Beacons in the waveforms

Image of the month - April 2006

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.

Beacons marking Malacca Strait channel ( ~3-4 km wide). Three of them in red, green and blue are close to Topex/Poseidon and Jason-1 theoretical tracks. (Credits Ifremer)

One Topex, two Poseidon-1 and two Jason-1 (Poseidon-2) Ku band high rate waveforms over the beacons. The signatures of the beacons estimated from the beacons and satellite data coordinates are superimposed on the waveforms, in red, green or black depending on the beacon. Note the very good agreement between the signature of the beacons estimated from the coordinates of the beacons (red, green and black lines) and of the satellite ground tracks and the parabolae clearly seen in the waveforms. (Credits Ifremer)

See also:

References:

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