Inside CryoSat

Platform

CryoSat is a satellite with a single mission objective – therefore the selection of its orbit and basic characteristics have been driven by the scientific needs.

In order to make comprehensive measurements of the polar regions, a radar altimeter needs to have a more specialised design than those currently in orbit. It also has to be carried on a satellite in an unusually high-inclination orbit, to take it very close to the poles.

Until the launch of NASA’s ICESat, with its laser altimeter, no remote-sensing satellite had ever flown in such an orbit. CryoSat-2 will go even further than ICESat’s latitude limit of 86°, reaching latitudes of 88° north and south on every orbit.

This presented some challenges to satellite design: all parts would at some time be exposed to the full heating power of the sun, while at other times half the satellite would have been in permanent shadow for weeks at end.

CryoSat's 'roof'

Unlike most satellites, CryoSat does not have any deployable solar panels; in fact the satellite has no moving parts at all, except for some valves in the propulsion system. This enables a very significant cost saving, but does pose some problems for the provision of adequate solar power in CryoSat's unusual orbit.

The solar panels are rigidly fixed to the satellite body, forming a 'roof' with a carefully optimised angle, which will provide adequate power under all orbital conditions and still fit within the launch vehicle.

The other area that received particular attention has been the mounting of CryoSat's two main instrument antennas. Any distortion in the support of the antennas would have caused errors to creep into the calculated angle of arrival and so the elevation of the surface.

CryoSat's orbit

To stay within the permitted error limits for the mission, such distortions have to be less than 30 arcseconds: approximately the same as the size of a football seen from two kilometres away. This represented a significant challenge, given CryoSat's unusual orbit – one that was met by designing the structure to be intrinsically stable and providing auxiliary attitude measurement sensors directly mounted onto this structure.

An S-band helix antenna mounted on the satellite's underside will receive telecommands from the ground and downlinks status and monitoring information.

Next to it is an X-band antenna that will transmit the satellite's enormous volume of measurement data whenever the satellite lines up with its assigned ground station of Kiruna – the antenna being designed to provide the same signal strength when the satellite is at the horizon as when it is overhead.

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