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GPS “The Next Utility”

GPS “The Next Utility”. Who What Where When Why How. GPS: Global Positioning System US System: NAVSTAR ( NAV igation S ystem with T iming A nd R anging) Managed by: US Dept. of Defense The Russian Federation system: GLONASS ( GLO bal N avigation S atellite S ystem).

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GPS “The Next Utility”

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  1. GPS “The Next Utility”

  2. WhoWhat Where When Why How GPS: Global Positioning System US System: NAVSTAR (NAVigation System with Timing And Ranging) Managed by: US Dept. of Defense The Russian Federation system: GLONASS (GLObal Navigation Satellite System)

  3. Who What Where When Why How 24 orbiting satellites (solar powered, radio transmitting) 4 satellites in each of 6 orbiting planes 3 of the 24 are considered “spares” Orbiting speed: 3.87 km/s Orbiting angle: 55o angle from equator Size: 9m width with solar panels extended Control stations are positioned near the equator, around the globe.

  4. Who What Where When Why How • A GPS system is comprised of several components: • GPS satellites • The satellite ground control system • A GPS antenna and receiver • Data loggers and/or computers • Software for processing

  5. Who What Where When Why How Geosynchronous orbit (communications satellites) 13,000 km Zone of 20,200 km space junk 35,420 km GPS satellite orbit

  6. Who What Where When Why How Origin of problem: shipping in the 1800’s Predecessor of GPS technology: WWII radar and later, shipping radio transmitters. 1978: First GPS satellite launch 1983: GPS revealed (kept secret until now) 1994: All 24 satellites operational 1996: Investment so far - $12 billion 1999: “Washington, DC -- Vice President Gore announced today a $400 million new initiative in the President's balanced budget that will modernize the Global Positioning System (GPS) and will add two new civil signals to future GPS satellites, significantly enhancing the service provided to civil, commercial, and scientific users worldwide.”

  7. Who What Where When Why How Originally develop by the military, for the military. Now, civilian uses have far exceeded military uses, but the DoD maintained strict control… leading to a long political battle. (GPS was used extensively in Desert Storm.)

  8. Who What Where When Why How The Fundamental Principal: Speed * Time = Distance Radio waves are electromagnetic radiation, and travel at a constant speed: 299,792,458 meters/sec Thus, if we can measure how long it takes for a signal to reach us, we know the distance to the satellite.

  9. Who What Where When Why How If we know our distance from one satellite, we know we lie somewhere on a theoretical sphere, with a radius equal to that distance. x x = known distance from satellite = GPS satellite (sphere)

  10. x x = known distance from satellite = GPS satellite Who What Where When Why How (spheres) If we know our distance from two satellites, we know we lie somewhere on a theoretical circle, that is the intersection of the two spheres.

  11. (spheres) x = one of two possible positions x = known distance from satellite = GPS satellite If we know our distance from three satellites, we know we lie on one of two points, one of which is impossible. Who What Where When Why How

  12. Who What Where When Why How In theory, only three satellites are needed to acquire an accurate position fix. In practice, we need four satellites because of error that arises from a variety of sources. The core of a good understanding if GPS is understanding these error sources.

  13. Who What Where When Why How Sources of error: Distance of error: Satellite clock errors < 1m Ephemeris errors (satellite position) < 1m Receiver errors (fraction arithmetic) < 2m Ionosphere (charged particles) < 2m Troposphere (the dense part) < 2m Multi-path errors Variable Selective Availability (when active) (< 33m) Satellite Geometry (PDOP) * 4 - 6 PDOP = Position Dilution of Precision

  14. Who What Where When Why How How does satellite geometry influence accuracy? A telemetry example (2D): Receiver 90o 30o Shape of area which may contain transmitter Shape of area which may contain transmitter

  15. Who What Where When Why How Without correcting for these errors, we can achieve about a 5m accuracy with parallel tracking units (good ones), or 10m with serial tracking units (cheap ones). Several sources of error are very difficult to correct for. Fortunately, the largest error sources can be corrected for using differential processing. With differential processing, we can achieve accuracies of < 1m, and even centimeter accuracy with the right receiver.

