PROPAGATION IN S AND X BANDS IN LOW-LEVEL OCEAN DUCTS 



43 



Figure 17 is a plot of total M deficit versus wind 

 speed, with isopleths of total slope, that is, the duct 

 height divided by the total M deficit. Again, the exact 

 pattern of the isopleths is not definitely determined. 

 With the inclusion of more data in the form of 

 smoothed individual soundings, this chart and the 

 previous ones may jjrove to he more conclusive. If this 

 is the case, it may then be possible to estimate tlie 

 values of duct height and eifective M deficit simply 

 from single observations of air temperature, air hu- 

 midity, sea temperature, and wind. Psychrometric 

 observations taken at a height of from 30 to 60 ft 

 above the water would provide the value of M at the 

 top of the duct to ±1 or 2 M units at the most. An 

 observation of sea temperature leads directly to the sea 

 surface value of M, and the wind speed can be obtained 

 from the ship's anemometer. Thus with the aid of the 

 charts three important points on the M curve can l.)e 



5 10 15 20 



SURFACE WIND SPEED IN KNOTS 



Figure 16. Height of M deficit versus wind sjieed. 



obtained, namely, the values of M at the sea surface 

 and at 1 ft and the minimum value of M and its 

 height. 



These preliminary results may be summarized as 

 follows : 



1. A surface duct between 40 and 50 ft high with 

 a slightly transitional-type layer extending above the 

 duct to between 100 and 150 ft exists most of the 

 time over the water in this area. 



2. The duct is destroyed over land in the daytime 

 within about 1/2 mile of the shore. 



3. Islands comparable in size to Antigua have little 

 effect on the duct on the leeward side at a distance 

 greater than 2Y:> miles off shore. 



4. The higher the wind speed the thicker the duct 

 becomes and the less the effectirr M deficit becomes. 



5 10 15 20 



SURFACE WIND SPEED IN KNOTS 



Figure 17. Total .1/ deficit versus wind speed. 



5. Changes in wind speed have little effect on the 

 total M deficit, which is determined essentially by 

 the temperature and humidity of the air mass as a 

 wliole in relation to the surface water temperature. 



0. These conditions probably prevail over ocean 

 areas having comparable climates. 



4.1.3 Preliminary Results of Radio and 

 Radar Measurements" 



The main purpose of the experiment was to estab- 

 lish what operational use could be made of low-lying 

 ducts and to confirm observation of the effects of such 

 ducts on radio and radar propagation made in various 

 parts of the world. The data accumulated have been 

 available for study only 3 weeks, and there has been 

 insufficient time for a complete analysis. As a con- 

 sequence only the highlights of the agreement between 

 experiment and theory have been determined. 



Ducts were present all the time, and trapping on 

 both X and S bands, which increased the signals to 

 levels considerably above standard propagation values, 

 was found to exist all the time. The general conclusion 



"By M. Katzin, U. S. Naval Re.search Laboratory. 



