2-11 



AEW RADAR BEAMWIDTH AS DICTATED BY PROBLEM 



89 



The updating process has another advantage. It produces an estimate of 

 target position on the next scan (see Fig. 2-23). This estimate greatly 

 facilitates the problem of maintaining the identity of a target from scan to 

 scan because it provides a better idea of where each target is going to be the 

 next time the AEW radar looks at it. The heading and velocity information 

 is used to obtain these predictions. 



Prediction must be paid for, though, and the longer the prediction time, 

 the larger the errors in the predicted position. Fig. 2-24 illustrates how the 



Error in Present 

 Position 



Errors in Future 

 Positions 



-• 8n.mi. — ^- Sn.mi. — A 



Vj = 800 f ps 

 a, - 200 fps 



2n.mi. 



Fig. 2-24 Growth of Position Error with Prediction Time. 



indeterminacy volume of the predicted position expands with the prediction 

 time. This figure was determined on the basis of the following expressions 

 for the future parallel and normal rms errors app and aNF in terms of 

 prediction time T and the present position, velocity, and heading errors. 



(2-20) 



(2-21) 



(JPF 



ctnf 



+ av'T- 



A&M^V^^^^ 



With the same target velocity and system characteristics used previously, 

 the position error expands from 2 n.mi. rms to 4.46 n.mi. rms with a pre- 

 diction time of 2 minutes. 



2-18 



AEW RADAR BEAMWIDTH AS DICTATED BY THE 

 TACTICAL PROBLEM 



On the basis of target resolution requirements (Paragraph 2-13) a value of 

 5° was selected for the fan beamwidth of the provisional AEW radar design. 

 Subsequent estimates of accuracy and information handling characteristics 

 were based on this value (Paragraphs 2-15 to 2-17). 



