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[aprssig] GPS Jamming On The Rise

Stephen H. Smith wa8lmf2 at aol.com
Fri Jun 27 21:34:11 UTC 2008

Lloyd M. Mitchell wrote:
> If you're using a Garmin GPS V, I can jam it with my FM broadcast 
> radio on 94.3MHz at 50 ft.  I imagine the IF freq's are mixing. Took 
> me the longest to figure out why in the morning drive I could not get 
> a fix, but the afternoon drive no problem (different stations morning 
> and evening) of course this was pre-Sirus...
> Lloyd KO4L

Most likely, it's the 15th harmonic of the FM broadcast radio's local 
oscillator landing on the GPS operating frequency.    

94.3 + 10.7 =  105 MHz      [10.7 MHz is the standard IF frequency of 
most FM receivers; thus the LO will be above the operationing frequency 
by this amount.}

105 MHz  x 15 = 1575 MHz   which is the center frequency of the GPS 

I had this kind of problem years ago during some GPS experiments with 
Los Angeles Sheriff Dept vehicles.  My GPS-driven radio-coverage mapping 
system worked fine in my car, but would absolutely not work in LASD 
black&whites.   Finally discovered that the third harmonic of the local 
osc of the GE Ranger 506/507 MHz T-UHF mobiles were landing directly on 
the GPS frequency and wiping out GPS RX completely.   

[Bear in mind that GPS signals on the ground are INCREDIBLY WEAK (i.e. 
-130 to -140 dBm).  It is only the sophisticated spread-spectrum 
de-processing at the receive end that allows these signals (that are 
10-20 dB weaker than anything a conventional narrowband FM receiver 
could ever hope to hear) to be detected at all.  And with a very minimal 
no-gain antenna to boot.   It doesn't take much to cover them up. 

Aircraft of course do have a huge advantage over land vehicles.  Since 
the desired signal (the satellites) is coming from above, antennas 
mounted on the top of the vehicle with a hemispheric directional pattern 
upward can ignore a pretty hefty jamming signal originated on the 
ground.  On the other hand, I have no idea what sort of antenna pattern 
a JDAM (GPS-guided bomb) would have...  ]
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