Abstract The Radar Altimeter to be carried on ERS-1 will provide measurements of altitude, significant waveheight and windspeed over the oceans to a latitude of 82°. In addition, the instrument has a dedicated Ice Mode which is designed to ensure operation over the extreme terrain found at the margins of ice sheets, though with reduced performance. A description of the instrument, its operating environment, the mission objectives, and the particular problems of calibration and validation, are presented. Attention is also focussed on the synergystic nature of Radar Altimeter data with respect to data from other sources. Thus independent measurements are required to provide corrections to altimeter data, while altimeter data in turn may be combined with data from other sources to enable studies not possible with either sensor alone.