This set of Radar Multiple Choice Questions & Answers (MCQs) focuses on “Doppler Radar”.
1. What is vr in the Doppler frequency shift equation, fd = 2frvr/c?
a) Radar velocity
b) Displacement
c) Velocity
d) Radial velocity
View Answer
Explanation: vr is the radial velocity or the rate of change of range with time in the Doppler frequency shift equation, fd = 2frvr/c. The angle between the target’s velocity vector and the radar line of sight to the target is θ, then vr = v cos θ, where v is the speed or magnitude of the vector velocity.
2. Which of the following equation is correct when the Doppler frequency is in hertz, the radial velocity in knots and the radar wavelength in meters in the Doppler frequency shift equation?
a) fd (Hz) = 2vr (kt) / λ (m)
b) fd (Hz) = 1.03vr (kt) / λ (km)
c) fd (Hz) = 1.03vr (kt) / λ (m)
d) fd (Hz) = 2vr (kt) / λ (cm)
View Answer
Explanation: If the Doppler frequency is in hertz, the radial velocity in knots and the radar wavelength in meters in the Doppler frequency shift equation, the equation can be written as fd(Hz) = 1.03vr(kt) / λ(m). The Doppler frequency in hertz can also be approximately expressed as 3.43vrft, where ft is the radar frequency in GHz and vr is in knots.
3. A continuous wave (CW) radar transmits while it receives.
a) True
b) False
View Answer
Explanation: Unlike a pulse radar, a CW radar transmits while it receives. Without the Doppler shift produced by the movement of the target, the weak CW echo signal would not be detected in the presence of the much stronger signal for the transmitter.
4. In a simple continuous wave Doppler radar, the negative sign applies when the distance between radar and target is decreasing.
a) True
b) False
View Answer
Explanation: In continuous wave Doppler radar, when the distance between radar and target is decreasing, then it is indicated by positive sign i.e. at that time both the quantities are added. Similarly, when the distance between a radar and a target is increasing, then it is indicated by a minus sign.
5. Which signal acts as a reference to determine that a frequency change has taken place in continuous wave Doppler radar?
a) Transmitter leakage
b) Clutter leakage
c) Mixer signal
d) Echo signal leakage
View Answer
Explanation: The transmitter leakage acts as a reference to determine that a frequency change has taken place in continuous wave Doppler radar. The detector, or mixer, multiplies the echo signal at a frequency ft ± fd with the transmitter leakage signal ft.
6. Subtraction of the echoes from two successive sweeps is accomplished in a ____________
a) A/D converter
b) Subtractor
c) Delay line canceler
d) D/A converter
View Answer
Explanation: Subtraction of the echoes from two successive sweeps is accomplished in a delay line canceler. The delay T is achieved by storing the radar output from one pulse transmission, or sweep, in a digital memory for a time equal to the pulse repetition period so that T = Tp = 1/fp.
7. What is the Doppler Effect?
a) Process to separate the radar echo and clutter using a device
b) Change of frequency of the radar echo
c) Methods to reduce the clutter signals
d) Method to increase the strength of echo signal
View Answer
Explanation: Doppler Effect is the change of frequency of the radar echo signal due to the relative velocity between the radar and the moving target. The use of the Doppler frequency shift with a pulse radar for the detection of moving targets in clutter is one of the examples.
8. Which out of the following is not an application of Doppler frequency shift?
a) Detecting moving targets in clutter echoes
b) Find the nature of the target
c) Measure synthetic aperture radar
d) To measure radial velocity
View Answer
Explanation: Finding the nature of the target is not an application of Doppler frequency shift. The important application includes detecting moving targets in the midst of large clutter, measuring radial velocity, synthetic aperture radar and inverse synthetic aperture radar for producing images of targets, and meteorological radars concerned with measuring wind shear.
9. What is the purpose of Doppler filter in a simple CW Doppler radar?
a) Passes signals with Doppler frequency within its pass band
b) Removes Doppler frequency from the signal
c) Increases the strength of the Doppler frequency
d) Increases the strength of the clutter signal
View Answer
Explanation: The Doppler filter passes signals with a Doppler frequency fd located within its pass band, but the sign of the Doppler is lost along with the direction of the target motion. It has a lower frequency cutoff to remove from the receiver output the transmitter leakage signal and clutter echoes.
10. When the received signal is heterodyned with the reference signal Aref sin 2πfdt and the difference is extracted in a pulse radar that extracts the Doppler frequency shifted echo signal, which of the following equation is suitable for the outcome?
a) Vd = Ad cos (2πfdt)
b) Vd = cos (2πfdt – 4πR0/λ)
c) Vd = Ad cos (4πR0/λ)
d) Vd = Ad cos (2πfdt – 4πR0/λ)
View Answer
Explanation: The equation can be represented as Vd = Ad cos (2πfdt – 4πR0/λ) where Ad = amplitude, fd = 2vr/ λ = Doppler frequency. For stationary targets fd = 0 and the output signal is constant. Since the sine takes on values from +1 to -1, the sign of the clutter echo amplitude can be minus as well as plus.
Sanfoundry Global Education & Learning Series – Radar.
To practice all areas of Radar, here is complete set of Multiple Choice Questions and Answers.
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