Roadster may I refer you to the GPS used by the crew on the HMS Cornwall Lynx. This is identical to the one I bought on eBay and have in my drawer. The one in my office is made by the 'other' manufacturer, is in a fetchind shade of green, has a lot of numbers on it and is decorated with a stylish NATO logo. It is no different from a commercial product.
There are other comments that I might make but I believe I got that information from protected sources.
What you might deduce from what I said is that there are GPS and GPS.
Remember too that GPS is a proprietary name whose full name is Navstar GPS. It is the US owned Global Navigation Satellite System. This link gives some useful background and the bit on Selective Availability is relevant.
http://en.wikipedia.org/wiki/Global_Positioning_System
There is a rival Russian system, GLONASS, with about 14 funtioning satellites:
http://usinfo.state.gov/xarchives/di...ellep0.5061609
There is then a possibility that the European system Galileo will become operational too.
http://www.military-geospatial-techn...cfm?DocID=1379
I believe the Chinese have bought in to Galileo.
With 3 available systems and multi-channel capable receivers an exo-atmospheric nuclear burst would provide a 100% ECM solution. For local GPS denial you would need a battery of jammers to cover each satellite frequency and system.