2-29] REQUIREMENTS BY MISSILE GUIDANCE CONSIDERATIONS 135 



The required detection range is found by specifying a mean lock-on time 

 and adding the range closed between the target and the interceptor during 

 this time. For example, the closure rate in a head-on attack is 2000 fps 

 (0.33 n. mi. /sec). For mean lock-on times of 6 and 12 seconds the required 

 detection ranges are therefore 12 and 14 n.mi. respectively. In each of 

 these cases, the required cumulative probability of detection is defined as 

 90 per cent.'^ 



2-29 AI RADAR REQUIREMENTS IMPOSED BY MISSILE 

 GUIDANCE CONSIDERATIONS 



The requirements dictated by missile guidance considerations can be 

 derived from Fig. 2-6 and the previous analysis of the tactical situation. 



The AI radar illuminated the target continuously during the missile flight 

 time; the missile seeker tracks the reflected signal and homes on the target 

 on a proportional navigation course. From Fig. 2-6 it is seen that if the 

 AI radar tracking accuracy is better than 0.35° rms, the AI radar will not 

 cause degradation of missile performance. The specified tracking accuracy 

 of 0.15° rms is well within these limits. However, dynamic lag errors pose 

 an additional complication. The data of Fig. 2-45 show that very rapid 

 changes in angular rate and range occur near the end of missile flight 

 (T^O). The dynamic responses of the range and angle tracking loops 

 must be sufficient to maintain AI radar range lock-on and limit the angle lag 

 error. The exact determination of the allowable lag error would require 

 a more detailed study of interrelations between the missile seeker and the 

 AI radar. However, a value of about 0.25° would represent a reasonable 

 estimate. 



The maximum range to the target for which illumination must be 

 provided is obtained on the head-on attack (4.4 n.mi.). Fig. 4-6 shows that 

 120 kw of peak pulse power is required to ensure seeker lock-on at this range 

 with a 24-inch antenna. A larger antenna would reduce the power require- 

 ment and vice versa. 



The frequency of the seeker (X band) and the type of seeker (pulse radar 

 semiactive) are major factors governing the choice of AI radar frequency 

 and type, since a separate illuminating system would have to be provided 

 if the two were different. As will be indicated in Chapter 6, the choice of a 



'^The derivation of the radar detection and lock-on requirements made no mention of 

 probability. As discussed in Paragraph 2-12, it is customary to express ranges so derived as 

 the range at which the radar should have 90 per cent cumulative probability of detection 

 (on lock-on). This assumption puts a safety factor into the analysis, since a radar which meets 

 this requirement will yield a slightly better probability of conversion than a radar which 

 always locked on at exactly 10 n.mi. This comes about because 90 per cent of the lock-ons 

 occur at ranges greater than 10 n.mi.; the resulting improvement in conversion probability 

 for these cases more than offsets the decreased conversion probability of the 10 per cent which 

 occur at ranges less than 10 n.mi. 



