Satellite Orbits

Remote sensing satellites use two types of orbits:

The map shows the coverage of the TOPEX/Poseidon radar altimeter, but all polar orbiting satellites would show a similar geometry. TOPEX has a 10 day repeat cycle, with 127 compete orbits during the cycles. Each orbit has an ascending (odd numbers) and a descending (even numbers) portion, numbered from 1 to 254. The coverage of all 254 passes is indicated with the gray color, with the first eight highlighted.

The Sentinel-2 satellite has the same 10 day repeat pattern, but its inclination allows imaging higher latitudes.

Note the lack of coverage for high latitudes in both hemispheres, determined by the orbit's inclination.

 

The track width is imaged on each pass, and occurs on either side of the point directly beneath the satellite ("nadir")s. Greater track width allows greater coverage, but the geometric distortion at the edges of the track increases because of the earth's curvature. If track width is decreased for greater geometric precision, more orbits will be required for complete global coverage, which decreases the time between coverage.

Each orbit of a polar orbiting satellite will have an "ascending" pass when the satellite moves northward toward the equator and then into the northern hemisphere, and then a descending pass on the other side of the word when then satellite moves southward. Because of the geometry, one of the passes will occur during daylight hours and the other will be during the night because it occurs on the opposite side of the earth with a very short delay.

Typical orbits and oribital velocities.  Rotational periods range from 100-110 minutes in sun-synchronous orbit, about 12 hours for the GPS constellation, and 24 hours for geostationary. (from Nasa)

 

Orientation of the satellite and sun over the course of the year.  A sun-synchronous satellite like Landsat or Sentinel-2  always passes at the same time, so the position of the satellite with respect to the ground will be same same, with small differences from one corner of the scene to anothter, and potential small difference if the orbit is not precisely constant.

The solar illumination in the scene, and and the angle between the sun and the spacecraft, will change with the seasons.  Both the elevation and azimuth to the sun change with the seasons.  The most prominent effect will be greater shadows in winter.







(https://www.usgs.gov/media/images/earth-s-axial-tilt-and-seasonal-effects)

 


Last revision 5/6/2020