# Avionics MCQs – Doppler Radar

1 - Question

What is the primary function of the Doppler radar?
a) Determining true airspeed
b) Determining ground speed
c) Determining altitude
d) Detecting stealth aircraft
Explanation: The primary function of a Doppler radar is to determine the velocity vector of an aircraft with respect to the ground. The velocity component can be integrated into distance travelled and hence Doppler radar can be used as a primary sensor for dead reckoning.

2 - Question

The Doppler radar works by measuring the ________ of the reflected radio wave.
a) Intensity
b) Frequency
c) Doppler amplitude
d) Doppler shift
Explanation: the velocity is determined by measuring the Doppler shift of microwave transmitted from the aircraft in several narrow beams pointed toward the surface at relatively steep angles, backscattered by the surface and received by the Doppler radar receiver.

3 - Question

Which of the following is not true with respect to Doppler radar system?
a) Measures the ground velocity
b) Transmitter power is high and requires ground assistance
c) Velocity is extremely accurate
d) All weather system
Explanation: The Doppler system is self contained, it requires no ground based stations or satellite transmitters. The airborne transmitter power requirements are extremely small, which leads to low weight, size, and cost of equipment.

4 - Question

Doppler radar cannot be used in helicopters.
a) True
b) False
Explanation: Helicopters fly at a very slow speed where the conventional pitot tube system may fail. In order to measure accurate airspeed below 40 knots, other systems such as the Doppler radar is used. It is suitable for three dimensional velocity and low velocity measurement, as required for helicopter navigation.

5 - Question

What is the change in the observed frequency called?
a) Doppler shift
b) Differential frequency
c) Delta frequency
d) Delta shift
Explanation: In Doppler radar, there is a change in reflected frequency when there is relative motion between the transmitter and the receiver. This change in frequency is called the Doppler shift and is proportional to the relative speed between the transmitter and receiver.

6 - Question

The configuration in which the Doppler radar beams are both forward and backward looking is called as?
a) Janus
b) Dual
c) Redundant
d) Wide beam
Explanation: Since the three orthogonal components of velocity are of interest, a minimum of three noncoplanar beams are required to measure the three components. Since such a beam configuration has both forward and rearward looking beams it is called Janus configuration, named after the roman god who has the ability to look backward as well as forward.

7 - Question

What is the number of beams that a Doppler radar uses?
a) 4
b) 2
c) 1
d) 3