Regarding GPS jamming:
Last year I was flying with another pilot near Lossiemouth. We had three GPSs that day and were navigating primarily with GPS. (We had a current, marked up chart and plog before anybody asks!)
A Garmin Pilot III, A Garmin Etrex Ventrue and a Windows CE sat-nav unit running SkyDemon. When I called Lossie Radar the Etrex and Pilot III both lost signals, but the box running SkyDemon continued as expected.
So was this GPS jamming? Nope!
The cause? The chap I was flying with also mentioned he's seen this happen near Coventry with the Pilot III. Our VHF Com set when tuned to Lossie was on 119.35, and I understand Coventry is 119.25. It seems there's an intermediate frequency in the radio that interferes with either the GPS signal itself, or another IF in the Garmin GPS chipset.
Ground testing identified that moving the GPS a metre or so from the Com radio fixed the issue.
Just another potential issue to be aware of if you're using a handheld/portable GPS without an airframe mounted antenna.
-j34-
Morale of the story as I take it: Aviation GPS is *not*, as a whole package, some simple, dumbed-down consumer-grade "turn key" operation...one has to know his/her stuff.