The doppler navigation system is so named because it utilizes the doppler shift principle. The doppler shift is the difference in frequency which occurs between a radar signal emitted from an aircraft radar antenna and the signal returned to the aircraft.

If the signal is sent forward from an aircraft in flight, the returning signal will be at a higher frequency than the signal emitted. The difference in the frequencies makes it possible to measure speed and direction of movement of the aircraft, thus providing information which can be computed to give the exact position of the aircraft at all times with respect to a particular reference point and the selected course.

In the doppler navigation system, flight information is obtained by sending four radar beams of continuous wave, 8,800-MHz energy from the aircraft to the ground and measuring the changes in frequencies of the energy returned to the aircraft.

The change in frequency for any one beam signal is proportional to the speed of the aircraft in the direction of the beam. The radar beams are pointed forward and down at an angle of approximately 45 deg to the right and left of the center of the aircraft and rearward and down at a similar angle.

When the airplane is flying with no drift, the forward signals will be equal. The rearward signals will be equal to the forward signals, but opposite in value.

The difference between the frequencies of the forward and rearward signals will be proportional to the ground speed, hence this difference is used to compute the ground speed and display the value on the doppler indicator.

If the airplane drifts, there will be differences in the frequencies between the right and left beam signals, and these differences are translated into drift angle and displayed on the doppler indicator.

Figure 20.20 is a drawing showing how the radar beams are aimed with respect to the aircraft. The doppler indicator is shown in Fig. 20.21.

The advantage of a doppler system is that it is completely contained in the aircraft and requires no external signals. At the start of a flight, the course or courses to be flown are programmed into the system. Therefore, continuous information regarding the position of the aircraft will be displayed on the doppler indicator and the computer controller.

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