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Mark Borgerson

In article ,
says...
Mark Borgerson wrote:
In article ,
says...
Mark Borgerson wrote:
In article ,
says...
Jack Linthicum wrote:
[snip]

I think that a spread spectrum burst type transmission can be
intercepted and given a rough bearing. The money to do this is
miniscule in comparison with making Trident missiles into hand
grenades.
The command post does not move between transmissions. Spread
spectrum/frequency hopping systems return to previous frequencies every
few seconds. Just use several bursts to home in on the transmitter.

Why are you assuming that the command post does not move? I see no
reason that a mobile command post and multiple mobile transmitters
could not be used.

This comes down to the definition of mobile. If the command post stays
in the same place for half an hour it is static. A constantly moving
command post would need a vehicle the size of a bus to hold the
operators and long range transmitters, possible but hard to camouflage.

So you don't think the Iranians have buses or semi-trailers? Suppose
there are 100 semis on the coastal road. Which one do you target?

The one with the big aerial.

Small aerial to small aerial on moving objects gives a short range.

You can put a pretty large antenna (at 900Mhz or higher) inside the
back of a semi trailer. (Probably best to use fiberglass panels
rather than aluminum on the outside, though!)

Spread spectrum and frequency hopping systems do use a finite number
of frequencies---but the sequence of freqencies used may not repeat for
many hours. That leaves you with a broadband collection problem
and having to sort out multiple emitters on the same bandwidth with
different hopping schedules. I suspect that is a problem handled
offline and after-the-fact, and not in real time. However, the
technology has probably advanced a bit in the 30 years I've been
out of the sigint world. ;-)
If we are trying to destroy the command post we do not need to receive
the entire message we can simply wait until that frequency is reused by
that transmitter. If the equipment is hopping over 100 frequencies it
should be back within the next 200 transmissions.

With spread-spectrum transmitters, the time spent at one particular
frequency may be only a millisecond or two. If you can provide a link
to a system that can accurately track a moving spread-spectrum
transmitter, I'd be interested in reviewing its specifications.

Try
http://klabs.org/richcontent/MAPLDCon98/Papers/d3_haji.pdf

Thanks for the link. It looks very interesting. I couldn't find
any data on the angular resolution of the DF, though.

For DFing you do not need to accurately track a spread-spectrum
transmitter's hops. You only need to guess one of the frequencies.
To intercept and decode a signal you need (almost) all the frequencies,
providing it can tell the difference between static and modulated signal
the above machine may be able to reconstruct the signal by listening
on hundreds of frequencies simultaneously.

The problem with intercepting spread-spectrum signals is that the
receiver KNOWS where the next signal will arrive. It can tune it's
receiver software for that frequency. The intercept receive has to be
able to recieve ALL frequencies---and thus cannot use the same signal
processing techniques as a receiver that knows the sequence.
The computers will need programming to treat transmissions from two
widely separated locations as two targets. Home in on them one at a time.

How do you work with one continuously moving target transmitting on
256 different frequencies? I suppose it could be done with large
enough antennas and enough processing power on a number of different
ships. It's not going to be easy, cheap, or widely available, though.

You can deal with frequency hopping by listening on hundreds of
frequencies simultaneously. When one of the frequencies is known very
accurate direction finding equipment can tune to that frequency and wait
for the transmitter.

How accurate is 'very accurate'? Back when I was working with HFDF,
one or two degrees angular resolution was considered reasonable.
The system I worked with is described he

http://www.nrl.navy.mil/NewsRoom/images/75awards.pdf
(look at the award citation for high frequency direction finding)

Where the target is physically moving whilst transmitting something like
a radar display is needed. PCs can be programmed to act in this
fashion. Five years ago the army was working on things like this.

Mark Borgerson