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Height for GPS.


Most satellites are in Low Earth Orbit for the simple reason that it is cheaper to get them there than further up. Some other satellites are in Geostationary Orbit because their function requires it.

The GPS (and other GNSS, e.g. GLONASS) satellites are in a much higher (more expensive) MEO orbit (sub-GEO):

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Why to they need to be in such a high orbit? Clearly the GPS design does not require that they need to be in GEO.

The GPS wikipedia page mentions that with this orbit, the satellites have an orbital period of about 12 hours, and thus follow the same track over the earth - this was useful for debugging when the system was first being set up. But surely a similar effect could have been achieved with an 8 or 6 hour orbital period (or some other divisor of 24) for much less expense.

Possible, though unconfirmed reasons I can think of for the high orbit:

  • Initially (and still) a military project, having the satellites in such a high orbit makes them harder for the enemy to shoot down.
  • Being higher up means more satellites are in line-of-sight to any given point on the surface of the earth. I don't know how many satellites would be required for the same level of service if they were at the 8 or 6 hour period orbit, though I'd be interested to see how the costs compare to put more satellites in lower orbits.
  • LEO satellites are more affected by atmospheric drag, so will need to perform more regular station-keeping maneuvers. Presumably they need to be temporarily taken out of GPS service when performing these maneuvers - perhaps this is unacceptable within the GPS design. Also more fuel is required for station-keeping, or there will be shorter service lives which perhaps offsets the extra expense of the higher orbit.
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Article details
Article ID: 27
Category: GPS Module
Date added: 2013-11-06 18:29:53
Views: 769
Rating (Votes): Article rated 3.4/5.0 (5)

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