146 



REFLECTION COEFFICIENTS 



tion the experimental values of reflection coefficient 

 generally fall well below the theoretical values based 

 on the assumption of a smooth sea. Whereas over sea 

 a regular interference pattern existed, over land 

 (Orlando, Florida) no specular reflection was ob- 

 served. The lobe structure was absent in the observa- 

 tions over land. 



Another experiment*' carried out over laud was 

 performed using X-band waves between Beer's Hill 

 and Deal, New Jersey. The reflection coefficient of 

 the ground is expected to change with the seasons 

 on account of seasonal vegetation changes on the path. 

 One series of measurements lead to reflection coeffi- 

 cients of 0.17 and 0.20 for horizontally and vertically 

 polarized radiation respectively. 



'•^^ Specular Reflection and Scattering 



Ordinarily neither the sea nor the land are ideally 

 smooth, and one would expect always nouspecular re- 

 flections which tend to perturb the interference pat- 

 tern of the direct and reflected rays from a smooth 

 surface. 



It has been pointed out" that the condition which 

 has to be fulfilled for specular reflection to occur is 

 that the grazing angle xj/ be such that sin xp ^ \/g, 

 where g is the wavelength of the sea waves. Clearly 

 this is a kind of limiting condition and assumes the 

 perfect regularity of the sea waves. It is seen that 

 the above condition expresses the fact that the smaller 

 the grazing angle, the smaller are the apparent irregu- 

 larities of the sea and, if these apparent irregularities 

 are much closer than the wavelength of the incident 

 radiation, it is to be expected that specular reflec- 

 tion should i^redominate. 



A direct consequence of this condition is that the 

 echoes from a target, that is, a ship, will not be 

 drowned by the clutter from the sea waves for large 

 distances between the target and observer. Whereas at 

 closer distances (large grazing angles) the echo from 

 the target might be drowned by the irregular reflec- 

 tion, i.e., scattering from the sea. To this efliect, a report 

 is quoted in which it is stated that ships could only 

 be detected beyond a certain distance from the shore. 



It is also thought^" that the discrepancies observed 

 between the theoretically predicted and measured sea 

 reflection coefficients (horizontal polarization) could 

 be attributed to scattering. The irregular reflections 

 have the effect of decreasing considerably the ratio of 

 the successive maxima and minima of the interference 

 pattern de\cloped. The discrepancies referred to are 



those discussed by the Radiation Laliuratoi'y workers.' 

 In this connection, Eckersley mentions some experi- 

 ments by Hoyle on sea reflections in which no corre- 

 lation could be observed on the voltage registered by 

 two aerials a few inches apart. This tends also to sug- 

 gest the existence of scattering from the sea. 



In another series of transmission experiments^" it 

 ^^•as ol)ser\ed that the contrast between maxima and 

 minima was poor. Here the experiments were carried 

 out at 200 nic over sea at a distance of 100 miles be- 

 tween an airjilane and a ground station. The diver- 

 gence of the observed from the calculated values of 

 reflection increases as the grazing angle increases. 

 This seems to be in agreement with the results accord- 

 ing to which the sea surface may be considered as 

 formed by a number of corrugations which, for snuill 

 grazing angles, appear to be so close together that the 

 reflection is mostly specular. 



As to the frequency \ariatiou of scattering one 

 would expect more and more scattering with increas- 

 ing frequency. 



The eifect of uneven ground on the reflection co- 

 efficient was investigated by the British workers al- 

 ready mentioned.^'- The reflecting ground consisted 

 of an artificially prepared series of uniform ridges, 

 placed along, across, and at 4.5° to, the direction of 

 propagation. These ridges simulated waves, and their 

 wavelength D lay between 0.6 and 1.2 m, whereas the 

 double amplitude h varied between 5 and 15 cm, and 

 the wa\elength of the radiation used was 9 cm. 



Tables 8 and 9 give the reflection coefficients of 

 the uneven ground when the direction of propagation 

 is at 45° with the ridge and along or across the ridge 

 system. The tables also gi\e an estimation of the re- 

 flection coefficient of level ground of the same moisture 

 content as that under test. 



The results given in these tables show how a rela- 

 tively small irregularity in the ground surface is 

 sufficient to prevent regular reflection. The reflection 

 coefficient becomes erratic when it has fallen below 

 a value of about 0.1. The values given for level ground 

 refer only very approximately to the state of the 

 ground in the ridged condition. Since the measure- 

 ments extended over several days, those relative to 

 level ground may not correspond necessarily to the 

 same degree of moisture as those referring to the 

 ridged ground. The level reflection coefficients in the 

 two preceding tables differ from each other because 

 tliose of Table 8 refer to drier ground than those of 

 Table 9. 



