Not that hard to get a GPS signal
Since my original posting which started off this particular thread ages ago, I've used a bottom-of-the range handheld GPS as a passenger quite a bit. It's always worked nicely, even if it takes a while for the thing to work out what's happening to it. I've used it on A320s, 737-800s, 757s (Dublin-Keflavik), and MD81/82s. And also the fibreglass K-21 glider, although that's for checking the track log afterwards - busy with the business of (learning to) fly while in that craft!
As SLF, I always double-check with the cabin crew as we board, mainly to put them at their ease in case they think I'm using a mobile phone during the flight. The answer is nearly always yes - sometimes after checking with the flight crew. I'm surprised they never seem phased by the request - like the professionals they are.
Obviously, you need to position it at the window, and sometimes it's a bit tricky to get a signal. The GPS should show the location overhead and elevation of the satellites it expects to see. That can be a help in trying to work out where to position it to receive the required three satellites for a fix (or four, if you want altitude as well).
It's taught me some things I didn't know - like how frequently direct routings are given, and how long they can be. Flying between Ireland and Scandinavia, for example, generally involves some pretty long "directs" over the North Sea - more than 200 nm in one case. (News to SLF - not to others here, I realise).
As far as speed restrictions are concerned, it's clocked close to 500 kts groundspeed, so that's not a problem. It crosschecks nicely with info from the flight deck, including altitude.
But not always. Last month, on a LH Airbus en route DUB-FRA, we were told we had "just passed Birmingham". This was way off the route, and didn't match the compass readings which were within about 10 degrees of East since brake release on runway 10. Turned on the GPS again, and found we had just passed Manchester, and not Birmingham. Similar, but different, I guess.
It all may seem a bit sad, but I reckon it's harmless and very educational - I still have much to learn about flying and navigation.
Shamrock