report of the action team on global navigation satellite systems (gnss)

• Planet Quest:http://planetquest.jpl.nasa.gov/• U.S. Coast Guard Navigation Center:http://www.navcen.uscg.gov/• U.S. Mission to <strong>the</strong> European Union: GPS and <strong>the</strong> EU's GALILEO Systemhttp://www.useu.be/Galileo/• U.S. Space Objects Registry:http://www.usspaceobjectsregistry.state.gov/Global Positioning System description22. GPS is a Satellite Navigation System with many millions <strong>of</strong> civil users worldwide. GPSprovides specially coded satellite signals that can be processed in a GPS receiver, enabling <strong>the</strong>receiver to compute position, velocity and time. At least four GPS satellite signals are necessaryto compute positions in three dimensions and <strong>the</strong> time <strong>of</strong>fset in <strong>the</strong> receiver clock. With aReceiver Autonomous Integrity Monitoring (RAIM) capable receiver and six in view GPSsatellites allow <strong>the</strong> users to achieve RAIM. The Space Segment <strong>of</strong> <strong>the</strong> system consists <strong>of</strong> <strong>the</strong> GPSsatellites. These space vehicles (SVs) send radio signals from space. The nominal GPSOperational Constellation consists <strong>of</strong> 24 satellites plus on-orbit spares that orbit <strong>the</strong> Earth in 12hours. There are <strong>of</strong>ten more than 24 operational satellites as new ones are launched to replaceolder satellites.23. The satellite orbits repeat almost <strong>the</strong> same ground track (as <strong>the</strong> Earth turns beneath <strong>the</strong>m)once each day. The orbit altitude is such that <strong>the</strong> satellites repeat <strong>the</strong> same track and configurationover any point approximately each 24 hours (4 minutes earlier each day). There are six orbitalplanes (with nominally four SVs in each), and inclined at about fifty-five degrees with respect to<strong>the</strong> equatorial plane. This constellation provides <strong>the</strong> user with between five and eight SVs visiblefrom any point on <strong>the</strong> Earth.Control Segment24. The Control Segment consists <strong>of</strong> a system <strong>of</strong> tracking stations located around <strong>the</strong> world,including <strong>the</strong> GPS Master Control and Monitor Network. The Master Control facility is located atSchriever Air Force Base (formerly Falcon AFB) in Colorado. These monitor stations measuresignals from <strong>the</strong> SVs, which are incorporated into orbital models for each satellite. The modelscompute precise orbital data (ephemeris) and SV clock corrections for each satellite. The MasterControl station uploads ephemeris and clock data to <strong>the</strong> SVs. The SVs <strong>the</strong>n send subsets <strong>of</strong> <strong>the</strong>orbital ephemeris data to GPS receivers over radio signals.25. The GPS User Segment consists <strong>of</strong> <strong>the</strong> GPS receivers and <strong>the</strong> user community. GPSreceivers convert SV signals into position, velocity, and time estimates. Four satellites arerequired to compute <strong>the</strong> four dimensions <strong>of</strong> X, Y, Z (position) and Time.26. GPS receivers are used for navigation, positioning, time dissemination, remote clockcomparison and for o<strong>the</strong>r research purposes. Navigation in three dimensions is <strong>the</strong> primaryfunction <strong>of</strong> GPS. Navigation receivers are made for aircraft, ships, ground vehicles and for handcarrying by individuals. Enhanced positioning is possible using GPS receivers at referencelocations providing corrections and relative positioning data for remote receivers. Surveying,geodetic control, and plate tectonic studies are some examples.8