. Airborne radar. Airplanes; Guided missiles. 14-2] BASIC PRINCIPLES OF DOPPLER RADAR NAVIGATION 731 in a gimbaled and track-directed antenna) can then be obtained by adding the doppler shift obtained from the two beams. The cross-heading velocity component is obtained by taking the dif- ference between the doppler shift from the two beams. In a gimbaled antenna system, the drift angle can be obtained by servoing the antenna until equal doppler shifts are obtained from the two beams. It is clear that the measurement of along-heading and cross-heading velocity components is equivalent to the me
. Airborne radar. Airplanes; Guided missiles. 14-2] BASIC PRINCIPLES OF DOPPLER RADAR NAVIGATION 731 in a gimbaled and track-directed antenna) can then be obtained by adding the doppler shift obtained from the two beams. The cross-heading velocity component is obtained by taking the dif- ference between the doppler shift from the two beams. In a gimbaled antenna system, the drift angle can be obtained by servoing the antenna until equal doppler shifts are obtained from the two beams. It is clear that the measurement of along-heading and cross-heading velocity components is equivalent to the measurement of ground speed and drift angle, since the former two are simply components of the ground velocity vector, of which the ground speed is the magnitude and the drift angle is the angle of the vector with reference to the aircraft heading line. The hyperbolas appearing on the right side of Figs. 14-6 and 14-7 are lines of constant doppler shift on the ground ("isodops") for a velocity vector Isodops. Fig. 14-7 Three-Beam Doppler System Configuration. which is coincident with the heading line of the aircraft. By considering these lines, it is easy to see how the doppler shifts from the left and right antenna (of a fixed antenna system) will be different if the velocity vector is not coincident with the heading line, when a drift angle exists. The difference between these two doppler shifts is then proportional to the cross-heading (or drift) component of velocity. For a number of reasons it turns out that the use of two beams is not optimum for most applications, but rather the use of three or four beams, as shown schematically in Figs. 14-7 and 14-9. This configuration has been given the name Janus, after the Roman god who was said to have the facility of looking forward as well as backward. Such a Janus system has the following advantages over the one-way looking (or non-Janus) system: 1. It provides for inherent cancellation of the vertical velocity comp
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