FIRE-CONTROL RADARS FOR A'.-ir.4Z, VESSELS 31 



tliose received from the target being tracked by the range oj)erator, all 

 others being blanked out in the indicator circuits. 



Accurate Range Measurement 



The second major problem which required s:)lution to adapt radar to the 

 tire control problem was the provision of means for accurate and continuous 

 range tracking. It was obvious that what was required was some sort of 

 electronic range mark on the indicator sweejis, the position of which could 

 be varied by a rotary device whose motion could be used to transmit range 

 information to a remote point over a synchro system. The range mark 

 could then be aligned with the target "pip" on the oscilloscope. For 

 accurate data transmission it was necessary to obtain a linear relationship 

 between angular rotation of the range handwheel and corresponding range 

 to the marker on the radar indicator screen. 



One method which was first employed by the Signal Corps made use of 

 the fact that the transmitted pulses were generated at a periodic rate from a 

 sine wave oscillator of fixed frequency; the pulse being produced at a fixed 

 point in each cycle. By transmitting this same sine wave through a linear 

 phase shifter a new pulse could be generated whose position in time, rel- 

 ative to the transmitted pulse, could be varied by rotation of the phase 

 shifter. In the Signal Corps equipment a special goniometer was used to 

 produce the phase shift and the accuracy obtained was considered adequate 

 for the intended purpose. However, non-linearity of the phase shifting 

 device, though small, was much greater than could be tolerated in the 

 Navy fire control system. A study indicated that large scale manufacture 

 of special phase shifters, hand adjusted to meet the stringent accuracy re- 

 quirements was out of the question. It was therefore decided that a two 

 speed system be used, in which the phase shifter errors would be divided by 

 the gear ratio to the high-speed unit in much the same way that accurate 

 synchro information is transmitted by a "coarse" and a "fine" synchro. 

 The manner in which this was worked out by Bell Telephone Laboratories 

 and applied to Radars Mark 3 and 4 is described below. 



The method of range measurement can perhai)s best be understood In- 

 first examining the method of presentation used on the cathode ray tube 

 indicator for the range operator. This presentation is shown in Fig. 21 

 in which it will be noted that a Class A sweep is used to display the trans- 

 mitted pulse and received echoes. This horizontal sweep, however, differs 

 from the simple sweep of earlier radars in several respects. First, the 

 central portion of the sweep is expanded to permit more accurate viewing 

 of signals appearing within this region; second, a downward deflection 

 called the range "notch" is produced in the approximate center of the ex- 

 panded section; and third, the circuits are so arranged that the notch 



