Re: [time-nuts] The Demise of LORAN (was Re: Reference oscillator accuracy)
From a friend and GPS expert:
====
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.
====
-John
Lux, Jim (337C) wrote:
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.
The delay attack is known (it's actually more of a spoofing attack), but
most jamming uses just simple CW or noise signals. A problem with the
delay attack is that you have a risc of creating a feedback loop which
creates an unstable oscillation, which may or may not be what you would
like. Among other things, directional finding on an oscillation is much
easier than the delayed signal. However, a good hint is to use a
directional L1 antenna and point it to the signal and just hook a
receiver up and the position you have is that of the attackers antenna.
Again that reveals the position. I guess this is why the attack isn't
particularly used.
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.
Actually, even the simpler receivers has some resistance to it, the
higher amplitude is the main problem.
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.)
The CW attack is well known and has been analyzed fairly deeply. You
don't need the sampling rate to make it efficient. The one-bit receiver
is dead in the sea compared to even 1.5 bit receivers with suitable AGC
loop detection.
Cheers,
Magnus
2009/11/16 Didier Juges didier@cox.net:
Not all GPSs are on airplanes, but those I am really worried about are.
I don't care if the GPS receiver in my cell phone stops working in some
locations, it does not work everywhere to begin with (like in the house, I
can't use it to find the bathroom in the dark...), but I worry that the one
guiding a missile or a fighter plane might not work. I also worry that the
one abord a large passenger aircraft might not work. Most of those that are
stationary or move on the ground or on water, I am not too worried about.
And I'm really worried about statements like this that seem to
indicate just how vulnerable we are if something seeming as fragile as
GPS is compromised. People found the bathroom, drove from place to
place and flew vast distances without GPS in the past but now have we
taken a step too far. Surely all commercial and non-commercial pilots
are able to navigate without such systems, this must be part of their
training and are people not driving round with a set of maps in the
glove compartment any more. If Starbucks is really reliant on GPS as a
means for getting it's customers, perhaps they should review their
business model.
For instance, the local garbage pickup people use GPS to locate the houses
of customers who have unusual (large or unsafe, or possibly contaminating)
garbage to pickup. I have had a dead TV on the side of my house for 3 weeks,
three times I called and 3 times they missed it because the GPS guided them
to my front door (my normal mailing address), but I live on a corner lot and
the garbage pickup is on the side of the house, by the garage. If that
particular GPS receiver stopped working and forced the employees to look
around, it would not bother me (the TV ended up in the trash can. After 3
times and 3 weeks, I assumed I had done a reasonable attempt at avoiding
putting too much lead in the garbage, and it had to go.)
Maybe your local garbage company should look at increasing the
intelligence level requirements in it's recruitment process. Surely
they could have looked around a bit to find it or were they lazy and
gave up when the item was not at the exact spot as recorded by the
GPS. That is surely a ludicrous system if you ask me. Perhaps the guys
need more training, a course in map reading and the inclusion of maps
in the vehicle.
Steve
Didier
-----Original Message-----
From: time-nuts-bounces@febo.com
[mailto:time-nuts-bounces@febo.com] On Behalf Of Magnus Danielson
Sent: Sunday, November 15, 2009 7:03 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] The Demise of LORAN (was Re:
Reference oscillator accuracy)
Francesco Ledda wrote:
Considering that the GPS antenna in aircrafts is mounted on top of
fuselage, and that its radiation pattern is upward, it seems that a
ground jammer will have an uphill battle.
Not all GPS receivers is sitting on flying airplanes. Far from it.
Cheers,
Magnus
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Steve Rooke - ZL3TUV & G8KVD
A man with one clock knows what time it is;
A man with two clocks is never quite sure.