14-4] MAJOR CHARACTERISTICS AND COMPONENTS 743 



latter is smoothed in an integrator and used to control the frequency of the 

 tracking oscillator. The tracking oscillator can then provide the frequency 

 tracker output. Mechanically tuned discriminators have also been em- 

 ployed, although they appear to have less accuracy capability. 



The type of computation system used in conjunction with the doppler 

 radar determines the form which the output of the frequency tracker must 

 have. Most analogue computation systems require 400-cps voltages 

 proportional to the velocity components. In such cases, the frequencies 

 must be converted to shaft positions or voltages; the basic components used 

 for this purpose have been the motor-tachometer, which has a linearity 

 error proportional to its full-scale value, and, more recently, various types 

 of more accurate electronic circuits, such as the so-called "bucket counter." 

 Radars operating in conjunction with digital computers that accept pulse 

 trains whose rates are proportional to the desired velocity components do 

 not have this requirement for frequency-to-voltage conversion. 



In a well-designed system, the frequency tracker measurement error and 

 the conversion error are the two primary sources of random errors in the 

 radar. (As previously shown, the stabilization error can be made negligibly 

 small.) Typically, the frequency tracker is of the order of better than 0.05 

 per cent of the velocity, while the conversion error ranges from negligible 

 to small values for digital and bucket counter outputs to 0.1 per cent of 

 the full-scale velocity for tachometer outputs. 



Ground Speed and Drift Angle Determination. In general, the 

 ground speed or the along-heading velocity components are obtained by 

 adding the one-directional (rearward) left and right doppler components in 

 the non-Janus case, or by subtracting the forward and aft-looking doppler 

 components in the Janus case. The drift angle or drift (cross-heading) 

 velocity component is generally obtained by subtracting the left and right 

 doppler components in either the Janus or the non-Janus case. In the 

 antenna-stabilized system, however, the drift angle can be obtained by 

 servoing the antenna until the left and the right doppler components are 

 equal. Practically, the two techniques are equivalent. 



Furthermore, Janus systems can be subdivided into three categories, 

 RF Janus, IF Janus, or JF Janus, depending on whether the Janus mixing 

 process is done at radio, intermediate, or audio frequencies. All three t^-pes 

 are represented in modern doppler radars. The type used has an effect on 

 whether or not directional sense of motion is obtained (as required in 

 helicopters), on the signal-to-noise ratio obtained, on cancellation of 

 incidental FM in the transmitter, and on the dynamic characteristics of the 

 system. Generally speaking, Janus mixing after tracking results in slightly 

 improved signal-to-noise ratios at the higher altitudes, but at the same time 

 it results in some degradation in dynamic performance where fixed antennas 



