Doppler Radar Systems		5

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.

Doppler Radar Systems 5

Figure 5.2 Janus Arrays

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5		Doppler Radar Systems

Systems Radar Doppler 5

Doppler Operation

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 ground speed.

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.

Figure 5.3 Racal RNS 252 Navigation Computer Unit

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		Questions		5
Questions
1.	Doppler operates on the principle that ...... between a transmitter and receiver will
	cause the received frequency to ...... if the transmitter and receiver are moving ..... .
	a.	apparent motion, decrease, together
	b.	relative motion, decrease, apart
	c.	the distance, increase, at the same speed	5
	d.	relative motion, increase, apart
2.	Due to ‘Doppler’ effect an apparent decrease in the transmitted frequency, which is			Questions
	proportional to the transmitter’s velocity, will occur when:
	a.	the transmitter and receiver move towards each other
	b.	the transmitter moves away from the receiver
	c.	the transmitter moves towards the receiver
	d.	both transmitter and receiver move away from each other
3.	The change in frequency measured in an aircraft from a radio transmission reflected
	from the ground is used to determine:
	a.	the drift and ground speed of the aircraft
	b.	the aircraft’s track and speed
	c.	the across track wind component and heading
	d.	track error and ground speed

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5 Answers

Answers

1 2 3

b b a

Answers 5

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