Radar altimeters are active sensors which use the ranging capability of radar to measure the surface topography profile along the satellite track. They provide precise measurements of a satellite’s height above the ocean and, if appropriately designed, over land/ice surfaces by measuring the time interval between the transmission and reception of very short electromagnetic pulses.
To date, most spaceborne radar altimeters have been non-imaging, wide-beam (pulse-limited) systems operating from low Earth orbits. Such altimeters are useful for relatively smooth surfaces such as oceans and low relief land surfaces, but are less effective over high relief continental terrain as a result of their large radar footprint (of the order of 25 km).
The Ocean Surface Topography Mission (OSTM) is built around the series of Jason satellites that will collect global ocean surface data on a continuous basis for several decades. Its aims are to measure the global sea surface height to an accuracy of a few centimetres every 10 days, and to determine ocean circulation and the mean sea level trend in support of weather forecasting, climate monitoring and operational oceanography. Launched in June 2008, Jason-2 continues to operate in parallel with the Jason-1 mission. Jason-3 is currently being developed for launch in the first half of 2014, with the aim of securing the continuity of high accuracy satellite altimetry observations.
Successful exploitation of the height data is dependent upon precise determination of the satellite’s orbit. A number of precision radar altimetry ‘packages’ are available which contain:
— a high precision radar altimeter (with basic measurement accuracy in the range 2 cm to 4 cm);
— a means of correcting errors induced in the height measurements by variations in the amount of water vapour along the path (for example, by means of a microwave atmospheric sounder or radiometer);
— a high precision orbit determination system (typically based on the GPS, the DORIS beacon/satellite receiver system and/or a lidar tracking system).
Radar altimeters have been flown on a number of satellites. Seasat was the first ocean-oriented mission carrying an altimeter package (including a precise orbit determination system) for the measurement of ocean circulation. Asatellite altimetry revolution happened with the launch in 1992 of the US-French Topex/Poseidon mission. Carrying two high-precision altimeters, a multichannel microwave radiometer, and several precise orbit determination devices on a dedicated, high-altitude (1,336 km), low inclination (66°), non-Sun-synchronous orbit, it enabled the large-scale ocean circulation to be accurately measured. The European ERS-1 (from 1991) and ERS-2 (complete July 2011) also provided long time-series of complementary altimetric observations from a Sun-synchronous polar orbit. These observations were continued with Jason-1 (launched in 2001), Envisat (launched in 2002), and Jason-2 (launched June 2008).