It will happen on various occasions, especially with the handhelds. Most of them only have a small reciever, which even if you put on top of the panel you only have a small portion of the sky available to receive.
There could be all 24 satellites running, of which 18 are over the horizon, leaving 6 covering where you are flying. Presume your reciever wont pick up the 2 sats that are below 15 degrees elevation and the two behind you are blocked by your own aircraft... That leaves two, which is not sufficient to get a position.
Normally in the UK we have between 4-10 satellites available to use with GPS.
Rather than a GPS outage, you will probably find you might of had signal if you turned round and flew back the other way
