Basic Information• GPS is used tofind your exactlocation on theworld, to helpyou navigate.• It is a complexsystemdeveloped bythe military fortheir use.
What GPS Needs to Work• A receiver.• Three or foursatellites“visible” for theGPS receiverto use to collectdata.• The correcttime.
Control SegmentSpace SegmentUser SegmentThree Segments of the GPSMonitor StationsGroundAntennasMaster Station
Four Basic Functions of GPS Position and coordinates. The distance and direction between anytwo waypoints, or a position and a waypoint. Travel progress reports. Accurate time measurement.
GPS System• 24 satellites + spares.• 6 orbital planes 55°inclination.• Each satellite orbits twiceevery 24 hours.• At least 4 satellites visibleany time of day,anywhere in the world.
A 2 Dimensional Example• Time for the signal to reachGPS receiver is determined.• Distance is computed bymultiplying by the speed of light.• Distance from two satellites defines 2 points(in 2 dimensional space).
A 2 Dimensional Example• The distance from a thirdsatellite narrows thelocation to an“error triangle.”
Now for 3 Dimensions• Distance from a single satellite locates aposition somewhere on a sphere.
Now for 3 Dimensions• Two measurements put thelocation somewhere on acircle at the intersectionof the two spheres.
Now for 3 Dimensions• Three measurements putthe location at one of twopoints at the intersectionof the three spheres.
Now for 3 Dimensions• A fourth measurement selectsone of the two points, andprovides enoughinformation to solve forthe constant error.
Time Difference is Distance• Timing of the signals transmitted by thesatellites is very accurate due to the dualatomic clocks on board each satellite.• The time difference between the two PRNcodes represents the time it took the radiosignal to travel from the satellite to the GPSreceiver.• The distance or “range” to the satellite isgiven by the equation.range = time difference X speed of light
Satellite Position is Known• The position of each satellite is known withgreat accuracy. Current orbital position data istransmitted by each satellite.• Orbits are monitored by ground controlstations. Corrected orbital information isuploaded several times a day.• Given the position of each satellite and thedistance from the GPS receiver to eachsatellite, the position of the GPS receiver canbe computed.
GPS Limitations –Fewer than 4 satellites visible• Your GPS needs to be able to receive a strongsignal from at least 4 satellites to report anaccurate position• Problems could be caused by…The sky is obscured by canyon walls, mountains,or tall buildings.Dense tree canopy. Especially if it’s wet.Antenna is shielded by metal from a car, aircraft orbuilding.Low batteries may reduce receiver sensitivity.