The Ocean Topography Experiment (TOPEX/Poseidon) mission was a collaboration between NASA and the Centre National d'Etudes Spatiales (CNES) of France to obtain global measurements of sea-surface heights using radar altimetry. Topex/Poseidon was a core element of the World Ocean Circulation Experiment (WOCE), an international study designed to improve our understanding of global climate predictions conducted in the early 1990's. TOPEX/Poseidon, the second element in NASA's Mission to Planet Earth (MTPE) program, was designed to measure shifts in the global climate system that account for global warming or cooling, and the El Nino phenomena in the Pacific basin. The TOPEX/Poseidon spacecraft design was based on the existing Multimission Modular Satellite (MMS) bus flown on the French/CNES SPOT 2 spacecraft. The TOPEX/Poseidon spacecraft consisted of the MMS and the instrument module housing the sensors. The MMS consisted of: (1) a command and data handling subsystem which contained the main on-board computer; (2) the attitude determination and control subsystem, for maintaining the spacecraft attitude; and, (3) the electrical power subsystem, which contained the solar array and three batteries. The command and data handling subsystem housed three tape recorders for collecting engineering telemetry and instrument data. This subsystem also provided telecommunications using a steerable high-gain antenna dish and two omni antennas. During normal operations, the satellite communicated with the Tracking and Data Relay Satellite System (TDRSS). The 8.7 m by 3.3 m solar array was deployed about two minutes after separation from the launch vehicle. The batteries provided power before deployment of the solar array and during the nighttime part of the orbit. The TOPEX/Poseidon spacecraft carried the following instruments: (1) the Dual-Frequency TOPEX Radar Altimeter (ALT), provided by NASA, the first 2-channel radar altimeter to be used in space and a follow-on to the radar altimeter flown on Seasat in 1978; (2) the TOPEX Microwave Radiometer (TMR), provided by NASA, a three-channel passive microwave radiometer for the detection of atmospheric water vapor to correct the altimeter data; (3) the Single-Frequency Poseidon Altimeter (SSALT), provided by CNES, an experiment to validate improved radar altimetry technology; (4) the Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) experiment, provided by CNES, for precise orbit determination (a DORIS system was also on the French/CNES SPOT 2 spacecraft); and, (5) the Global Positioning System Demonstration Receiver (GPSDR), an experimental receiver provided by NASA for continuous orbit determination. The spacecraft also carried a ring of laser corner-reflectors (LRA) around the altimeter antenna for precise satellite tracking by a network of twelve ground stations. The spacecraft orbit was designed to cover 90% of the global ocean every 11 days. The lifetime of TOPEX/Poseidon is expected to be 3 to 5 years.
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