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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