790 BELL SYSTEM TECHNICAL JOURNAL 



which means precise coordinate data necessary for most military applica- 

 tions are determinable. As indicated previously, the optical tilterscommonly 

 employed over the screen of the radar indicator often serve as a medium for 

 display of range, azimuth or elevation coordinate markings: however, 

 these methods are seldom completely satisfactory in military fire-control 

 radar systems because of errors introduced by the ever-present size or posi- 

 tion variations in the electronic display field. Their use has been strictly 

 limited to search or reconnaisance radar systems. 



2.61 Electronic Bearing Marker Circuits 



The bearing marker methods reviewed here are applicable generally to 

 both azimuth and elevation angle determination. A method of azimuth or 

 elevation bearing determination which can be associated with a lobing 

 antenna system and an A-type indicator has been mentioned previously. 

 This method remains an extremely precise system which has the desirable 

 advantage of simplicity. During the latter part of the war, automatic 

 tracking was applied to this method where the actual comparison of the 

 lobes of the selected target signals was carried out electronically and the 

 resultant antenna steering information utilized as the final bearing data. In 

 a strict sense, however, only an indication of error in antenna training is 

 observable to the operator on the radar equipment proper. The exact 

 bearing data must be obtained from a measurement of the position of the 

 antenna itself. 



In the case of the continuous scanning systems employing B, C, or PPI 

 presentations, it is common practice to provide a steerable electronic marker 

 which can be superimposed upon the display field and by which means rela- 

 tively exact azimuth and elevation angles can be determined by target and 

 marker coincidence. This electronic marker method has the advantage that 

 it is subject to the same size and position display field distortion influences 

 as the received pulse signal, thus eliminating this source of error. 



A circuit arrangement which has been employed in connection with a 

 naval vessel radar search system to brighten a selected and variable range 

 trace of the PPI indicator to serve as an electronic azimuth marker is given 

 in Fig. 58. In this example the rotating antenna structure is equipped 

 with a small permanent magnet j)ole piece whose cyclic excursions past a 

 sealed magnetic reed relay cause a jxiir of contacts to close indicating coin- 

 cidence. The relay structure is likewise mounted on a ring which can be 

 rotated with respect to the scanning axis of the antenna. The relative bear- 

 ing of a target is thus determinable by a knowledge of the angular position 

 of the relay ca])sule with respect to the lubber line of the vessel. The cir- 

 cuit of Fig. 58 i^roduces one brightened range trace for each revolution of 

 the antenna upon closure of the bearing marker relay contacts and is ar- 



