20 FXEMENTS OF AIRBORNE RADAR SYSTEMS DESIGN PROBLEM 



Various types of transmitter frequency modulations are commonly 

 employed. Pulse-width modulation and pulse-time modulation are used to 

 transmit information on a train of pulses. In Chapter 6 it will be seen 

 how range can be obtained from a continuous-wave (CW) radar by fre- 

 quency modulation of the transmitted frequency. 



Frequency modulation of the transmitted signal often occurs inadvert- 

 ently owing to the characteristics of the transmitter. The magnitude of 

 this effect must be carefully controlled by the designer. 



Phase Modulation. Phase modulation is similar to frequency 

 modulation in that the instantaneous phase angle is varied from some mean 

 value. With phase modulation by a single cosinusoid of frequency coi and 

 phase deviation <^, the transmitter output is 



E{t) = A cos (coo/ + A0 cos CO i/). (1-18) 



The difference in the arguments of the cosine functions of Equations 1-16 

 and 1-18, while not important for audio systems, is important elsewhere 

 where the waveshape must be controlled. 



Subcarriers. The foregoing discussion has shown that it is possible 

 to modulate the transmitted radar signal in four basic ways • — space, 

 amplitude, frequency, and phase. At this juncture, it is appropriate to 

 consider just why one would want to modulate the transmitted signal. 

 The purpose of these modulations is to create information subcarriers, i.e., 

 an angle information subcarrier, a range information subcarrier, etc. The 

 target information is contained in modulations of these subcarriers (and 

 also the carrier frequency) that are created by the target itself and is 

 derived upon return of the signal to the receiver by correlation with the 

 transmitted subcarriers. 



Target Modulations. In order to understand the basic processes 

 involved, it is now appropriate to investigate the modulations of the main 

 carrier and its associated subcarriers that are created by the target. First 

 of all, the amplitudes of the transmitted radar signals that are reflected 

 back to the transmitting location are vastly reduced — perhaps by a factor 

 of 10^" on a power basis. Moreover, the reflecting characteristics of the 

 target are, in general, a function of frequency. Thus, the amplitudes of 

 each carrier frequency in the reflected wave may not be modulated by 

 equal amounts. 



Additional amplitude modulations are created by characteristic time 

 variations of the target reflective characteristics. Chapter 4 will cover 

 this phenomenon in detail. It will suffice for the moment to state that this 

 effect introduces additional modulation which broadens each of the returned 



