Introduction to GPS presentation
- 1. Introduction to GPS/GNSS Basics Vince Cronin (Baylor University) & Shelley Olds (UNAVCO) Revisions by Beth Pratt-Sitaula (UNAVCO) Version July 2019
- 2. GPS receivers all around us
- 3. The Global Positioning System • 24–32 satellites • 20,200 km altitude • 55 degrees inclination • 12-hour orbital period • Need 4 satellites to be accurate • Ground control stations • Each satellite passes over a ground monitoring station every 12 hours
- 4. GPS Satellite Artist’s conception of a GPS Block II-F satellite in Earth orbit (public domain from NASA)
- 5. http://en.wikipedia.org/wiki/Global_Positioning_System Ground control stations The tracking information from the monitoring stations is sent to the Air Force Space Command, which is operated by the 2nd Space Operations Squadron (2 SOPS) of the US Air Force. 2 SOPS contacts each GPS satellite regularly with a navigational update using the dedicated ground antennas. These updates synchronize the atomic clocks on the satellites to within a few nanoseconds of each other and adjust the ephemeris of each satellite's internal orbital model.
- 6. Almanac & Ephemeris data • GPS satellites include almanac and ephemeris data in the signals they transmit • Almanac data are coarse orbital parameters for all GPS satellites • Ephemeris data are very precise orbital and clock correction for that particular GPS satellite—necessary for precise positioning
- 7. Antennas receive data streams Your location is: 37o 23.323’ N 122o 02.162’ W The time is: 11:34.9722 (UTC) Works the same… ERRORS Horiz: +/- 10 m (30 ft) Vert: +/- 15 m (45 ft) ERRORS (after 24 hrs) Horiz: +/- 1–2 mm (<1/8 in) Vert: +/- 5 mm (1/4 in)
- 8. • Radio signal from satellite tells GPS receiver the satellite- clock time and provides the most recent corrections to the satellite’s position relative to Earth (ephemeris) • GPS receiver compares the satellite-times to receiver- time to determine the distance to each satellite How satellite–receiver distance is measured
- 9. How actual location is determined Antenna position is determined by calculating the distances to at least 4 satellites. This enables the solving for four variables: x, y, z and time using trilateration. http://spaceplace.nasa.gov/gps-pizza/en/
- 10. Anatomy of a High-Precision Permanent GPS Station 10 GPS antenna inside of dome Monument solidly attached into the ground with braces. If the ground moves, the station moves. Solar panel for power Equipment enclosure • GPS receiver • Power/batteries • Communications/radio/modem • Data storage/memory
- 11. High-Precision GPS • Stable monuments • Multiple stations • Sophisticated processing • Collecting lots of data • Using the carrier phase • Dual-frequency receivers • High-precision orbital information (ephemeris) with several years of data can determine velocities to 1–2 mm/yr
- 12. GPS & Atomic Clocks Each GPS satellite has 4 atomic clocks, to be sure that one is always working. Each costs ~US$100,000 and is accurate to 1 billionth of a second (1 nanosecond).
- 13. Sources of Error 13 Some GPS Error Sources • Selective Availability (ephemeris data encrypted by military—turned off in 2000) • Satellite orbit irregularities • Satellite and receiver clock errors • Atmospheric delays—speed of light is affected by water content and other variables in the atmosphere • Multi-path—GPS signals can bounce off the ground and then enter the antenna, rather than only entering from above only • Human errors
- 14. GNSS/GPS Stations Globally • >10,000 stations, with more added all the time • Some data freely available, some not • GNSS = Global Navigation Satellite System
- 15. Network of the Americas (NOTA) NOTA involves installation, operation, and maintenance of >1000 continuously operating high-precision GPS stations (plus >170 other instruments: strainmeters, borehole seismometers, and tiltmeters) http://www.unavco.org/instrumentation/networks/status/all
- 16. Two NOTA stations in California • Twenty-nine Palms,(BEMT) • Mission Viejo (SBCC) Where is that chunk of crust going? • Example: using GPS velocities to understand plate motions
- 17. http://www.unavco.org/instrumentation/networks/status/nota/overview/SBCC • Station position over time North–South East–West Up–Down GPS Time-Series Data
- 18. • From the changing position velocity can be calculated using slope (rise-over-run) http://www.unavco.org/instrumentation/netwo rks/status/nota/overview/SBCC GPS Time-Series Data 19 years 510 mm north 26.8 mm/yr 475 mm west 25.0 mm/yr ~0.4 mm/yr ~8 mm down
- 19. 19 years 510 mm north 26.8 mm/yr 475 mm west 25.0 mm/yr ~0.4 mm/yr ~8 mm down NOTA also supplies “detrended” data with the average velocity subtracted out to observe other phenomena. In that case official velocity is given. Detrended GPS Time Data
- 20. What is a site’s 3D speed? Using the Pythagorean Theorem (high school math...), Speed = (27.8)2 + (25.7)2 + (1.3)2 = 37.9 mm/yr
- 21. Using the horizontal components of velocity, and a bit of high school trigonometry… In what compass direction is the site moving? 42.8°west of north or 317.2°azimuth θ = tan-1(25.7/27.8) = 42.8° θ 27.8 mm/yr 25.7 mm/yr North West
- 22. Map View of Velocity 20 mm/yr
- 23. Two Different Velocities Same process yields much slower velocity at BEMT 20 mm/yr Why the difference?
- 24. San Andreas Fault! 20 mm/yr
- 25. Reference Frames All velocities are RELATIVE to a given reference frame • Velocities compared to International Terrestrial Reference Frame 2014 (IGS14 is GPS reference frame name) • Hot spot constellation as “stable”
- 26. Reference Frames All velocities are RELATIVE to a given reference frame • Velocities compared to stable North America (called NAM14 reference frame) • Eastern North America as “stable”