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.
Unlike conventional radar altimeters, where the interval between pulses is about 500 μs, the CryoSat-2 altimeter will send a burst of pulses with an interval of only 50 μs between them. The returning echoes will be correlated, and by treating the whole burst at once, 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 the Synthetic Aperture Radar, or SAR mode.
In order to measure the arrival angle, a second receive antenna is activated to receive the radar echo with two antennas simultaneously. When the echo comes from a point not directly beneath the satellite there will be a difference in the path-length of the radar wave, which will be 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 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:
The DORIS radio receiver detects and measures the Doppler shift of signals broadcast from a network of over 50 radio beacons spread around the world.
A small laser retroreflector reflects light back in exactly the direction it came from. A global network of laser tracking stations fire short laser pulses at CryoSat-2 and time the interval before the pulse is reflected back – providing independent reference measurements of CryoSat-2's position.
- The International Laser Ranging Service (ILRS) provides tracking from its global network of laser ranging stations to support the project.
Last update: 23 February 2010