Satellite altimetry has proven to be a valuable source of data for a broad range of applications. Looking beyond the missions in operational service today, future satellites will need to provide better spatial and temporal coverage so that we can study mesoscale variations and other phenomena more closely. For the medium term, consideration is now being given to altimetry missions capable of ‘scanning’ the ocean surface to acquire data at scales of a few tens of kilometres, passing over the same spots every few days. Other projects on the drawing board are based on constellations of dedicated, low-cost microsatellites. The use of ‘opportunity signals’ is also being considered, with the possibility of retrieving reflected signals transmitted by satellites in the Global Navigation Satellite System (GNSS).

Ka-band altimetry

A Ka-band (35 GHz) altimeter would be much less affected by the ionosphere than one operating at Ku-band, and would have greater performance in terms of vertical resolution, time decorrelation of echoes, spatial resolution and range noise. With the design of an adapted tracker algorithm, near-continuous altimetric tracking over all kinds of surface could be performed, which is especially important when approaching or leaving coasts. The main drawback is that Ka-band electromagnetic waves are sensitive to rain. However, this does not prevent them from acquiring a fairly high percentage of measurements, except for strong rain rates.

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An altimeter/interferometer would include several altimeters mounted on masts which would acquire measurements simultaneously, thus providing continuous, single- or multi-altimeter wide-area coverage.
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One of the ways to improve altimetry resolution is to use several satellites at the same time. Until now, this has been done with very different types of satellite. The use of several identical satellites in constellation could reduce costs (development costs, and launch costs too for micro-satellites launched by the same rocket).
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GNSS altimetry

One approach being pursued to achieve maximum altimetry data coverage is to receive reflected signals transmitted by satellites in the Global Navigation Satellite System (GNSS), in particular from the Global Positioning System (GPS) constellation and its European civil counterpart, Galileo. This concept is based on a satellite in near-polar orbit (at an altitude of 400 to 500 km) retrieving signals emitted by multiple satellites and reflected by the ocean surface, then analysing these signals to compute sea surface height. This concept is currently still only in the study phase.
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Further information:

Special Issue on Satellite Altimetry: New Sensors and New Application, Sensors, Ge Chen and Graham D. Quartly ed., 2006
Guijarro, J., R. Santoleri, B.B. Nardelli, L. Borgarelli, R. Croci, R. Venturini, G. Alberti, A. Caramagno, F. Pirondini, Innovative radar altimeter concepts, Geoscience and Remote Sensing Symposium, 2003. IGARSS ’03 Proceedings, IEEE International, 2, 1080- 1082, 2003.