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

Jim Lux jimlux at earthlink.net
Tue Oct 8 09:42:20 EDT 2013


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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