2-19] FACTORS AFFECTING HEIGHT-FINDING RADAR 95 



It will be assumed that the same height-finding information rate (one 

 measurement on each target aircraft every 2 minutes) will be maintained 

 during vectoring. 



Requirements Dictated by the Surface-to-Air Missile System. 



Height-finding data can be used to direct the search and tracking system 

 associated with the surface-to-air missiles to those regions of the airspace 

 where targets are most likely. For the system of the example, such informa- 

 tion can be most useful, since the primary target for the ground-to-air 

 missile system is a missile launched from the hostile aircraft at a range of 

 about 50 n.mi. The relatively smaller size of the missile makes knowledge 

 of where to look for it most desirable. For such an operation, provision 

 must be made for the proper transfer of data within the CIC system. It is 

 not likely, however, that the requirements of this function are more severe 

 than the interceptor vectoring height-finding requirements. For the 

 purposes of the example, this will be assumed to be the case. Once again, 

 this is an area which deserves more detailed scrutiny in an actual design 

 study. 



Requirements Dictated by the Stabilization Problem. Height- 

 finding — even when the requirements are as coarse as indicated for the 

 hypothetical problem — involves measuring rather small angles. The 

 assumed system logic requires that elevation angle of the target be meas- 

 ured with respect to the horizontal plane. In addition, the height-finder 

 must be commanded to the measured space azimuth position of a particular 

 target. 



Some idea of the problem may be obtained by translating the derived 

 0.5-n.mi. rms interceptor vectoring height-finding requirement into an 

 equivalent angle for 75 n.mi. range. This angle may be expressed 



^, = ^ = 0.067 rad = 0.38° (rms). (2-22) 



This is a total error — including the accuracy of the radar, the stabiliza- 

 tion errors, mechanization errors, and quantization errors. If the latter 

 two errors are assumed negligible and if the stabilization and radar meas- 

 urement rms errors {au and dhm respectively) are assumed equal, normally 

 distributed, and independent then 



c^a™ = <r,, = 0.38/V2 = 0.27°rms. (2-23) 



Thus, to meet the height-finding requirement of 75 n.mi., the height- 

 finding system must be stabilized to within 0.27° of true vertical. This 

 accuracy must be maintained despite aircraft pitching or rolling motions. 



In addition, the azimuth beamwidth of the height-finding radar must be 

 large enough to include the uncertainty of the azimuth fan beam. It should 



