[AU] Light plane sparked terror alert

On Mon, 1 Sep 2003 11:51:53 +0100, "Keith Willshaw"
wrote:


"matt weber" wrote in message
.. .
On Sun, 31 Aug 2003 11:11:10 +0100, "Keith Willshaw"
wrote:


"matt weber" wrote in message
.. .
On Sat, 30 Aug 2003 12:18:13 +0100, "Keith Willshaw"
wrote:


They show up on most ATC radars only because they have a transponder.

Nonsense they show up on primary radar very well

Note the difficulties US ATC had in locating 757's and 767's on 9/11
after the transponders were turned off, and 757 or 767 has a far far
larger radar cross section than a single engine cessna.


Again this is nonsense, the radar tracks of those aircraft have been
produced in evidence primary radar is more than adequate
It is more then adequate, as long as they never get more than about 35
miles away from the RADAR.


Actual range depends on height, RCS , transmitted power and
the sensitivity of the radar, However its a matter of historical
fact that during WW2 the primitive Chain Home system could
detect aircraft out to 200 miles

http://www.radarpages.co.uk/mob/chl/chl.htm

However your original claim was that they couldnt be seen
at all unless they had a transponder !

After that, the combination of the inverse square law, and the very
limited RCS of many light aircraft makes them just about impossible to
see. that is one of the reasons that ATC in the USA also receives data
from far more capable military RADAR systems that are not limited by
Civilian energy exposure limits.


Come now make up your mind , are they impossible to see
or is it just that the range is limited ?

Most of the track data for both JFK Jr's crash, and the EgyptAir crash
came not from civilian ATC radars, but Military Radars which share
data with ATC.

I suggest you do the arithmetic sometime on what sort of power you
need to radiate to be able to get a reliably detectable return on a 1
m^2 RCS at 50km. 1m^2 is fairly typical of Cessa single. Some of the
older aircraft with fabric instead of metal are considerably smaller
RCS.


A Cessna in the head on aspect may indeed have an RCS as small
as 1 m2 , this is around the same as an F-16 !


After you have done that calculation, decide how near you would like
to live to that particular radar.

ATC radars generally only see either very large targets, or very
cooperative targets (transponders).

Wrong, ATC radars track light aircraft every day.
Only at short range.


Take a look at the free space coverage diagram for the
Raytheon ASR-23SS surveilance radar at

http://www.raytheon.com/products/asr...docs/asr23.pdf
And how may cars have you owned that have actually achieved the fuel
economy advertised?

You'll find that this civil aradt ssytem is quite capable of detecting
a 2 sq m target at 10,000 ft at 40 nautical miles
Which means 1 m^2 target at 28 nm...

Range isnt the problem, height is, if the aircraft is down in the
weeds you will indeed be limited to 20 miles or less

Thats why we have AWACS

Keith

Keith


Hell the radars of WW2 had no problem tracking aircraft of the
same size, its for damm sure that modern radars are better
My father assures me that was not the case, and he WAS the Radar
officer on a US Carrier in WW II. I'll take his word on that subject
over yours anytime


Yet 1930's era radar could detect an Me-109 over France from
the UK, the Me-109 has a wingspan roughly the same as a Cessna
at 32 ft
You are missing a very suble, but very important point that is
involved in your argument, and the Chain home argument. Microwave
radars are a relatively late development in WWII.


The US Army SCR-270 could detect aircraft at around 120 miles
out. One such set detected the Pearl Harbor raid 30 minutes before the
attack.

Neither Chain home, or Ship, or airborne radars or the SCR-270 were
microwave radars by any stretch of the imagination for most of WWII.
VHF RADARS relied on much more 'interesting' effects to work.


With a Microwave radar, the whole game is RCS, with a VHF radar
(typically 40-70Mhz), if you have to depend upon RCS you are blind.
VHF radar relies on picking a wavelength that produced a resonance
with one or more parts of the airframe, turning them into very
efficient re-radiators, making them appear many many times larger then
the real RCS.

Later in WWII, as Microwave radars became available, chaff was
dispensed as 1/4 wave aluminum foil. It produces such strong
reflections that it blinded the radar. It drove the AGC to the point
where the radar couldn't see anything that wasn't the chaff. The point
is the resonance caused the chaff to appear to have much much larger
RCS then it really had, just as various parts of the airframe did for
VHF radar.



Such technology was often used in towed reflector arrays, where a
few such tuned reflectors towed behind a frigate gave it the radar
signature of an Aircraft carrier! So while you could pick a frequency
that might be able to see a Cessna single well, it would be almost
blind to anything that didn't have similar size, or odd multiple sizes
of the airframe feature being used.

A TBD or a Betty could be seen at about 100 miles, but they are a
whole lot bigger than a Cessna 172


A Cessna Skylane has a wingspan of 35 ft, a TBD had a wingspan of
50ft and an Aichi Val a wingspan of 47.1 ft
Perhaps, but the wing probaly isn't the feature they were relying on.
For a VHF radar you need something that is an odd multiple of 1/4 wave
for it to work well. AT 40Mhz, that is roughly 1.9, 5.7, or 9.5 meters

The Radar in an F16 in Air to Air mode has a 50% probability of
detecting a 1 m^2 RCS at 40km..


We arent talking about an F-16 radar, we are talking about the
more capable search radars at ground stations.
I think you'd be surprised at just HOW incompetent many search radars
are. Have you ever used one.

I've operated both a Raytheon and a Siemens search radars. The Siemens
was part of a NATO installation, it was 120Kw, and we were about 60nm
from CPH. It had trouble seeing anything smaller than a D9 at that
range, and the doppler speed information at that range wasn't real
good, +/- about 80kt...

They show up on most ATC radars only because they have a transponder.

Nonsense they show up on primary radar very well

As posted, I seemed to be invisible to the PSM tower when I asked
permission to descend through the Delta airspace earlier this summer.
Flying a rag & tube Cub with no transponder. Can't speak for spam
cans--all the local ones have transponders anyhow.

all the best -- Dan Ford
email: www.danford.net/letters.htm#9

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