166 



PROPAGATION ASPECTS OF EQUIPMENT OPERATION 



data: P^ = Pf= 60 kw, G = 22.5 db, and <t = 7,440 

 square meters. 



In Figure 6, the maximum range versus distance 

 curves for a set are presented. This set has a beam- 

 width of 60 degrees so that it is effective for tracking 

 aircraft coming in toward the set (increasing angle 

 of elevation). The near coverage then approximates 

 the free-space coverage which is dependent on the 

 angle of inclination of the antenna. 



In Figure 7, curves are presented for a 3-cm set. 



Figure 7. Ma.ximum range versu.s height curves for 

 various targets. 



7.3.6 



Estimating Ship Size 



The fact that the strength of a signal recei\'ed 

 from a target depends on the size of the target can 

 be utiUzed to estimate ship size. If signal strength 



versus distance curves are obtained for surface craft 

 of various sizes, then by plotting readings from an 

 unknown vessel, its size may be estimated. A field 

 procedure for obtaining these curves is to use a 

 number of surface craft of assorted sizes and follow 

 them on a radar ^dth a calibrated A scope, recording 

 signal strength versus distance. If a number of such 

 runs are taken during periods judged to be standard 

 and the results averaged, a fairly reliable chart 

 should result. 



The curves can be obtained by calculation using 

 measured power output and antenna gain and 

 empirically determined values of a in equation (5) 

 in Chapter 5. The shape of such a curve for a "point" 

 target is given in Figure 26 in Chapter 5. The radar 

 curve can be obtamed from this cur^'e by changing 

 the ordinate back to 20 log A [equation (3) in 

 Chapter 5] and then using equation (5) in Chapter 5. 



Since an actual surface target is generally large 

 and at short distances intercepts a considerable 

 part of the energy, the oscillatory part of the point 

 curve of Figure 26 in Chapter 5 is smoothed out. 

 The value of A for short distances is sometimes 

 taken as 6ylo, or 15 db above the free-space value. 



In Figure 8, an example is given of curves com- 

 puted for various ship sizes. Most of the plotted 

 points fall in the 200- to 600-ton region. However, 

 the precaution of a performance check is necessary 

 in order to use this method most effectively (see 

 Section 7.3.7). Since shape plays an important part 

 in radar cross section, a ship of unusual shape may 

 gi\'e a signal outside its class. 



7.3.7 



Performance Check 



Since the performance of a set varies, before a 

 chart described in Section 7.3.6 is used to detect 

 unknown ships, it should be checked to determine 

 whether set performance is normal. The curves 

 can be used for this check by using a known ship. 

 Such a check is shown in Figure 9 for two different 

 days. On one day, the points for the ship fell in 

 the correct region. On the other day, they fell too 

 low by about 20 db. Accordingly, on the latter day 

 the curves should have been used with an ordinate 

 correction of —20 db. 



Other types of performance check may be used, 

 such as permanent echoes, free-space range of air- 

 craft, or a signal generator. 



