218 REFLECTION AND TRANSMISSION OF RADIO WAVES 



Fig. 4-40 Geometrical Relations for Doppler Spectrum of Sea Return. 



Thus it is evident that the spread in cos x, and hence the width of the 

 induced doppler spectrum, will be minimum along the ground track 

 (00 = 0)- As an example, we consider an airborne X-band pulse radar 

 (9400 Mc/sec) with a horizontal beamwidth of 3° (A</> = 1.5°), and an 

 aircraft speed of 200 knots. Then from Equation 4-69, /i = 6.44 kc. At 

 grazing depression angles along the ground track {do = 0), the induced 

 doppler spectrum has a half-power width of 0.000343/i = 2.2 cps, while at 

 45° to the ground track the half-power width is 0.0037 /i = 238 cps. 



In principle, the induced spectrum is known from information available 

 at the radar and thus may be compensated, in part, by appropriate (though 

 complicated) circuitry. There still remains the intrinsic spectrum, and a 

 knowledge of this is necessary in order to determine the capabilities and 

 limitations of doppler radar in target detection and tracking through 

 clutter. 



Measurements of the intrinsic spectrum of sea clutter have been made by 

 the Control Systems Laboratory of the University of Illinois. ^^ These were 

 made with a coherent airborne radar operating on a wavelength of 3.2 cm. 

 By making measurements along the ground track, the width of the induced 

 spectrum was made small relative to that of the measured spectrum, so that 

 the measurements yielded the intrinsic spectrum directly. By multiplying 

 the frequencies by X/2 (see Equation 4-69) the results were converted to a 

 velocity spectrum. 



^iThe information on the intrinsic doppler spectrum of sea clutter was furnished through the 

 courtesy of the Control Systems Laboratory, University of Illinois. 



