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We use satellites to calculate our position. How do satellites know their position if it is right or wrong ? I have heard that the base station records and corrects the satellite path but how do they know what is right path what is wrong path ?

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  • Orbital characteristics are fairly predictable. Combine that and measurements from fixed places, and you have the fine correction details you need. Can you ask a more specific question (as to what problem you are trying to solve) if you have one.
    – BradHards
    Feb 7, 2014 at 9:22
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    @NathanW: You need the satellite position. Trilateration needs distance measurement from 4 satellites with known positions to solve for the three position terms and time error term.
    – BradHards
    Feb 7, 2014 at 9:30
  • Ahh true. Ignore me, been a long day :)
    – Nathan W
    Feb 7, 2014 at 9:33

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There are two different issues here.

The position of the satellite is a complex calculation based on a network of 400+ ground stations (IGS) and orbital characteristics (from keppler's laws). The orbite is computed with a precision of a few decimeters based on 25 stations, and this information is sent back to the satellite.

The base stations are not used to send information to the GPS satellites. However, it allows for the correction of the errors of propagation through the ionosphere and the troposphere, as well as the residual errors on the orbites. Those errors are systematic and can be considered as identical if the base station is close to the receiver. Because the position of the base station is known, you can infer the errors and remove them from the mobile receiver signal.

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    It additionally uses calculations from both Special and General Relativity. In fact, GPS is famously one of the few examples that uses both.
    – rsegal
    Feb 7, 2014 at 14:54
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I recently came across an article in GPS World stating that some of the more recent vehicles launched into space had small special mirrors attached to the satellite's exterior. Repeated laser beams sent from the earth's surface would reflect off the mirror, providing a more accurate estimate of the vehicle's location.

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