Targets for Microwave Radar Navigation 



By SLOAN D. ROBERTSON 



The effective echoing areas of certain radar targets can be calculated by the 

 methods of geometrical optics. Other more complicated structures have been 

 investigated experimentally. This paper considers a number of targets of practi- 

 cal interest with particular emphasis on trihedral and biconical comer reflectors. 

 The possibility is indicated of using especially designed targets of high efficiency 

 as aids to radar navigation. 



Introduction 



IT NOW seems likely that radar, developed during the war, will find in- 

 creasing application as a navigational aid for aircraft and surface vessels. 

 In fact there are good reasons for expecting that peace-time radar can be 

 made even more efficient than its war-time prototype. 



There are two ways of improving radar performance. One may concen- 

 trate on the radar set proper with the object of increasing either the power of 

 the transmitter or the sensitivit}^ of the receiver. Or, one may take steps to 

 improve the echoing efficiency of the targets. The latter was, of course, not 

 possible during the war since most of the targets of interest were controlled 

 by the enemy. It is a purpose of the present paper to consider the design 

 of various targets of high echoing efficiency and wide angular response which 

 may be placed at strategic points as aids to radar navigation. The ideal 

 reflector to serve as a "beacon" or "buoy" for guiding radar-equipped air- 

 craft or ships would present a highly effective area to incident radiation over 

 a full 360° in azimuth, and would also be operative over a fairly broad verti- 

 cal angle. The value of a particular target for navigational purposes may 

 therefore be considered in terms of two factors: effective area, and angular 

 response. 



The echo received by a radar from a particular target can be calculated 

 by the formula:^ 



W. = Wr^^^ (1) 



where Wr = echo power available at the terminals of the radar antenna. 

 Wt = power launched by radar. 



Ar = effective area of radar antenna assuming that the same an- 

 tenna is used both for transmission and reception. 



' This equation follows directly from Equation (1) of a paper !)>• H. T. Friis, "A Note 

 on a Sim])le Transmission Formula," J'yoc. I.K.E., Vol. 34, pp. 2.S4-256, May 1946. The 

 radar transmission formula is ol)tained by applying Friis' formula twice; first to the trans- 

 mission from the radar to the target, then to the transmission from the Uirget to the radar. 



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