SIRAL
type: Concept
change note: 2019-02-13 08:32:55.0 [mmorahan] insert WeightedRelation (id: null related concept uuid: a915ab2f-46c5-493b-9f18-aeb3383ee72b relationship type: null relationship value: null generated by: null);
change note: 2017-05-08 13:08:19.0 [mmorahan] insert Definition (id: null text: CryoSat’s primary instrument is the Synthetic Aperture Interferometric Radar Altimeter (SIRAL). It was designed to meet the measurement requirements for ice-sheet elevation and sea-ice freeboard, which is the height of ice protruding from the water. Conventional radar altimeters send pulses at intervals long enough that the echoes are uncorrelated; many such echoes can be averaged to reduce noise. At the typical satellite orbital speed of 7 km/s, the interval between pulses is about 500 microseconds. However, the CryoSat altimeter sends a burst of pulses at an interval of only about 50 microseconds. The returning echoes are correlated and, by treating the whole burst together, 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 known as the Synthetic Aperture Radar – or SAR – mode. In order to measure the arrival angle, a second antenna receives the radar echo 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 then provides the angle between the 'baseline', joining the antennas and the echo direction. In addition to the altimeter, knowledge of the precise orientation of the baseline of the two receiving antennas is essential. CryoSat measures this baseline orientation using the oldest and most accurate of references: the position of the stars in the sky. Three star trackers mounted on the antenna support structure each takes five pictures per second. The ima…
change note: 2019-02-12 14:46:19.0 [mmorahan] insert WeightedRelation (id: null related concept uuid: e377ef25-1612-4b8d-ac98-54e3977d7e31 relationship type: null relationship value: null generated by: null);
change note: 2017-05-05 09:02:09.0 [mmorahan] insert AltLabel (id: null text: Synthetic Aperture Interferometric Radar Altimeter language code: en);
change note: 2017-04-14 05:40:13.0 [mmorahan] Insert Concept add broader relation (SIRAL [30787b9f-a407-47a5-b69b-5b9e1d1b1144,278797] - Radar Altimeters [92ddaaca-40b2-4936-bc1a-28cc5743e8b8,259513]);
in scheme: instruments
definition: CryoSat’s primary instrument is the Synthetic Aperture Interferometric Radar Altimeter (SIRAL). It was designed to meet the measurement requirements for ice-sheet elevation and sea-ice freeboard, which is the height of ice protruding from the water. Conventional radar altimeters send pulses at intervals long enough that the echoes are uncorrelated; many such echoes can be averaged to reduce noise. At the typical satellite orbital speed of 7 km/s, the interval between pulses is about 500 microseconds. However, the CryoSat altimeter sends a burst of pulses at an interval of only about 50 microseconds. The returning echoes are correlated and, by treating the whole burst together, 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 known as the Synthetic Aperture Radar – or SAR – mode. In order to measure the arrival angle, a second antenna receives the radar echo 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 then provides the angle between the 'baseline', joining the antennas and the echo direction. In addition to the altimeter, knowledge of the precise orientation of the baseline of the two receiving antennas is essential. CryoSat measures this baseline orientation using the oldest and most accurate of references: the position of the stars in the sky. Three star trackers mounted on the antenna support structure each takes five pictures per second. The images are analysed by the star trackers’ built-in computers and compa…

Radar Altimeters
type: Concept