Aviation In Canada

Friday, March 18, 2005

GPS Elevation and Barometric Altimeter

I recently flew Westjet, and was lucky enough to get in one of their B737-700s with the entertainment system. After I put thoughts of Swissair 111's inflight fire aside (thinking of the entertainment system), I had a look at it. The "start up" screen was a picture of Canada with an icon representing the aircraft's position. At the bottom of the screen was an altitude and groundspeed readout. While I don't understand why they would post speed in mph (knots or km/h would make more sense in Canada), the altitude measured in feet did. Sort of...

The pilot reported shortly after levelling off that we had reached our cruising altitude of 40,000 feet. To me, that meant FL400. The altitude readout read ~38,800. Hmmm. I checked my GPS, and it concurred, within about 200 feet. I called my altimetry guru, George Dewar, and we talked out the answer.

There are several issues in altimetry that combine to increase discrepancies between the two. I say discrepancies, because the word 'error' wouldn't really be right. An error that would be of concern would be a difference between where the aircraft should be, and where it is. In the case of an aircraft's barometric altimeter, it very rarely reads how high the airplane really is due to a variety of differences between actual conditions and the ICAO standard atmosphere, the set of conditions for which the barometric altimeter is calibrated. It is subject to a number of errors, but they are all pretty consistent, allowing each aircraft to experience the same amount of error. This makes the barometric altimeter useful for altitude guidance in aircraft. Though they are in error, the consistency becomes the important part. GPS is not subject to the same errors (it has its own bugaboos), so mixing the two is not practical, especially under certain circumstances.

Cold weather altimeter error is the most significant error faced in my example above. Surface pressure was near standard, so the difference between standard pressure and surface pressure is minimal (remembering we set 29.92 at 18,000 in Canada). The temperature at the surface was about 25C° colder than standard. A quick figuring of the cold weather altimeter correction needed ends up accounting for the vast majority of the difference between barometric altimeter reading and GPS calculations for my example. The extreme discrepancy is largely due to the height of the aircraft. At lower elevations, this discrepancy would be significantly lower, though significant if on approach. GPS is subject to its own issues, but temperature will not affect the altitude readout. This would make GPS calculations a more accurate determination of altitude compared with the barometric altimeter, and our actual altitude in this flight probably was closer to the 38,800 mark than to 40,000.