[time-nuts] GPS jamming susceptibility

jimlux jimlux at earthlink.net
Tue Nov 23 14:48:10 UTC 2010


John Green wrote:
> I read that Phrack article and their jammer is much more sophisticated
> than mine. Mine is just a sweeper. I don't even know yet the sweep
> rate. I was thinking more along the lines of theirs. Something that
> actually put out a signal that contained at least some aspects of the
> signal they were intending to jam. My next experiment will be to get a
> clock oscillator that works at some submultiple of the GPS frequency
> and see if one of its harmonics can jam a handheld GPS if held close
> by.
> 


Yes, it will.. It doesn't even have to be a submultiple.  Most GPS 
receivers have a pretty wideband front end filter feeding a single bit 
ADC.  Why a wideband filter?  Because it's hard to make a 1 MHz wide 
filter at 1500 MHz that is temperature stable.  Much easier to make a 
10,15,20 MHz wide filter (1-2% bandwidth) and let it move around. So, I 
would venture that ANY signal within a few MHz of the GPS center at 1575 
  would jam it.

Bear in mind that if you are doing a comb generator, the power in each 
comb spur is pretty small.  1 mW at 10 MHz with harmonics up to 2 GHz 
means you're spreading that mW over 200 teeth of the comb, so each one 
is -23dBm.

For more jamming fun.. What about a 200kHz spacing comb mixed with a FM 
modulated carrier at, say, 99.9 MHz... Enjoy..



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