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

September 25th 03, 09:54 PM

In article ,
(phil hunt) writes:

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?

On the SINCGARS family it can be the entire region of 30-88 MHz.
With FHSS enabled, the hop rate is 10 per second. The hop
frequency positioning and increments are set by the touch-screen
entry; all the settings go in that way and that is called the "hop set."

One of my work tasks a decade or so ago was to TRY to locate a
SINCGARS emitter using a vehicular model's high power setting
(not the small output of the manpack PRC-119). While I can't say
much about the test details, I think it is safe to say that it was
damn difficult! [my little group was really trying to grab that emitter]

In this case the emitter was on all the time. With the manpack
version as well as the vehicular version there is a power output
selector switch to effectively go "QRP" to a minimum level necessary
to maintain a linkage.

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.

The "SIP" (SINCGARS Improvement Program) resulted in ITT Fort Wayne
making the basic R/T in half the original size. Also added was the time
synchronization via a front panel connector to an AN/PSN-11 GPS
receiver...plus full internal voice or data encryption/decryption...voice is
digitized.

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).

Transmissions lasting only a 100 mSec are VERY hard to "see" over
a bandwidth larger than an octave. Trust me on that. :-)

The hop timing is internally coordinated with voice/data transmission
so that you can't really use fixed-carrier narrowband rules of thumb.
The SINCGARS family isn't really in the "battlefield internet" since all
nets are considered separate entities and many can exist in the same
geographical area. The choice of settings and interoperability is up to
the unit Signal Officer.

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.

It is safe to say that even the most modern cryptanalytic attack could
not get much useful information from a SINCGARS net in time to
counter any communications-coordinated military task. SINCGARS is
designed for small-unit land-air tactical operations...it isn't designed or
intended for top-level sensitive communications. In its assigned task,
the System is VERY good.

ITT Fort Wayne, IN, is the contracted manufacturer of SINCGARS. About
a quarter million (!) systems have been built since 1989. The General
Dynamics Land Division in FL made some SINCGARS but GD has since
downsized its Land Division.

There's an amazing amount of publicly-available information on SINCGARS
externals. That includes the frequency range, hop rate, bandwidth, hop
increment and so on. What you will NOT get publicly is the details on
the essential algorithms hardwired into the ICs within.

One of the good fall-outs of the military electronics programs begun in the
1980s is the improved stability of crystal oscillator packages over
temperature and other nasty military environment extremes. That includes
things like temperature-compensated voltage-controlled crystal oscillators
and more specialized crystal cutting for better control over temperature.
Major quartz crystal producers have put that improved technology on the
market. Accurate, stable time control is essential to systems like the
SINgle Channel Ground-Air Radio System.

Len Anderson
retired (from regular hours) electronic engineer person