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

[Bob Camp]

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….

Bob

On Oct 7, 2013, at 9:11 PM, Jim Lux <jimlux at earthlink.net> wrote:

> On 10/7/13 8:31 AM, Chris Albertson wrote:
>> OK so let's say you have a receiver and detect a certain about of power at
>> the right frequency.  How do you determine which of three cases you have
>> (1) an actual GPS signal from a satellite. (2) a spoofer (who tries hard to
>> look like #1) or (3) a jammer.
> 
> 
> The jammers put out many milliwatts and have enormous signals that are obvious on a spectrum analyzer.  GPS signals are invisible on a spectrum analyzer, normally.  IN fact, most GPS receivers don't work very well if there are signals above the noise floor: they depend on the noise to make them work with their mighty 1 bit quantizers.
> 
> 
>> 
>> 
>> Spoofers are a real problem.
> 
> I doubt anyone is selling spoofers on eBay.
> Sure, one can probably find some  code to run on a USRP from some grad student's project.
> 
>> So the easiest thing to detect would be a cheap, GSP jammer that is moving.
>>   You could use multiple receivers to triangulate the location and then
>> determine it is not in orbit and is not a reflection from a metal roof or
>> something.    The problem is the jammer's very low power.  These things are
>> inteneded to only cover a tiny area
> 
> They are not designed with coverage area in mind.  They are basically "whatever power the VCO puts out coupled to the antenna"  From a jamming standpoint, they're not very sophisticated.
> 
> As a result they dump out something like +10dBm.
> So running a quick Friis formula link budget, and assuming you want to have a Prec of around -100dBm (10 MHz BW, kTB)
> 
> 110 = 32+ 20*log10(1575) + 20*log10(d)
> 110-32 - 25 = 20*log10(d)
> d = 400 km...
> 
> This is why they are such a problem
> 
> 
> 
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