2-17] VELOCITY AND HEADING ESTIMATES 85 



AEW aircraft and the CIC and the data-processing computer at the CIC 

 will have limited capacities for handling data. The maximum per channel 

 capacity of the data link has been specified (Fig. 2-3) as 1000 bits^ per second. 



There will be 60 objects (40 interceptors and 20 targets) in the field of 

 the AEW radar. To identify each object requires 6 bits as shown in the 

 footnote. The azimuth location of each target is determined to the nearest 

 multiple of 1.25°. This will require 9 bits per object. Range information to 

 the nearest 0.25 n.mi. from zero to 150 n.mi. requires 10 bits per object. 

 If we add these items and multiply by 60, we find that the amount of 

 information needed to specify the range and azimuth of the 60 objects on 

 a single scan of the radar is 1500 bits. In addition, the elevation of the 20 

 targets must be determined. The accuracy and quantization level of the 

 target altitude has not yet been specified. Here, we shall assume that target 

 altitude is determined to the nearest 0.25 n.mi. = 1500 ft, the same as in 

 range. To specify a target altitude from zero to 50,000 ft, then, requires 

 6 bits, and all the altitude data for 20 targets comprise 120 bits. The total 

 information load on one scan, then, is 1500 + 120 = 1620 bits. In order 

 to incorporate self-checking codes and message redundancy in the data link 

 to increase reliability, this figure should be about doubled. Thus, in a round 

 figure, the data link must transmit about 3000 bits per scan to the CIC. 



The actual information rate will, of course, depend upon the scan time. 

 It is generally desirable to make the scan time relatively short in order to 

 increase the accuracy of the heading and velocity estimates. A study in 

 Chapter 3 indicates that the cumulative detection range tends to be 

 relatively independent of scan time, although a broad optimum may exist. 

 Yet the scan time cannot be made indefinitely small, because of the 

 limitations of mechanical design and the increase in the data rate. We 

 have chosen a provisional scan time of 6 seconds for the basic AEW radar. 

 This radar then scans at a rate of 60° /sec. The information rate which the 

 data link must handle is 500 bits /sec. This figure is well within the capacity 

 of the defined data link system. 



2-17 VELOCITY AND HEADING ESTIMATES 



The position data are used in the CIC to compute estimates of target 

 heading and velocity. This may be done in a variety of ways. One of the 

 simplest can be illustrated with the aid of Fig. 2-22. 



^A "bit" represents a binary digit, i.e. either zero or one. Transformations from decimal to 

 binary are made in the following manner: the number 60, for example, may be expressed 



60 = (1 X 25) + (1 X 24) + (1 X 23) + (1 X 22) + (0 X 2^) + (0 X 20) 



In binary form, the number 60 is the six-digit number formed by the multipliers of the powers 

 of2: 



60 decimal = 111100 binary. 



