Someone please stop me. I'm so weak. I can't resist.
> On Tue, 8 May 2001, Sandy Sandfort wrote:
>
> > The "total frontal area" is irrelevant if it bounces the radar signal
> > anywhere except more-or-less directly back at the radar gun, which is rare.
> > (Think F-117A.)
>
> Think radiator; a flat plate reflector of much larger area beats out a
> corner reflector every time.
>
The radar cross section of a car dead-on is actually much less than its
geometric cross section for lots of reasons. Here's a fun one from the
past - a howitzer projectile has roughly the same radar cross section as
a bumblebee.
A corner reflector has a very high reflectivity but its effective area
varies as cosA. If you're off-axis it appears smaller but it still
reflects directly back to the source.
A flat plat should be a pretty simple diffraction problem but for the
case where the dimensions of the plate are much larger than the
wavelength ( true for many areas of a car but not all ) the plate is
like a mirror. Off axis you're going to get precious little signal. Go
figure it out : beam divergence, angle of incidence, distance to target
( X 2 ). I would think the beam would be off the receiver long before
the effective area cosA would be much less than 1.
There's a corner reflector up on the moon that has been used to reflect
laser signals. The reason they used it was its directional behavior. The
flat plate would've sucked since it would only work over a REALLY SMALL
angle. Too bad it's not practical for Cypherpunks to make their own
cross-country optical nets using the moon. Or the various satellites for
that matter. Might be worth a try bouncing some light off one of the
geostationaries. Anyone want to have a go at that sort of experiment?
Temporary private links. On topic. Almost.
Funny this topic keeps coming up - we were drawing sketches of stealth
cars with absorbers, reflectors and thin metallic films on the
windshield years before rumors of the 117 were out. The basic ideas are
really old. Maybe one major accomplishment was making the damn thing
fly.
> The goal of beating a doppler beam is to kill the reflection back into the
> gun or 'burn through' it's S/N ratio.
>
> > Corner cut reflectors, on the other hand, reflect almost all of the signal
> > impinging on them directly back at the source, the radar gun.
>
> It's worth noting this is true of "Square Corner" reflectors, there are
> other kinds (think F-117)...
>
> > Even a small (6-12") corner cut radar reflector on a yacht, makes a big fat
> > blip on a radar screen.
>
> Not if it's doppler radar, it filters the primary frequency out on
> receive. It's the CHANGE in frequency we're looking at, not just absolute
> magnitude.
>
Something sounds wrong here. I'm no radar guy ( I have written code that
sorts targets ) but I would guess that a doppler radar has two
measurements to make :
1) location of reflected signals. In which case Sandy's corner reflector
will make the Yacht stand out nicely - a good thing for the recreational
boater, not so nice if you're smuggling.
2) the frequency shift of the reflected signal. This allows a partial
velocity measurement with a single return rather than multiple samples.
If you always filtered out the primary beam frequency then anything that
sat still would disappear. Reminds me of experiments with human vision
and muscle paralysis.
Anyway I think the single return gives you two pieces of info :
_ _
r and d|r|/dt
So if your goal is to defeat the system then you want to :
reduce the cross section of the target.
if
the cross section is below what is required for the equipment to make
a measurement
then
done
else
add cross section that is vibrating parallel to the motion of the
target.
The benfits of very low cross section are obvious. The benfits of the
vibrating reflector is that it will smear out the return spectrum and
make it more difficult to measure the doppler shift.
I may be wrong but the option of swamping the front end in order to
reduce the available S/N sounds like it needs active transmission from
the target. That's generally a no-no in a polite flock.
All in all the options involving corner reflectors with motion parallel
to the vehicle axis sound like they will be functional countermeasures
as long as they represent a large enough percentage of the cross
section.
What the hell "burn through" is I have no idea except that it sounds
like it has something to do with large collections of neurons that have
long been unsuccessfully attempting to act in concert finally producing
a coherent thought.
Mike
