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GPS

GPS. A guide to GPS theory …. www.usace.army.mil. … and practice. Recreational GIS inputs Surveying Transportation and of course, the military. A brief history of GPS….

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GPS

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  1. GPS CS 128/ES 228 - Lecture 10b

  2. A guide to GPS theory … www.usace.army.mil CS 128/ES 228 - Lecture 10b

  3. … and practice. • Recreational • GIS inputs • Surveying • Transportation • and of course, the military CS 128/ES 228 - Lecture 10b

  4. A brief history of GPS… • Forerunners- LORAN-C: marine navigation. Used radio beacons along shoreline. Localized coverage and low accuracy (CEP ~ 200 m) - TRANSIT: developed by U. S. Navy. Used 6 satellites, low orbits. Global coverage & high accuracy (sub-meter) but “fix” took hours to days • Department of Defense: 1970s study showed ~120 navigation systems in use. Proposed a single system, called NAVSTAR CS 128/ES 228 - Lecture 10b

  5. The NAVSTAR system • Military wanted portable, fast, passive positioningsystem • Navstar I launched Feb ’78. Now 24 satellites + “spares” • Global, 24/7 coverage by at least 4 satellites • Satellites carry atomic clocks GPS for Land Surveyors. J. Van Sickle. Ann Arbor Press, 1996 CS 128/ES 228 - Lecture 10b

  6. GPS basics Both: Differential GPS Explained. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10b

  7. 1, 2 … Both: Differential GPS Explained. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10b

  8. 3! • Technically 4 satellites are necessary, but normally only 1 of the 2 points is on the geoid • But, as we’ll see later, a 4th satellite is still useful Differential GPS Explained. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10b

  9. How to time the signal? • Standard EDMs bounce a signal off a reflector and measure time of return • But, GPS requires a low-power transmission and passive receivers • Imagine 2 people with synchronized stop watches, standing 1 mile apart. A gun fires near 1, and each records the time when they hear the shot. What do the 2 times tell you? CS 128/ES 228 - Lecture 10b

  10. Pseudocode • A string of pseudo-random bits • Predetermined sequence – can be generated by the satellite and the receivers GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10b

  11. Code correlation GPS for Land Surveyors. J. Van Sickle. Ann Arbor Press, 1996 CS 128/ES 228 - Lecture 10b

  12. But wait: for $19.95 you get all these extra codes … • C/A (coarse acquisition) code:- short (1 ms) & slow (1.023 Mbps)- meant to enable receivers to get a crude “fix” quickly • P (precision) code:- long (267 d) & fast (10.23 Mbps) - permits sub-meter accuracy CS 128/ES 228 - Lecture 10b

  13. Why use the P code? • C/A code bits are ~ 1 µs wide • Even a good receiver can be out of phase with the code by 1-5% • 1% phase error ~ 3 m position error Differential GPS Explained. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10b

  14. Still not sure? There’s more … • Each code is broadcast on 2 frequencies, the L1 and L2 bands • “Dual frequency” receivers utilize the frequency difference between L1 & L2 to compensate for atmospheric distortions – more on that later • Mucho expensive CS 128/ES 228 - Lecture 10b

  15. Sources of error • [Selective availability] • Clock errors • Ephemeris errors • Atmospheric delays • Multipath effects • Receiver errors Differential GPS Explained. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10b

  16. A small clock error matters! • Microwaves travel at approximately the speed of light: 300,000 km per second • A clock error of only 1 µs could produce a positional error of ~ 300 m! • How can a $100 GPS receiver have a clock that stays accurate to the µs?? CS 128/ES 228 - Lecture 10b

  17. Voila – the 4th satellite! A 2-D example: Clocks synchronized Clocks not synchronized Both: GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10b

  18. If the 4 ranges don’t overlap: • Receiver adjusts its clocks until they do • [Actually: done by algebra, not trial & error] • The time on a $100 GPS unit is really accurate! Both: GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10b

  19. Ephemeris errors Ephemeris: mathematical description of an orbit Trimble Navigation GPS for Land Surveyors. J. Van Sickle. Ann Arbor Press, 1996 CS 128/ES 228 - Lecture 10b

  20. Atmospheric delays • Signal slowed by:- charged particles in the ionosphere- water vapor in the troposphere • Dual-frequency receivers can correct for ionosphere delays Differential GPS Explained. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10b

  21. Multi-path errors • Worse:- near buildings, other obstructions- satellites near horizon: use “elevation mask” • Better:- more sophisticated antenna- ground plane to block low-angle reflections Top: GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993Bottom: www.gpsw.co.uk CS 128/ES 228 - Lecture 10b

  22. Typical error “budget” (m)* Differential GPS Explained. J. Hurn. Trimble Navigation * Horizontal position; vertical errors typically 2x or greater CS 128/ES 228 - Lecture 10b

  23. Improving GPS accuracy • DOP and mission planning • Differential GPS • Surveying-grade GPS CS 128/ES 228 - Lecture 10b

  24. DOP & “mission” planning • Dilution of precision: a small number of satellites or poor positions degrades accuracy • Advance software lets you plan when to use GPS for maximum accuracy Both: GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10b

  25. “mission” planning software Pathfinder Office. Trimble Navigation CS 128/ES 228 - Lecture 10b

  26. Differential GPS • Fixed base station:- knows its location- records any shifts in its readings in correction file • Differential corrections:- real time- post-processing Differential GPS Explained. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10b

  27. Real-time DGPS • Radio link with base station • U. S. Coast Guard beacons • WAAS GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1989 CS 128/ES 228 - Lecture 10b

  28. Coast Guard beacons Trimble’s Beacon on a Belt www.navcen.uscg.gov/dgps/coverage/NYork.htm www.trimble.com CS 128/ES 228 - Lecture 10b

  29. WAAS • Wide Area Augmentation System • Designed by FAA &DOT for generalaviation • 25 ground basestations collect DGPS data & uplinkto 2 geostationary satellites, 1 over east coast, the other over the west coast www.garmin.com/aboutGPS/waas.html CS 128/ES 228 - Lecture 10b

  30. Effectiveness of WAAS • Corrects for satellite orbit & clock errors, plus ionosphere & troposphere distortions • Capable of improving accuracy to < 3 m for WAAS enabled receivers • Vertical accuracy is not yet sufficient for landing airplanes at uninstrumented airports, the original program objective • The eastern satellite (#35) is low on the SE horizon and signal can be hard to receive CS 128/ES 228 - Lecture 10b

  31. Post-processing • GPS receiver logs all signals received during data collection phase • Data log compared to similar record from a base station receiver • U. S. NGS operates a series of public base stations and an internet file processing system (OPUS) for free postprocessing www.ngs.noaa.gov/OPUS/What_is_OPUS.html CS 128/ES 228 - Lecture 10b

  32. Survey-grade GPS • Uses high-quality, dual-frequency receivers and DGPS • Carrier phase processing- utilizes the L1 or L2 carrier signal, 1.2 / 1.5 GHz • Can achieve accuracies of < 1cm • Expensive, complex, and requires long periods of data collection at each station Differential GPS Explained. J. Hurn. Trimble Navigation, 1993 CS 128/ES 228 - Lecture 10b

  33. The Future of GPS www.garmin.com/aboutGPS/waas.html • Everyday utilities (cell phones, car navigation systems) • Integrated GIS/GPS units • GPS stalking?? • ??? CS 128/ES 228 - Lecture 10b

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