the Global Positioning System components and operation

1 GPS/GNSS FundamentalsIntroduction:the Global PositioningSystem componentsand operationNavigation HistoryCelestial Navigation Satellite SystemGNSS NAVSTAR GPS (USA) Since early 1980 s Galileo (European Union) by 2019 GLONASS (Russian) 2012 (started in 1993) Global 'naya Navigatsionnaya Sputnikovaya Sistema Global Orbiting Navigation Satellite SystemBEIDOU China s GNSS (under development)IRNSS India s regional System (under development) /GNSSD escription:A Global Positioning System (GPS/US*) is based on observations of signals transmitted from satellitesSource: *Owned and operated by the Department of *Owned and operated by the Department of DefenseSource: /GNSSD escription.

2 A Global Positioning System (GPS/US*) is based on observations of signals transmitted from satellitesGPS SegmentsNAVSTARR ockwell International20,190 km1900 lbs17ftwith solar panelsGPS Satellite Segment (US only)30 satellites 6 orbital planes12 hour return interval for each satellite 4 8+ satellites above the horizon during most ground stations on Russian territory, eight in neighboring countries, three in Antarctica, and one in time ViewerGPSUser SegmentTrimble GeoXH 6000 Users with a device that records data transmitted by each satellite and processes this data to obtain three dimensional coordinatesGarmin eTrex 30 Satellite circle the earth twice dailyFour to twelve satellites should be visible from any unobstructed satellite continuously broadcasts signals on two carrier frequencies, L1 and also transmit coded signals.

3 Modulations of the carrier transmit coded signals at various frequenciesGPSL1, L2 & L5 are carriers modulated signals, almanac, ephemeridesThe C/A (Coarse Acquisition)code is a series of plus and minus 1 values at (Precision) code is a sequence of plus and minus 1 values at MHz ( Military encrypted) .GPS L1 also carries a Navigation Message. Each GPS satellite broadcasts data about the status and location of all of these data are anAlmanac, data used to determine the location of every satellite in the GPS Almanac includesEphemeris data for the broadcasting satellite health, clock corrections, etc.(allows a GPS receiver to accurately calculate the position of the broadcasting satellite)GPSR eceiver has a precise internal clock so it knows when it generated a signal, and when the satellite signal was received.

4 Subtraction yields the time, which can be converted to we assume satellite receiver clocks are in synch. Must make this assumption to calculate travel time. Unfortunately, this is not the case. Basically, receiver clock may be biased, so we need extra have extremely accurate atomic clocks, (cost $100k each).These are monitored and synchronized among satellites. GPSP osition is estimated based on range measurementsRange = speed of light x travel timeRange = c(t1 t2)(c =299,792,458 meters per second)The speed of sound750 mph (1,129 ft/s) or 1,224 km/h (344 m/s)The speed of light:186,000 mp/s or 300,000 km/sIf you count the gap between the lighting and the thunder, you can tell the distance to the every kilometer count 3 seconds, for every mile count 5 seconds.

5 Two Types of Range MeasurementsCoded, or C/A measurements are rapid (need only a few seconds per satellite) robust, and inexpensive, but relatively coarse ( to 10 meters).Carrier phase measurements require longer time periods, an uninterrupted signal, an expensive receiver, but provide high accuracies (millimeters to centimeters) Coded (or C/A code) MeasurementsMeasurements to multiple satellites determine positionGPSP ositional Uncertainty Errors in range measurements and satellite location introduce errors Creates a range of uncertainty around the GPS receiver position rangeuncertaint yrangeuncertaint yrangeuncertatintySources of ErrorSeveral factors can result in erroneous location determination with GPS (besides blunders)SourceTypical Range Error (m)

6 Satellite clock error1 Satellite position error1 Receiver effects4 Total path additional error, depending on conditions and received qualitySource: of a receiver error: multipath signalMask or limit satellites too low on horizonPDOPs Position Dilution of PrecisionIdea (one overhead and three all at 120 intervals)Positional Dilution of Precision (PDOP) PDOP is a measure of satellite spacing a wider spaced constellation is better PDOPs typically range from 2 to 10, but can go as low as 1, or as high as 100 s Lower PDOPs are better, because they represent signals from widely spaced satellites, and hence a smaller area of errors and DOP s combine to affect GPS position C/A code receivers typically provide accuracies between 3 30 meters for a single possible when multiple fixes can be averagedAccuracies with carrier phase receivers can be a few centimeters or millimetersWithin each type, C/A and Carrier Phase.

7 There are differences in accuracies due to the technology appliedRealized Accuracies Averaging at least 300 fixesRealized Accuracies Averaging at least 300 fixesGPS Differential CorrectionUsing two unitsto improve accuracyStation set up over known position (Base).1. Station set up at unknown Collect data Calculate new points from known Base Station and CorrectionEach rover observation is shifted back by the observed errorTwo Types of Differential GPSPost processedHighest accuracy, but a posterioriReal time differential correctionLower AccuracyBetter navigationRequires a radio link (typically FM)Requires a transmitting base stationReal time differential correctionSlightly lower accuracyBetter location while in the fieldRequires a radio link (typically FM)Requires base station.

8 Can use US Coast Guard GPS radio beaconsOr requires WAASGPS WAASWide Area Augmentation System (WAAS), for civilian aircraft on a network of ground Beacons at NGS CORS sites(Continuously Operating Reference & GLONASSGPS Applications TrackingGPS Applications TrackingGPS Applications NavigationGPS Applications NavigationOn the groundAirborneGPSA pplicationsField digitization(carried by car,boat,helmets, hand,etc.,)Field Digitization with GPSD irect occupationField Digitization with GPSM easured offsetsCombine:GPS ReceiverLaser RangerfinderElectronic Compassmxu, yuxn, yndydx xu = xn - dxazimuth, ady = m. cos( )yu = yn - dy180o90o270o0oxn, xy are known,the azimuth anddistance to xu, yuare known.)

9 Whatare xu, yu?dx = m. sin( )If xn = 12, yn=3,m= , and =62ody = cos(62) = = sin(62) = = 12 - 6 = 6yu = 3 - = xn, ynwith GPSmwith laser rangefinder with electronic compassCombine all three usingCoordinate geometry toCalculate xu, Summary Space based Positioning System with satellite, control, and user segments Carrier phase (accurate) and code phase Positioning Range measurements from 4 or more satellites to estimate position Range errors from ionosphere, atmosphere, System , and receivers Satellite geometry affects position accuracy low PDOPs are betterGPS Summary We can also average, and differentially correct to improve accuracy Many differential correction approaches Real time beacon, WASS, post processed Positional accuracies higher in open than sub canopy or in canyons Main application in GIS are for field digitizing