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

[Jim Lux]

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