This set of Radar Multiple Choice Questions & Answers (MCQs) focuses on “Probabilities of Detection and False Alarm”.
1. Which of the following combinations are used to obtain an envelope detector?
a) Local oscillator, matched filter & threshold decision
b) IF amplifier, matched filter & video amplifier
c) IF amplifier, second detector & video amplifier
d) Mixer, IF amplifier & second detector
View Answer
Explanation: The combination of IF amplifier, second detector & video amplifier acts as an envelope detector which is used to pass pulse modulation and reject the carrier frequency i.e. the high frequency components of the signal.
2. If a square law characteristic is easier to handle mathematically, which out of the following is preferred in practice?
a) Power law
b) Inverse square law
c) Linear law
d) Lanchester’s square law
View Answer
Explanation: A linear law is preferred in practice because unlike square law characteristic which is assumed when the effect on the detection probability is relatively insensitive to the choice, linear law allows a larger dynamic range than the square law.
3. What does ψ0 stands for in the equation of probability of false alarm?
a) Bandwidth of radar
b) Mean square value
c) False alarm time
d) Probability of finding the noise voltage
View Answer
Explanation: ψ0 is used to represent mean square value of the noise voltage (mean noise power) in false alarm. Here the Gaussian pdf is used to describe the receiver noise at the input to the IF filter. So when Gaussian noise is passed through the IF filter, the pdf of the envelope R is given in the form of Rayleigh pdf.
4. Which out of the following equations correctly represent the probability of a false alarm?
a) Pfa = exp (- VT2 / 2ψ0)
b) Pfa = exp (VT2 / 2ψ0)
c) Pfa = exp (- VT / 2ψ0)
d) Pfa = exp (- VT2 / ψ0)
View Answer
Explanation: The probability of a false alarm is represented by the equation Pfa = exp (- VT2 / 2ψ0). Even though this equation is used to represent false alarm, however it won’t indicate whether or not a radar will be troubled by excessive false indications of targets which can only be done by false alarm time.
5. The average duration of a threshold crossing by noise <tk>av is approximately the reciprocal of ______
a) IF bandwidth
b) RF bandwidth
c) 2nd detector
d) Threshold
View Answer
Explanation: It is approximately the reciprocal of IF bandwidth in false alarm time where tk is the time between crossings of the threshold VT by the noise envelope because it’s the envelope of IF amplifier output which is applied to the threshold detector due to which the signal noise is able to cross it.
6. The false alarm probabilities of radars are generally quite large and bandwidth is usually small.
a) True
b) False
View Answer
Explanation: The false alarm probability of radar is generally small since a decision is made as to whether a target is present or not every 1/B seconds. The bandwidth is usually large so there are many opportunities during one second for a false alarm to occur.
7. The probability that the envelope R will exceed the threshold VT is called as ______
a) probability of indication
b) probability of false alarm
c) probability of increased bandwidth
d) probability of detection
View Answer
Explanation: The probability of detecting the signal is the probability that the envelope R will exceed the threshold VT (set by the need to achieve some specified false alarm time), and in order to evaluate the probability of detection Pd, numerical and empirical methods were used.
8. The empirical formula which is given in order to relate S/N, Pd, and Pfa is called as ________
a) Albersheim’s detection equation
b) Taylor’s detection equation
c) Goldman’s detection equation
d) Dixon’s detection equation
View Answer
Explanation: Albersheim’s detection equation was used to relate S/N, Pd and Pfa which is S/N = A + 0.12AB + 1.7B where A = ln [0.62/Pfa] and B = ln [Pd / (1-Pd)]. Albersheim’s detection equation is quite simpler to use compared to lengthy computer programs with all sorts of quantitative information.
9. It is more easy to use signal to noise power ratio S/N instead of A2/2 ψ0 in radar system analysis.
a) True
b) False
View Answer
Explanation: In radar system analysis it is more convenient to use signal to noise ratio S/N rather than A2/2 ψ0 because the probability of detection Pd can then be expressed in terms of S/N and the ratio of the threshold to noise ratio VT2 / 2ψ0.
10. During probability of detection for a sine wave in noise, why is it accurate to take the values in dB?
a) To provide greater values of Pd and lower values of Pfa
b) To provide lower values of Pd and greater values of Pfa
c) To provide greater values of Pd and greater values of Pfa
d) To provide lower values of Pd and lower values of Pfa
View Answer
Explanation: The probability of detection for a sine wave in noise as a function of the signal to noise ratio and the probability of false alarm is given in dB since its suitable for rough calculations for greater values of Pd and lower values of Pfa and this applies for a single pulse only.
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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