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From: TOHALL@MARS.LERC.NASA.GOV
Newsgroups: sci.military
Subject: Re: Radar counting turbine blades (was Re: Airborne Radar)
Message-ID: <1991Mar14.041632.29149@cbnews.att.com>
Date: 14 Mar 91 04:16:32 GMT
Sender: military@cbnews.att.com (william.b.thacker)
Organization: AT&T Bell Laboratories
Lines: 32
Approved: military@att.att.com



From:    TOHALL@MARS.LERC.NASA.GOV


	>>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.  


       What you are actually measuring is the blade passing frequency.
This will be equal to the fan shaft speed times the number of blades. In
the case of fighter aircraft engines, the number of blades differs from
one to another, and the fan shaft speed will vary with throttle setting.
You have a real mess if you try to use blade passing freq to ID a threat:
	
Was that a 10 bladed fan running at 10KRPM,  or a 20 bladed fan running at
5K RPM (for example)? Additionally, the fan will produce harmonics at 
multiples of the blade passing frequency: A 16 bladed fan running at 12000
RPM produces signals at 3200 Hz, 6400 Hz, 9600 Hz ....... These signals 
could be misinterpreted as well! The 6400 Hz signal which corresponds to the 
2nd harmonic of the 16 bladed fan at 12000 RPM could be confused with the
blade passing fundamental tone of a 32-bladed fan running at 6000 RPM, etc.
	
   This discussion ignores issues of the F-15 radar sensitivity, 
angular resolution capability, or max range, which are probably classified.


              Regards, 


                              Dave Hall