The Doppler principle can be used to determine the relative speed between moving objects by measuring the difference between transmitted and received frequencies; for example, police forces all over the world use a form of Doppler radar to check vehicle speeds.
A Doppler navigation system uses the Doppler principle to measure an aircraft’s ground speed and drift. The most modern systems combine the inherent accuracy of Doppler measurements with information from other navigation systems (for example: IRS, VOR/DME or GPS) in various configurations to suit customer requirements.
Using these additional navigation inputs helps to eradicate the problems associated with early Doppler Navigation Systems, such as inaccurate heading references, and degradation (or loss) of Doppler inputs when flying over large expanses of water.
The Doppler principle is utilized in many navigation systems, such as Radar, Doppler VOR and VDF.
The Doppler Principle
The Austrian physicist, Christian Doppler, predicted the Doppler Effect in connection with light waves in the 19th Century, but it also holds true for sound and radio waves: a received frequency will only be the same as the transmitted frequency when there is no relative movement between the transmitter and receiver.
A simple analogy would be a visit to the beach. Standing still in the water, the waves rolling in splash you at, for example, four waves per minute. If you walk into the sea, you are progressively reducing the space between each wave and therefore they splash you more frequently than four times per minute. The rate at which the waves are produced has not changed, but you perceive that the rate has increased. The faster you walk into the sea, towards the waves, the greater the rate at which they will strike you.
Conversely, if you walk back towards the shore, you are effectively stretching out the distance between each wave and therefore the waves will strike you less frequently.
The result is that you (as a receiver) perceive an increase in the frequency of the waves when there is relative movement towards the waves (the sea as transmitter), and a decrease in the frequency when the relative movement is away from the waves; there has been no actual change in the frequency of the waves.
The difference between the frequency you perceive the waves striking you and the actual frequency at which they roll in to shore is the ‘Doppler Shift’ or ‘Doppler Frequency’. That difference varies with the speed at which you walk into or out of the sea – the relative motion.
The same effect occurs at radio frequencies: whenever there is relative motion between a transmitter and a receiver, the receiver will perceive a Doppler frequency shift that is proportional to their relative motion.
A typical airborne Doppler installation employs a slotted waveguide antenna in which the transmitter and receiver elements are screened from each other but share the same aerial. It is arranged that an array of beams is transmitted downwards towards the earth’s surface as shown in Figure 5.1.
The diagram shows a commonly-adopted configuration: there are four beams, two pointing forward and two pointing aft. This is known as a 4-Beam Janus Array, named after the Roman God of Doorways who was reputed to be able to face both ways simultaneously.
Janus Array System
A Janus array normally comprises 3 or 4 beams. Figure 5.2, below, illustrates various ways that the beams can be configured.
The Doppler functions by continuous measurement of the frequency shift in the reflected signal caused as a result of the aircraft’s motion over the ground. The equipment converts the measured values into the aircraft’s speed along track (ground speed) and speed across track (used to determine drift).
The frequency shifts detected in a four-beam Janus array of an aircraft travelling forwards with zero drift will be equal (but opposite for fore and aft beams). In other words, the forward beams detect an upward shift in the received frequency and the aft beams detect a downward shift in the received frequency from the beams pointing aft; the magnitude of the shift will be equal but opposite. The shift in both sets of beams is proportional to the aircraft’s groundspeed.
If the aircraft is drifting left or right, then there will be a difference in the frequencies received from port and starboard beams. In a modern, fixed aerial system the differences in frequencies are electronically processed to provide a continuous indication of drift and ground speed; the information (together with a heading input) can also be provided to a navigation system that can determine the aircraft’s position.
In earlier, mechanical systems (using pitch-stabilized, rotating aerials) the difference in frequency shifts was converted to an electrical signal that actuated a motor. The motor then drove the aerial until it was aligned with aircraft track, at which stage the port and starboard frequency shifts would be equalized. A pick-off then measured the difference between the aircraft’s fore/ aft axis (representing heading) and the alignment of the port and starboard beams (track); the difference being drift.
Doppler Navigation Systems
The Doppler continuously updates the values of aircraft drift and ground speed. In early systems, the aircraft’s departure point was loaded into a navigation computer. The values of drift and ground speed, together with an input of aircraft heading, were also fed into the computer, which converted them into aircraft position. The calculated position was then displayed as latitude and longitude or as distances (in nautical miles) along and across track.
Figure 5.3 is the Control and Display Unit (CDU) for the B-52 system mentioned above.