[time-nuts] GPS jamming

Robert Atkinson robert.atkinson at genetix.com
Thu Jul 27 03:57:48 EDT 2006


Hi,
While not related to Timing systems, I personally know of two other
causes of GPS Jamming. They both relate to aviation (my background). The
first was leakage of receiver local oscillator signals from VHF
communication receivers. For example a Rx tuned to 132.525 MHz with a
10.7MHz I.F and high side local oscillator would have the L.O. at
143.225. The 11th harmonic is 1575.475. 
Garmin sell a 1.575.42 MHz notch filter Part No.330-00067-00 to fit in
the antenna coax of the radio.
The second is the jamming of mobile GPS's near our local airfield. The
cause is unknown but is believed to be receiver overload caused by the
"L" band (roughly 1 to 1.4GHz) ground movement radar.

Robert G8RPI.

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of SAIDJACK at aol.com
Sent: 27 July 2006 02:12
To: time-nuts at febo.com
Subject: Re: [time-nuts] time-nuts Digest, Vol 24, Issue 47

Hello Faisal,
 
a small annecdote about GPS jamming from the design of our FireFox GPS  
Disciplined synthesizers:
 
We have a broadband synthesizer driven by a GPSDO on the same PCB. This

synthesizer would completely swamp out the GPS receiption if you set it
to  
1574MHz CW output, the on-board noise was so powerfull (the output of
the unit can 
be set from DC to 1640MHz). Our output can go up  to +18dBm, millions of
times 
more power than the GPS signal itself...
 
The effect was that the M12+ receiver would just loose lock within a +-5
-  
10MHz bandwidth around the GPS carrier. The receiver would show 0
sattelites  
being received. As soon as you set the frequency outside of this  band, 
everything was fine. 
 
We improved this by putting the GPS board into a metal shield. So the
effect 
of noise generated inside the enclosure was greatly mitigated.
 
But the CW power radiated by the BNC connector itself on the unit is
still  
enough to find its way to the antenna 10 meters away and about 3m above
it, and 
 swamp the signal!
 
Putting a small paperclip into the BNC RF output connector at +18dBm at

1574MHz would probably cause a couple of blocks in our neighbourhood to
loose  GPS 
lock :)
 
Never tried this and never will of course.
 
bye,
Said
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