134 



SITING AND COVERAGE OF GROUND RADARS 



It should be noted that 



1. Fresnel's theory is valid when the wavelength is 

 small compared to the dimensions of the diffracting 

 object (as in optics). 



2. Fresnel's theory should not be used: 



a. For large angles of diffraction. 



b. Close to the diffracting edge. 



c. For apertures or obstacles of the order of a 

 wavelength. 



d. When the diffracting edge is not parallel to 

 the direction of polarization of the wave. 



In spite of these shortcomings the theory is useful 

 because it provides simple solutions for the majority 

 of the diffraction problems encountered in the field, 

 and, considering the difficult nature of the general 

 problem, it is still the most manageable treatment 

 that has been developed. 



antenna cause permanent echoes to appear at several 

 other azimuths in addition to that of the main lobe. 

 Although the signal intensity of the side lobes is 

 much reduced, the echoes may still be strong enough 

 to obscure targets. 



4. Strong permanent echoes causing considerable 

 trouble may be obtained from distant mountains in 

 the rear as a result of back radiation. Again, the 

 weakness of the radiation and distance of the moun- 

 tains are often compensated for by the large extent 

 of the reflecting surface. 



5. Antennas with wide beams cause permanent 

 echoes to be much wider than the object that 

 produces them. 



6. Diffraction over intervening ridges is often 

 sufficient to nullify their screening action so that 

 objects behind the ridge are visible. 



15.5 

 15.5.1 



PERMANENT ECHOES 

 Introduction 



Permanent echoes are due to reflection from terrain 

 features such as mountains, islands, or even smooth 

 surfaces near the antenna (ground clutter). Nearby 

 hills and surfaces produce strong echoes which 

 obscure the indicator and widen the main pulse so 

 that the minimum range of detection is increased. 

 More important are the distant hills, especially those 

 in the operating sector which obscure areas of tactical 

 importance. Permanent echoes are a prime considera- 

 tion in siting, as many otherwise excellent sites are 

 rendered worthless by excessive fixed echoes. A care- 

 ful analysis of the terrain will enable an approximate 

 prediction of such echoes. In this section is presented 

 a systematic method of preparing permanent echo 

 predictions so that the suitability of sites may be 

 determined without actual field tests. 



Several factors combine to make permanent echoes 

 more troublesome than might be expected on first 

 thought. 



1. Hills and land surfaces are so much greater in 

 extent than the target which the equipment is 

 designed to detect, that strong echoes may be 

 obtained from distances where an ordinary target 

 would give an echo far below normal detection levels. 



2. The low elevation of the land surfaces places 

 them in regions most subject to nonstandard propa- 

 gation effects where extreme ranges and large 

 responses are frequently obtained. 



3. Side lobes of the horizontal pattern of the 



Permanent Echo Diagrams 



The permanent echoes associated with a radar 

 station may be plotted on a chart and their extent, 

 location, and strength represented. Permanent echo 

 diagrams should be prepared for each unit of a radar 

 system using a standard procedure for the taking 

 and presentation of data. These diagrams are very 

 useful for: 



1. Indicating blind areas in a station's coverage. 



2. Assigning the operating area of a station. 



3. Checking the range and azimuth accuracy. 



4. Checking the transmitter output and receiver 

 sensitivity. 



5. Estimating nonstandard propagation. 



6. Planning test flights. 



While methods used in different theaters vary as 

 to detail, the typical permanent echo diagram is 

 prepared about as follows. The equipment should 

 be in normal operating condition: that is, the trans- 

 mitter output and receiver sensitivity should be as 

 recommended by the instruction manual; the range 

 and azimuth calibrations should be accurate; and 

 the weather conditions that affect propagation should 

 be average. The receiver gain should be set to some 

 standard level, usually maximum, or to some definite 

 noise height. The value of the data taken will depend 

 to a considerable extent on the skill and judgment 

 of the operator. The station would normally be taken 

 out of operation for about an hour while data are 

 taken, although it is possible to take observations 

 during normal scanning by stopping momentarily. 

 Where antenna switching is provided, the low-angle, 



