signal. This method of presentation provides the best graphic illustration 

 of what the radar operator sees while approaching a target. The slope of 

 the least squares best fit for the two runs is —4.3 in both cases. This is in 

 good agreement with the theoretical given by the free space radar equation : 



Pr_ (? 2 \V 



p~ t ~ (^vrj 



(2) 



Where P r is the reflected power received, P t the power output, G the 

 antenna gain, A the wave length, a the radar cross-section, and R the 

 range. This equation indicates that the power received should follow an 

 inverse fourth power law. The validity of this equation is well known for 

 most targets, and was first demonstrated to be approximately true for 

 ice targets in 1945 and L946. To provide a means of observational com- 

 parison between iceberg reflected power and ship reflected power, the 

 echo signal strength from the retreating stern of the Evergreen was also 

 measured (fig. 25). In remarkable agreement with the theoretical, the 

 best fit locus for that run has a slope of —4.0 to 18,000 yds and then 

 changes abruptly to approximately a —8 slope. This slope change in at- 

 tenuation rate is also in conformance with the theoretical. If perfect re- 

 flection from the sea is assumed at low grazing angles, then the free space 

 radar equation (2) becomes 



P r= W^)V 



p t \ 2 R 8 



for targets at low angles (i.e. 2hih 2 /R\<l) where the terms have the 

 same meaning as in (2) and hi and h 2 are the heights of the antenna and 

 target, respectively. This effect is also quite apparent on most of the 

 iceberg echo strength observations of this year and of 1945 and 1946. The 

 few cases where the change is not noticeable can be attributed to the 

 rough sea surface and concomitant diminished reflection reinforcement 

 when the iceberg echo measurements were made. 



The free space radar equation (2) can be further modified by a path- 

 gain factor which is a function of the geometry of the transmission path, 

 the electromagnetic properties of the sea, the reflective and refractive 

 properties of the atmosphere, frequency, etc. This factor is dependent 

 upon the specific conditions at a particular time, and in view of the gen- 

 eralized approach of this paper, dealing with a wide variation of propaga- 

 tion and sea conditions, no further consideration will be given to the 

 path-gain factor. However, a treatment of the prevalent meteorological 

 conditions and expected anomalies in radar propagation over the (band 

 Banks will be given in a later section. 



Fluctuating Echoes 



There was considerable concern over the rapidly fluctuating signal and 

 early in the field work a study was made to determine the time variations 



58 



