48 OCEAN ELECTRONIC NAVIGATIONAL AIDS 



The 5-centimeter band has been allocated in order that the opportunity 

 be provided to determine whether Radars operating in this band might com- 

 bine many of the desirable features of those in the other two bands. Equip- 

 ment operating in the 5-centimeter band is not yet generally available, and 

 no advisory specifications have been prepared for equipment in this band. 



ADVANTAGES AND LIMITATIONS OF RADAR 



Radar is definitely not a "cure-all" to replace other devices and methods 

 of navigation, but is rather a supplement to such devices and methods. The 

 chief advantage of Radar is that it succeeds in those conditions where other 

 methods are impossible; i. e., in fog, heavy rain, and other conditions of poor 

 visibility. These conditions, however, do have a decided effect upon any 

 Radar set and it is well to have'an understanding of these effects in order to 

 utilize the Radar to the fullest extent when it is most needed. An under- 

 standing of the effect of wind on Radar is also important. These and other 

 conditions, of course, tend to impose limitations on any Radar, and are there- 

 fore discussed in some detail in the following paragraphs. The extent to 

 which these things affect the usefulness is, of course, dependent upon the 

 design of the particular equipment and the experience of the user, but all 

 Radars are affected to some extent. 



In the open water the effects of wind are most pronounced. The wind by 

 itself gives no trouble but the attendant sea results in an obscuration of the 

 Radar known as "sea return." The waves present myriads of targets for 

 the Radar signals to detect, with the most pronounced effect being in the 

 direction of the sea. (See fig. 4-6.) 



Sea return may be visible up to 10 miles, depending upon the sea conditions 

 and the design of the Radar set. Merchant marine Radar sets are now 

 equipped with devices for minimizing the effect of sea return and permitting 

 more or less normal operation of the set. While such devices are quite ef- 

 fective they do not wholly remove the sea clutter in bad weather. With 

 careful conning of the ship it is usually possible to pick up large targets such 

 as ships, before they get close enough to get into the sea return. It is also 

 possible in most cases to properly manipulate the receiver gain control and 

 sea return suppressor to detect ships inside the range of the sea return be- 

 cause a ship normally gives a larger concentrated echo than do waves. The 

 Radar set in this condition is operating at reduced sensitivity and will, of 

 course, miss small targets which may still be a source of potential danger to 

 the ship. 



As an example of failure to pick up a small target, a ship on a southerly 

 course standing into a harbor with a southerly wind of 30 to 40 miles per 

 hour, observed that the small buoys at the breakwater entrance could not 

 be detected, regardless of how the Radar controls were manipulated. The 

 breakwaters themselves and the shoreline, however, were easily visible, thus 

 permitting safe navigation. The effects of wind are somewhat reduced on 

 the Great Lakes, when compared to ocean travel, in that the ship master 

 usually can and does lay his course to take advantage of any lee afforded by 

 the surrounding land. 



Rain, snow, sleet, and clouds are generally observed to have a somewhat 

 similar effect on the picture observed on the scope. If the ship is in the 

 midst of a general rain. Radar operation will probably be normal or there 

 will be a slight haze on the screen. In the case of heavy concentrations of 

 precipitation, usually local in nature, the actual area and location of the 

 storm will be seen on the scope. During this time the Radar detects nor- 

 mally in the other areas of the scope and will probably see targets on the 

 same azimuth as the storm but either closer to or beyond it. (See fig. 4-7.) 



Present experience on the Great Lakes indicates such storms to be of 

 relatively short duration. Radar detection of clouds, heavy precipitation, 

 cold fronts, etc., is being exploited by meteorologists in weather predictions. 



The operation of Radar in fog is usually good and can be relied upon al- 

 though there may be a reduction in the range at which targets are first de- 

 tected. It is reiterated, however, that all other safety precautions must be 

 continually used by the nagivator. 



It is apparent, therefore, that the navigator must always, be particularly 

 vigilant during periods of inclement weather, and must use more care in 

 operating the Radar and in studying and using the data obtained from it. 



