of fresh water. From the upper portion of figure 27 it is readily apparent 

 that the reflection coefficient increases rapidly with addition of molt 

 water. A wet iceberg might be considered to have a surface consisting of 

 approximately 15 percent water by volume. This corresponds to a re- 

 flection coefficient of 0.32. In the above arguments the reflection from the 

 surface veneer of melt water is neglected as field observations indicate 

 that the thickness of this melt water rarely exceeds that of j wave 

 length (0.3 inches for A'-band and f .0 inches for -S-band). The theoretical 

 results shown in figure 27 are in good agreement with the computed 

 value of 0.33 based on the maximum range of detection. 



It follows from the foregoing that insofar as the mean temperature is 

 considered a function of latitude the reflection coefficient and therefore 

 the maximum range of detection might decrease with increasing latitude 

 and cloud cover. It should be noted, also, that the precipitation of mois- 

 ture from fog on a cold iceberg might lead to increased ranges. 



Qualitative Comparison Between Iceberg and Ship Reflection 



Both approaches to the determination of the reflection coefficient pre- 

 sented above either are purely theoretical or make assumptions which 

 cannot readily be proved valid or otherwise. The most straightforward 

 approach is to compare the reflection from a ship to that from an iceberg, 

 the comparison being made under identical atmospheric or propagation 

 path conditions. On three occasions the echo from a ship and an iceberg 

 at nearly the same range were photographed on the PPI presentation. 

 Two of these occasions are shown in figure 28. 



Figure 28 (top) illustrates the relative intensities of an echo from an 

 iceberg (010°T; 18,200 yds) and a merchant ship, SS Mormaepenn (047°T; 

 19,000 yds) on the 20-mile range scale. The illuminated cross section of 

 the iceberg (90 by 460 feet; 22,080 sq. feet) was approximately 40 times 

 that of the ship, however the blip intensity was considerably less. On 

 another occasion shown by figure 28 (bottom), the FSXS Alatna (341°T; 

 6,000 yds) was photographed with a large iceberg (150°T; 7,000 yds). 

 Plere again, although the area of the iceberg (150 by 400 feet; 20,600 sq. 

 feet) was 34 times that illuminated on the Alatna (6,000 sq. feet), the 

 berg blip was less intense. 



Quantitative Comparison Between Berg and Ship 



Although the qualitative evidence supports the reflection coefficient 

 computations it remained to make a quantitative measurement of the 

 comparison between an iceberg and a merchant vessel. The reflection 

 coefficient could not be determined due to the difference in geometry and 

 therefore gain of the two types of targets; however, direct measurement 

 of reflected power from a ship and iceberg would afford a means of com- 

 paring the radar cross sections of the two objects. 



67 



