Nearly 50,000 measurements per day
How altimetry works
The radar altimeters on the satellite send very short impulses, which are reflected by the ocean's surface. Measurements of the time that it takes for the return trip permit to calculate the distance, or range, between the satellite and the sea surface. See how altimetry works ?
How altimetry works. The radar altimeter beams down a signal, which is reflected by the surface back to the antenna. Crédits : CNES, ill D.Ducros
Electromagnetic waves are slowed down when travelling through the atmosphere. Once the necessary corrections have been applied, the altimeter is accurate to within 2 cm. These date are used to calculate the precise speed and direction of the currents, wind speed and wave height.
Topex/Poseidon instruments
| Instrument | Origin | Purpose |
| NRA, dual-frequency radar altimeter | NASA | Measures the altitude of the satellite above the sea surface, wind speed, wave and the ionospheric correction |
| TMR, Topexmicro-wave radiometer | NASA | Measures the water vapour content along the trajectory of the altimeter for correction purposes |
| Doris | CNES | Calculates the precise orbit and the ionospheric correction to within 3 cm |
| LRA, laser retroreflector | NASA | Calculates the orbit and calibrates other orbitography systems |
| Poseidon, altimeter | CNES | Measures the altitude of the satellite above the sea surface, wind speed and wave height |
| GPS receiver | NASA | Calculates the precise orbit |
Ground segment
NASA and CNES share responsibility for the ground segment. It consists of 2 centres for controlling and programming the satellite as well as for processing the data. Both centres regularly produce geophysical data for distribution to users for research work.
