There are restrictions on the capabilities of consumer GPS receivers, otherwise they're considered to be munitions. I think it's something like not working above 60000ft (so not a problem for most of us) and there's also a speed limit on them, which I vaguely remember as 200mph but that seems a bit slow and I might be getting it confused with the GSM phone limit (which is a technology limit, not a legal one). It's obviously to stop the bad guys from fitting them to guided missiles and rockets. Also, most of them ignore satellites below about 5 degrees elevation which will rule out some that are visible from five miles up.
You need to know how the satellite constellation is aligned to get best results - in the UK a south-facing window is generally best because the satellites spend most of their time south of our latitudes, obviously not so true elsewhere. If you can get a fix in the terminal before taking off then your satellite search window is much narrowed and you stand a chance of finding some of them quickly. You'll need quite a few satellites in view to get a decent 3D fix, otherwise you'll confuse it because for a 2D fix it'll assume you're at ground level and so will be a bit inaccurate at flight levels. However, once in the clear then anything above the horizon will be line-of-sight, subject to aircraft metalwork.
OK, it's only a problem on Concorde, speed limit appears to be about 1000mph.
Last edited by llondel; 10th May 2007 at 17:56.
Reason: Update on speed limit