(I'm not an electronic engineer, so I've cross-posted this to some
newsgroups which might be able to give informed comment on a number
of points.)

On 23 Sep 2003 05:51:41 -0700, Kevin Brooks wrote:
(phil hunt) wrote in message ...
[regarding battlefield internet]
The signal must be such that the extended receiver can hear it. So
others can too, in principle. (Though detecting the signal and
knowing where it's from aren't the same thing). I'm not a radio
engineer but I can imagine a few ways how direction-finding might
work; for example place two (or 3) detectors a few meters apart
and calculate the time delay between each one receiving the signal.

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.

over a
pretty wide band of freq's (this is why synchronization of the radios
on a time basis is critical to succesful operation of the net).

So the frequency changes are pre-determined on a time basis?

If there is a radio receiver, is it better able to detect/deceive a
signal whgen it knows the frequency in advance? Or can it "sniff"
for lots of frequencies at a time and pick out what looks
interesting?

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.

Alternately, would something like a pinhole camera work? What I mean
here is: imagine a cubic metal box, 1 m on its side, with a vertical
slit, about 1 cm wide down one of its vertical faces. On the
opposite face, there are detectors for detecting radio waves. If the
elevctromatnetic ratiation coming into the box can only go in
through the slit, and goes in a straight line, then knowing which
detectors are lit up would allow someone to tell where the
radiation was coming from. It may be that, depending on the
wavelength, the incoming radiation would be diffracted by the slit
and would get spread all over the detectors. If this is the case,
perehaps multiple slits could be used, and the diffraction pattern
would differ dependent on the angle with which the radiation strikes
the slitted face? (because the radation at each slit would be
out-of-phase with the radiation at other slits). Has anything like
this been tried?

It is
hard enough for the average "rest of the world" intel unit to DF an
old fashioned non-hopping transmitter if the radio operator uses good
RTO procedures--trying to pluck enough of these random
fractional-second bursts out of the ether to determine a direction is
more difficult by a few orders of magnitude.

What methods are used to do DF?

--
"It's easier to find people online who openly support the KKK than
people who openly support the RIAA" -- comment on Wikipedia

(I'm not an electronic engineer, so I've cross-posted this to some
newsgroups which might be able to give informed comment on a number
of points.)

On 23 Sep 2003 05:51:41 -0700, Kevin Brooks wrote:
(phil hunt) wrote in message ...
[regarding battlefield internet]
The signal must be such that the extended receiver can hear it. So
others can too, in principle. (Though detecting the signal and
knowing where it's from aren't the same thing). I'm not a radio
engineer but I can imagine a few ways how direction-finding might
work; for example place two (or 3) detectors a few meters apart
and calculate the time delay between each one receiving the signal.

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.

over a
pretty wide band of freq's (this is why synchronization of the radios
on a time basis is critical to succesful operation of the net).

So the frequency changes are pre-determined on a time basis?

If there is a radio receiver, is it better able to detect/deceive a
signal whgen it knows the frequency in advance? Or can it "sniff"
for lots of frequencies at a time and pick out what looks
interesting?

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.

Alternately, would something like a pinhole camera work? What I mean
here is: imagine a cubic metal box, 1 m on its side, with a vertical
slit, about 1 cm wide down one of its vertical faces. On the
opposite face, there are detectors for detecting radio waves. If the
elevctromatnetic ratiation coming into the box can only go in
through the slit, and goes in a straight line, then knowing which
detectors are lit up would allow someone to tell where the
radiation was coming from. It may be that, depending on the
wavelength, the incoming radiation would be diffracted by the slit
and would get spread all over the detectors. If this is the case,
perehaps multiple slits could be used, and the diffraction pattern
would differ dependent on the angle with which the radiation strikes
the slitted face? (because the radation at each slit would be
out-of-phase with the radiation at other slits). Has anything like
this been tried?

It is
hard enough for the average "rest of the world" intel unit to DF an
old fashioned non-hopping transmitter if the radio operator uses good
RTO procedures--trying to pluck enough of these random
fractional-second bursts out of the ether to determine a direction is
more difficult by a few orders of magnitude.

What methods are used to do DF?

--
"It's easier to find people online who openly support the KKK than
people who openly support the RIAA" -- comment on Wikipedia

(phil hunt) wrote in message ...
(I'm not an electronic engineer, so I've cross-posted this to some
newsgroups which might be able to give informed comment on a number
of points.)

On 23 Sep 2003 05:51:41 -0700, Kevin Brooks wrote:
(phil hunt) wrote in message ...
[regarding battlefield internet]
The signal must be such that the extended receiver can hear it. So
others can too, in principle. (Though detecting the signal and
knowing where it's from aren't the same thing). I'm not a radio
engineer but I can imagine a few ways how direction-finding might
work; for example place two (or 3) detectors a few meters apart
and calculate the time delay between each one receiving the signal.

