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CS 128/ES 228 - Lecture 11a1 GPS

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CS 128/ES 228 - Lecture 11a2 Global Positioning System www.usace.army.mil

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CS 128/ES 228 - Lecture 11a3 GPS in practice. Recreational GIS inputs Surveying Transportation and of course, the military

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CS 128/ES 228 - Lecture 11a4 The NAVSTAR system GPS for Land Surveyors. J. Van Sickle. Ann Arbor Press, 1996 Military wanted portable, fast, passive positioning system Navstar I launched Feb ’78. Now 24 satellites + “spares” Global, 24/7 coverage by at least 4 satellites Satellites carry atomic clocks

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CS 128/ES 228 - Lecture 11a5 GPS basics Both: Differential GPS Explained. J. Hurn. Trimble Navigation, 1993

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CS 128/ES 228 - Lecture 11a6 1, 2 … Both: Differential GPS Explained. J. Hurn. Trimble Navigation, 1993

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CS 128/ES 228 - Lecture 11a7 3! Differential GPS Explained. J. Hurn. Trimble Navigation, 1993 Technically 4 satellites are necessary, but normally only 1 of the 2 points is on the geoid But, as we’ll see later, a 4 th satellite is still useful

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CS 128/ES 228 - Lecture 11a8 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?

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CS 128/ES 228 - Lecture 11a9 Pseudocode GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993 A string of pseudo-random bits Predetermined sequence – can be generated by the satellite and the receivers

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CS 128/ES 228 - Lecture 11a10 Code correlation GPS for Land Surveyors. J. Van Sickle. Ann Arbor Press, 1996

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CS 128/ES 228 - Lecture 11a11 But wait: for $19.95 you get … 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

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CS 128/ES 228 - Lecture 11a12 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 Survey-grade receivers – mucho $$

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CS 128/ES 228 - Lecture 11a13 Sources of error 1.[Selective availability] 2.Clock errors 3.Ephemeris errors 4.Atmospheric delays 5.Multipath effects 6.Receiver errors Differential GPS Explained. J. Hurn. Trimble Navigation, 1993

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CS 128/ES 228 - Lecture 11a14 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 <<1 µs??

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CS 128/ES 228 - Lecture 11a15 Voila – the 4 th satellite! Both: GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993 A 2-D example: Clocks synchronized Clocks not synchronized

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CS 128/ES 228 - Lecture 11a16 If the 4 ranges don’t overlap: Both: GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993 Receiver adjusts its clocks until they do [Actually: done by algebra, not trial & error] The time on a $100 GPS unit is really accurate!

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CS 128/ES 228 - Lecture 11a17 Ephemeris errors GPS for Land Surveyors. J. Van Sickle. Ann Arbor Press, 1996 Trimble Navigation Ephemeris: mathematical description of an orbit

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CS 128/ES 228 - Lecture 11a18 Atmospheric delays Differential GPS Explained. J. Hurn. Trimble Navigation, 1993 Signal slowed by: - charged particles in the ionosphere - water vapor in the troposphere Dual-frequency receivers can correct for ionosphere but not troposphere errors

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CS 128/ES 228 - Lecture 11a19 Multi-path errors Top: GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993 Bottom: www.gpsw.co.uk Worse: - near buildings, other obstructions - satellites near horizon: use “elevation mask” Better: - more sophisticated antenna - ground plane to block low-angle reflections

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CS 128/ES 228 - Lecture 11a20 Typical error “budget” (m)* Source: Standard GPS Differential GPS Satellite clocks1.5~ 0 Ephemeris errors2.5~ 0 Ionosphere delays5.00.4 Troposphere delays0.50.2 Multi-path (reflections)0.6 Receiver errors0.3 Typical totals10 – 151-2 Differential GPS Explained. J. Hurn. Trimble Navigation * Horizontal position; vertical errors typically 2x or greater

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CS 128/ES 228 - Lecture 11a21 Improving GPS accuracy DOP and mission planning Differential GPS Surveying-grade GPS

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CS 128/ES 228 - Lecture 11a22 DOP & “mission” planning Both: GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1993 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

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CS 128/ES 228 - Lecture 11a23 “mission” planning software Pathfinder Office. Trimble Navigation

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CS 128/ES 228 - Lecture 11a24 Differential GPS Differential GPS Explained. J. Hurn. Trimble Navigation, 1993 Fixed base station: - knows its location - records any shifts in its readings in correction file Differential corrections: - post-processing - real time

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CS 128/ES 228 - Lecture 11a25 Real-time DGPS GPS: A guide to the next utility. J. Hurn. Trimble Navigation, 1989 Radio link with base station U. S. Coast Guard beacons WAAS

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CS 128/ES 228 - Lecture 11a26 Coast Guard beacons www.navcen.uscg.gov/dgps/coverage/NYork.htm Trimble’s Beacon on a Belt www.trimble.com

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CS 128/ES 228 - Lecture 11a27 WAAS Wide Area Augmentation System Designed by FAA & DOT for general aviation ground stations collect DGPS data & uplink to 2 geostationary satellites, 1 over east coast, 1 over the west coast www.garmin.com/aboutGPS/waas.html

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CS 128/ES 228 - Lecture 11a28 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 < 1 cm Expensive, complex, and requires long periods of data collection at each station Differential GPS Explained. J. Hurn. Trimble Navigation, 1993

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CS 128/ES 228 - Lecture 11a29 The Future of GPS Everyday utilities (cell phones, car navigation systems) Integrated GIS/GPS units GPS stalking?? ??? www.garmin.com/aboutGPS/waas.html

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