Lecture_02_segments.pptx

GE 354 : SATELLITE SURVEYING S. A. Andam-Akorful (PhD) Dept. of Geomatic Engineering KNUST  2 GPS OVERVIEW  3 Recap: GNSS Principle  4 What do w...

GE 354 : SATELLITE SURVEYING S. A. Andam-Akorful (PhD) Dept. of Geomatic Engineering KNUST  2 GPS OVERVIEW  3 Recap: GNSS Principle  4 What do we need? Satellites and their positions signals receivers: tracking and processing the signals i.e. measurements observation equations take into account propagation errors, measurement noise estimate position, velocity, time  How does GPS work?  Stations on earth, and a GPS receiver, the distances between each of these points can be calculated.  The distance is calculated based on the amount of time it takes for a radio signal to travel between these points.  Using satellites in the sky, ground allows the GPS receiver to know where you are, in terms of latitude and longitude, on the earth.  The more satellites the GPSr (GPS receiver) can “see”, the more accurate your reading.  The GPSr must “see” the satellites, so it does not work well in dense forests, inside caves, underwater, or inside buildings. GPS System Description GPS (Global Positioning System) also called NAVSTAR (NAVigation System, Timing And Ranging) The GPS consists of 3 main segments: Space Segment: the constellation of satellites Control Segment: operation and monitoring of the GPS System User Segment: all GPS receivers and processing software's  7 GPS Segments Satellites  GPS satellites are controlled and operated by the US Dept. of Defense.  The system is designed to maintain the availability of at least 24 operational GPS satellites, 95% of the time.  6 satellites are within view of any location at one time, provided that physical terrain, or structures do not block them.  To ensure this, a constellation of 31 operational satellites are in orbit  Satellites constantly transmit their locational information and time data. GPS Constellation 9 10  11 Constellation Arrangement  GPS satellites fly in medium Earth orbit (MEO) at an altitude of approximately 20,200 km  The high altitude insures that the satellite orbits are stable, precise and predictable, and that the satellites' motion through space is not affected by atmospheric drag  Each satellite has an orbital period of 11.5 hours  The satellites the constellation are arranged into six equally-spaced orbital planes surrounding the Earth  Each plane is inclined 55 degrees relative to the equator, which means that satellites cross the equator tilted at a 55 degree angle  12  The GPS satellites are powered primarily by sun-seeking solar panels, with nicad batteries providing secondary power.  On board each GPS satellite are four atomic clocks, only one of which is in use at a time.  These highly accurate atomic clocks enable GPS to provide the most accurate timing system that exists.  13  In June 2011, the Air Force successfully completed a GPS constellation expansion known as the "Expandable 24" configuration.  Three of the 24 slots were expanded, and six satellites were repositioned, so that three of the extra satellites became part of the constellation baseline.  As a result, GPS now effectively operates as a 27-slot constellation with improved coverage in most parts of the world.  14 Orbit ground tracks for 1 GPS satellite  15 Orbit ground tracks for three GPS satellites  16 Space Segment Function  Maintain accurate time by means of rubidium and cesium clocks  Broadcast the time information to the user by means of Two/three carrier frequencies: L1 (1575.42 MHz) ; L2 (1227.60 MHz) ; L5 (1176.45 MHz) Pseudo random noise (PRN) codes  Broadcast its position (satellite ephemeris), clock corrections and health information by means of data message  Receive and store satellite ephemeris and other data regularly uploaded by the Control Segment  17 Current and Future Satellite Generations  The GPS constellation is a mix of old and new satellites.  The following table summarizes features of the current and future generations of GPS satellites, including Block IIA (2nd generation, "Advanced"), Block IIR ("Replenishment"), Block IIR(M) ("Modernized"), Block IIF ("Follow-on"), and GPS III. 18 Control segment 19  GPS control segment consists of a global network of ground facilities that Monitor satellite orbits Monitor and maintain satellite health Maintain GPS time Predict satellite ephemerides and clock parameters Update satellite navigation messages Command maneuvers, relocations of satellites  The current operational control segment includes a master control station, an alternate master control station, 11 command and control antennas, and 15 monitoring sites  20 Control Segment - U.S. DoD Monitoring Control Segment Elements 21 Master Control Station (MCS)  Located in Colorado  Performs the primary control segment functions, providing command and control of the GPS constellation  The MCS generates and uploads navigation messages and ensures the health and accuracy of the satellite constellation.  It receives navigation information from the monitor stations, utilizes this information to compute the precise locations of the GPS satellites in space, and then uploads this data to the satellites.  22 Monitor Stations  Monitor stations track the GPS satellites as they pass overhead and channel their observations back to the master control station.  Monitor stations collect atmospheric data, range/carrier measurements, and navigation signals.  The sites utilize sophisticated GPS receivers and are operated by the MCS.  There are 15 monitoring stations located throughout the world, including six from the Air Force and nine from the National Geospatial-Intelligence Agency (NGA).  23 Ground Antennas  Ground antennas are used to communicate with the GPS satellites for command and control purposes.  These antennas support S-band communications links that send/transmit navigation data uploads and processor program loads, and collect telemetry.  The ground antennas are also responsible for normal command transmissions to the satellites.  There are four dedicated GPS ground antenna sites co-located with the monitor stations at Kwajalein Atoll, Ascension Island, Diego Garcia, and Cape Canaveral.  24 User Segment - Military and Civilian GPS Users  All GPS receivers on land, on sea, in the air and in space.  The U.S. military uses GPS for navigation, reconnaissance, and missile guidance systems.  Extremely broad civil user community with applications of the GPS for: Navigation Surveying and Mapping Geodynamics and geophysics Remote sensing (troposphere and ionosphere) Time and frequency transfer