An instrument that transmits a microwave pulse and measures the return echo. Earth-observation satellites use them to gather information on the characteristics of the Earth's surface. Measurements of the echo's delay, power, and shape will indicate land features, wind speeds, sea elevation, and sea ice. This is used to monitor global climate changes. The first spaceborne radar altimeter flew on the Skylab space station in 1973. The European Space Agency installed the instrument on ERS-1 and ERS-2, and put its RA-2 instrument, which has improved measurement performance, on the Envisat.