(Credits NASA/CNES)
The Topex/Poseidon satellite was an adaptation by Fairchild Space of the existing Multimission Modular Spacecraft (MMS) which successfully carried the payloads of the Solar Maximum Mission, Landsat 4 and Landsat 5. The MMS design was modified to meet Topex/Poseidon’s requirements: a 2,400 kg mass carrying six science instruments – four operational and two experimental.

Topex (Topography Experiment) altimeter, or NRA (NASA Radar Altimeter)

The NRA is a radar altimeter that emits pulses at two frequencies (13.6 and 5.3 GHz, the second frequency is used to determine the electron content in the atmosphere) and analyses the return signal reflected by the surface. The signal’s round-trip time is estimated very precisely in order to calculate the range, after applying the necessary corrections (Instrument supplied by NASA).
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Poseidon-1 altimeter, or SSALT (Single frequency Solid State radar ALTimeter )

Poseidon-1 is an experimental instrument. It is a compact, low-power, low-mass instrument offering a high degree of reliability. Poseidon-1 is a radar altimeter that emits pulses at a single frequency (13.65 GHz – Ku-band) and analyses the return signal reflected by the surface. It shares the same antenna as the NRA; thus only one altimeter operates at any given time. It operates about 10% of the time, or one cycle in ten (Instrument supplied by CNES).
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TMR (Topex Microwave Radiometer)

This instrument measures radiation from the Earth’s surface at three frequencies (18, 21 and 37 GHz). Measurements acquired at each frequency are combined to determine atmospheric water vapour and liquid water content. Once the water content is known, we can determine the correction to be applied for radar signal path delays (Instrument supplied by NASA).
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Location systems

Altimetric measurements are referred to geodetic coordinates by means of a precise orbit determination system. Two precise orbit determination teams, one operated by CNES and one by NASA, use a combination of LRA and DORIS tracking data.

The location systems on board Topex/Poseidon complemented each other to measure the satellite’s position on orbit to within two centimetres on the radial component. The LRA is highly accurate, but it requires ground stations that are complex to operate, and its use can be restricted by adverse weather conditions. It is used to calibrate the other two location systems so that the satellite orbit can be determined as accurately as possible. The TRSR (a GPS receiver) acquires data that complement DORIS measurements in order to determine the orbit in real time and to support precise orbit determination.

DORIS (Doppler location)

The DORIS system uses a ground network of orbitography beacons spread around the globe, which send signals at two frequencies to a receiver on the satellite. The relative motion of the satellite generates a shift in the signal’s frequency (called the Doppler shift) that is measured to derive the satellite’s velocity. These data are then assimilated in orbit determination models to keep permanent track of the satellite’s precise position (to within three centimetres) on its orbit (Instrument supplied by CNES).
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GPSDR (GPS location)

The GPSDR uses the Global Positioning System (GPS) to determine the satellite’s position by triangulation, in the same way that GPS fixes are obtained on Earth. At least three GPS satellites determine a mobile object’s (in this case, the satellite’s) exact position at a given instant. Positional data are then integrated into an orbit determination model to track the satellite’s trajectory continuously (Instrument supplied by NASA).
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LRA (laser tracking)

The LRA (Laser Retroreflector Array) is an array of mirrors that provides a target for laser tracking measurements from the ground. By analysing the round-trip time of the laser beam, we can locate where the satellite is on its orbit (Instrument supplied by NASA).
Further information

Further information on Topex/Poseidon (Aviso website)