[time-nuts] How hard is it to detect a GPS Jammer?

[Bob Camp]

Hi

Well finding a +10 dbm 1.5 GHz transmitter isn't very hard to do at all. I've got several of those….

Bob

On Oct 8, 2013, at 9:42 AM, Jim Lux <jimlux at earthlink.net> wrote:

> On 10/8/13 4:17 AM, Bob Camp wrote:
>> Hi
>> 
>> But there's obviously something wrong with the 400 KM number.
>> 
>> 1) If +10 dbm is good enough to burry a useful signal at that distance, it should be good enough to communicate at that distance. That's pretty impressive QRP without high gain / directional antennas involved.
>> 2) The radios (at least the modern ones) do have CW signal immunity. Weather that's 60 db or something else probably varies with the make / model of the GPS. How well that works with a VCO jammer - again, a that depends sort of thing.
>> 3) There's a (maybe) 40 db variation in GPS signals. To deny service you need to take out the strong ones, not just the weak ones.
>> 4) Even without specific anti-jam in the GPS, the code it's self does have some immunity to a jammer.
>> 
>> Of course you don't have to look very far into the archives to find wonderful examples of slipped decimal points in my posts….
>> 
> 
> 
> You're right, I forgot the process gain of the despreading.
> Assuming you're going from 1 Mchip/sec for the C/A code to 50 bps for the nav message, that's 43 dB
> 
> That alone gets you to 2km jamming range from 400km, but that also assumes that the jamming signal doesn't inhibit acquiring the code and despreading.
> 
> As Dixon's book on Spread Spectrum says, "acquisition is the hard part"; because you don't have the process gain yet. I suppose with a parallel acquisition strategy, you're basically trying all codes, and that might be able to work.
> 
> But even so, those sorts of process gain arguments don't necessarily work if the receiver has a hard limiter or quantizer in it.
> 
> I don't know that modern consumer GPSes have CW immunity. If they're using a 1 or 2 bit quantizer, a strong CW signal pretty much captures the front end.
> 
> Easy to try.  Let me just fire up my kilowatt 1.5 GHz transmitter here<grin>
> 
> 
> 
> 
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