In article ,
Bill Powell wrote:

Anyone with some level of technical knowledge might wonder why a
billion dollar (boondoggle) "radar system" can't discriminate between
a fixed, known "target" (like a repeater)and one that is moving, comes
from over the horizon which might be something nasty?

Sounds like some real shoddy engineering took place at taxpayer
expense. I can think of 3 or 4 ways to remove false targets w/o
loosing any system level accuracy or sensitivity. In fact, didn't
they perfect that during the cold war?

Take a look at this month's issue of CQ for a possible explanation of
the problem.

To sum it up briefly: PAVE PAWS is a phased-array radar system, with
a large number of individual turnstile antennas on each side. During
reception, the signals picked up by the various individual antennas
are combined electrically/electronically, in ways which cause them to
mix in-pase for signals coming from the desired direction and
out-of-phase for other directions.

Older-generation phased array antenna systems perform the phase
shifting by switching individual phase shifters (delay lines or
similar) in series with the feedlines from the individual antennas.
The delayed signals are then combined and detected. If you want to
point the beam in a different direction, you change all of the
phase-shifter delays.

The newer generation of phased-array radar systems actually digitizes
the incoming signal at each antenna, and then does the linear mixing
(addition/subtraction) entirely in the digital domain.

Why the change? I gather that it allows for both a finer degree of
control of the delays (allowing higher resolution in beam-pointing),
and also allows multiple different delay-and-combine operations to be
performed in parallel (just add banks of DSPs), allowing one to track
multiple targets simultanously.

The disadvantage of this new system (as stated in CQ): it has rather
less ability to reject off-axis signals than the older delay-line
method of phasing. In the delay-line system, off-axis interference
would tend mix out-of-phase *before* it was detected, and would
largely cancel out. In the new system, *every* individual antenna and
digitizer receives the interfering signal at full strength - there's
no phase cancellation in the analog domain.

This would leave the newer systems at a significant disadvantage with
regard to saturation and desensitization by strong off-axis signals.
It's not so much a question of false targets appearing, I think, but a
question of the system losing the ability to detect the real targets.
The digigal method of doing phase-shifting and beamforming is faster
and more precise than the switched-analog method, but apparently
somewhat less robust in this regard.

As Scotty said, "The more complicated they make the plumbing, the
easier it is to plug up the drains."

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!