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From: emory!Dixie.Com!jgd@gatech.edu (John G. DeArmond)
Newsgroups: sci.military
Subject: Radar counting turbine blades (was Re: Airborne Radar)
Message-ID: <1991Mar4.211709.8954@cbnews.att.com>
Date: 4 Mar 91 21:17:09 GMT
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From: emory!Dixie.Com!jgd@gatech.edu (John G. DeArmond)

>Rumor is that the radar on an F-15C is so good, it can actually
>count the number of blades on an oncoming jet's engines, and then
>determine what kind of plane it is based on this and profile.  This
>at 20 miles out!  Anyway, that is the rumor, but it comes from
>someone (not I) who should know.  Could anyone else elaborate?

I don't know anything about the F-15 but from my work with electronic
countermeasures more applicable to our streets and highways, :-) I know
that counting blads of a fan is easy.  In fact, I'd suspect that in many
instances, it can actually be a pesky side effect.  A rotating fan, 
especially fans such are are in jet engines and have space between the
blades, makes an excellent chopper.  This chopping action returns a 
radar reflection that to simple doppler radar looks like a high speed
doppler shift.  Any of my traffic radars will measure the "speed" of an
ordinary fan at surprising range.

>From looking at the doppler output of simple radars on a scope, I can
observe the wobble in the fan shaft from the periodic variation in the 
reflected amplitude.  It would be a trivial DSP application to look
at this signal, count the pulses between wobbles, and look up the 
blade count in a table, and figure out what class engine is involved.
How accurate your blade count vs aircraft table is is a measure of 
your intelligence agency's effectivness :-)

I don't think the ability of a radar to count turbine blades has anything
to do with the object size resolution of the radar.  I rather suspect that
someone observed the chopper return on some telemetry tape and realized
that the effect could be used for aircraft identification.  Like the
old saying goes, "given a choice between luck and brilliance, I'll take
luck every time" :-)

John

-- 
John De Armond, WD4OQC        | "Purveyors of speed to the Trade"  (tm)
Rapid Deployment System, Inc. |  Home of the Nidgets (tm)
Marietta, Ga                  | 
{emory,uunet}!rsiatl!jgd      |"Politically InCorrect.. And damn proud of it