SCIENCE AND THE SEA 



Preparation is therefore very desirable when approaching land 

 in poor visibility with the aid of radar, so that the mariner may make 

 a sound prediction of what should be seen and when. To do this, he 

 must acquire a thorough understanding of the capabilities and limi- 

 tations of his radar equipment, knowledge of the meteorological 

 factors, either favorable or unfavorable, which will cause anomalous 

 wave propagation, and some means of determining the distance off 

 at which features of various heights will begin to appear above the 

 radar horizon. 



Radars operate in the frequencies that are essentially line-of- 

 sight, but due to barometric pressure, relative humidity, and 

 temperature gradient variations the waves are subject to some 

 bending, either up or down, under certain atmospheric conditions. 



The normal radar horizon is approximately 15 percent greater 

 than the visual horizon at the same height. The approximate 

 distance at which a feature will be on the horizon of the radar set is 

 found by adding the distance of the radar horizon of the antenna to 

 that of the feature, or can be computed by the formula: — 



D = 1.23\/H^-|-1.23\/Hr 

 D=distance in nautical miles 

 Ho = height of antenna, in feet 

 Ht = hei9ht of target (feature), in feet 



Rodar horizon is 15°o greater than visual horizon. 



The following table gives the approximate distance to the radar 

 horizon for a standard 3-cm radar under normal conditions for 

 various heights of either Ha or Ht. 



APPROXIMATE DISTANCE TO RADAR HORIZON 



(3-cm radar and normal conditions) 



Inspection of the above table would indicate to the mariner whose 

 radar antenna was 60 feet above the water, that a coastal bluff 80 

 feet high would not be visible on his radar scope until the vessel was 

 within a maximum range 20.5 miles and the chances are that the bluff 

 would have to rise above this maximum radar horizon distance before 

 the reflected echo was strong enough to show upon the radar scope. 

 The knowledge of the probable distances at which various objects 

 can be expected to appear on radar will greatly assist in the accurate 

 identification of various landfall targets. A more graphic means 

 of showing target range can be had by constructing a simple curve on 

 a piece of graph paper using the height of the mariner's own antenna 

 based on the above table. 



As with all other aids to navigation, the use of radar in good 

 weather to check out target identification, ranges and skill of the 

 operator will return valuable dividends when visual identification 

 is not possible. 



Regardless of how simple and direct the radar presentation may 

 appear, it is essential that the navigator continue to employ all normal 

 navigational techniques when in coastal waters. The navigator can- 

 not afford the luxury of not maintaining a good dead reckoning 

 position plot, a sharp eye on the soundings and awareness of the 

 effects of set and drift conditions upon the vessel. 



ICEBERG DETECTION 

 While any suggested signs, warnings or proposed methods of 



detecting the proximity of icebergs may prove of great assistance, 

 they can be but supplementary to the eyes of an alert lookout. The 

 old phrase that the only sure sign of an iceberg is to see it is still a very 

 vaUd one. Over reliance in any other means could be extremely 

 dangerous. 



/V^ 



'1 '^^'' CLEAR NIGMT 



Vniii-E 



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Distance at u-hick a lookout can sight icebergs under various 

 atmospheric conditions. 



The distance at which a lookout can sight a berg varies, of 

 course, with the state of visibility, height of eye and height of berg. 

 On a very clear day a lookout stationed 70 feet above the water could 

 sight a large berg up to 18 miles; in clear weather, with low-lying haze 

 on the horizon, the top of a berg at 9 to 11 miles; in light fog 

 or drizzling rain at 1 to 3 miles; and in dense fog about 100 yards. In 

 light fog the lookout could sight a berg sooner if aloft, but in dense fog 

 a position in the bow would be best. On a clear starlit night, a look- 

 out will not sight a berg more than one-fourth of a mile away. If the 

 bearing is known, however, this distance could be increased to one 

 mile, with .the aid of binoculars, by picking up the occasional spots 

 of light as the swells break against it. On a bright moonlit night, a 

 berg could probably be sighted up to 3 miles away. 



As a genera! rule, there is no appreciable change in the air tem- 

 perature near a berg nor in the water temperature surrounding it. 



The presence of growlers and other pieces of detached ice usually 

 indicates that a berg is in the vicinity and probably to windward. As 

 growlers can cause considerable damage to a vessel, it is always best 

 to pass a known berg on its windward side, especially at night or in 

 low visibility. 



The use of radar to detect the presence of bergs and growlers is 

 certainly helpful, but often a large berg that has been sighted visually 

 will not appear on the radar scope. This is probably due to the berg 

 having a very smooth sloping side or because of sub-refraction which 

 often occurs in ice areas. Many times, a berg detected by radar will 

 disappear again f rojn the scope as the relative positions of the berg and 

 the ship change. 



The detection of growlers by radar is even less certain. The 

 echoes returned by these small bergs, which show only a few feet above 

 the surface, are difficult to distinguish from strong sea clutter on the 

 scope. 



The following table shows the approximate maximum range 

 that bergs can be detected by a 3-cm radar with the antenna located 

 50 feet above the water: 



HEIGHT ABOVE WATER RANGE (N.M.) 



The 



PIN POINT ANCHORING 

 following method of instantaneous plotting of a ship's 



position approaching and anchoring in an assigned berth has been 



30 



