Radar altimeters are active sensors that 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 by measuring the time interval between the transmission and reception of very short electromagnetic pulses.
A variety of parameters may be inferred using the information from radar altimeter measurements, such as time-varying sea-surface height (ocean topography), the lateral extent of sea ice and altitude of large icebergs above sea level, as well as the topography of land and ice sheets, and even that of the sea floor. Satellite altimetry also provides information for mapping sea-surface wind speeds and significant wave heights.
The following GMES Contributing Missions carry altimeters complement the altimetry instrument that will be carried on ESA's Sentinel-3 mission:
ESA has had radar altimeters in orbit since July 1991, when ERS-1 was launched, which was followed by ERS-2 in 1995, Envisat in 2002 and CryoSat in 2010. This will continue to launch radar altimeters on Sentinel-3.
The Radar Altimeter 2 (RA-2) carried on ESA's Envisat is the improved follow-on to earlier radar altimeters on the ERS-1 and ERS-2 spacecraft. From its 800 km-high polar orbit it sends 1800 separate radar pulses down to Earth per second then records how long their echoes take to return – timing their journey down to under a nanosecond to calculate the exact distance to the planet below.
From an altitude of just over 700 km and reaching latitudes of 88°, the CryoSat mission delivers precise data on changes in the thickness of ice. CryoSat carries the first radar altimeter of its kind to overcome the difficulties intrinsic to measuring icy surfaces. The instrument is able to determine the thickness of ice floating in the oceans and monitor changes in the vast ice sheets on land, particularly around the edges where icebergs are calved.
Launched in 2008, the Ocean Surface Topography Mission (OSTM) Jason-2 is an international satellite mission providing a record of sea-surface height measurements that began in 1992 by the joint NASA/CNES Topex/Poseideon mission and continued in 2001 by the Jason-1 mission.
Jason-2 is measuring the global sea-surface height to an accuracy of a few centimetres every 10 days to determine ocean circulation and mean sea level in support of weather forecasting, climate monitoring and operational oceanography.
Jason-3, planned to launch in 2013, is the follow-on to Jason-2. It is the result of close cooperation between CNES, NASA, Eumetsat and NOAA. It should carry the same kind of payload as its three predecessors for high-precision altimetry data. A follow-on mission, Jason-CS, is planned to launch in 2017 for GMES.
The Saral/Altika mission is the result of collaboration between France and India. AltiKa, the Ka-band altimeter and prime payload of the Saral mission, will be the first oceanographic altimeter to operate at such a high frequency. Along with Jason-2, Envisat and Sentinel-3, Altika will contribute to operational oceanography and forecasting. The mission is due to launch in 2011.
Last update: 13 November 2012