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GPS indications (without augmentation) may sometimes provide an inaccurate representation of altitude. Why is that?

  • A

    If there is an insufficient number of satellites in view, this can result in missing information and inconsistencies with the ephemeris data.

  • B

    The height of the ionosphere interferes with the path lengths of the radio waves.

  • C

    If there are too many satellites in view, the excess information fails to coincide with that in the Almanac.

  • D
    The mathematical model used by the GPS is a very close approximation to, rather than a complete match with, the actual shape of the Earth.
GPS ALTITUDE
GPS altitude measures the users' distance from the center of the SVs orbits. These measurements are referenced to geodetic altitude or ellipsoidal altitude in some GPS equipment. Garmin and most equipment manufacturers utilize a mathematical model in the GPS software which roughly approximates the geodetic model of the earth and reference altitude to this model. As with any model, there will be errors as the earth is not a simple mathematical shape to represent.

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