210 



TROPOSPHERIC PROPAGATION AND RADIO METEOROLOGY 



extension of the normal range by atmospheric 

 conditions is important primarily from a security 

 standpoint. It must always be borne in mind that 

 transmissions on VHF may frequently be propagated 

 hundreds of miles beyond the normal limiting range 

 and are subject to enemy interception. Superrefrac- 

 tion has also been observed to cause very objection- 

 able mutual interference between two control towers 

 attempting to use a common VHF channel, although 

 the distance between the airports was great enough 

 to prevent serious mutual interference under normal 

 conditions. Point-to-point VHF radio links are also 

 affected by refraction, over longer paths than optical. 



Radio Countermeasures 



The laws of radio propagation enter into the 

 problem of jamming the enemy communication and 

 radar equipment. Since it is rarely possible to locate 

 the jamming transmitter coincident with the enemy 

 transmitter whose signals it is desired to mask, the 

 efficiency of propagation of the signals from the 

 enemy transmitter relative to those of the friendly 

 transmitter enters into the problem. This has been 

 worked out in detail for the standard atmosphere. 

 When conditions are not standard, however, the 

 effectiveness of the enemy transmitter, as determined 

 for standard conditions, no longer applies. A case of 

 special interest occurs when an airborne jamming 

 transmitter is used as a countermeasure against an 

 enemy radio communication link operating between 

 two points on the ground. If the meteorological 

 situation is such as to be favorable to formation of 

 a ground-based duct the enemy signals may be 

 propagated with small attenuation, whereas the 

 signals from the jamming transmitter may be unaf- 

 fected or even weaker than would normally be 

 expected. 



Plans for the employment of ground-based jammers 

 against enemy radio and radar systems should take 

 into consideration the ability of atmospheric refrac- 

 tion to increase, or occasionally to decrease, the 

 signal propagated to the enemy's installation for 

 jamming purposes. However, there has been only 

 limited use of ground-based jamming so far. Unin- 

 tentional mutual jamming has occurred between the 

 spaced radar sets of a coastal system on the same 

 frequency, where nonstandard propagation condi- 

 tions caused strong signals to be propagated between 

 normally noninterfering radars. 



17 3 RADIO METEOROLOGY 



17.3.1 Temperature and Moisture Gradients 



Section 17.3 is devoted to a survey of the meteoro- 

 logical conditions which produce the various types 

 of propagation described in the preceding sections. 

 This brief outline is not intended to replace the 

 assistance of a professional meteorologist in analyzing 

 short and microwave propagation problems; but by 

 familiarizing radar or communications personnel with 

 the fundamental physical processes of low-level 

 weather it may open the way toward a more fruitful 

 consultation with the meteorologist. 



Duct formation is the most important phenomenon 

 for which a detailed knowledge of the physical state 

 of the lower atmosphere is required. Whenever a 

 duct is formed, M decreases with height within a 

 certain height interval. Since, according to Sections 

 17.1.5 and 17.2.1, M = (n - 1) • 10 6 + 0.157/i,the 

 existence of a duct presupposes that the refractive 

 index n decreases with height over at least a limited 

 range of altitudes at a rate more rapid than 0.157 

 MU per meter. Such a decrease can be produced by 

 two different meteorological conditions. 



1. A rapid increase of temperature with height. 

 This temperature inversion must be very pronounced 

 in order, by itself, to produce a duct. In practice, a 

 temperature inversion contributes to duct formation 

 when accompanied by a sufficiently strong moisture 

 lapse. 



2. A rapid decrease of humidity with height desig- 

 nated as a "steep moisture lapse." 



When ducts are produced by only one of these 

 causes, they may be designated as "dry ducts" and 

 "wet ducts," respectively. In the general case a 

 temperature inversion and a moisture lapse cooperate 

 in producing a duct, but one of the two factors will 

 be preponderant, thus facilitating the analysis of the 

 meteorological problem. 



Whether or not a duct occurs under given meteoro- 

 logical conditions and what the rate of change of M 

 is inside the duct may be determined by means of 

 the diagram, Figure 27. (This discussion is presented 

 for the purpose of illustrating the importance of 

 temperature and moisture gradients. The technique 

 more readily usable in practice is to compute the 

 values of M at various altitudes directly from tem- 

 perature and relative humidity data with the aid of 

 Figure 19.) The abscissa in Figure 27 is the rate of 

 decrease of humidity with height ( — de/dh), where e 

 is the water vapor pressure in millibars, (e can be 



