[time-nuts] GPS Jammer

Jim Lux jimlux at earthlink.net
Tue Oct 2 22:45:32 EDT 2012


On 10/2/12 7:33 PM, johncroos at aol.com wrote:
> In considering the effect of a simple jammer on a GPS receiver, a simple
> link analysis
> is insufficient.
>
> What must also be considered is the anti-jam capability of the receiver
> which due to spread spectrum processing gain will reject any simple
> jamming signal even though is it 10's of dB stronger than the desired
> signal.


not most simple GPS receivers which have very little AJ capability. They 
have a single bit quantizer (or maybe a 1.5 or 2 bit) after the LNA.  If 
the LNA doesn't saturate, then the quantizer is captured by the strong 
CW carrier.

This is a classic problem with DSSS receivers and led to a lot of 
research in the 80s on things like "adaptive excisers" to remove CW 
carriers.

If you built a linear receiver with a lot of dynamic range, then, yes, 
the process gain will suppress the CW tone, but you still have to 
acquire the code, and as Dixon says (paraphrasing) "acquisition is the 
secret sauce in spread spectrum systems".  Back when I was doing this 
kind of thing seriously (mid to late 80s), acquisition, particularly 
robust techniques, were literally SECRET (in the DoD sense).


There have been a nice series of articles in GPS World over the past few 
months about the variety of inexpensive GPS jammers out there. (and the 
problems they cause).






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