[time-nuts] The Demise of LORAN (was Re: Reference oscillator accuracy)

Magnus Danielson magnus at rubidium.dyndns.org
Mon Nov 16 00:56:23 UTC 2009


Hal Murray wrote:
> cfharris at erols.com said:
>> What makes you think it needs to be CW, and cannot be pulsed and
>> chirped?
> 
>> All it has to do is confuse the receiver enough so that you can't
>> trust its readings. 
> 
> Why is pulse or chirp likely to be more confusing per W of jamming power?
> 
> 
> I thought the GPS signal was spread spectrum so I wouldn't expect any simple 
> pulse or chirp to be better at jamming than noise over the appropriate 
> bandwidth.
> 
> If you know the spread spectrum details (which must be public or the receiver 
> can't listen to the signal), then you might be able to mimic a satellite 
> signal.  I think that needs the time, so you probably need to listen to the 
> signal you are jamming.
>
> Am I on the right track?  Is there a trick I'm missing?  (If so, how 
> complicated is it?)

I think you are running into the jamming/spoofing definition of the 
problem. When you are jamming, you are transmitting a signal which just 
overshadows the propper signal but when you are spoofing you try to send 
a signal which looks like the propper signal.

A spoofing attack is a different story altogether. For a spoofer to 
confuse a receiver, it either needs to be there when the receiver turns 
on or the spoofer needs to transmitt signals having a position so near 
the receivers current signals that it can't discriminate them and then 
track onto the wrong signals.

The counter-measure towards spoofers is to used a keyed signal, in which 
case only delayed signal is available to the attacker, but for most 
cases the receiver should be able to handle that.

For jamming I think chirps or phase-noise transmitters is effective 
since they select no particular feature of the spreading code, but 
induces error into both the weak and strong features, which is then 
shifted by the expected doppler frequency. The sensitivity of jamming 
energy for various offsets from the carrier for a pseudo-random sequence 
can be retrieved by Fourier-transform (DFT).

Cheers,
Magnus



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