GPS, RDS-TMC and writing apps for boats

Presentation I gave at NZ Gather 2013.

Transcript

1. GPS, RDS-TMC and writing apps for boats Tom Isaacson @parsley72
2. GPS predecessors • LORAN (1947-2010) – Ground based. • TRANSIT / NAVSAT (1964-1996) – Analyse Doppler shift of signals • Timation (1967-?) – Broadcast accurate time reference • Omega (1971-1997) – First global system, ground based.
3. GPS #1
4. GPS #2 • Completed in 1995 • At least 24 satellites, currently about 31 • Medium earth orbit NOT geostationary – 20,200 km, 12 hour orbit • Almanac data – List of all satellites – Valid for 180 days, broadcast every 12.5 mins • Ephemeris data – Specific to individual satellite – Valid for 4 hours, broadcast every 30 secs • 15 metre nominal accuracy
5. GPS Time To First Fix (TTFF) • Cold / Factory – No data • Warm / Normal – Estimate of current time ~20 seconds, position ~100km, velocity ~25m/s and valid almanac data • Hot / Standby – Valid time, position, almanac and ephemeris data
6. GPS protocol • NMEA 0183 (National Marine Electronics Association) $GPGGA,092750.000,5321.6802,N,00630.3372,W,1,8,1.03,61.7,M,55.2,M,,*7 6 $GPGSA,A,3,10,07,05,02,29,04,08,13,,,,,1.72,1.03,1.38*0A $GPGSV,3,1,11,10,63,137,17,07,61,098,15,05,59,290,20,08,54,157,30*70 $GPGSV,3,2,11,02,39,223,19,13,28,070,17,26,23,252,,04,14,186,14*79 $GPGSV,3,3,11,29,09,301,24,16,09,020,,36,,,*76 $GPRMC,092750.000,A,5321.6802,N,00630.3372,W,0.02,31.66,280511,,,A*43 $GPGGA,092751.000,5321.6802,N,00630.3371,W,1,8,1.03,61.7,M,55.3,M,,*7 5 $GPGSA,A,3,10,07,05,02,29,04,08,13,,,,,1.72,1.03,1.38*0A $GPGSV,3,1,11,10,63,137,17,07,61,098,15,05,59,290,20,08,54,157,30*70 $GPGSV,3,2,11,02,39,223,16,13,28,070,17,26,23,252,,04,14,186,15*77 $GPGSV,3,3,11,29,09,301,24,16,09,020,,36,,,*76 $GPRMC,092751.000,A,5321.6802,N,00630.3371,W,0.06,31.66,280511,,,A*45
7. GPS limitations
8. GPS improvements #1 • Differential GPS (DGPS) – Fixed, ground based reference stations – Broadcast the difference between calculated and actual position locally – Setup in US, Europe, Canada, Australia • Wide Area Augmentation System (WAAS) / European Geostationary Navigation Overlay Service (EGNOS) – Broadcasts the correction via satellite
9. GPS improvements #2 • Assisted GPS (aGPS) – Get almanac and ephemeris data over network – Server can assist in calculations • Generation of ephemeris data – Only valid for 3 days • Dead Reckoning – Wheel ticks
10. Downtown LA – GPS only
11. Downtown LA – GPS and DR
12. GPS in NZ • PositioNZ – 31 stations continuously tracking GPS at locations across New Zealand, two in the Chatham Islands and three in Antarctica – Real-time 1Hz streams from the stations are available in RTCM (Radio Technical Commission for Maritime Services) 3.1 format via NTRIP (Networked Transport of RTCM via Internet Protocol) http://apps.linz.govt.nz/positionz/rt/index.php
13. GPS interference http://www.newscientist.com/article/dn20202-gps-chaos-how-a-30-box-can-jam-your-life.html • January 2007, San Diego: – Air traffic control, pagers, traffic management for boats, cellphones, ATMs failed. – Two navy ships in harbour were conducting a training exercise. To test procedures when communications were lost, technicians jammed radio signals which also blocked radio signals from GPS across the city.
14. GPS interference #2 • In 2010 an experiment was conducted in the North Sea aboard the THV Galatea, a 500-tonne ship. They used a simple jamming device that blocked GPS. • The ship went haywire. According to the electronic display on the ship's bridge, the Galatea was suddenly flying at Mach speeds over northern Europe and Ireland. Then alarms sounded. The ship's navigation backup – its gyrocompass – crashed, because it uses GPS to provide corrections. The radar did the same. Even the ship's satellite communications failed, because GPS points the antenna in the right direction.
15. GPS combinations • Wifi positioning systems – Database of Wifi locations – Providers: Google, Navizon, Skyhook Wireless • Enhanced LORAN (eLORAN) – Advancement over LORAN – 8 meter accuracy – UK eLORAN in full service 2010-2022 – Unfortunately US shut down LORAN in 2010
16. GPS extensions • Carrier-Phase Enhancement (CPGPS) – Uses measurements of the phase of the signal′s carrier wave, rather than the information content of the signal – Uses a single base station that retransmits the phase of the carrier that it observices – Centimetre-level accuracy, but only in relation to base station
17. GPS alternatives • Galileo (European) – 2019? – 1 meter resolution – Search and rescue, relay distress signals • GLONASS (Russian) - 1995 • BeiDou / Compass (Chinese) – 2020? – Only covers Asia
18. RDS - Radio Data System • Embeds data in FM radio broadcasts • First specified in 1984
19. RDS information fields • AF (alternative frequencies) • CT (clock time) • EON (enhanced other networks) • PI (programme identification) • PS (programme service) • PTY (programme type) • REG (regional) • RT (radio text) • TA, TP (traffic announcement, traffic programme) • TMC (traffic message channel) Most car stereos will support at least AF, EON, PS, REG and TA/TP.
20. TMC - Traffic Message Channel • Each message consists of an event code, location code, expected incident duration, affected extent and other details. – 14: queuing traffic (with average speeds Q) – 100: vehicles slowing to look at (Q) accident(s). Traffic building up – 379: sports meeting. Danger of stationary traffic – 877: sewer overflow. Danger – 1071: swarms of insects (visibility reduced to Q)
21. RDS-TMC in NZ
22. AIS - Automatic Identification System
23. Footer 23 NMEA 2000 (N2K) • Successor to NMEA 0183 • Based on CAN • “Open” standard • Exchange of data between multiple manufacturers’ equipment simultaneously
24. Footer 24
25. Footer 25 NMEA 2000 costs • US$4,000 for complete specification (electronic) • N2K certification • Per-device fee, also applies to mobile apps
26. Footer 26 GoFree • Service discovery (proprietary, Bonjour, SSDP) • Remote control (iOS and Android apps) • NMEA 0183 stream • Device webserver for settings files • Websocket server
27. Footer 27 Websockets (using JSON) • Open specification (download from website) • Free to use • Sample source on Github (C#) • Access to some of N2K data, more to come • Apps featured on brand websites
28. Footer 28 Links • http://www.simrad-yachting.com/en- gb/Products/GoFree-Wireless-Technology/ • http://www.lowrance.com/Products/Wireles s/ • http://www.bandg.com/en-GB/Product- Groups/GoFreeWireless/