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Hello everybody, I read that GPS derived altitude should not be relied upon to determine aircraft altitude since the vertical error can be quite large and no integrity is provided. If I fly an aircraft with a GPS installed, do I follow the altimeter or GPS altitude and why? Thanks in advance!  

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2 Answers

  1. Best Answer

    John D Collins on Jul 07, 2015

    The reason that the GPS altitude is not used is not related to its accuracy. A barometric altimeter is based on a standard model of pressure for the atmosphere. Below the flight levels, the altimeter is adjusted for the barometric pressure. Since all aircraft are required to have a barometric altimeter to determine altitude, vertical separation is based on the barometric altimeter. These altimeters are adjustable for local differences in barometric pressure, but not for temperature. So the true altitude can vary substantially from the indicated altitude, particularly on warm or cold days and when the aircraft is far from the location where the barometric pressure setting is determined. In spite of this large potential for error in the true altitude, all aircraft in the same location and at the same altitude indication will be at the same height, within the tolerances of the altimeter. But for terrain and obstacles, one needs to add a substantial buffer, usually a 1000 feet in non mountainous areas and 2000 feet in mountainous areas.

    A GPS altitude is usually more accurate than a barometric altimeter with respect to the true altitude, and as such, it makes a better reference for terrain and obstacles. But in spite of its accuracy and the fact that the accuracy is independent of the altitude and temperature, it is not suitable for vertical separation from other aircraft. The only way it could be suitable if GPS with altitude were mandated to be required equipment for all aircraft, including those without an electrical system. This is not going to happen.

    Usually on the ground at an airport where the barometer setting is determined, the GPS and barometric altimeter are quite close, usually within 35 feet of each other. But on a warm or cold day, they may be hundreds and sometimes up to a thousand feet off from each other. You still use the barometric altimeter if you want vertical separation from other aircraft, in spite of its errors, but use your GPS altitude to determine your true altitude above an obstacle or mountain.

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  2. flyingpiggy on Jul 07, 2015

    Thank you John!

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