9-1] GENERAL PROBLEMS OF AUTOMATIC TRACKING 473 



of the target space vector. If the radar error signal from the target is 

 maintained near zero while the target moves, the antenna motion is that 

 of the target vector, and the required angular rate information about the 

 target is obtained from measurements of the antenna motion. Although 

 contaminated with noise, the radar error signal is a measure of the true 

 tracking error and it is held to a minimum value by automatic angular 

 tracking circuits. In a similar manner, the length of the vector between 

 the target and antenna is measured with automatic range tracking circuits 

 to an accuracy consistent with the physical equipment available, the 

 existent noise, and the rapidity with which the target position changes. 



As there are several types of radar systems that may be used to obtain 

 space error information, there are many types of tracking systems. Funda- 

 mentally, however, the radar must furnish not only the magnitude of 

 tracking error but its space direction as well. This may be obtained by 

 several different methods (conical scanning, monopulse, etc.), but basically, 

 the space direction must be used to direct mechanical actuators that move 

 the antenna about supporting pivots or gimbals in a space direction which 

 will reduce the error continuously and thereby direct the antenna toward 

 the target. The number of gimbals needed to move the antenna in space 

 is kept to a minimum to save space, to avoid excessive weight, to prevent 

 excessive structural flexure, and to minimize the number of rotary joints 

 in the radar waveguides. 



In theory the radar error signal could be separated into appropriate 

 components, modified to obtain a desired driving function, and applied 

 directly to the antenna actuators. In practice the antennas must be driven 

 by devices which amplify power, and generally power amplifiers have 

 transfer characteristics which vary considerably with environment and 

 operating conditions. To obtain constant antenna drive characteristics, a 

 feedback loop is therefore designed around the antenna actuators. This 

 may be a conventional tachometer feedback loop; or the stabilization loop 

 discussed in Chapter 8 may provide this function and thus serve a double 

 purpose. 



The angle tracking problem is further complicated in an airborne system 

 by the fact that the radar antenna is on a platform moving with respect to 

 the space reference system as discussed in Chapter 8. Fortunately, the 

 angular motion of the target vector does not have significant components 

 as high in frequency as the platform motion does, and the automatic 

 tracking loop may therefore have a much lower bandwidth than the 

 stabilization loop. If this were not the case, it would be difficult to design 

 a stabilization loop as discussed in Chapter 8 that would result in a suitable 

 system when operating with the tracking loop. As implied earlier in this 

 discussion and as shown later in this chapter, the tracking loop bandwidth 

 is limited to prevent excessive transmission of noise and to provide the 



