a good probability exists that it would never have been picked up unless 

 continuous surveillance of radar was maintained or the minimum discern- 

 ible signal was lower than —92 dbm. The necessity of keeping a system 

 in near peak operating condition is strikingly illustrated by this example. 



Frequency and Polarization 



As shown in figure 40, ice and sea water behave similarly to both X- 

 and N-band frequencies with the exception that the reflection from 

 droplets in sea spray is frequency sensitive (greater reflection the greater 

 the frequency [o]). Less weather attenuation is found in the lower fre- 

 quencies, but a better display (less conducive to operator fatigue) is 

 found on the higher frequency sets. All things considered, there is no 

 preference to the frequency of radiation for an ice detecting radar. 



The rapid decrease in reflection coefficient with angle of incidence for 

 vertical polarization indicates that horizontal is the preferred polarization. 

 Circular polarization was not examined. 



Antenna Rotation 



An evaluation of antenna rotation was facilitated by the use of con- 

 tinuous rotation and 30° and 60° sector scans. It had been observed on 

 many occasions that sector scan does provide a more definite detection 

 and would probably be of value in detecting a weak ice target. The value 

 of sector scan was qualitatively determined by a series of PPI photographs 

 taken under the same propagation, target, and radar conditions. The 

 favorable results to be expected in defining a weak target by sector scan 

 are shown in figure 43. Figure 43 A is a photograph of the PPI scope with 

 the antenna in continuous rotation. This photograph shows, very faintly, 

 two targets indicated by open circles. Immediately after this picture was 

 taken the antenna was placed on sector scan. Although the rotation was 

 only about two times faster on 60° sector scan, the two previously barely 

 discernible targets appear many more times brilliant and a third target, 

 previously undetected, became visible. The exposure for these pictures 

 was exactly the same in all cases; however, photograph 43 A suffered in 

 development. There is little doubt that this antenna control device is ad- 

 vantageous, and assuming continuous rotation will be employed peri- 

 odically, sector scanning ahead of the ship is recommended for iceberg 

 navigation. The difference between 30° and 60° sector scan was negligible; 

 therefore, ()0° scan should be used. These observations suggest another 

 device which might prove very effective in the discernment of a persistent 

 weak echo amidst random echoes from the sea. This electronic integrating 

 device, known as a "memory tube," might prove to be a definite asset in 

 commercial radar sets. 



Anticlutter Devices 



Certainly the quest for a reliable radar should not exclude anticlutter 

 devices; however, the proper use of any device involves the competency 

 of the operator and the use of these devices present on many radar instal- 



95 



