1-4] FUNCTIONAL CHARACTERISTICS OF RADAR SYSTEMS 7 



Angular bearing of the target is measured by utilizing a directive beam 

 like that shown in Fig. 1-1. With this arrangement a target return is 

 obtained only when the beam is pointed in the direction of the target. 

 Thus by measuring the angular position of the beam with respect to some 

 reference axis when a target return is present, a measure of relative target 

 bearing from the radar system is obtained. The accuracy of this measure- 

 ment depends to a large extent on the parameters associated with the 

 detailed design. The nature of this dependence and the means that may 

 be used to improve the accura.':y of angular measurement will be developed 

 in later portions of this book. 



Target motion relative to the tracking radar platform may be computed 

 with measured range information and the time derivatives of the measured 

 range and angle information. Analysis of the two-dimensional case dis- 

 played in Fig. 1-3 illustrates the basic principle. 



The relative velocity of the target, 

 Vtr can be represented by two com- 

 ponents — one parallel to the line-of- 

 sight, Vtrp, and the other normal to 

 the line-of-sight, Vtru- These quan- 

 tities may in turn be expressed 



Line • of • Sight 

 to Target 



Vt 



V, 



(R) = R (1-3) 



Target Velocity 

 Relative to Target 



R(f) 



(1-4) 



where R = range rate along the line- 

 of-sight and 4> = space angular rota- 

 tion of the line-of-sight. 



Range-rate information can be ob- 

 tained by differentiation of the radar 

 range measured. It can also be 

 measured directly by doppler fre- 

 quency shift as previously indicated. 



Commonly, the space angular rate 

 of the line-of-sight is measured by 

 an angular-rate gyroscope mounted 

 on the antenna of a tracking radar. 



The relative target velocity information may be utilized in several ways. 

 For example, this information coupled with a knowledge of the tactical 

 situation can provide a means for identifying targets of tactical interest. 

 In addition, the computation of the components of relative target velocity 

 makes it possible to predict the future target position relative to the radar 



Fig. 1-3 Relative Target Motion: Two- 

 Dimensional Case. 



