Chapter 16 

 VARIATIONS IN RADAR COVERAGE* 



Variations in coverage of radio and radar 

 equipment are caused by atmospheric factors 

 which influence propagation of very short radio 

 waves. 



The rapid and accurate evaluation of radar signals 

 is dependent to a great extent upon our knowledge 

 and understanding of the effects produced by the 

 variable conditions of the lower atmosphere. 



Evaluation of radar signals influenced by weather 

 introduces problems of identification, actual range 

 determination with second or third sweeps, and radar 

 coverage characteristics, each having a direct bearing 

 on the tactical situation. 



Enemy ships far beyond the horizon have been 

 located by radar and sunk by radar-controlled gun- 

 fire. United States warships in the Pacific, in several 

 instances, have picked up targets by radar at ranges 

 four to five times those obtained under standard 

 conditions. 



Army coastal radars have tracked convoys on 

 some occasions to 20 or 30 miles beyond normal 

 radar ranges. The same radars, a few hours later, 

 may have failed entirely to pick up targets clearly 

 visible to the eye. 



Allied forces are employing radar and VHF (very 

 high frequency) equipment with steadily increasing 

 effectiveness. But we are forced to revise and improve 

 our early conceptions of the capabilities and limita- 

 tions of these useful instruments of World War II. 

 Serious errors and false evaluation of radar presenta- 

 tion may result if we do not take into consideration 

 the effects of weather and atmosphere on radar 

 ranges and VHF coverage. 



Complete reports of the variability of radar cover- 

 age show that certain weather and atmospheric 

 conditions prevailing along the transmission path 

 may greatly modify the normal range characteristics 

 of radar and VHF radio. The operator, at certain 

 times, can "see" targets or hear messages far beyond 

 the horizon, sometimes at unbelievable distances. At 



"This document was published June 1, 1944 and distributed 

 widely to Service personnel under the above title, and under 

 short title JANP 101, by authority of the Joint Communica- 

 tions Board. Originally prepared by the Columbia University 

 Wave Propagation Group, it was amended and improved by 

 representatives of both Services in an effort to prepare a brief, 

 qualitative but authoritative statement of the then known 

 facts concerning the factors contributing to nonstandard 

 propagation. 



other times he is unable to contact, by radar or 

 VHF, aircraft or surface craft well within the normal 

 range limit. 



These effects of a nonstandard atmosphere might 

 leave doubt in our minds as to the effectiveness of 

 radar and the usefulness of VHF radio. But we should 

 adopt the reverse view. We can, by understanding 

 and allowing for these phenomena, make a useful 

 instrument more effective — the weather will work for, 

 instead of against, radar and microwave equipment. 



Unusual ranges are caused by bending or refraction 

 of the radio waves by the atmosphere. A most import- 

 ant special case of refraction is the concentration of 

 the wave energy in ducts within the atmosphere. 

 This bending and duct formation is a direct result of 

 the meteorological factors involved — factors of 

 weather and atmosphere — peculiar, in many cases, 

 to the locality and the season. Such factors are dis- 

 cussed later. 



161 BENDING 



The VHF or radar operator usually assumes that 

 short waves and microwaves, at frequencies above 

 about 30 mc, travel along the line of sight from the 

 transmitter to the receiver and, in the case of radar, 

 to and from the target. Experience has shown that 

 this assumption, nearly true in many instances, may 

 lead to serious errors or false evaluation if applied 

 to radar operation and microwave communication. 



Radio waves are bent from a straight line path as 

 a result of refraction by the lower atmosphere. This 

 bending, or refraction, is generally recognized as a 

 property of light. It is equally a property of radio 

 waves. The underlying principles are exactly the 

 same in both cases. 



The quantity that determines refraction is called 

 the index of refraction. Refraction occurs whenever 

 there is a change of index of refraction, as at the 

 boundary of two substances. In the interior of a 

 material of constant refractive index, the rays travel 

 in a straight line. The change in angle at the bound- 

 ary is the larger, the greater the difference in refrac- 

 tive index from one material to the next. 



Radio waves are refracted or bent in the atmosphere 

 because the index of refraction of the atmosphere 

 changes with height. The properties of the atmos- 

 phere which determine the refractive index and which 



178 



