Natural Science Forum / Physics / General Physics / July 2008

Or one could use a phased array.

Congratulations!  You've just invented "side-looking radar" and in a
kind of upside down fashion you've just invented the "synthetic
aperture" antenna.  Too bad you are just a little bit too late.
Here's my idea: Instead of walking on a road, you use these
cylindrical disks whereby the outer circumference is laid down upon
the road again and again as the disks turn! What do you think?

Consult a current edition of Skolnik or Brookner.  As Jim pointed out,
phased array technics such as Pave Paws or Cobra Dane can track, and
meaure the dimension of anything in earths orbit down to the
dimensions of a large bolt, even through such systems implement 20 or
30 year old technology.

In more recent years the focus in more on pulse compression and spread
spectrum techniques. Also, look into the "backscatter" technique like
those used in Over-The-Horizon-Radars like ROTHR. For example, ROTHR
can track an object the size of an aircraft at ranges of up to 2,000
NM, through exploitation of radar backscatter from the surface, not by
reflections from the tracked aircraft.  Most OTH systems operate at
only vhf frequencies and employ atmospheric refraction for range
scanning, and phase comparisons for azimuthal determinations. If
suffices to say that to perform this trick, the receiving array length
is measured in miled, and the transmitting array is generally not
situated at the same location, although it can be. It also should be
obvious that such systems employ a 'considerable' array of signal
processors since the radar return is generally below the noise
threshold.  (Research the Kalman Filter and its ilk.)

Pulse compression techniques are, on the other hand, quite simply
impremented. In this techniques, rather than transmitting a short
pulse to perform a precise range measurment, you use what it called a
'chirp' pulse that allows the transmitted pulse spread out over a much
longer period of time to contain much more energy. The "chirp" pulse
is characterized by a frequency vs. time relationship that is very
carefully produced. On the receiving end, the usual practices is to
employ a surface acoustic wave device (SAW) to compress the
transmitter chirp into a very precise single pulse, with astonishing
resolution capability. (If you have an engineering background, the SAW
effectively performs an inverse Fourier tranform in real time,)

It would be indiscrete to say more becasue of security considerations.
Everthing that I have mentioned can be found on the Internet, or in
the pages of Scientific American and IEEE publications.

The Radar Range Equation that you cite still generally holds, but it
mostly relates to what are now regarded as brute force techniques,
that do no consider the improvments that can be achieved though the
use of today's digital techniques and the impact of spread specturn.
In fact, these techniques are the basis of all current research being
performed in radar, with some rather astonishing results.

I would highly commend this area to any graduating EE or physicist.
The only problem here is that this work is so important to national
defense that it is generally classified. Unfortunately this limits
admission to students holding active security clearances and about the
only way that you can obtain one of these is through the military or
though employment with a defense contractor who requires US
citizenship as a basic qualification for employment. (Although, there
are exceptions. If you are a Citizen of a NATO member country, you may
receive consideration.)

Harry C.