No subject

Fri Oct 9 21:52:43 UTC 2009


> A *much* more effective and cheap strategy is a repeater jammer..
> Receive
> the signal and retransmit it: two antennas and an amplifier. The
> victim sees
> the delayed retransmitted signal at a higher level than the direct
> one. It's
> sort of like creating fake multipath interference. No need for PN
> generators, oscillators, etc.
> Granted, a smart receiver that *understands* the relationship
> between SV to
> user geometry and doppler can beat it (because the carrier phase and
> PN
> phase of the repeated signal won't be consistent with the geometry),
> but the
> run of the mill PN tracking loop probably won't.

Jim Lux is smart and knowledgeable, so naturally :-) he suggested a
repeater jammer as I did.  The implementation that he describes is the
traditional one, in which separate transmitting and receiving antennas
are used to avoid self-interference.  I suggested a single-antenna
form because I wanted the jammer to be compact and self-contained.

> Most inexpensive receivers use a single bit sampler, so a suitable
> CW tone
> could also probably jam it effectively, but might require some
> knowledge of
> the victim receiver to pick an appropriate frequency (i.e. You'd
> need to
> know the sampling rate.)

Jim Lux is correct.  A one-bit sampler is just a hard-limiter.  Any
signal in the passband that is stronger than the normal receiver input
of background random noise plus GPS signals kills the sensitivity of
the receiver by holding the output of the limiter against its high and
low limits.  (Normally, the hard-limiter dithers randomly between its
limits, so that hard-limiting reduces the receiver's sensitivity by
rather little, less than 2 dB IIRC.)  The jamming signal does not have
to be CW, but CW works.  It is not difficult to excise CW signals by
means of anti-jamming DSP; and sophisticated military receivers have
such protection.  However, anti-jamming processing is somewhat costly
in terms of parts-count, complexity, and power-consumption.

AFAIK, no commercial or "civilian" GPS receiver manufactured within
the last 23 years has included such processing.  The very first
commercial/civilian GPS receivers, more than 25 years ago, _were_
quite immune to CW and other narrowband jamming; but this feature did
not survive market price competition.

Most inexpensive receivers are especially highly vulnerable to jamming
by CW at or near 1575.42 MHz, the L1 carrier frequency, even when the
jamming signal is not much stronger than a normal GPS-plus-random-
background input (so that one-bit sampling is not an issue), because
the C/A code correlation in these receivers is not 100% continuous.
Once per millisecond, there is a dead time of few microseconds during
which correlator outputs are read and correlator input parameters such
as delay and delay-rate are updated.  I have tested several makes and
models of GPS receivers and been amazed by how little CW power was
required to disable them -- typically 20 dB less than with full-time
correlation.  Again, this vulnerability is a consequence of cost-
cutting pressure.



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