CryoSat-2's primary payload is the SAR/Interferometric Radar Altimeter (SIRAL), which has extended capabilities to meet the measurement requirements for ice-sheet elevation and sea-ice freeboard. CryoSat-2 will also carry three star trackers for measuring the orientation of the baseline. In addition, a radio receiver called Doppler Orbit and Radio Positioning Integration by Satellite (DORIS) and a small laser retroreflector ensures that CryoSat-2's position will be accurately tracked.
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 the Synthetic Aperture Radar, or SAR mode.
Knowledge of the precise orientation of the baseline and the two receiving antennas is essential for the success of the mission. CryoSat-2 will measure this baseline orientation using the oldest and most accurate of references – the position of the stars in the sky. Three star trackers are mounted on the support structure for the antennas. Each containes 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 makes a measurement of the distance between the satellite and the surface of the Earth. This measurement can not be converted into the more useful measurement of height of the surface until the position of the satellite is accurately known. This requires that CryoSat-2 carry some specific equipment: