no 



RADAR SCOPES 



a little wider than this. With too narrow a 

 band pass, signal strength is reduced and 

 with too wide a band pass noise is admitted 

 in excessive amounts. Either way, the sig- 

 nal-to-noise ratio is reduced. If noise is 

 minimal or absent, presumably a wider band 

 pass could be used. Video bandwidth had 

 neghgible effect. 



In general, these results were confirmed 

 and extended by Ashby et al. (1) at M. I. T., 

 who also studied signal duration, focus, con- 

 trast, and detector law. Longer signals were 

 detected better. Defocussing had almost no 

 effect when parallel to the pip and very 

 minor effects when perpendicular. No effect 

 of trace brightness was evident unless the 

 contrast was \e,vy near threshold. Whether 

 a square law or linear detector was used 

 made no difference, unless the signal was 

 actually hmited by the amplifier. 



So far as is kno\vn, no current work is 

 under way in any laboratory on A-scope de- 

 tectabibty. Interest has shifted to the in- 

 tensity-modulated scopes. There undoubt- 

 edly exist many questions yet to be 

 answered, chiefly as to the pecuhar visual 

 abilities required in A-scope operation. 

 Judgments of length of lines, of small separa- 

 tions between triangular areas, and of varia- 

 tions in hnear extent have to be made in a 

 limited time. The psychological processes 

 involved are a rather different combination 

 than is required in the usual visual experi- 

 ment and have never been intensively 

 analyzed. 



Research on Visibility: Intensity- 

 Modulated Scopes 



Electrical Parameters 



In an attempt to develop a theoretical 

 integration. Miss Payne-Scott (44), of the 

 Austrahan Council for Scientific and Indus- 

 trial Research, carried out a w^hole series of 

 experimental studies on electrical parame- 

 ters. The series covered the following: 



/-/ Bandwidth.^ With a constant pulse 

 length, the bandwidth was varied and found 

 to have an optimal value very httle in excess 

 of the reciprocal of the pulse length. This 

 agrees with the A-scope data. 



Pulse Length. Increasing pulse length im- 

 proved detection very markedly with short 

 pulses but less and less with longer pulses. 

 The effect, of course, is partly dependent on 

 the minimum spot diameter of the tube. 



Pidse Repetition Frequency. With low 

 frequencies the phosphor screen is excited 

 inteiTnittently and there are fewer echoes 

 actually returned to it. Therefore, any in- 

 crease in PRF improves the probabihty of 

 detection up to the point where the scope 

 background becomes unif onn in appearance. 

 Beyond this critical frequency, the threshold 

 signal is said to be independent of PRF. 

 The critical frequency is a function of an- 

 tenna rotation rate. The lower the rotation 

 rate, the lower the critical PRF. It is also 

 a function of beamwidth. 



Antenna Rotation Rate. If the combined 

 PRF and rotation rate were sufficient to 

 paint a uniform background, detectability 

 was essentially independent of rotation rate. 

 If the background was discontinuous and 

 "grainy," the slower rotation rates were 

 slightly advantageous, over a range of about 

 1 to 70 rpm, 



Beamwidth, The beamwidth or lobe- 

 width of the antenna largely determines the 

 angular dimension (arc length) of the ob- 

 served pip. The data showed that from 2° 

 to 12°, detection improved as the f power of 

 beamwidth. 



Video Bandwidth. In agreement with the 

 previous A-scope data, no effect of video 

 bandwidth was found. 



Many of the same variables have been 

 studied by others (see below) and most of 

 the results have been confirmed. In con- 

 structing her equation for visibiHty, Payne- 



^ Although the author designates this parame- 

 ter r-f bandwidth, she apparently means i-f 

 bandwidth. 



