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GPS, RDS-TMC and writing apps for boats

GPS, RDS-TMC and writing apps for boats

Presentation I gave at NZ Gather 2013.

Tom Isaacson

July 13, 2013
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Transcript

  1. 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.
  2. 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
  3. 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
  4. 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
  5. 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
  6. 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
  7. 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
  8. 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.
  9. 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.
  10. 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
  11. 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
  12. GPS alternatives • Galileo (European) – 2019? – 1 meter

    resolution – Search and rescue, relay distress signals • GLONASS (Russian) - 1995 • BeiDou / Compass (Chinese) – 2020? – Only covers Asia
  13. RDS - Radio Data System • Embeds data in FM

    radio broadcasts • First specified in 1984
  14. 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.
  15. 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)
  16. Footer 23 NMEA 2000 (N2K) • Successor to NMEA 0183

    • Based on CAN • “Open” standard • Exchange of data between multiple manufacturers’ equipment simultaneously
  17. Footer 25 NMEA 2000 costs • US$4,000 for complete specification

    (electronic) • N2K certification • Per-device fee, also applies to mobile apps
  18. Footer 26 GoFree • Service discovery (proprietary, Bonjour, SSDP) •

    Remote control (iOS and Android apps) • NMEA 0183 stream • Device webserver for settings files • Websocket server
  19. 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