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May 06, 2013

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GPS satellites orbit the centre of gravity of the Earth and thus only measure heights relative to a geocentric reference ellipsoid (WGS).
For this reason modern GPS receivers have a SW inside where they obtain the geoid (EGM-96) height over the WGS ellipsoid from the current position. Then they are able to correct the height above WGS ellipsoid to the height above WGS84 geoid. And all this to get the sea "level" ;-)

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