RadioBanter - View Single Post - battlefield Internet (was: Stryker/C-130 Pics)

September 25th 03, 03:23 PM

(phil hunt) wrote in message ...
On 23 Sep 2003 20:00:32 -0700, Kevin Brooks wrote:

No. Paul is correct, DF'ing a "frequency agile" (or "hopping")
transmitter is no easy task. For example, the standard US SINCGARS
radio changes frequencies about one hundred times per *second*,

Bear in mind that I'm talking about automated electronic gear here,
not manual intervention. Electronics works in time spans a lot
quicker than 10 ms.

So what? Unless you know the frequency hopping plan ahead of time
(something that is rather closely guarded), you can't capture enough
of the transmission to do you any good--they use a rather broad
spectrum.

OK, I now understand that DF generally relies on knowing the
frequency in advance.

BTW, when you say a rather broad spectrum, how broad? And divided
into how many bands, roughly?

It uses the entire normal military VHF FM spectrum, 30-88 MHz. ISTR
that the steps in between are measured in 1 KHz increments, as opposed
to the old 10 KHz increments found in older FM radios like the
AN/VRC-12 family, so the number of different frequencies SINGCARS can
use is 58,000.

Both radios have to be loaded with the same frequency hopping (FH)
plan, and then they have to be synchronized by time. When SINGCARS
first came out the time synch had to be done by having the net control
station (NCS) perform periodic radio checks (each time your radio
"talked" to the NCS, it resynchronized to the NCS time hack); failure
to do this could result in the net "splitting", with some of your
radios on one hack, and the rest on another, meaning the two could not
talk to each other. I believe that the newer versions (known as
SINCGARS EPLRS, for enhanced precision location system) may use GPS
time data, ensuring that everyone is always on the same time scale.

That would make sense.

If two receivers, placed say 10 m aparet, both pick up a signal, how
accurately can the time difference between the repetion of both
signals be calculated? Light moves 30 cm in 1 ns, so if time
differences can be calculated to an accuracy of 0.1 ns, then
direction could be resolved to an accuracy of 3 cm/10 m ~= 3 mrad.

The fact is that the direction finding (DF'ing) of frequency agile
commo equipment is extremely difficult for the best of the world's
intel folks, and darned near impossible for the rest (which is most of
the rest of the world); that is why US radio procedures are a bit more
relaxed than they used to be before the advent of FH, back when we
tried to keep our transmissions to no more than five seconds at a time
with lots of "breaks" in long messages to make DF'ing more difficult.

So transmissions of 5 seconds tend to be hard to DF? Of course, with
the battlefield internet, a text transmission will typically be a
lot less than 5 s (assuming the same bandwidth as for a voice
transmission, i.e. somewhere in the region of 20-60 kbit/s).

ISTR the old guidance was to keep transmissions no longer than 5 to 7
seconds without a break (a break normally was announced as part of the
message, followed by release of the mic key, then rekeying and
continuing the message).

transmissions still very clear), and the use of FH combined with
crypto key makes it darned near impossible for the bad guy to decypher
it in any realistic timely manner.

Modern crypto is good enough to withstand all cryptanalytic
attacks.

Only if it were so...but thank goodness it is not. Otherwise we would
have lost the value of one of our largest and most valuable intel
programs, and NSA would no longer exist. Even the cypher keys used by
our modern tactical radios (said keys being generated by NSA at the
top end, though we now have computers in the field capable of "key
generation" using input from that source) are not
unbreakable--instead, they are tough enough to break that we can be
reasonably assured that the bad guys will not be able to gain any kind
of *timely* tactical intel; enough computing power in the hands of the
crypto-geeks and they can indeed break them, but it will probably take
them a while, not to mention the time to get the data into their hands
in the first place.

Brooks