Since data on the satellite position goes directly into the equation for sea level, it is vital to know the position (altitude) of the satellite as accurately as possible in order to identify and obtain accurate data on the sea level and its variations.
TOPEX/Poseidon utilizes three independent satellite tracking systems for precision orbit determination (POD). Two of them are primary systems and the other is an experimental system. One of the primary systems is called Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) developed by the French Space Agency CNES. The other primary system is NASA's Laser Retroreflector Array (LRA). Finally, the Global Positioning System (GPS) is an additional tracking system, which was experimental but has ended up being very reliable with a great accuracy.
The LRA is a ground-based satellite laser ranging (SLR) system. The LRA system has laser beams sent out from the Satellite Laser Ranging (SLR) stations (laser tracking stations) on the ground and reflected off the LRA (like a mirror) and measures the time that it takes for the beams to travel back to the ground, from which TOPEX/Poseidon's orbital radial position can be calculated. The LRA is accurate to ±2 cm for overhead ranging.
The DORIS is another ground-based satellite radio tracking system that utilizes microwave Doppler effect. The DORIS system has microwave radio signals sent out from a worldwide network of Doppler tracking stations on the ground to the DORIS dual-frequency receiver aboard the spacecraft. These beacon stations are equipped with meteorological sensors to measure temperature, humidity, and atmospheric pressure to correct for the atmospheric effects on the signals. The receiver measures the Doppler frequency shift in the signals as the satellite and the Earth moves with respect to each other, from which the satellite position can be determined. The DORIS signals are transmitted at two different frequencies (401.25 and 2036.25 MHz) to allow ionospheric correction on the tracking data. The DORIS system is accurate to within ±0.3 mm/s radial velocity.
The Global Positioning System (GPS) is an all-weather ranging system under development by the U.S. Department of Defense to determine unknown positions on land, sea, in air and space from known positions of satellites in space. TOPEX/Poseidon utilizes the GPS experimentally as a NASA's satellite tracking system for precision orbit determination to test its validity and accuracy, which had never been done before. While the LRA and DORIS system are limited in coverage due to the nature of a ground-based system, the GPS provides continuous tracking of the satellite.
The GPS consists of three segments: the space segment, the control segment, and the user segment.
The space segment consists of a constellation of 24 satellites deployed in six planes with an inclination of 55 degrees and with four satellites in each plane. The satellites are in nearly circular orbits with an altitude of about 20200 km (12552 miles) above the Earth and a period of 12 hours, which means the satellites complete one revolution around the Earth every twelve hours and overfly the same locations at the same time every day (every 24 hours).
The control segment comprises the Operational Control System (OCS) which consists of a master control station, worldwide monitor stations, and ground control stations. The task of the control segment is to operate the GPS.
The master control station is located at the Consolidated Space Operations Center (CSOC) at Falcon AFB, Colorado Springs, Colorado. CSOC collects the tracking data from the monitor stations, calculate the satellite orbit and some parameters, and send them to one of the three ground control stations. It is also responsible for the satellite control and system operation.
The worldwide monitor stations are located at five different places: Hawaii, Colorado Springs, Ascension Island in the South Atlantic Ocean, Diego Garcia in the Indian Ocean, and Kwajalein in the North Pacific Ocean. Each station tracks all GPS satellites in view and transmit the information to the master control station.
The ground control stations are collocated with the monitor stations at Ascension, Diego Garcia, and Kwajalein. They are equipped with the ground antennas and upload the data calculated at the master control station to each of the GPS satellites.
The user segment consists of GPS receivers and antennas. TOPEX/Poseidon has a receiver called the GPS Demonstration Receiver (GPSDR) inside the Instrument Module (IM) and the GPS antenna mounted on the zenith side of the IM. The GPSDR receives the broadcast signal from the GPS satellites through the antenna and calculates the range to each satellite in view by measuring the travel time of the signal and multiplying it by the speed of light. The receiver can calculate the position of the TOPEX/Poseidon satellite with an accuracy of 3 cm from the obtained ranges along with some additional data. At least four GPS satellites are always in view of the TOPEX/Poseidon, which is the minimum number of the GPS satellites that is required to obtain a position.
Refer to the site by Peter H. Dana, The Geographer's Craft Project, Department of Geography, The University of Texas at Austin for more detailed information on GPS.
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This page is created by Masaharu Suzuki The University of Texas at Austin
Last Modified: Wed Feb 11, 1999