[ntp:questions] GPS Jammers in Use by Criminals - Warping Time for Fraud Suggested

Jason bmwjason at bmwlt.com
Sat Feb 25 16:05:47 UTC 2012

On 22-Feb-12 17:28, Chris Albertson wrote:
> Should be easy to build a hand held jammer detector with a directional
> antenna that the user can sweep around.    Or they can put the
> detector at the car ferry toll both.    In theory jammer detectors
> could be cheaper to build than GPS receivers.
> Could you make one at home.  I think all that is needed is a long
> directional antenna.  Maybe a helix type, an RF amplifier, filter and
> a RF power meter.
> My guess is these jammers are not very sophisticated and simply blast
> RF hash all over L1 and adjacent bands.   A sophisticated jammer would
> know exactly when to transmit over one bit or two bits, and in a
> narrow band just enough to corrupt the GPS data and would have a very
> lower average power output.  That would be hard to detect
> I'd bet these jammers are the simplest type that can still work.
> On Wed, Feb 22, 2012 at 2:16 PM, David Woolley
> <david at ex.djwhome.demon.invalid>  wrote:
>> An article in the Metro, the free morning paper on the London commuter
>> transport network, suggests that criminals may be using GPS jamming
>> equipment to warp the time on financial systems to allow the commission of
>> fraud.
>> Although I can't find the source of that article, the BBC has an article,
>> presumably from the same underlying source, addressing another point in that
>> that article, that GPS jammers are increasingly being used to defeat GPS
>> based car tracking systems.
>> http://www.bbc.co.uk/news/technology-17119768
>> The Metro mentions a risk to car ferries from the denial of navigation data.
>> _______________________________________________
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So, what would be a characteristic symptom or tell-tale of jamming?

Consider a receiver that is connected to an NTP server, which receives 
the NMEA strings and PPS from the receiver? Would the server report loss 
of NMEA strings and/or loss of PPS? I know that would be dependent on 
the particular manufacturer, but would you expect that behavior as being 
a reasonable implementation, or something else?

Consider the hypothetical case of the delivery driver that is jamming 
the GPS receiver in his van to block his company from tracking the 
vehicle, 'cause he's napping along the way and blaming the delay on 
"traffic jam".

Would you expect two (or more?) similar but from different manufacturer 
devices/servers, located, say, in one large building, or adjacent 
buildings, to experience similar loss of signal at the same (or nearly 
so) time?

In another scenario, consider sequential deliveries by the same tired 
delivery driver: If the delivery truck (with a jammer to block vehicle 
tracking) visiting two adjacent buildings, would you expect one receiver 
to get wonky (highly technical term)(presumably the closest to the 
source) and recover after a few minutes , followed within a minute or so 
later, by the receiver in the next building going wonky? This might be a 
scenario where a delivery truck is stopping at building one's dock, 
making a delivery, then moving to the next building's dock and doing the 

With a pattern of disruptions, and by correlation of time of disruption 
with delivery logs, I would expect that you'd be able to ascertain if, 
indeed, there is a delivery van with a jammer.

But, what if the times are more random, and or outside what would be 
'normal' delivery times. Or of very short duration (such as the jammer 
driving by on the street outside the building).

Further, if two servers lost NMEA and PPS from two different antenna and 
receivers, nearly instantaneously. Would you consider that evidence of 
purposeful jamming? Would it be unintentional if it were very short (the 
nefarious tired delivery driver just driving by on his route).

What else might convince you that your receivers were being jammed?


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