GPS
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GPS: Global Positioning System Our next utility How do we know where we are? Line of sight Celestial Navigation LORAN DECCA Sat-Nav What is GPS? Developed by the Dept. of Defense $12 billion system Began construction in mid ’70s Developed for military operations, but with a provision for civilian use Consists of 24 (28) high orbit satellites sending out coded radio signals that are picked up by receivers that calculate position How does GPS work? GPS is based on satellite ranging 24+ satellites orbit 11,000 miles overhead Each satellite orbits the earth once every 12 hours Four satellites are required by the receiver for an exact position Usually with 5 to 8 SVs visible at any time (SVs= Space Vehicles) Precision timing Each satellite is equipped with an atomic clock, accurate timing is key Receiver clocks don’t have to be perfect because a trigonometry trick can cancel out receiver clock errors In reality 3 satellites can give a position (narrows it down to 2 points and the erroneous point is discarded) The fourth satellite allows for correction of the receiver’s clock Knowing where the satellites are is key Satellites have predictable orbits Minor variations in their orbits occur Base stations monitor satellite atomic clocks and position Corrections are broadcast from the satellites GPS Satellites NAVSTAR (NAVigation Satellite Timing And Ranging) Rockwell International Orbit is 10,900 nautical miles above Earth Satellites weigh 1900 lbs 17 ft with solar panels extended 12 hour orbital period Orbital plane is 55* to equatorial plane Six orbital planes with 4 SVs in each plane Life span 7.5 years 24+ satellites at any given time (only 17 are required) Measuring the distance from a satellite Speed of light x time = distance Radio waves are electromagnetic waves, like light, and travel at 186,000 miles per second A satellite overhead will transmit its signal to us in 6/100ths of a second Most receivers measure in nanoseconds (0.000000001 second) All satellites generate the same “psuedorandom” code every second These codes are compared by the receiver to calculate position GPS Errors (typical) Satellite clock error 2 feet Ephemeris error 2 feet Receiver error 4 feet Atmospheric delay 12 feet Selective Availability (if on) 25 feet Multipath errors and obstructions* Multiply by Geometric Dilution of Precision Error = 60 to 100 feet in most cases, up to 350 with selective availability implemented Selective Availability Turned off May 1, 2000 Limits civilian accuracy while allowing military to use GPS full potential Alters satellite’s atomic clocks according to a specific code However the “civilians” got around this quickly with Differential GPS Can get accuracy of less than a meter with inexpensive equipment Specialized receivers can get within a centimeter So why have Selective Availability? [It’s turned off now] It was turned off during the Persian Gulf War and the invasion of Haiti because the military did not have enough classified equipment to go around and bought off the shelf GPS units by mail order. The Russians have their own GPS and it is not encoded So why bother with selective availability when it cost us millions of taxpayers money? Because the DOD is dithering on Earth and in space Differential GPS A receiver placed at a known location calculates the combined error in the satellite range data That correction can be applied to all other receivers in the same locale, to eliminate virtually all error in their measurements. This can be done in “real time” or by “postprocessing” the data after collection Selecting a GPS Do you need an occasional position fix or accurate steering? Do you need accurate velocity measurements? Is economy more important than accuracy? Is power consumption an important factor? Is size and weight a factor? Will the receiver operate in high dynamic conditions (ex. open ocean)? Two broad groups of receivers are those that can track four or more satellites simultaneously and those that sequence between satellites. Satellite Health Good GPS receivers will carry an “Almanac” that they download from the satellites This means the GPS receiver is programmed to know where in the sky each satellite will be at a given moment An addition to the psuedo-random code each satellite broadcast a “data message” indicating the satellite’s health and will also broadcast minor corrections. GeoExplorer III 12 Channels Post processing and real-time differential collection Stores an Almanac Capable of receiving “Carrier Signals” GeoExplorer II by Trimble [The GeoExplorer III’s are simplier] There are five GeoExplorer II settings: Logging Interval Position Mode Elevation Mask Signal-To-Noise Ratio Position Dilution of Precision (PDOP) Logging Interval Logging interval defines the frequency at which a position is stored. Point features – Set a 1 second Line/Area features – should match the base station logging interval. Logging intervals may also depend on speed of travel: If walking – 5 seconds If driving – 1 second Position Mode For the best accuracy with GEII avoid 2D data collection Manual 2D: you must enter the altitude value Auto 2D/3D: uses 3D unless only 3 SVs available than 2D is collected Manual 3D: uses 4+ SVs Overdetmined 3D: uses 5+ SVs Elevation Mask Elevation masks restrict your receiver to using only those satellites above a certain elevation in the sky. This ensures that a base station can always see all the satellites used by the rover. The default elevation mask for a rover is 15* Lowing masks increase # of satellites, increases atmospheric delay and increases effects of multipath. Signal to Noise Ratio Mask As the proportion between the signal and the noise decreases, data is distorted by the noise. The higher the value of the SNR, the better. Quality is degraded if it falls below 6.0 Typical SNRs range between 10 and 25 Position Dilution of Precision (PDOP) Is a value that indicates when the satellite geometry can provide the most accurate results. The wider the angle between satellites the better the measurement. It measures satellite’s location relative to other satellites. A low PDOP, such as 3, indicates a higher probability of accuracy, a high value of 7, indicates a lower probability of accuracy. Vertical Error Depending on satellite geometry the vertical error can be up to 3 times the horizontal error. It is difficult to calculate because SVs have a limited perspective in which to measure height. If the receiver could use signals form underneath it would be much better but the Earth blocks that Autonomous GEII operation No differential applied Errors now seem to be about 5 to 10 meters with atmospheric delay causing the largest errors. In the past Selective Availability caused the largest error. SA error can be up to 100 meters in the horizontal and 156 meters in the vertical Postprocessed Differential Done with Pathfinder Office software You need 2 receivers to correct data One that operates as a base station at a known location One that operates as a rover and collects data at the same time as the base station The correction factor is applied to the rover Accuracy is 2-5 meters in the horizontal Real-time Differential GPS receiver is linked to the base station by radio and corrections are continuously broadcast Additional equipment is needed Can be less accurate than postprocessing but usually not by more than 5 meters High Accuracy In addition to determining a satellite’s range by measuring code, the GEII can measure carrier waves between the satellite and the receiver. The carrier wave is a much finer measuring tool High accuracy requires more rigorous data collection Only point features can by collect in this mode, best to collect data for 10 minutes Accuracies are submeter, will not work on the water. Where next? Weather forecasting (atmospheric moisture) GPS in rental cars Land planes Emergency medical services Science Lead the blind Commercial applications far outnumber the military Annual GPS services are worth about $1 billion in the US
