area solution in most locations on the earth’s surface

18. 4. Landstar Coverage  Receiver 

19. 4. Landstar • Benefits • good coverage all over Australia and overseas • good accuracy in Australia • Disadvantages • expensive

20. 5. Private Radio • In areas where public DGPS coverage is non-existent or unreliable, and where it is cost-effective to do so, a private DGPS radio network can be established • Where a base station is set up, a radio base transmitter must also be established • Need a licence for frequency • Baseline distances will determine power of transmitter

21. Real time DGPS applications • Most recreational GPS receivers will accept an RTCM DGPS signal and correct the displayed position to + 2-5 metres (not as useful since SA was turned off) • GIS level receivers will accept RTCM DGPS and provide positions corrected to + 1 metre • Higher grade receivers will accept RTCM DGPS and provide positions to better than one metre • Survey grade receivers use RTCM RTK and provide positions to centimetre level

22. Real time DGPS applications • Depending upon the DGPS service used, a GPS receiver that has an internal DGPS receiver may be available: • CSI receivers can include built-in beacon receivers, or Omnistar/Landstar receivers or BOTH! • Omnistar receivers can include FM or Beacon in addition to their satellite based correction receivers • Suitable for: • asset data collection • agriculture mapping • mapping/location applications

23. Real time communications to GIS • GPS data can be output to an external device to expand the range of applications available • Some GPS receivers can accept real time DGPS input and output differentially corrected position data • The standard for GPS data output is one or more of the NMEA messages

24. NMEA • NMEA - National Marine Electronics Association • a communications standard established by the marine industry • used by a variety of electronic devices, including GPS and beacon receivers • 8 common messages used by GPS • most useful is the GGA message

25. GGA • The GGA message contains detailed GPS position information. The string takes the following form: $GPGGA,hhmmss.ss,ddmm.mmmmm,s,ddmm.mmmmm,s,n,qq,pp.p,saaaaa.aa,M,+xxxx.xx,sss,aaaa*cc<CR><LF> a = start of string i = position status b = from a GPS receiver j = no. of sats c = string type k = HDOP d = UTC time l = antenna altitude e = latitude m = height units f = hemisphere n = geoidal separation g = longitude o = age of differential corrections

26. GGA • A real example: $GPGGA,034547.00,3730.709706,S,14507.090090,E,2,5,3.9,197.753 • This data can be fed in through the Comm port of a computer, if the communication protocols have been set • HyperTerminal is one program that can be used to set/check this • baud rate - speed of data transfer, correspond with GPS • bits/hardware/etc - all standard communication settings • comm port - which port the GPS is plugged into (serial)

27. GGA • Being ASCII format, many programs can use this data and with some modification, transform this textual data into something spatial.

28. Real time GPS programs • Palm top • ArcPad - ESRI • StarPal - independent, but takes MapInfo table files • Midas - Sokkia • Pen computer • AssetMap - MapInfo • FieldNotes - independent

29. Real time GPS programs Software demonstrations…..

30. Conclusion • RTCM source needed for real time differential GPS • Choose the most appropriate for location and cost • NMEA output needed for real time communication to computer (laptop, pen, palmtop) • With appropriate receivers or cabling, can have real time DGPS input and real time data output • Offers greater flexibility for GIS mapping or data capture