On May 4, 11:13 pm, 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?

I think the radar system can indeed discriminate. One problem, I
think, may be this:

Radar that uses a single antenna for both transmit and receive cannot
"see behind" a
reflecting obstacle, nor an interfering RF source. So the amateur
repeater casts a "shadow"
as it were.

To make it more of a sporting course, the amateur signal is
intermittent, and FM. Which may look
like all kinds of things on the radar display.


Sounds like some real shoddy engineering took place at taxpayer
expense.

Maybe, but probably not. Some things are fundamental limitations of
the physics involved.

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?

Of course the processing system may be able to be programmed to ignore
the amateur repeater - which would make it the perfect place to hide
something.

Remember that the radar system in question is probably being used in
roles it was not originally designed for. That's probably why the
problem didn't show up before. For example, if the radar was meant to
look for high-altitude intruders, things like ground clutter and RF
sources below a few hundred feet could simply be ignored. If the
system is now being adapted to look for low-altitude and water-bourne
intruders, those RF sources become a big headache that the system
wasn't designed to handle.

Gee... Thinking about it some. All Abdulah (or Ivan or whoever)
needs to do is buy a 440 rig, an amp and a yagi and go out as a
"rover"; 3 or 4 kW ERP down the bear's craw for a while then move.

Maybe. But the result would probably be just the opposite: firing up
that setup would announce his exact bearing and altitude. IOW,
announcing "HERE I AM" to the radar system. With no legitimate sources
of RF in the area, and no "shadows" to hide behind, finding the
intruder would be easier and faster.

This sort of thing isn't new. When you don't know the exact threat,
you try to plug every possible hole.

Way back in WW2, the Allies spent a lot of time and expense developing
receivers that had extremely low local-oscillator radiation. Only
approved receivers could be used aboard Allied vessels. The concern
was that enemy U-boats could detect and find Allied convoys by
listening for the local-oscillator radiation. When you have dozens of
ships all monitoring the same frequency using big antennas and
unshielded receivers, the total LO radiation could be heard a long way
off. And while radio silence could be maintained in a convoy most of
the time, the receivers were needed for weather reports, U-boat
warnings, distress calls and such.

It turned out that the U-boats did not listen for the LO radiation
after all. But this was not known until after the war.

73 de Jim, N2EY