Chapter 6 

 ELEMENTARY THEORY OF NONSTANDARD PROPAGATION 



6.1 



HISTORICAL 



Dtjking 1941 and 1942, short and microwave 

 radar sets became available in England and 

 were installed along the Channel and North Sea 

 coast. Very soon it was found that at certain times 

 these sets were able to pick up targets such as ships 

 and fixed echoes from the French coast which were 

 well below the line of sight and which under the 

 conditions of standard propagation would have given 

 entirely negligible responses. A relationship with 

 the weather soon became apparent. In 1942, enough 

 had become known to establish most of the correla- 

 tions between excessive ranges and meteorological 

 conditions which have remained fundamental and 

 which are based on the picture of refraction in the 

 lower atmosphere that is now generally accepted. 



Later on similar effects with radar sets were dis- 

 covered all over the world. An example in point is 

 in the Mediterranean where nonstandard propa- 

 gation, during certain seasons, is the rule rather 

 than the exception. These conditions will be dis- 

 cussed in more detail in the chapter on radiometeor- 

 ology. The most extraordinary ranges, perhaps, were 

 found in the Indian Ocean where radar sets operat- 

 ing at frequencies of 200 mc were found on occasion 

 to record fixed echoes from as far away as 1,500 

 miles. The mechanism of this phenomenon is not yet 

 fully understood. 



In the Pacific theater extended ranges have also 

 been observed; but, on account of the vast territory 

 covered, the technical difficulty of all operations, and 

 the inadequacy of meteorological coverage, it is 

 difficult to evaluate the results systematically. Up 

 to the present, reports on the conditions responsible 

 for nonstandard propagation have been received 

 from many parts of the world which vary widely in 

 their characteristic features and dependence upon 

 season, weather, time of day, properties of the 

 ground, etc. It is possible to lay down certain general 

 rules, but on the whole the phenomena are exceed- 

 ingly complex. 



During 1943 and 1944, a number of systematic 

 experiments on nonstandard propagation were car- 

 ried out by the British and American Services and 

 affiliated organizations. Most of these were one-way 



transmission experiments that have a number of 

 advantages over radar experiments, but some of the 

 latter also were undertaken. Extensive transmission 

 experiments were conducted by the British in the 

 Irish Sea and the Americans in Massachusetts Bay, 

 the state of Washington, southern California, and 

 Arizona, and in the West Indian Ocean. 



These experiments will be described in the next 

 chapter. Because of the nature of the subject, it will 

 be profitable to discuss the theory before the experi- 

 ments and to give, in this chapter, an outline of our 

 present conceptions of the theory of nonstandard 

 propagation. 



62 REFRACTIVE INDEX 



Nonstandard propagation takes place whenever 

 the rate of variation of the refractive index in the 

 lower atmosphere deviates considerably from the 

 "standard" linear slope defined by equation (11), 

 Chapter 5. The variation might consist either in a 

 deviation from linearity, which is the most common 

 case, or in a linear slope in the lowest layers that is 

 widely different from the value assumed for the 

 standard. The refractive index is a function of tem- 

 perature, pressure, and the partial pressure of water 

 vapor, given by equation (9), Chapter 5. The de- 

 pendence of the refractive index on pressure leads 

 to a regular decrease with height, but the change of 

 barometric pressure with the weather produces only 

 an insignificant effect on the gradient. The variations 

 of refractive index in the lower atmosphere owe their 

 existence to stratifications in which the temperature 

 and moisture changes rapidly with height. 



In order to express refraction in quantitative 

 terms Snell's law for a curved earth is used as given 

 by equation (14), Chapter 5: 



nr cos a = nor a cos a. ■ (1) 



Now let 



n = 1 + (n - 1) with n - 1 « 1 



r = ail + -) with-«l (2) 



V a a 



cos a = 



-H 



with a«l 



42 



