Cryosat-2’s primary payload will be the SAR/Interferometric Radar Altimeter (SIRAL), which has extended capabilities to meet the measurement requirements for ice-sheet elevation and sea-ice freeboard.
- Low-resolution mode like a conventional altimeter, on land ice or sea which are composed of few rough surfaces,
- SAR mode operating a high resolution measurement on sea ice,
- SAR interferometer (SARIn) mode operating on rough surfaces like on the sea ice/land limit.
Unlike conventional radar altimeters, where the interval between pulses is about 500 microseconds, the Cryosat altimeter sends a burst of pulses with an interval of only 50 microseconds between them. The returning echoes are thus correlated, and by treating the whole burst of pulses in one operation, the data processor can separate the echo into strips arranged across the track by exploiting the slight frequency shifts (caused by the Doppler effect) in the forward- and aft-looking parts of the beam. Each strip is about 250 m wide and the interval between bursts is arranged so that the satellite moves forward by 250 m each time. The strips laid down by successive bursts can therefore be superimposed on each other and averaged to reduce noise. This mode of operation is called Synthetic Aperture Radar, or SAR mode.
Phase difference between returning radar waves: in order to measure the arrival angle, a second receive antenna is activated so that the radar echo is received by two antennas simultaneously. When the echo comes from a point not directly beneath the satellite there is a difference in the path length of the radar wave, which is measured. Simple geometry provides the angle between the baseline joining the antennas and the echo direction.
Knowledge of the precise orientation of the baseline and the two receiving antennas will be essential for the success of the mission. Cryosat is to measure this baseline orientation using the oldest and most accurate of references – the position of the stars in the sky. Three star trackers will be mounted on the support structure for the antennas. Each will contain a camera, which will take up to five pictures per second. The images will be analysed by a built-in computer and compared to a catalogue of star positions.
The altimeter will measure the distance between the satellite and the surface of the Earth. This measurement cannot be converted into the more useful measurement of surface height until the position of the satellite is accurately known.