A little fun from France (approach procedure)

There is a particular piece of digital signal processing used in radar (or at
least, it was in use when I was an undergraduate electrical engineering student
25 years ago) called the "chirp Z-transform." The singularities in the function
are called the "poles," after an analogue computing technique used in the 1930s
for finding those points. It may be that pilots are taught to figure out the
poles in the particular function in use and to fly a pattern that corresponded
to the singularity, or perhaps their onboard computers do this.

Just a guess. Perhaps there are some more current EEs reading who can fill in?

David

Paul Tomblin wrote:

I'm told that fighter planes use the same technique to evade enemy radar
guided missiles, flying an arc around the in-coming missile (although they
use terminology like "putting your z-pole on the target" or something). I
believe fancier radars can apply the speed gate to your speed even if it's
parallel to the antenna, even if it's primary only, but I don't believe
either ATC radar (which is, after all, designed to track cooperative
targets) or older missiles like Sparrow have that capability. I wouldn't
be surprised if AMRAAM and other newer missiles do.

David Kazdan wrote:
The singularities in the function are called the "poles," after an
analogue computing technique used in the 1930s for finding those
points.

I thought they were called poles because if you made a surface plot of
z(x,y), it looked like a big tent, and the sigularities were like the
spikes where the tent poles stuck through.

I'm sure I'm dating myself, but I actually used analog computers in
college. The EE lab had a bunch of them for control systems work. It
was kind of fun programing systems of differential equations by plugging
patch cords into a big plugboard and watching the answer get drawn on a
scope. I wouldn't be surprised if the aerodynamics of most of the light
planes we fly today were worked out on exactly such machines.

Hmmmm. Just did some googling. The more I look at the picture, the
more I'm convinced it was a TR-20 we must have been using. We also had
a TR-48 in the lab, but most students didn't get to use that.

http://dcoward.best.vwh.net/analog/eai.htm

Disappearance of the primary target when it has no radial velocity relative
to the radar antenna is due to zero range rate and zero doppler. Radar
signal processing usually eliminates returns with very low dopplers to
reduce ground clutter. Radar data processing frequently eliminates targets
that show zero range rate even though they may have a doppler signature.
Flying an arc relative to an enemy radar is one technique sometimes employed
to reduce detectability, but it has to fit in with other tactics. As for
the "z-pole on the target", I have never heard that expression, but my best
guess is that it refers to the z-axis of a 3-dimensional Cartesian
coordinate system, which would be the vertical axis. Flying a circular
pathway in the x-y plane relative to the origin of this coordinate system,
and positioning the origin at the radar (z-axis vertical through the radar)
would result in zero doppler of the main body return.

Stan

"David Kazdan" wrote in message
...
There is a particular piece of digital signal processing used in radar (or
at
least, it was in use when I was an undergraduate electrical engineering
student
25 years ago) called the "chirp Z-transform." The singularities in the
function
are called the "poles," after an analogue computing technique used in the
1930s
for finding those points. It may be that pilots are taught to figure out
the
poles in the particular function in use and to fly a pattern that
corresponded
to the singularity, or perhaps their onboard computers do this.

Just a guess. Perhaps there are some more current EEs reading who can
fill in?

David

Paul Tomblin wrote:

I'm told that fighter planes use the same technique to evade enemy radar
guided missiles, flying an arc around the in-coming missile (although
they
use terminology like "putting your z-pole on the target" or something).
I
believe fancier radars can apply the speed gate to your speed even if
it's
parallel to the antenna, even if it's primary only, but I don't believe
either ATC radar (which is, after all, designed to track cooperative
targets) or older missiles like Sparrow have that capability. I wouldn't
be surprised if AMRAAM and other newer missiles do.