Basic measurement of an altimeter is the power of the radar wave received after reflection on the surface. The radar emits a short electromagnetic pulse, which is reflected by the sea surface. The waveform, i.e. the shape and strength of the return echo is recorded on the satellite. Those echoes are usually averaged (per second), but they can also be used singly or averaged over 1/20th of a second. Those high rate altimeter echo waveforms are particularly sensitive to smallscale changes of surface backscatter and they contain a wealth of information on the sea surface. A detailed analysis of these waveforms often reveals the signatures of objects that emerge from the sea, such as beacons, lighthouses and small islands.

The most striking example of punctual target signatures found in the Topex/Poseidon and Jason-1 archive is located in the Strait of Malacca. There, several beacons signal shoal and sea lanes and luckily, the first orbit per cycle of Topex/Poseidon and Jason-1 passes over a triangle formed by three beacons about 11 m high. These beacons are equipped with radar reflectors so that, in spite of their small size, their signature is often detectable in the high rate altimeter waveforms. The permanence of the presence of echoes at an almost constant latitude and the good agreement between the waveforms and the expected beacons' signatures shows that the echoes are certainly caused by the beacons. This allows to "check" that the satellite ground track positioning is within 250 m.

See also:

• Image of the Month, January 2002: From Poseidon-1 to Poseidon-2

References:

• Tournadre, J., 2007: Signature of lighthouses, ships and small islands in altimeter waveforms, 24(6): 1143-1149, J. Atmos. Ocean. Tech.