MICROWAVE BEACONS AND OTHER RADAR AIDS 



INTRODUCTION 



Radar, to the layman, may appear to be the "cure-all" for short range navi- 

 gational problems. It is commonly known that Radar is an electronic means 

 by which a vessel or aircraft may see through fog, darkness, or in generally 

 reduced visibility. To see or detect objects in advance permits safe rendez- 

 vous or altering of course, whichever is desired. If these were the only 

 considerations of the problem, the solution would be simple indeed; however, 

 in areas of considerable traflBc a more important consideration is the problem 

 of target identification. The problem is well presented by several quotations 

 from letters from Merchant Marine Masters in the Hydrographic Bulletin, 

 April 24, 1948: 



"The most difficult problem when using Radar for navigation is the identi- 

 fication of the target. 



"With a good navigational plot, most objects can be identified by their 

 relative position. But, if the ship's position is not accurately fixed it is usually 

 very difiBcult to positively identify the Radar target. Lightships are an 

 important aid in entering port or in coasting in fair weather or foul, but, 

 without a visual bearing, identification of a lightship by Radar is often very 

 diflacult because of the focusing of ship traffic near them. Also, in fog many 

 ships will anchor in the vicinity of the lightship, causing additional confusion. 

 There should be an identification system to positively identify every Radar 

 navigational aid. 



"It is earnestly recommended that a type of signal generator be devised for 

 installation on major navigational aids that will give a positive identifying 

 characteristic on the conventional PPI scope. 



"Radar reflector beacons are a great step forward and should be increased 

 in number. But due to the small reflecting surface on the masts of lightships, 

 it is recommended that Radar reflectors be installed to increase the range 

 at which lightships can be detected." 



The problem of positive Radar target identiflcation is not a new one but is 

 one which came into being with the first use of Radar. The problem has 

 been met to date by two means : (1 ) By increasing the strength of the refiected 

 signals returned to the navigators Radar, and (2) the transmission of addi- 

 tional Radar signals to the navigator. In the first category is the passive 

 beacon, or Radar reflector, which is simply a mirror method of reflecting a 

 strong portion of the Radar wave back to its source. The second category 

 is further subdivided into (a) responding beacons (those which are triggered 

 by the navigators Radar) and (b) those which operate continuously without 

 being triggered by the user. 



RADAR REFLECTOR 



The simplest form of these Radar aids, the one which offers a great deal 

 in return for very little power consumption, is the "passive radar beacon," 

 or Radar reflector. This device is one which requires no electronic equipment 

 or electrical power, and little maintenance. It is merely a physical arrange- 

 ment of such material and design as to reflect back to an ordinary Radar 

 most of the energy which strikes its surface. To the user then, this has the 

 effect of presenting a much clearer and larger target than any random 

 geometric configuration of similar size, and one which can be picked up at 

 relatively longer ranges. The Radar reflector is not an amplifying device, 

 an therefore cannot improve the reflecting eflBciency of any object which is 

 itself a good Radar target. It will, however, make a good Radar target out 

 of an object that normally reflects little or no Radar energy. Its primary 

 application is for mounting on buoys, important landmarks, etc., so as to 

 facilitate their identiflcation. 



Groups of these corner reflectors are shown in figure 3-1. Several of the 

 individual reflectors in the circular arrangement shown insure that they will 

 reflect adequately in any direction from whence a Radar wave may arrive. 

 These reflectors are constructed of lightweight aluminum alloy, and do not 

 materially affect the buoyancy characteristics of the buoys on which they are 

 mounted. The United States Coast Guard is now in the process of deter- 

 mining the best possible reflector types and mountings for lighted, can, and 



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