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Asked by: 3389 views Aircraft Systems

Do GPS altimeters compensate for hot and cold errors of which are found on the basic altimeter found in most aircraft?

The AIM has guidance on procedures if the pressure is greater than 31.00''. Why are most altimeters not produced from the factory to be able to be adjusted greater than 31.00'' (such as 34.00'')?


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3 Answers

  1. Michael Lepkowski on Mar 31, 2014

    GPS “altimeter” is a position in space measured by electronic signals between a receiver and satellites. An altimeter is based on barometric pressure, namely, from the atmosphere it is surrounded by. Hot and cold do just about nothing to an electromagnetic wave, but, they do influence the atmosphere and must be compensated for.

    The highest recorded barometric pressure was 1083.8 mb (32″) in Siberia in 1968.

    Why build something to account for an event that will almost never ever happen?

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  2. Wes Beard on Mar 31, 2014

    Most GPS altimeter readouts on a standard GPS (those that don’t have an input from the ADC) don’t use pressure altimeter. Instead they use GPS triangulation with four or more GPS satellites. This give the GPS unit its height (with some error) above its ellipsoidal plane. This plane is close to modeling the sea level surface of the Earth but is not exact. This results in large deviations from what the GPS altitude reports versus what a pressure altimeter reports.

    I would expect that even on a standard day the two (pressure altimeter and GPS altimeter) would not agree.

    Does GPS use non standard or pressure and temperature? No.

    Some advanced GPS’s with an ADC input will give a vertical glidepath that can be adjusted for non standard pressure and temperature called temperature compensation.

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  3. Dan Chitty on Apr 01, 2014

    Thank you for the feedback Michael and Wes. Much appreciated.

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