Slide 1Slide 2Slide 3GPS – User Segment (Receivers)Slide 5GPS - Satellite SignalsSlide 7Slide 8Slide 9Slide 10Slide 11GPS - Sources of ErrorGPS - Selective AvailabilityGPS - Error BudgetGPS - Error CorrectionGPS - Point AveragingGPS - Differential CorrectionSlide 18Slide 19GPS ApplicationsGarmin’s cheapest receiversGarmin’s Outdoor GPS Receivers: etrex seriesGarmin’s Outdoor GPS Receivers:Bluetooth GPS ReceiversHP’s Ipaqs and other PDAs with GPS softwareGPS BasicsWhat is GPS?GPS stands for Global Positioning System which measures 3-D locations on Earth surface with the aid of satellites • Created and Maintained by the US Dept. of Defense and the US Air Force • System as a whole consists of three segments satellites (space segment) receivers (user segment) ground stations (control segment)SatellitesSatellites (space segment)24 NAVSTAR satellites (21 operational and 3 spares)orbit the Earth every 12 hours~11,000 miles altitude positioned in 6 orbital planes orbital period/planes designed to keep 4-6 above the horizon at any time controlled by five ground stations around the globe•Ground-based devices read and interpret the radio signals from several of the NAVSTAR satellites at once•Determine their position using the time it takes signals from the satellites to reach the hand-held unit •Calculations result in varying degrees of accuracy that depend on:•quality of the receiver •user operation of the receiver•local & atmospheric conditions•current status of systemGPS – User Segment (Receivers)Ground stations (control segment)Five control stationsmaster station at Falcon (Schriever) AFB, Coloradomonitor satellite orbits & clocksbroadcast orbital data and clock corrections to satellitesMap from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin.Ground Stations (control segment)GPS - Satellite SignalsSatellites have accurate atomic clocks and all 24 satellites are transmitting the same time signal at the same timeThe satellite signals contains information that includesSatellite numberTime of transmissionReceivers use an almanac that includesThe position of all satellites every secondThis is updated monthly from control stationsThe satellite signal is received, compared with the receiver’s internal clock, and used to calculate the distance from that satelliteTrilateration (similar to triangulation) is used to determine location from multiple satellite signalsHow It Works (p. 1)How It Works (p. 2)Start by determining distance between a GPS satellite and your positionAdding more distance measurements to satellites narrows down your possible positionsHow It Works (p. 3)Three distances = two pointsIntersection of Four spheres = one pointNote: • 4th measurement not needed• Used for timing purposes instead (discussed later)How It Works (p. 4)Distance between satellites and receiversdetermined by timing how long it takes the signal to travel from satellite to receiver. How?Radio signals travel at speed of light: 186,000 miles/second Satellites and receivers generate exactly the same signal at exactly the same time Signal travel time = delay of satellite signal relative to the receiver signal Distance from satellite to receiver = signal travel time * 186,000 miles/second 1secReceiver signalSatellite signalHow It Works (p. 5)How do we know that satellites and receivers generate the same signal at the same time? satellites have atomic clocks, so we know they are accurate Receivers don't -- so can we ensure they are exactly accurate? No!But if the receiver's timing is off, the location in 3-D space will be off slightly...So: Use 4th satellite to resolve any signal timing error insteaddetermine a correction factor using 4th satellite(like solving multiple equations...will only be one solution that satisfies all equations)Error SourcesSatellite errorssatellite position erroratomic clock, though very accurate, not perfect.AtmosphereElectro-magnetic waves travels at light speed only in vacuum.The ionosphere and atmospheric molecules change the signal speed.Multi-path distortionsignal may "bounce" off structures nearby before reaching receiver – the reflected signal arrives a little later.Receiver error: Due to receiver clock or internal noise. Selective AvailabilityGPS - Sources of ErrorPoorIdealSatellite Coverage in SkyPosition Dilution of Precision (PDOP)GPS - Selective AvailabilityA former significant source of errorError intentionally introduced into the satellite signal by the U.S. Dept. of Defense for national security reasonsBased on Clinton’s order, Selective Availability turned off early May 2, 2000GPS - Error BudgetTypical observed errorssatellite clocks 0.6 metersorbit error 0.6 metersreceiver errors 1.2 metersatmosphere 3.7 meters Total 6.1 metersMultiplied by PDOP (1 - 6) 6.1 - 36.6 metersMetersAtmosphereReceiversOrbit ErrorSatelliteClocks0 6 12 18 24 30•Example of some typical observed using a consumer GPS receiver:GPS - Error Correction2 Methods:Point AveragingDifferential CorrectionGPS - Point AveragingAveragedLocation•This figure shows a successive series of positions taken using a receiver kept at the same location, and then averagedGPS - Differential CorrectionDifferential correction collects points using a receiver at a known location (known as a base station) while you collect points in the field at the same time (known as a rover receiver)Any errors in a GPS signal are likely to be the same among all receivers within 300 miles of each other~ 300 miles (~ 480 km) or lessBase station (known location)Rover receiverGPS - Differential CorrectionThe base station knows its own locationIt compares this location with its location at that moment obtained using GPS satellites, and computes errorThis known error (difference in x and y coordinates) is applied to the rover receiver (hand-held unit) at the same momentTime GPS Lat GPS Long Lat. error Long. error3:12.53:13.03:13.53:14.03:14.53:15.035.5035.0534.9536.0035.3535.2079.0578.6579.5580.4579.3079.35.5.05-.051.0.35.20.5-.35.551.45.30.35Example: Base Station FileGPS - Differential CorrectionGPS error when using differential correction: 1 – 3 metersThere are two ways that differential correction can be applied:Post-processing differential correction•Does the error calculations after the rover has collected the pointsReal-time differential correction•Done in real time by