14-4] MAJOR CHARACTERISTICS AND COMPONENTS 741 



discussed. Fig. 14-9b is a typical four-beam configuration particularly- 

 suited for certain RF-mixed Janus systems. Fig. 14-9c is a configuration 

 particularly suitable for a helicopter doppler system, since a complete 

 ±180° drift angle range is required for this application; this configuration 

 can also be applied to fixed-wing systems. 



Antennas. Antennas used for doppler radars include linear arrays 

 producing partial fan-shaped beams and a large variety of other antennas 

 producing pencil beams or near pencil beams such as those shown in Fig. 

 14-9. Among the latter are parabolas and cut parabolas, dielectric and 

 metal plate lens antennas fed by horns, and planar slotted arrays. Linear 

 and planar arrays are normally considerably thinner than equivalent lenses 

 and parabolas; they are also more adaptable to beam shaping and can be 

 designed to make the doppler calibration constant completely independent 

 of transmitter frequency. Essentially the desired beamwidth dictates the 

 aperture or cutout size of the antenna or antennas. The size of the beam- 

 width largely determines the velocity fluctuation error, the over-water bias 

 error, and hence the overall system error of the doppler navigation system. 

 One-way beamwidths chosen for modern doppler radars range from 3° to 7°, 

 the majority of navigation systems having adopted a value near 5°. 



Misalignment of the antenna can contribute a fixed (or systematic) 

 navigation error. Janus-type systems are generally much less sensitive to 

 this alignment error than non-Janus systems because of the nature of 

 Janus self-compensation. Also, since the antenna misalignment error is 

 systematic it can be "biased out" in a flight calibration procedure. This 

 applies as well to any error in antenna bore-sighting. 



Stabilization. As indicated in the discussion in the previous sections 

 all doppler radars are sensitive in some degree to the attitude (pitch and 

 roll) of the aircraft — Janus systems less so than non-Janus systems. Some 

 correction or stabilization for the pitch and roll of the aircraft must there- 

 fore be made by means of data from a vertical reference. The two ways of 

 accomplishing this are called antenna stabilization and data stabilization. 

 Also, systems using fan-shaped beam configurations require physical drift 

 angle stabilization, because of the shape of the constant-doppler hyperbolas 

 shown in Figs. 14-6 and 14-7. For the same reason if such systems are roll 

 and pitch stabilized, this stabilization is best done physically by the 

 antennas. This form of stabilization is therefore termed antenna stabiliza- 

 tion. Data stabilization., on the other hand, as the name implies, constitutes 

 pitch angle and roll angle correction of the data in some form of computer 

 and allows the use of fixed antennas, usually requiring less cutout size and 

 weight than stabilized antennas. Antenna stabilization is capable of 

 somewhat greater accuracy in some cases, particularly for drift-angle 

 determination over water. While fan-beam systems require the use of 



