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2 Answers

Accuracy of GPS

Asked by: 1906 views Airspace, General Aviation, Student Pilot


please anyone guide me whether the altitude as shown by the GPS is accurate or not.Once when flying above 5000 feet density altitude i compared my altimeter with GPS and there was lot of difference. Altimeter was set to standard QNH.

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2 Answers

  1. Wes Beard on Jul 24, 2012

    The altitude the GPS generates has errors.  For example, the mathematical geoid equation is only an approximation of the Earth’s surface.  The GPS signals can provide a very precise location above the geoid but that is only an approximation of the altitude above mean sea level.
    We know the pressure altimeter, if the kollsman window is set to the current altimeter, measures mean sea level.  Unfortunately, there is also errors in the pressure altimeter to include distance both horizontally and vertically from the weather station for pressure and temperature differences from standard temperature.  You may have remembered the saying, “From High to Low, look out below”; it deals with both pressure and temperature changes.  The canadian pilots are required to adjust their altitude for really cold weather; something US pilots are not required to do (but should).
    These errors present in the GPS equation and the pressure altimeter make it impossible to completely rely on the GPS altimeter for altitude in flight. 

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  2. Bob Manning on Jul 24, 2012

    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.  What this means is that if you are walking on the seashore,  and see your altitude as -15 meters,  you should not be concerned.  First,  the geodetic model of the earth can have much more than this amount of error at any specific point and Second,  you have the GPS error itself to add in.  As a result of this combined error,  I am not surprised to be at the seashore and see -40 meter errors in some spots.
    DGPS operation (where available) will dramatically improve the performance of even low cost GPS receivers. Horizontal accuracy of +/- 5 meters and altitude accuracy of +/- 10 meters (relative to the WGS-84 geode) with suitable DGPS receivers and low cost GPS receivers such as the Garmin GPS-12XL can be expected.
    In any case, it is extremely unwise to overly depend on the altitude readout of a GPS. Those who use GPS altitude to aid in landing their small plane should have their insurance policies paid up at all times

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