208 



TROPOSPHERIC PROPAGATION AND RADIO METEOROLOGY 



approximately Y% of a cycle of an approximate sine 

 wave, followed by an exponential decrease. Higher 

 modes have multiples of half cycles added to the 

 sinusoidal part. 



How these modes must be combined to give the 

 total field strength and its vertical distribution is a 

 question which depends on the height of transmitter, 

 the distance out to the point where the total field 

 strength is to be obtained, the rate of attenuation of 

 each mode as a function of the distance, and its 

 phase velocity. Since the attenuation and the phase 

 velocity are different for the various modes, the 

 vertical distribution of the total field changes with 

 the distance from the transmitter, and the number 

 of modes composing the total field decreases with 

 increasing distance. 



1727 Reflection from an Elevated Layer 



This phenomenon has been studied extensively at 

 San Diego. The meteorological situation there is 

 rather unique in that the warm and extremely dry 

 upper air overlies a cooler and very moist lower 

 stratum. The transition between the two layers is 

 very sharp. This gives rise to an elevated duct of 

 the type exhibited by the M curves of Figures 24D 

 and 24E. Often the reversal of the M curve takes 

 place over an even narrower interval of height than 

 shown in these graphs. In such cases there is a 

 reflection analogous to the reflection of waves at a 

 true discontinuity between two media and which 

 cannot be accounted for by the bending of rays. 



At an interface between two media of different 

 refractive indices there is partial reflection of radia- 

 tion for any angle of incidence, but when the 

 phenomenon (partial reflection and partial trans- 

 mission) takes place in a layer of finite thickness, 

 the reflected radiation is appreciable only at angles 

 near grazing (less than 1° under the conditions found 

 at San Diego). Furthermore, other things being 

 equal, the reflection coefficient increases with increas- 

 ing wavelength. This feature distinguishes the reflec- 

 tion by a layer from the duct effects produced by 

 this layer, as the latter generally tend to become less 

 pronounced for longer waves. The reflection gives 

 rise to an additional field strength near the ground, 

 often well beyond the optical horizon. 



Transmission experiments carried out at San Diego 

 at frequencies between 50 and 500 mc gave results 

 that are explained satisfactorily on the basis of 

 reflections of the type just described but not on the 



duct theory. Thus most of the ducts caused by 

 reversals of the M curve of the type shown in Figure 

 24D will be beyond cutoff for a frequency of 50 mc, 

 according to Section 17.2.6. No guided propagation 

 should therefore be expected, whereas the observed 

 field at the receiver, located well beyond the optical 

 horizon, was consistently very high. 



At a frequency of 500 mc the reflection is found 

 to be highly critical with respect to the angle of 

 incidence at the reflecting layer. When meteorological 

 conditions are such that the layer is high (3,000 to 

 4,000 ft), and therefore the angle of incidence large, 

 the intensity of the reflected radiation is found to 

 be very low; when the layer forms at a low level 

 (a few hundred feet only) the reflected radiation 

 becomes very strong. This behavior agrees with the 

 predictions of electromagnetic theory. 



So far, the experiment at San Diego is the only 

 instance where a clear-cut case of reflection by an 

 elevated layer has been found, although indications 

 of similar effects have been observed elsewhere. 

 Whether or not this phenomenon will occur at other 

 places in or near the subtropical belt is not conclu- 

 sively known since our knowledge of meteorological 

 conditions in these climates is far from complete. 

 If it does occur, it will obviously be of great opera- 

 tional significance. 



17.2.8 Operational Applications 



Radar 



Ground radars have experienced most of the effects 

 of propagation in nonstandard atmospheres so far 

 observed operationally. Phenomenal ranges on ship 

 and low-flying airplane targets have been observed, 

 especially in the Mediterranean area, the Arabia- 

 India area, in Australia, and the Southwest Pacific 

 theaters. In the United States and Europe ground- 

 based ducts over land have occasionally produced 

 fixed echo clutter seriously interfering with the 

 plotting of aircraft targets over land. This ground 

 clutter interference is especially troublesome with 

 microwave early warning sets plotting targets over 

 land. On ground radars with high pulse repetition 

 rates, echoes from large distances frequently return 

 on the second or later traces. Such echoes interfere 

 with first sweep echoes and sometimes are misinter- 

 preted as having ranges appropriate to the first 

 sweep, with serious tactical consequences. 



One of the most serious operational consequences 

 of superrefraction is a secondary effect, that of 



