GPS vs ADIRU Altitudes
Along with the torch and the digital camera, I recently added a handheld GPS to my toy box.
I have used the handheld regularly on hiking treks and found it provides accurate elevation/altitude readings.
Once in a while I have switched it on in he cruise and found it very accurate in the lateral mode but just noticed it invariably gives an altitude reading substantially higher than the Air Data & Inertial Reference Units that drive the airshow readout.
In the past two weeks I have bounced this question off a number of colleagues and got many differing answers..all from guys with 15,000+ hours.
Yesterday the ADIRU indicated 37,000' while the GPS showed about 39,200'.
Likewise the on board GPS read about 39,200'.
FL vs true altitude differences are accounted for by pressure and temp differences.
The most plausible reason I have heard to date is that the GPS has the 'true' surface of the earth in its memory...it being a flat reference point This varies from location to location, and it is from this non-spherical surface that it takes its height reading.
Can someone put me out of my misery because this is an airine with 85 heavies that doesn't seem to know the answer?
Last edited by Traffic; 9th July 2004 at 15:22.