  16. Who What Where When Why How Averaging (no correction) Actual location Single GPS location Averaged GPS location + 1-2 m + + + + + Averaging increases accuracy to around 2-4m. The more points you average, the better your accuracy. + + + + + + + + + + + +

  17. Who What Where When Why How Differential GPS 1. Using post-processing Data from the known location is used to identify the error. The post-processing removes this error from the remote data. Known location data (base station) Remote location data collected simultaneously

  18. Who What Where When Why How Differential GPS 2. Real-time differential Data from the known location is sent via radio signals to the remote receiver, which removes the error using real-time processing. No post-processing is needed. We can obtain sub-meter locations in about 5 seconds. Known location data (transmitting base station) Remote location data collected simultaneously

  19. Who What Where When Why How Differential GPS US Coast Guard transmitting station coverage for real-time DGPS in Wisconsin.

  20. Who What Where When Why How Obstacles to GPS signals: GPS signals are high frequency because low frequency signals tend not to travel in a straight line through the atmosphere. The cost is that high frequency signals have very little penetration through matter. They are also easily reflected. The signals pass through: thin plastic, cloth, canvas, etc. They do not pass through: anything metal, or anything containing a high degree of water (flesh, deciduous leaves, very heavy rain, etc).

  21. Who What Where When Why How Obstacles to GPS signals (cont.): Smooth surfaces act as mirrors to GPS signals. “Smooth” to a high frequency radio wave means anything as smooth or smoother than a coarse gravel road. Open water is particularly reflective. Reflectance leads to multipath errors...

  22. Who What Where When Why How Multipath error: Increases the length of time taken for a signal to reach the receiver.

  23. Who What Where When Why How • Types of GPS unit: • Low end (fishing boat GPS) • Single channel • Track in serial • $100-$400 • Accuracy: 10m • High end (University / surveying / photogrammetry) • 6-10 channels • Track in parallel • $2500 - $25000 • Accuracy: 5m, sub-meter real-time, centimeter accuracy with post-processing. • Military: ? Real-time centimeter accuracy?

  24. TEST 1. Why is this equatorial rainforest wildlife biologist sitting on a horse in the middle of a river? 2. What problems is she likely to be having (with her GPS unit)?

  25. The Physics of GPS

  26. Who What Where When Why How • How is the signal sent? • The signal consists of two parts: • the carrier - on all the time • the modulation - carries the information • Signals are broadcast on 2 frequencies (only one of which is for civilian use). • Coarse/Acquisition code: • Frequency: 1575.42 MHz (FM radio is around 100 MHz) • Wavelength: 20 cm (short, hence difficulty with obstacles).

  27. Who What Where When Why How What is in the signal? 10011101101110001010101010 1023 bits repeated 1000 times/second Called “pseudo-random noise” Contains information about the satellite, the contents of the code, the time the code was sent, etc.

  28. Who What Where When Why How How is time measured? Each satellite keeps accurate time using four atomic clocks ($50,000 each). The receiver has a much less accurate (and cheaper) clock (this is one source of error). The receiver and the satellite generate the same code at the same time. The receiver determines range by matching the satellite code to its’ own code to calculate how long the signal took to reach it, and therefore the distance of the satellite (time x speed = distance).

  29. Who What Where When Why How How does satellite geometry influence accuracy? (cont.) GPS positioning involves 4 dimensions (3D space plus time) The influence of geometry is measured with “Dilution of Precision” (DOP). A DOP value of 1 is perfect geometry. DOP’s > 6 indicate poor geometry and readings are not taken.

  30. Who What Where When Why How DOP North DOP (NDOP) East DOP (EDOP) Vertical DOP (VDOP) Time DOP (TDOP) The 4 dimensions Horizontal DOP (HDOP) consists of NDOP and EDOP Position DOP (PDOP) consists of HDOP and VDOP Geometric DOP (GDOP) consists of PDOP and TDOP

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