As AZcachemeister said, receivers also typically transmit, usually on some harmonic frequency of the transmit frequency. AM and FM radios are dangerously close in the spectrum to airline frequencies (close to FM, but the signal has AM characteristics in the carrier method, as I understand it--doubtless ShadowAce or somebody will fine tune this if it isn't accurate).

There are reports that some spies have been detected based on detection of the transmitter elements of their receivers.

So the "logic" probably goes like this: Since radio receivers also transmit, they might interfere with aircraft electronics or communications. A GPS receiver is a radio receiver, so it too must be capable of interfering. Ergo, we can't allow it. (Never mind the details of the frequency and modulation....) Perhaps now somebody is doing some actual RF emissions testing and deciding GPSr's are acceptable.

On Fri, Mar 21, 2008 at 5:00 PM, Bill Brightman <techwbb@yahoo.com> wrote:

Does anyone have an idea of the logic behind banning use of GPS units in flight?

I can see why devices which emit a signal, like a cell phone, could be suspected of possibly interfering with the plane's electronics. But it seems like a GPS would be more along the lines of an MP3 player or something. Just wondering if I'm missing the point here...

Bill B.