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Exploring Orbits

A ground track comparison of Sun-synchronous satellites in low Earth orbit and geosynchronous satellites in high Earth orbit.

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  1. Most of the examples in my previous post about making Ground Track Generator show satellites or stations in low Earth orbit. For example, here is a ground track of WorldView-2 (mislabeled WorldView-1 in the image). It crosses the equator at an altitude of about 770 kilometers and has an orbital period under 100 minutes.
  2. A satellite in a sun-synchronous polar orbit like this can observe much of the globe in detail every day, with consistent illumination on each pass. This is an important property for Earth science and imaging platforms; indeed, the "revisit time" time touted by commercial operators (1.1 days for WorldView-2) is a measure of the minimum time between consecutive passes over the same point.

    (Not all satellites in low Earth orbit are sun-synchronous, of course, but it's common among the imaging satellites I'm most interested in.)

    Contrast that orbit to the geosynchronous orbit of a communications satellite like Nilesat 101. (I've chosen Nilesat as an example because of recent news about allegations of jamming in the aftermath of the Arab Spring - knowledge is power.) It barely strays from a single point.
  3. This satellite orbits at an altitude above 35000 kilometers and has a correspondingly longer orbital period of 24 hours. Because it is orbiting in the same plane as the Earth is rotating (the equatorial plane) and it completes an orbit in the same time the Earth completes a rotation (one day), it appears to remain stationary relative to the Earth's surface. This is the key property of geosynchronous satellites.

    Geosynchronous satellites are useful for communication purposes because receivers on the ground - like your satellite TV or radio antennas - don't have to move to track them across the sky. Since geostationary satellites necessarily orbit around the equator, this also explains why satellite TV antennas in the northern hemisphere must face to the south (and to the north in the southern hemisphere).

    (Note: Although the ground track of Nilesat 101 appears to form a ring, it's not "orbiting" some point above the Atlantic. I don't presently know how to explain the motion. I suppose if the satellite's orbit is not perfectly aligned with the equator, as few must be, it will appear to wobble above and below the equatorial plane over the course of each day – and indeed, the same pattern is exhibited by other geosynchronous satellites.)

    For more information about different kind of orbits, including very clear descriptions and diagrams, see these two pages at NASA's Earth Observatory site.
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