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.


over a
pretty wide band of freq's (this is why synchronization of the radios
on a time basis is critical to succesful operation of the net).

So the frequency changes are pre-determined on a time basis?

Yes.


If there is a radio receiver, is it better able to detect/deceive a
signal whgen it knows the frequency in advance? Or can it "sniff"
for lots of frequencies at a time and pick out what looks
interesting?

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.

Yes, you can set up to scan various nets (we did so for command post
operations where we wanted to monitor multiple nets), but they all
have to be on that same time hack, and you have to have each net's FH
plan loaded; you can't just decide to operate it like a police scanner
and listen in on whoever you choose to.


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.


Alternately, would something like a pinhole camera work? What I mean
here is: imagine a cubic metal box, 1 m on its side, with a vertical
slit, about 1 cm wide down one of its vertical faces. On the
opposite face, there are detectors for detecting radio waves. If the
elevctromatnetic ratiation coming into the box can only go in
through the slit, and goes in a straight line, then knowing which
detectors are lit up would allow someone to tell where the
radiation was coming from. It may be that, depending on the
wavelength, the incoming radiation would be diffracted by the slit
and would get spread all over the detectors. If this is the case,
perehaps multiple slits could be used, and the diffraction pattern
would differ dependent on the angle with which the radiation strikes
the slitted face? (because the radation at each slit would be
out-of-phase with the radiation at other slits). Has anything like
this been tried?

Hey, I just *used* the critters and was fortunate enough to attend new
equipment training from the manufacturer when we got it; suffice it to
say that use of FH makes DF'ing a remote concern, pretty much
eliminates any concern over jamming (even broad band jamming can only
take down a small percentage of the available spectrum, making voice
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.


It is
hard enough for the average "rest of the world" intel unit to DF an
old fashioned non-hopping transmitter if the radio operator uses good
RTO procedures--trying to pluck enough of these random
fractional-second bursts out of the ether to determine a direction is
more difficult by a few orders of magnitude.

What methods are used to do DF?

You'd have to find a signals intel puke to answer that one (but you
can rest assured that any really good methods/systems remain
classified).

Brooks

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?

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).

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's easier to find people online who openly support the KKK than
people who openly support the RIAA" -- comment on Wikipedia

"phil hunt" wrote in 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.

Thank you Admiral Doenitz...

L'acrobat wrote:

"phil hunt" wrote in 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.

Thank you Admiral Doenitz...
------------
He's right. Major breaththrough of all possible barriers, the RSA
algorithm. Uncrackable in the lifetime of the serious user, and
crack is entirely predictable with improved computing power and
can be lengthened to compensate.

-Steve
--
-Steve Walz ftp://ftp.armory.com/pub/user/rstevew
Electronics Site!! 1000's of Files and Dirs!! With Schematics Galore!!
http://www.armory.com/~rstevew or http://www.armory.com/~rstevew/Public

"R. Steve Walz" wrote in message
...
L'acrobat wrote:

"phil hunt" wrote in 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.

Thank you Admiral Doenitz...
------------
He's right. Major breaththrough of all possible barriers, the RSA
algorithm. Uncrackable in the lifetime of the serious user, and
crack is entirely predictable with improved computing power and
can be lengthened to compensate.

The fact that you and I think it is unbeatable, doesn't mean it is.

"lifetime of the serious user" what ********, you and I have absolutely no
idea what sort of tech/processing power will be available 10 years from now,
let alone 30.

"and crack is entirely predictable with improved computing power" of course
it is...

Ask the good Admiral how confident he was that his system was secure.

Damn near as confident as you are and that worked out so well, didn't it?

"R. Steve Walz" wrote in message
...
L'acrobat wrote:

"phil hunt" wrote in 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.

Thank you Admiral Doenitz...
------------
He's right. Major breaththrough of all possible barriers, the RSA
algorithm. Uncrackable in the lifetime of the serious user, and
crack is entirely predictable with improved computing power and
can be lengthened to compensate.

The fact that you and I think it is unbeatable, doesn't mean it is.

"lifetime of the serious user" what ********, you and I have absolutely no
idea what sort of tech/processing power will be available 10 years from now,
let alone 30.

"and crack is entirely predictable with improved computing power" of course
it is...

Ask the good Admiral how confident he was that his system was secure.

Damn near as confident as you are and that worked out so well, didn't it?

L'acrobat wrote:

"phil hunt" wrote in 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.

Thank you Admiral Doenitz...
------------
He's right. Major breaththrough of all possible barriers, the RSA
algorithm. Uncrackable in the lifetime of the serious user, and
crack is entirely predictable with improved computing power and
can be lengthened to compensate.

-Steve
--
-Steve Walz ftp://ftp.armory.com/pub/user/rstevew
Electronics Site!! 1000's of Files and Dirs!! With Schematics Galore!!
http://www.armory.com/~rstevew or http://www.armory.com/~rstevew/Public

"phil hunt" wrote in 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.

Thank you Admiral Doenitz...