742 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



antenna stabilization, pencil beam systems may use either antenna stabili- 

 zation or data stabilization. 



Receivers. Doppler radar receivers can be classed into the two 

 categories o{ crystal video (zero frequency IF) and the intermediate frequency 

 (IF). There are further differences, determined largely by whether the 

 system is CW or pulse, non-Janus, RF Janus, IF Janus, or AF Janus, as 

 discussed later in this paragraph. 



Crystal video receivers heterodyne a portion of the transmitted energy 

 directly with that of the received energy to obtain an audio doppler 

 component at the crystal of the receiver. While this system is by far the 

 simplest and also eliminates one of the carrier leakage problems, it suffers 

 from the poor noise characteristics of crystals at low audio-frequencies 

 (i.e. low velocities) resulting in an inherent reduction of signal-to-noise ratio 

 by 15-20 db. Intermediate frequency systems require the necessary local 

 oscillators, mixers, and filters, but exhibit a 15-20 db signal-to-noise ratio 

 improvement over crystal video receivers. 



In incoherent pulse systems the local oscillator source is usually a 

 conventional low-power klystron. In CW, coherent pulse, and FM-CW 

 systems the local oscillator power is usually derived directly from the 

 transmitter or a continuously-running driver (in the pulse case) by means 

 of a so-called side-step local oscillator crystal mixer. The latter method 

 cannot be used in the incoherent transmitter case, since local oscillator 

 energy would then not be present during the receiving period. 



Frequency Trackers. As mentioned earlier, the function of the 

 frequency tracker is to determine the center (of area) of the noiselike 

 frequency spectrum obtained from the ground, which in turn results from 

 the random nature of the radar scattering phenomenon. 



The generic forms of the frequency tracker are the axis-crossing counter 

 and the closed-loop frequency filter ox frequency discriminator. 



Practically all modern doppler radars use some form of closed-loop 

 frequency discriminator as the frequency tracking device. Such a frequency 

 tracker can further assume different forms, depending on whether the 

 tracking operation is done at audio (d-c) or at some intermediate frequency 

 and whether or not the frequency discriminator employs a sweeping 

 operation, a comparison of two nearby filters, some form of autocorrelation 

 technique, or a combination of these methods. Practice appears to show 

 that there is little difference in the performance of any of the existing 

 frequency trackers either as regards accuracy or signal sensitivity, although 

 conclusive comparisons have not been made to date. The doppler signal is 

 usually fed to one or more modulators which modulate it with a signal from 

 a variable frequency oscillator (tracking oscillator) and feed the output to 

 the device which performs the discriminator function. The output from the 



