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Topex Poseidon

ocean TOPography EXperiment


Designation 22076 / 92052A
Launch date 10 Aug 1992
Country of origin France and United States
Operator NASA & CNES
Mission Remote sensing
Perigee/Apogee 1330/1342 km
Inclination 66°
Period 112.4 min
Orbital cycle 10 days
Launch vehicle Ariane V52

Mission: global ocean circulation, ocean currents, tides and surface topography
Cost: $480 million

The inclined orbit samples from 66 North to 66 South latitude: it allows the satellite to observe 90% of the ice-free oceans and all major straits. The orbit is not sun-synchronous and is prograde. The repeat-cycle is about 10 days, ie the satellite passes vertically over the same location in a 1-kilometer bandwidth every 10 days. It has been chosen to optimize the study of the temporal large scale oceanic variability. The distance between successive tracks is on the order of 310 km at the Equator. The local time of successive passes shifts by nearly two hours.

In late 1998 the satellite was used to experiment an autonomous orbit control mode. A program was uploaded to enable the satellite to control its orbit on its own. The experiment was successful.

Since Mar 1999 it is known that the satellite operates with the backup altimeter.

The satellite's data helped in discovering, and then forecasting, the El Niño/La Niña phenomenon and aided in ship routing, fisheries management, and marine mammal research, among other things.

External resources

sat-index articles

End of life

Out of service 9 Oct 2005
Cause Reaction wheel failed

Technical data


Prime contractor Jet Propulsion Laboratory (+ bus: Fairchild, Poseidon: Alcatel Espace)
Platform MultiMission Spacecraft bus
Mass at launch 2100 kg
DC power 1500 W
Stabilization 3 axis
Design lifetime 4 years

Telemetry: 2287.5 MHz (realtime 1/16 kbps)
Command: 2106.5 MHz (realtime 1/0.125 kbps)

ALT (dual frequency radar altimeter) / NRA (NASA Radar Altimeter) (Topex)

Frequencies 5.3 GHz (C-band)
13.6 GHz (Ku-band)
Resolution 2-3 cm

NRA (206 Kg including redundance, 237 W) is the primary sensor for the TOPEX/POSEIDON mission. It has been provided by NASA. It is the fifth generation of altimeter (see Seasat and Geosat altimeters). The measurements made at the two frequencies are combined to obtain altimeter height of the satellite above the sea (altimeter range), the wind speed magnitude, the significant wave height and the ionospheric correction.

SSALT (Single-frequency Solid-state Radar Altimeter) (Poseidon)

Frequency 13.65 GHz (Ku-band)
Resolution 2-5 cm

SSALT (23 Kg without redundance, 49 W) validates the new technology of a low-power, light-weight altimeter for future Earth observing missions. It has been provided by the French Space Agency (CNES). It shares the same antenna as the NRA, thus only one altimeter operates at any given time. SSALT operates about 10% of the time that is one cycle in ten. The measurements give the same geophysical information as NRA's. However since this sensor uses a single frequency, an external correction for the ionosphere must be supplied (see the DORIS instrument).

TMR (Topex Microwave Radiometer)

Frequencies 18, 21 & 31 GHz

TMR (50 Kg including partial redundance, 25 W) measures the sea surface microwave brightness temperatures at three frequencies to provide the total vapor content in the troposphere along the altimeter beam. The 21 GHz channel is the primary channel for water vapor measurement. The 18 GHz channel is used to remove the effects of wind and the 37 GHz channel to remove cloud cover in the water vapor measurements. The measurements are combined to obtain the error in the satellite range measurements caused by pulse delay due to water vapor.

LRA (Laser Retroflector Array)

LRA (29 Kg) reflects laser signals from a network of 10 to 15 ground laser tracking stations to provide the baseline tracking data for precise orbit determination and to calibrate the altimeter bias.

DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite)

Frequencies 401.25 MHz
2036.25 MHz
Altitude accuracy 10 cm

DORIS (43 Kg including redundance, 21 W) is a French system that uses a two-channel receiver to observe the Doppler signals from a network of 40 to 50 ground transmitting beacons, spaced around the world. It provides all-weather global tracking of the satellite for precise orbit determination and an accurate correction for the influence of the ionosphere on both the Doppler signal and altimeter signals.

GPSDR (Global Positioning System Demonstration Receiver)

Frequencies 1227.6/1574.4 MHz

GPSDR (28 Kg without redundance, 29 W) uses a new GPS differential ranging technique for precise orbit determination. It receives signals from up to 6 GPS satellites. The GPS antenna is mounted on a long boom to reduce multipath effects which can severely corrupt the measurements. These data plus tracking of GPS from ground sites (3 minimum, 6 planned) allow nearly geometrical solutions.

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