On Mon, 19 Jul 2004 15:20:40 +0200, Nele VII wrote:

At a rough guess the cost for a 1W jammer using cellphone components would
be ~$5, just keep sending them up with balloons (add $1).

Well, it is a bit under-estimated. I don't know much about how GPS works
(electronically), but I know it has commercial and military channels/
frequencies and commercial and military encoding. For civilian use, optimum
accuracy is receiving three GPS signals from three satellites (somebody
correct me if I am wrong, but I read it somewhere for more expensive GPS
devices for use, let's say in aviation) for elevation and position.

You need to receive signals from four satellites to obtain a 3-D
position. Most GPS receivers can make do with three satellites, but
when they do they give only a 2-D position, and assume your elevation
is constant.

Modern consumer GPS receivers can receive and use signals from up
to twelve GPS satellites simultaneously. With the current GPS
constellation, it's not unusual to have ten satellites above the
horizon and useable. Most receivers will use the additional
satellite signals (above the four needed for 3-D position) to
determine your position more accurately.

As you travel, the sattelites "switch" (just like GSM) control.

Actually, the satellites are most likely traveling faster
than you are. Their orbital period is just under 12 hours, so
you'd have to be moving more than 2000 mph to stay directly
underneath one. The GPS receiver computes which satellites are
above the horizon, and listens for their signals. As satellites
rise and set, the receiver updates the group of satellites it's
listening for.

[...]

What method are you going to use-noise or deceptive jamming? Read the
history of updates of "AN-ALQ" devices-or just one! Noise? In that case, you
are a flashbulb, and you will be attacked with alternate weapon on jamming
station. The band is narrow AFAIK, so if your opponent knows what to
"listen", you're toast.

[...]

Deception jamming might be possible if you have a civilian receiver,
but you'd need to crack the encrypted signal to spoof a military
receiver.

There are a number of introductory descriptions of how the GPS system
works on the web -- for example, check out

http://www.aero.org/publications/GPSPRIMER/


ljd