Cryosat is a satellite with a single mission objective – the selection of its orbit and basic characteristics have therefore been entirely driven by the scientific requirements. Consequently, the orbit will have a high inclination of 92°, taking it just 2° short of the poles. This orbit will be non-sun-synchronous (commonly used for remote-sensing satellites) and will drift through all angles to the Sun in eight months. This has presented some challenges in the satellite design; all parts will at some time be exposed to the full heating power of the Sun, while at other times half the satellite will be in permanent shadow for weeks on end.
The science requirements for sea ice demand spatial sampling at 105 km² (~ 300 km by 300 km). The effect of snowfall variability requires that the measurements of sea ice for the primary mission goal fall within one month in any one year. This places an upper limit on the temporal sampling. This sampling is also suitable for the secondary sea ice objectives. For land ice, the requirements dictate spatial sampling at 104 km² (~ 100 km by 100 km). There is no specific temporal sampling requirement, although the accuracy of the trend estimate will depend on the number of temporal samples. To maximise coverage over ice caps and glaciers, dense spatial sampling consistent with the temporal sampling for the primary mission goal should be achieved.
The spatial pattern of samples need not repeat, provided it retains a constant temporal and spatial sampling density. Sea ice is a moving mass field. Were the sample pattern to repeat itself, measurements would still observe different ice. The land ice measurement uses crossovers of the orbit, and in doing so removes the effect of the topography.
Why doesn’t won’t Cryosat fly in a 90° inclination orbit, which would take it directly over the poles to observe all of the polar regions? The choice of orbit is a compromise, as a 90° orbit would be beneficial for the survey ofsurveying Arctic sea-ice, but would seriously degrade the monitoring of the Greenland and Antarctic ice masses. Such measurements are made at orbit crossovers, where the north-going satellite track crosses over an earlier (or later!) south-going track. With a 90° inclination orbit, crossovers would be few, only occurring due to the Earth’s rotation, since the orbit tracks are otherwise directly along the lines of longitude and do not cross. The 2° offset from a true polar orbit is enough to ensure an adequate density of crossovers over the ice sheets.