DEPENDENCE OF SIGNAL POWER ON PARAMETERS 



197 



zoiital polai-izatiuii Avas available. It was found that 



— = 3 to 5 db. 



Heuce under these sea conditions the increase in a 

 on going' from S to X is about the same as when going 

 from X to K. 



An interesting by-product of these measurements 

 was the comparison of polarizations, keeping the wave- 

 length the same. This ratio was quite variable, rang- 

 insr from 





= — 9db Xband 



on X band under stormy conditions, to 



11 



> 4-25 db Sband 



on S band with a calm sea. In general the ratio de- 

 creases as sea becomes rougher and is almost always 

 less on X than on S band. 



It is too early in the investigation to attempt a de- 

 tailed interpretation of the results. It does seem that 

 scattering from small spray drops is not the sole 

 mechanism, despite the popular observation that sea 

 echo seems to increase rapidly with the appearance 

 of whiteeaps. Other evidence also seems to coniirm 

 this. Under favorable conditions sea echo appears as 

 discrete signals, moving with the wind, that can be 

 tracked for 15 to 20 sec. This seems longer than one 

 would expect from a breaking wave. On the other 

 iumd, reflection from large wave surfaces cannot be 

 the whole story either. This is indicated by the fairly 

 rapid increase of o- with frequency and by the com- 

 plicated changes with polarization. If is jirobable 

 that we are dealing with a combination of mech- 

 anisms, and it will be a difficult task to unscramble 

 the contribution of each to the total signal. 



It should be emphasized that these measurements 

 were taken near the coast, though outside the break- 

 ers. C'onditions on the high seas might conceivably be 

 quite difl'erent. 



Discussion 



It was stated that individual sea echoes which 

 persist for many seconds cannot be caused either by 

 specular reflection from an inclined water-air inter- 

 face or by random (Eayleigh) scattering from in- 

 dividual drops of spray. Instead, an aerated surface 

 layer created by a breaking whitecap may jDersist for 

 many seconds and may be responsible for persistent 

 echoes. Such a layer constitutes an irregular network 



ol- aii'-water interfaces and may give rise to consider- 

 able scatter of microwaves. The actual mechanism by 

 which such a layer gives rise to a sea echo is likely to 

 be dift'erent at different sea states. If a large area is 

 covered with foam, then in the presence of strong 

 swell the chief return should be expected from a wave 

 crest, and the radar signal would appear to travel 

 slowly on the radar screen as the wave crest pro- 

 gresses. 



The author stated that so far no consideration had 

 been given to such involved mechanisms as the one 

 suggested, but added that data already collected might 

 well lead to such an investigation. 



It was suggested that several mechanisms, includ- 

 ing scattering from droplets, were probably respon- 

 sible for sea echo in rough weather. Experiments in 

 Britain reported by the British Army Operational 

 Eesearch Group showed that echoes from shell splashes 

 viewed on an S-band gunnery radar could be resolved 

 into two parts. One was from the "boil," a solid wall 

 of water with enclosed air bubbles, which could be 

 readily distinguished from the superimposed response 

 from the larger portion of the splash called the 

 "plume," which is a region of isolated water drop- 

 lets. Echoes from the droplets in the "plume" region 

 were of many seconds duration, and it seemed likely 

 that an investigation of the frequency dependence of 

 such scatterers would produce useful I'esults. 



12^ THE DEPENDENCE OF SIGNAL 

 THRESHOLD POWER ON RECEIVER 

 PARAMETERS'^ 



This paper deals with the effect on the signal thres- 

 hold power of various parameters in the receiving 

 systems of radar sets, i.e., with the minimum signal 

 power necessary for visibility. Altliough this is a diffi- 

 cult problem and all the important factors entering it 

 are not known, it is felt that at least qualitatively, 

 and sometimes quantitatively, a fairly good answer 

 can be given at present. First of all it is necessary to 

 define some of the parameters involved in ordinary 

 radar reception. When a signal is reflected from a 

 target the power entering the receiving system may 

 be written in the following form : 



where G, X, and u are the antenna gain, radar wave- 

 "By J. L. Lawson, Radiation Laboratory, MIT. 



