🚀 go-pugleaf

In <careyj.727992636@spot.Colorado.EDU> careyj@spot.Colorado.EDU (CAREY JOSEPH M) writes:

>np4@prism.gatech.EDU (Nick Pomponio) writes:

+>The ECM device can be a simple repeater applying amplitude modulation to
+>the received signal. The AM frequency would be a function of the actual
+>velocity, the desired (deception) velocity, and radar signal wavelength.
+>Simple AM will produce two false velocity "targets"; the undesired one
+>could be supprssed by using a single sideband AM scheme.

+>-- 
+>POMPONIO,NICHOLAS A

>Okay, in theory I'll buy this, but lets see how it would work in
>practice.  You'd need to detect (very quickly) the signal, have some
>kind of a system wired up that would determine how much frequency
>(i.e. apparent doppler) shift to apply, and retransmit that signal
>(presumably with something that radiates independently of phi, like
>a dipole).  Further, you would need to make sure that you would not
>detect the signal that you would be transmitting, which is only
>a few hundred Hz (if memory serves we're talking about 30 Hz/mph,
>but I could be wrong) off the one you received, or your repeater would
>lock up.  And keep in mind that you'll be transmitting significantly
>more power than that which is received.

A straight-through repeater (receive, amplify, modulate, transmit)
could do the job.  In fact, the jammer could be electro-mechanical
(i.e. passive), with no active RF components whatsoever.  No detection, in
the classical sense, is necessary, unless you want to gate the modulation
source based on the presence of a pulse.  Note that the modulation
frequencies applicable to the stated problem are in the _audio_ spectrum.

The choice of modulation frequency for a given received signal depends on the
velocity of the car and the desired (deception) velocity.  Both of these
vary slowly (compared to pulse-to-pulse times), and they are available as
inputs to the jammer (the jammer doesn't have to measure any radar parameters
to determine their values). This, of course, assumes that the radar
frequency is known ahead of time or is measured once at the start of the
jamming cycle.

Isolation in a broad-band system (i.e. 2 to 18 GHz) is tough, but for a
narrow-band jammer designed to counter a single known radar, high-
isolation RF components are much easier to find.

>Yeah, I'll believe that it can be done.  It would even provide some
>interesting engineering challenges, but this strikes me as the kind
>of problem people solve for the fun of solving it, not because they
>really need ECM for their CAR (an F-16, yeah, but not my Plymouth).
>Also, to design this properly, you would need some pretty expensive gear.

My interest is with military-type ECM.  Jamming a single, simple radar
is one thing; designing an ECM system to jam a plethora of disparate,
complex operator-in-the-loop radar systems is a challenge, to say the least.
That's one reason that military ECM systems aren't too cheap!

>Just a thought.  I'd sure like to see the results somebody else comes
>up with, but for me this reminds me of hacking computers -- an interesting
>problem that I don't want to spend too much time on.


>Sure is an interesting discussion, though.

Agreed!

>Joe Carey
>joe@lobos.colorado.edu
-- 
POMPONIO,NICHOLAS A
Georgia Institute of Technology, Atlanta Georgia, 30332
uucp:	  ...!{decvax,hplabs,ncar,purdue,rutgers}!gatech!prism!np4
Internet: np4@prism.gatech.edu