[time-nuts] GPS Jammer

[Tom Miller]

We don't know that they modulate the jamming signal some what. I bet 10 mW 
would do a good bit of harm to GPS systems even a block away.


----- Original Message ----- 
From: "Jim Lux" <jimlux at earthlink.net>
To: <time-nuts at febo.com>
Sent: Tuesday, October 02, 2012 10:45 PM
Subject: Re: [time-nuts] GPS Jammer


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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