CryoSat-2 technology: anatomy of a satellite
The theory sounds relatively straightforward, but examination of the CryoSat-2 satellite highlights the practical challenges faced by Astrium in Friedrichstafen, Germany (A-D) and Thales Alenia Space in Toulouse, France (TAS-F).
“CryoSat-2 is very similar to the first CryoSat in design, but features a number of improvements,” said Richard Francis, CryoSat-2 Project Manager. “The most obvious is that the SIRAL radar instrument is now fully redundant, and this induced a number of knock-on changes. We are also flying the next generation of DORIS, the radio-frequency positioning system, as well as brand-new S-band transponders. We increased the battery capacity and made lots of changes to the software, to make CryoSat-2 much easier to 'drive' than its predecessor.
“But fundamentally the satellite is really designed around the radar, the key requirement being to keep the radar antenna and startracker system as stable as possible.”
So the satellite has a minimal number of moving parts to avoid ‘rocking the boat’ as much as possible, while its distinctive dog-kennel shape is a consequence of CryoSat-2’s unusual orbit, which is the best compromise between high latitude coverage (88°) and maximising the number of orbital track cross-overs – key to obtaining the rates of change of ice sheet elevation.
Unlike standard ‘Sun-synchronous’ orbits, this means the satellite experiences every conceivable Sun angle in the course of 450 days. A diagonal rooftop of solar panels was the best method available within budget of producing sufficient power needed to keep CryoSat-2 running at all times. In a further boost to power availability, very high-performance triple junction gallium arsenide solar cells are used, technology originally developed for high-power communication satellites.
Doing without fold-out solar panels also has the virtue of constraining CryoSat-2’s overall surface area, cutting the amount of attitude correction needed due to solar radiation pressure.
Its direction and orientation in space must be maintained very precisely – the measurement accuracy of the orientation of its twin antennas is about 30 arcseconds, equivalent to a football seen from 2 km – so it is equipped with startrackers, laser retro-reflectors for ground-based position measurements and also radio-based navigation.
For active attitude correction, CryoSat-2 carries magnetorquers and cold gas thrusters for fine-tuning – the latter producing a correcting force equivalent to a cubic centimetre splash of water.
