Doppler Radar SystemsRadio Navigation — DGCA CPL practice questions
Question 1 of 3
Doppler operates on the principle that ______ between a transmitter and receiver will cause the received frequency to ______ if the transmitter and receiver are moving ______.
All 3 questions — Doppler Radar Systems
Radio Navigation · DGCA CPL. The correct option is marked on each.
Q1. 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
- D.relative motion, increase, apart
Why: The Doppler effect occurs due to relative motion between transmitter and receiver. When they are moving apart , the received frequency decreases (wavefronts are stretched, longer apparent wavelength, lower perceived frequency). See Section 2 . — The trap in this question is option (d) — it gets "relative motion" and "apart" right but flips the frequency effect. Moving apart = frequency decrease (like ambulance siren fading). Moving together = frequency increase (like approaching siren). Internalize the analogy before the exam.
Q2. Due to 'Doppler' effect an apparent decrease in the transmitted frequency, which is 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
Why: A frequency decrease occurs when there is increasing separation — i.e. the transmitter moves away from the receiver (or vice versa). This stretches the wavefronts, reducing the perceived frequency. Note option (d) would also produce a decrease but is more complex — (b) is the cleaner, direct answer covering the fundamental case. See Section 2 . — Q1 and Q2 both test the same fundamental rule from different angles. Master this: toward = increase; away = decrease. The Doppler frequency is proportional to the component of relative velocity along the line between transmitter and receiver.
Q3. 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
Why: The Doppler system converts measured frequency shifts to: (1) ground speed — from the magnitude of the fore/aft beam frequency shifts; (2) drift — from the difference between port and starboard beam frequency shifts. Heading must be supplied separately. See Section 5 . — The precise outputs of an airborne Doppler system are: ground speed and drift . Track requires adding drift to heading (external input). Position requires ground speed + drift + heading + time. The textbook is explicit: "The equipment converts the measured values into the aircraft's speed along track (ground speed) and spee…