4-15] ATTENUATION IN THE ATMOSPHERE 227 



strength. Depending on the transmitted power and other radar parameters, 

 therefore, the useful limit of region 3 will be set by the minimum power 

 required to produce a signal detectable above the noise. This minimum 

 power level is indicated by the horizontal dashed line labeled P^in in Fig- 

 4-45. Since Pm\n depends on receiver bandwidth, effective antenna scanning 

 rate and beamwidth, and other factors, changes which are made in /^/L in 

 order to increase Pt /Pc will also increase P.nin- Although Pt /Pc (in regions 

 1 and 3) does not contain an explicit frequency factor, both Pt and Pc 

 contain the factor X~-, and so increase with frequency. 



Fig. 4-45 relates to a specific target area and sea condition. Obviously, 

 one must consider a whole family of such curves, relating to various possible 

 combinations of interest, in order to arrive at the optimum choice of param- 

 eters. Some of these parameters depend on the operational philosophy 

 (e.g. barrier patrol, hunt-and-kill). In addition, the effect of the earth's 

 curvature, which will steepen the rates of signal decrease in region 3, will 

 have to be taken into account. 



The above discussion refers to non-doppler radar. Doppler radar offers 

 the additional possibility of increasing the target-to-clutter ratio by 

 exploiting differences in the target and clutter spectrums. In order to 

 achieve a gain in target-to-clutter ratio, it is necessary that the target 

 doppler frequency spectrum lie outside the range of the induced doppler 

 spectrum of the clutter. For the example given in Paragraph 4-11 (Vr = 

 200 knots, A0 = 1.5°) each doppler component of the intrinsic doppler 

 spectrum would be broadened by about 2 cps along the ground track and 

 about 350 cps at right angles to the ground track. The corresponding 

 effective velocity broadening would be about 0.1 and 11 knots, respectively. 

 Thus, no significant improvement will be obtained at large angles to the 

 ground track unless the radial component of target velocity exceeds 10-15 

 knots, for the 3° beamwidth assumed. Smaller beamwidths would reduce 

 this figure proportionately. 



In principle it is possible to improve the target-to-clutter ratio by 

 exploiting the difference between the widths of the received target and 

 doppler spectrums. This requires a "velocity" filter (or a set of them). A 

 system employing such techniques is described in Paragraph 6-6, below. 



4-15 ATTENUATION IN THE ATMOSPHERE 



The atmosphere is almost perfectly transparent to radio waves until 

 frequencies in the microwave region are reached. Attenuation of radio 

 waves in the atmosphere is due to absorption by gases (oxygen and water 

 vapor) and absorption and scattering by suspended particles (precipitation, 

 dust). The first effect will be discussed here, and the second in Paragraph 

 4-16. 



