[Milsurplus] Re: GPS jammers?

[Bob Camp]

Hi

The problem of GPS jamming is actually a bit more complex than you 
might think. The whole GPS process has a couple of steps to it:

1) You figure out roughly where and when (what time) you are
2) You look for sat's in view
3) You either find some sat's or you make a new guess in 1
4) You download a bunch of stuff from the sat
5) You improve your estimate of what time it is
6) You guess at some more sats

Once you have data from multiple sat's you are *much* more immune to 
jamming. The problem is that if you can't make it past step three then 
you can't do much.

If you are a military user once you make it past step 6 you can go into 
an even more secure mode than the normal commercial receivers use. Of 
course if you can't make it past step 3 this isn't much good. Some 
systems get around this by keeping very good track of both time and 
location. That way they can get past step 3 even in the presence of 
jamming.

Broadband jammers are easy to build and set up. A Viet Nam era HARM 
missile  is a cheap and effective counter to that sort of problem. A 
switched broadband jammer is not as effective because it will let the 
user get past step 3 when the jammer is asleep.

None of this is to say that GPS is secure or immune to jamming. It is 
*lot* easier to jam the GPS in my car than a full blown high end 
military setup. Of course a lot of the military uses the same 
commercial stuff as I have in the car (commercial is cheaper ....)

Which still gets us back to the question of surplus. Some of the 
original military GPS stuff dates back to the early 1980's. Has it 
shown up for disposal yet?

	Take Care!

		Bob Camp
		KB8TQ