Satellite Orbits and the Observation of the Earth - examples

  Summary of Advantages and Limitations

Advantages :

  The low altitude orbit favours a good ground resolution. It also enables easier active measurements with radar or lidar.

  The circular orbit implies a constant satellite velocity, which is important for having a regular scanning resolution along the satellite ground track (See in the same module: 3.1 Space-time sampling: low-altitude orbiting satellites).

  The near polar orbit allows a global coverage for the observation of the whole Earth. A good compromise for having both a large ground swath offering a daily global coverage and a good ground resolution, is obtained by using orbit altitudes of between 700 and 900 km.

  Sun-synchronism produces time-constant illumination conditions of the observed surfaces, (except for seasonal variations). This property is useful for many remote-sensing applications in Earth observation. Another property of interest is the nearly constant sunlight ratio of the satellite on each orbit, which implies a near constant solar energy supply for the satellite platform.

Sources.

Limitations :

  A continuous temporal observation is not possible with only one sun-synchronous satellite. It passes over polar regions on every orbital period, but much more rarely over equatorial regions (2 times a day for most current meteorological satellites; more generally it depends on the drift and the ground swath).

A possibility to ease this difficulty could be to use a constellation of satellites. At present this is only envisaged for telecommunication applications and not for Earth observation.

Consequences :

Most of the Earth observing missions use sun-synchronous satellites in low near polar orbits (NOAA polar orbiting meteorological satellites, Landsat, SPOT, ERS, etc...).

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