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Ha!
Here is my expalnation to the GPS question.
The GPS receives signals from up to 12 satellites all putting out the same time signal and triangulates it's position based on the differences in time lags between the received signals.
This would give the GPS unit it's position relative to the satellites and also calculates the distance to the ground from the relative angles of the satellites. The GPS's "ground" in reference is close to mean sea level (MSL) with average barometric pressure. (This is not the same as the elevation as measured by a radio altimeter which bounces a radar signal off the ground to measure the distance to the surface). Since the barometric pressure affects a barometric altimeter and has to be adjusted accordingly the GPS can be corrected also so it reflects flight levels closer to reality because the flight levels also change with barometric pressure and everyone else is flying them.
Alan Fairclough
Disclaimer:
All opinions offered by the above member should be considered as "straight from the horse's mouth". However, from time to time it may seem like the horse may be turned around.
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