as far as I know

来源: 于 2011-02-23 13:45:39 [博客] [旧帖] [给我悄悄话] 本文已被阅读：次

They all use air pressure to measure the altitude. It means they all use barometer. It takes a reference point at ground level, then measure the difference.

Aircraft altimeter converts the air pressure to altitude. Capable GPS does the same.