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

[Chris Albertson]

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.

You can't just go by the amount of power received because the
spoofer/jammer can have any amount of power because they can be at any
distance from you and the GPS satellites have variable power because of the
distance from the horizon.  I think you have to look at the signal itself.
 You can scan a directional antenna to determine direction but you will get
many false positives when real GPS satellites are near the horizon.   You
would have to track the orbits of all of them.

Spoofers are a real problem.   A sophisticated spoofer tries very hard to
look like the real thing.  I think you would have to compare the position,
velocity and time you get from GPS with your known position, velocity and
time and if there is enough difference assume you are being spoofed.  But
again a good spoofer will create a subtle error.  For example if the
spoofer is protecting a building from GPS guided bombs it only needs to
create a 200 yard position error.  The spoofer may be tracking the location
of the falling bomb and may start by transmitting a GPS signal with no
error and then add more and more error so as to guide the falling bomb off
target.   This is actually how the some radar jammers work, they send out a
signal designed to be "believable" so that the guidance system in the
anti-air missile dose not detect that it is being jammed an simply goes off
target.     I think the only way to detect a sophisticated spoof is to have
multiple receivers with known relative positions.   A change in relative
position would indicate a spoofed GPS signal.

Jammers can be sophisticated as well the better ones will use a directional
antenna aimed at the target and will adjust their power output based on
distance to the target.  A truck driver likely would not use such a device.
 A national military might deploy a very sophisticated GPS jammer as part
of an air defense system. It would by design be hard to detect.

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


On Mon, Oct 7, 2013 at 7:46 AM, Collins, Graham <CollinG at navcanada.ca>wrote:

>
> Indeed, the inexpensive DVB-T dongles are showing up in many places
> including as David noted, decoding GPS.
>
> For some details:
>
> This gets you to the start of their web site:
>
> http://gnss-sdr.org/
>
> This is an interesting document they have published on their project:
>
> http://www.cttc.es/wp-content/uploads/2013/09/Turning_TV_into_GNSS_Rx1.pdf
>
>
> The AMSAT Fun Cube Dongle is a very capable and interesting device.
> Interestingly it uses the same Elonics E4000 front end chip that many of
> the inexpensive DVB-T devices do. Apparently Elonics is no longer in
> business and the inexpensive DVB-T devices using this chip are becoming
> less common. The DVB-T devises using the R820T chip are becoming the
> preferable versions when those with the E4000 cannot be found. I wonder if
> the Fun Cube Dongle will be likewise changed (perhaps it already has).
>
>
> Cheers, Graham ve3gtc
>
>
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of David J Taylor
> Sent: October-07-13 10:21 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] How hard is it to detect a GPS Jammer?
>
> Many "scanners" now go to that frequency e.g AOR AR-8600. (100kHz to 2GHz
> ) They are hardly state-of-the-art receivers but should be capable of
> detecting jammers driving past. However a new unit is quite pricey $1000
> equivalent in the UK as little as $300 for a used version. Also the AMSAT
> FCD Pro+ USB SDR at $200 and free software.
>
> Alan
> G3NYK
> =========================
>
> Or even the DVB-T dongles at just a few pounds (and with a wider
> bandwidth), covers up to around 1.7 GHz, I have red.  For example:
>
> UK & Europe:
>   https://www.cosycave.co.uk/product.php?id_product=288
>
> US:
>
> http://www.nooelec.com/store/software-defined-radio/sdr-receivers/tv28tv2.html#.UlLDKRBE0a4
>
> Admittedly, you aren't supporting AMSAT compared to the FUNcube Pro+, but
> for a quick experiment....
>
> 73,
> David GM8ARV
> --
> SatSignal Software - Quality software written to your requirements
> Web: http://www.satsignal.eu
> Email: david-taylor at blueyonder.co.uk
>
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-- 

Chris Albertson
Redondo Beach, California