The first European Space Agency's (ESA) Remote Sensing Satellite (ERS-1) uses radar altimetry to measure sea-surface height. The spacecraft payload consists of five instruments: an active microwave instrument (operating in C-band either as a synthetic aperture radar, SAR, or as a wave and wind-scatterometer), a radar altimeter, a precise range and range rate equipment (PRARE), a laser retroreflector array, and an along track scanning radiometer (ATSR). SLR and PRARE were designed to provide two independent tracking systems for ERS-1; the PRARE system, however, failed soon after launch.