[time-nuts] GPS jamming susceptibility

[Bob Camp]

Hi

The same basic equations apply (as you point out) to any receiver. Lay down
enough on channel noise power and the receiver doesn't have much of a
chance. Nothing magic or terribly hard to figure out. Easy to do wideband at
"I could use a baseball bat" to "I could use a bow and arrow" type ranges.

Because it's easy to do, it happens all the time. Fire up some hair dryers
at close range and they do a fine jamming job. Some of the jamming is
electrical, some of it is the fact you can't hear anything over the noise
they are making. 

An axe (or good sling shot) is a fine way to take out a cell site and you
can get them just about anywhere. Jamming GPS isn't even going to make them
drop a call for at least a day or more. It won't make airplanes drop from
the sky either. 

Taking out cell sites one at a time isn't a real big deal. It's a rare area
that's covered by only one site. They go in and out of service now and then,
it mostly goes un-noticed. If it is noticed, it's by a small number of
people. I have a nasty suspicion that sites that fail don't get fixed as
fast as they might for just those reasons.  

The timing setups are indeed fragile. They could be improved quite a bit.
They aren't so fragile that they melt down immediately when a single fault
occurs. Lots of axes (or a big hurricane) are by far the more likely source
of full system failure.

Bob


 

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of jimlux
Sent: Tuesday, November 23, 2010 9:37 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPS jamming susceptibility

John Green wrote:
> 
> jamming anyone's GPS. A while back, I was looking at one of those
>  It doesn't look capable of putting out
> more than 50 milliwatts or so into a 2 inch antenna

  The GPS antenna is perhaps 35 feet away
> with a cinder block wall, a brick wall, and a metal roof in between. I
> also put a 15 Db attenuator between it and the antenna with almost the
> same result.

> down. Has anyone here had any actual experience testing GPS receivers
> for susceptibility?
> 


OK... typical received signal at a GPS receiver (L1) is on the order of 
-130dBm.  Thermal noise floor (assuming noiseless receiver and no 
losses) is -114 dBm in 1 MHz BW.

Remember, the typical GPS is a single bit quantizer, which works just 
fine considering the signal is 20dB below the noise floor.

So, let's do a little link calculation: 32+20log10(1500)+20log10(.010) 
between isotropic antennas (which is not a bad starting point for your 
jammer and the GPS)..
32+64-40 -> a link loss of 56 dB.. you're radiating +17dBm, so let's 
call it -40dBm into the GPS..  Yep, jamming is almost assured..

But at that kind of power, you'd jam almost ANY receiver that's trying 
to receive a signal at -130dBm.  90dB instantaneous dynamic range is 
pretty good when you can't use a filter to remove the interfering signal 
(e.g. a HF receiver has a narrow band filter in the IF to solve this 
problem).

realistically, you need about, say, 10 dB J/S so you'd need -120 dBm 
into the receiver from the jammer.  A microwatt 10 meters away would do 
it nicely.  Inverse square helps a bit.. if you were 1 km away, your 
interfering 50mW signal would be down another 40 dB.. -80dBm.

10km away, your jammer is down into the area where it probably won't jam 
all the time.


Obviously, *real* radios that need reliable GPS reception do things to 
make life easier.  Aside from using 1.5-2 bit detection, or signal 
excisers, etc.  There are also techniques that rely on looking at the 
post correlation signal (where the interferer is suppressed to a certain 
extent): with modern signal processing, you can correlate against all 
possible phases of the code in one shot, for instance.

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