5-MAR 15:32  Listening
     Scanner vs Hopping
     From: KA5S         To: ALL

Frequency hopping systems have found use in secure law enforcement
and military applications for some years now. Such systems may be
susceptible to interception by modern scanning radios.

A FH system may (for example) use 10 channels in the UHF band.
(These channels do not have to be licensed to the department or
orgaization using them). Power is restricted to low levels.  Time on
each channel will be .1 seconds before hoppping to another
frequency.  Hopping time is ignored. Ten channels will be visited in
one second.  Modern scanners are available that can scan 100
channels per second. This is  enough to compromise the system above.

If the FH system begins transmitting at the same time the receiver
starts scanning, then given a 100 chan/sec scan rate, it will take
between 0 and .1 seconds (actually, .09 sec -- there's never more
than a 9 channel difference) before the scanner settles on the
channel selected by the FH system.  The FH system remians on this
channel for .1 sec before hopping off, and again, the scanning
receiver, being able to scan all ten channels in .1 seconds, will
take -- on average -- .05 seconds to find the next channel.

The example above shows that the scanning receiver is technically
capable of receiving at least 50 percent of the FH transmission. In
reality, the presence of other signals on the sytem will prevent the
scanner from tracking the FH system, but it might happen that a
lightly used system might allow interception at some times of the
day or night.

The obvious solution for those interested in secure communications
is to increase the hopping rate, but since we can seriously degrade
the S/N ratio before spoken words become unintelligible, it follows
the hopping rate would have to increase by at least a factor of ten.
This can be done, but now the equipment becomes prohibitively
expensive.  Since there is probably a considerable invenstment in
such equipment already, it's likely that those of us with fast
scanners will be able to hear some frank, unguarded converation
between surveillance and investigative teams.

Anyone done this yet?


Cortland

Disclaimer: The file contained in the box above or displayed in a separate window from a link in the box above is NOT owned nor implied to be owned by BeYoND THe iLLuSioN. Most files at BeYoND THe iLLuSioN are originally from public Bulletin Board Systems (BBS) which were popular in the days before the Internet or from gopher, web, and FTP sites from the early days of the Internet which no longer exist today. Essentially, all files were acquired from the public domain in one for or another.

However, there have been occasions when copyright protected material has appeared on BeYoND THe iLLuSIoN without permission of the copyright holder. In these instances, we have and will continue to remove the copyright protected file as soon as it is brought to our attention. This can now be done using our Report Copyright Material form. Fill out the form, and the webmaster will be notified of the situation.

There are also times when files found on BeYoND THe iLLuSioN have a real home somewhere else on the Internet. In these instances, we will gladly replace the file with a link to its true home whenever it is brought to our attention. If you know of the true home of any of these files, you can use our Report Original URL form to bring it yo our attention.