Re:

[stevenw@best.com (Steven Weller)]

>On Fri, 22 Dec 1995 andr0id@midwest.net (Jason Rentz) wrote:
>>
>>The problem with the Interceptor is that I think it can only receive one
>>freqency at a time, and it is adjustable by a thumb wheel, not digitally.
>>This would tend to make changing frequencies at high rates VERY hard. :)
>>Also it has no frequancy readout, so this means that if you know what freq.
>>you should be at it is hard to tune in that freq. without searching a
>>little.
>>                        Dr0id
>
>The demo of the Interceptor I saw seemed to show it jumping from 150 MHz
>to 450
>MHz without any external adjustments, it locked on the strongest signal.
>Granted, it can only receive one frequency at a time, but there shouldn't
>be any
>significant delays when the phone hops a frequency.  The Interceptor's
>frequency readout is a bargraph style LED.  The Scout, from what I can
>see in Optoelectronics' ad doesn't have any frequency controls on it, it does
>have a digital frequency readout.  As far as timing is concerned, the Scout
>might be less useful because I think it feeds the frequency into the
>scanner for
>reception.  Most scanners take a significant amount of time to change
>frequencies.

I used to work for a company that made frequency-hopping military radios.
It's a catch-me-if-you-can game, a bit like the amusement arcade toy where
stuffed rats pop up through holes and you try to whack them with rubber
hammers. You can sure *see* where the next one is, but you can't get there
fast enough to make contact. If you knew the pseudo-random pattern, you
could anticipate and be there every time.

Thus in a frequency-hopping radio you can push the retuning (read RF
phase-locked loop) technology to its limit and build transmitters and
receivers around them. These typically hop in the order of 100 times a
second. The adversary has to find the uncorrelated signal very quickly
indeed *and* have PLL technology at least as good as yours to recover
anything from it. Finding the signal generally means listening to all
frequencies at once, requiring huge amounts of hardware parallelism and/or
realtime computing power. Once you throw ten or so radios onto the same
band, it's no longer any use looking for the strongest signal, making that
approach useless.

The primary reason for FH is not to hide information, however. Encryption
can be used for that. It's to prevent the enemy from hiding the information
from the intended recipient through jamming. Radio jammers work by simply
drowning out all other traffic so that the receivers either clip and
distort everything or have to attentuate the input signal so far that the
interesting stuff is undetectable. Like trying to have an intellectual
conversation at a bad rock concert. By employing FH you require the enemy
to have enormous and impractical jamming capacity. There are FH radars too.



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Steven Weller                      |  "The Internet, of course, is more
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stevenw@best.com                   |       -- Time Magazine, 3 July 1995