182 



VARIATIONS IN RADAR COVERAGE 



ALTITUDE IN FEET 

 7000 



3000 

 2000 



' ELEVATED OUCT r 1000 



TRANSMITTER HEIGHT-100 

 FREQUENCY- 200 MC 



BLIND ZONE DETECTION ZONE 



^MAL L^MITiNG~COVERAGE 



RANGE IN NAUTICAL MILES 



Figure 6. Standard and nonstandard coverage diagrams. 



to the lowest lobe. In diagrams 2, 3, 4, 5 the lower 

 part of the same diagram is drawn as it appears 

 under various conditions of guided propagation. The 

 bottom part of the "standard" main lobe is shown 

 by a broken line. The lines which separate the "blind 

 zones" from the "detection zones" represent the 

 range at which a medium bomber would just become 

 visible to this particular radar set. 



The diagrams clearly indicate the great extension 

 of ranges in the duct and also the moderate change 

 in ranges — sometimes an extension, sometimes a 

 reduction — above the duct. Another feature of some 

 of these diagrams is the appearance of "skip-ranges." 

 A plane flying at an altitude of 500 ft, for instance, 

 would be detected early under the conditions shown 

 in diagrams 4 and 5. As the plane approaches, the 

 echo will disappear from the scope and reappear only 

 at a range less than 20 miles. Similar conditions will 

 prevail for ground clutter. In diagram 3 there would 

 be ground clutter close in and also from beyond 33 

 miles but not from the space between. For conditions 

 shown in diagram 5, there would be echoes from very 

 remote ground targets but not from targets at inter- 

 mediate ranges. 



A change in echo strength from day to day is not 

 necessarily caused by the weather but might simply 

 be caused by a variation in performance of the set. 

 Cases have occurred where there was extensive trap- 

 ping, but because of lowered set performance there 

 was no corresponding increase in fixed echo strength. 

 The set then will appear to be in good operating con- 

 dition, and the operator will be deceived about ranges 

 of detection for craft flying above the duct. Equip- 

 ment for checking set performance is not usually 

 available in the field. The change in intensity of 

 nearby fixed echoes may be, in some cases, a measure 

 of set performance, but in the absence of more 

 elaborate checks this method can be misleading and 

 should not be relied upon entirely. 



Failure of detection of targets is not necessarily 

 due to weather influences. Electrical failure of the 

 set or inadequate adjustment may be the difficulty 

 and may be far more troublesome to identify than 

 meteorological effects which should not be used as 

 a "scapegoat" to be indiscriminately blamed for 

 poor coverage. 



16.3 



METEOROLOGICAL FACTORS 



The atmosphere is responsible for bending and 

 duct formation. To understand the "why" of non- 

 standard ranges of radar and radio with respect to 

 the weather, it is necessary to consider the meteoro- 

 logical factors involved. 



The strong refraction which results in guided 

 propagation is caused by a rapid decrease of index 

 of refraction with height within certain layers. The 

 decrease depends upon distribution of moisture and 

 temperature in the atmosphere, particularly in the 



