[time-nuts] GPS jamming susceptibility

Bob Camp lists at rtty.us
Mon Nov 22 23:19:14 UTC 2010


Hi

There is *very* little signal hitting the ground from a normal GPS bird. Even a few mili watts close at hand is going to be an enormous overload. The typical GPS does not use a lot of bits in the front end A/D. 

I suspect that if you tuned your little gizmo down to the FM broadcast band, it would take out your favorite FM station quite nicely. Same would be true of your cell phone if you tuned it there. Jamming from close by isn't all that hard to figure out, or to implement. There are switching power supplies that make wonderful jammers for low frequency signals. If it's RF, it can be jammed. The real question is can you jam it from a reasonable distance? 

Bob


On Nov 22, 2010, at 12:43 PM, John Green wrote:

> Given that this is an extremely sensitive topic and completely illegal
> also, let me just state at the outset that I have no interest in
> jamming anyone's GPS. A while back, I was looking at one of those
> Chinese discount electronics websites, I'm sure we all have, and
> noticed a GPS jammer for sale. I had been wanting to do some jamming
> susceptibility testing for quite some time but had never got around to
> building a generator to test with. The thing was cheap so I ordered
> it. After it arrived, I opened it up, first thing, to see how it was
> made. It has a dual 555 oscillator, a couple of analog switches, a 1.9
> GHz VCO, a single amplifier. It doesn't look capable of putting out
> more than 50 milliwatts or so into a 2 inch antenna. I was somewhat
> dubious that it would do anything, so I took it to the bench where the
> Z3801 lives and turned it on. Within 2 seconds, the holdover LED lit.
> I immediately turned it off and within a few more seconds, the
> holdover LED was back off. The GPS antenna is perhaps 35 feet away
> with a cinder block wall, a brick wall, and a metal roof in between. I
> also put a 15 Db attenuator between it and the antenna with almost the
> same result. I am going to do more testing with it wired into the GPS
> downfeed an an adjustable attenuator in line just to see how much
> signal it takes. That way, there will be little danger of messing with
> anyone's reception. It is just a simple sweeper so it must do its job
> by brute force. I am amazed that it took so little to shut my Z3801
> down. Has anyone here had any actual experience testing GPS receivers
> for susceptibility?
> 
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