[time-nuts] GPS "tests" by the DoD

jimlux jimlux at earthlink.net
Fri Jan 21 14:56:11 UTC 2011

On 1/20/11 5:59 PM, Chris Albertson wrote:
> On Thu, Jan 20, 2011 at 5:25 PM,<lists at lazygranch.com>  wrote:
>> Another time I was driving I-5 near Lemoore Navy airstation and got a different type of jamming. The whole gps display rotate back and forth. No idea how that jamming was done.
> Not just with GPS but in general that is the more sophisticated and
> useful method of jamming.  Think of an airplane being targeted by an
> radar guided missile.  It could simply broadcast while noise but then
> it would become a very bright target to a radio homing missile.  And
> you can bet $$$ that all radar guided missiles have a radio homing
> mode build into their software.    The smarter jammer sends a faulse
> signal back that is only "off" enough to cause the missile to miss but
> not "off" enough that the missile can know it is being jammed.

I haven't done jammers for almost 20 years now, but back then, the 
scheme was a digital RF memory (DRFM).. after all, the radar pulse is 
typically coded, and a good "fake return" has to be coded the same. 
Furthermore, the radars are smart enough to be able to match up the 
Doppler profile and the range rate, so you need to put a frequency shift 
on the fake return that is appropriate. The usual scheme is to light up 
the fake target exactly matching the real target, wait for the radar to 
track, then "walk" the fake return away slowly, while keeping the return 
power just a bit bigger than the real skin return.  Then, you shut off 
the jammer, forcing the radar to go through the whole acquisition 
process again (much easier to track a target in clutter than it is to 
find it in the first place).

Most simple jammers (for, e.g. GPS) don't necessarily try to spoof the 
receiver with a fake signal, but attack some other aspect of the signal 
processing chain; a classic older approach for radars is to transmit two 
strong signals separated by the IF frequency, and hope they make it 
through the front end filters. A pulsed signal that's in band attacks 
the AGC loop. GPS receivers with their 1 or 1.5 bit samplers are quite 
vulnerable, especially if you know the sampling rate, because they're 
pretty "wide open" as far as the front end goes.

> A smart GPS jammer would slowly "move" the location reported by any
> GPS receiver in the area just enough that weapons would miss their
> targets

That's actually quite difficult.  Consider that for GPS, each receiver 
is seeing a different set of signals (in terms of relative time/phase) 
from SVs that are moving.  The code phase and doppler have to be 
consistent, etc.

I spent some time over the last few weeks thinking about how hard it 
would be to build a cheap GPS simulator with an FPGA for testing a 
single radio.  Generating the NAV messages and the spread codes is easy. 
  Sliding them around and keeping range, range rate, and Doppler 
consistent, not so easy.  (some sort of funky chain of NCOs was what I 
came up with, but I don't know if that would work)

> Jamming is a very sophisticated science because for every trick there
> is are counter measures and so on for several layers
Yes, yes indeed..

Very intellectually stimulating and fascinating work.. Too bad that 
death and destruction is the object.  (Yes.. Andrew Undershaft and Major 
Barbara can have that discussion, too)

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