Altimetry theory
Altimetry measurements are acquired by an altimeter on a non-synchronous satellite in a repeating low-Earth orbit designed to fly over the same points at regular intervals. The altimeter is a radar instrument that emits a signal at very high frequency—typically 2,000 pulses per second—vertically beneath the satellite. This signal propagates until it meets an obstacle, which reflects a radar “echo” back to the altimeter’s antenna.
By calculating the round-trip time of the signal, we can determine the distance—or range—from the obstacle (in this case, the ocean surface) to the satellite. This calculation is performed simply by multiplying the time by the speed of light at which electromagnetic waves propagate (see panel).
| Formula |
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General formula for velocity (in kilometres per second): v = d/t where d is the distance in km and t the round-trip time in s. therefore: d = v.t In this case, the velocity v is equal to the speed of light c = 300,000 km/s |
One major drawback of ocean-observing altimeters is that radio waves cannot penetrate below the sea surface. Consequently, we have to estimate the sea-surface height with respect to an arbitrary terrestrial reference surface.
In practice, to achieve the required level of accuracy, this theoretical measurement is corrected for perturbations of the satellite on its orbit and propagation of radio waves as they pass through the atmosphere.
In practice, to achieve the required level of accuracy, this theoretical measurement is corrected for perturbations of the satellite on its orbit and propagation of radio waves as they pass through the atmosphere.
Main altimetry missions currently in operation
| Satellites | Origin | Altitude | Exact revisit interval * | Ground track separation between two passes ** |
| ERS-1 (1991), ERS-2 (1995), ENVISAT (2002) | Europe | 800 km | 35 days | 80 km |
| TOPEX/POSEIDON (1992), JASON-1 (2001) | France/U.S. | 1,330 km | 10 days | 315 km |
| GFO (1998) | U.S. | 880 km | 17 days |
The orbit of an altimetry satellite is a compromise: the shorter the orbital period, and therefore the revisit interval, the narrower the coverage area.
* The exact revisit interval corresponds to the time it takes the satellite to complete a full cycle and pass exactly above the same point.
** The ground track separation corresponds to the distance at the equator between two consecutive passes over the same area.
