Radar and Navigational Aids Unit I Prepared by R.

Rajesh, PEC

UNIT 1 – QUESTION BANK

PART A

1. What is radar? List a few applications of radar.

 RADAR means Radio Detection And Ranging.
 Radar is an electromagnetic system for the detection and location of objects. It operates by
transmitting a particular type of waveform and detects the nature of the echo signal.

The major areas of radar application are described below

 Air traffic control
 Ship safety
 Remotely sensing
 Law enforcement
 military

2. Define range to target.

The range to the target is given by
c TR
R=
2
where
T R = round-trip time. It is the time it takes for the radar signal to travel to the target and back.
c = speed of light = 3 ×108 m/s

3. A radar signal takes 100 µs to travel towards and back. Find the range to the
target.
Solution:
The range to the target is given by
c TR
R=
2
where
c = speed of light = 3 ×108 m/s
Round-trip time, T R =100 μs

So the range to the target is

c T R 3× 108 ×100 ×10−6

R= = R=15,000=15 km
2 2

4. Define second-time-around echoes.

 Echoes that arrive after transmission of the next pulse are called second-time-around echoes.
 Echoes that arrive after pulse repetition period are called second-time-around echoes.

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Radar and Navigational Aids Unit I Prepared by R. Rajesh, PEC

5. Define maximum unambiguous time.

The range beyond which the target appears as second-time-around echo is called maximum
unambiguous range
cT p c
Run= =
2 2f p
where
T p= Pulse Repetition Period
f p= Pulse Repetition frequency, prf
c = speed of light = 3 ×108 m/s

The maximum distance at which the return trip to a target is completed before the next pulse is
sent by the radar is called maximum unambiguous range.

6. A radar transmits the radar pulse signal with pulse repetition frequency of 1
kHz. Calculate the maximum unambiguous time.
solution
Pulse repetition frequency, f p=1000 Hz
Maximum unambiguous range is given by
cT c 3 ×108
Run= p = = =150,000=150 km
2 2 f p 2× 1000

7. Draw the simple block diagram of pulse radar.

8. Write the expression for the simple form of range equation.

The fundamental form of the radar equation is
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Pt G Ae σ
Rmax =
[ ( 4 π )2 S min ] 4

where
Pt = Transmitter Power (Watts)
G = Maximum gain of the antenna (no unit)
Ae = Effective area of the antenna ( m2 )
σ = Radar cross-section of target ( m2 )

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Radar and Navigational Aids Unit I Prepared by R. Rajesh, PEC

Smin = Minimum detectable signal

9. Define radar cross section (RCS) of target. List the three distinct regions of
scattering behavior for the RCS of target.
The radar cross section of target is defined as

power reflected towards /unit solid angle

σ=
incident power density /4 π

The radar cross section of target represents the magnitude of the echo signal returned to the radar
by the target.

The three distinct regions of scattering behavior for the RCS of target.

 Rayleigh region: When the wavelength is large compared to the objects dimension,
scattering is said to be in the Rayleigh region.
 Optical region: When the wavelength is small compared to the objects dimension,
scattering is said to be in the optical region
 Mie or resonance region: In resonance region, the radar wavelength is comparable to
the objects dimension.

10. Define noise figure.

The noise figure F n of a receiver is defined by the equation

noise out of practical receiver N0

F n= =
noise out ideal receiver at std temp T 0 k T 0 B n Gn
where
N 0=¿ noise output from receiver
G n=¿ available gain

11. Define false alarm and missed detection.

False alarm: If the threshold level is set too low, noise might exceed it and be mistaken for a
target. This is called a false alarm.

Missed detection: If the threshold is set too high, weak echo signal may not exceed the
threshold and will not be detected. This is called missed detection.

12. Define probability of false alarm and probability of detection.

Probability of false alarm
The probability that noise will cross the threshold and be called a target when only noise is
present. The probability of a false alarm is
−V T 2
Pfa =exp ( )
2Ψ0
where
V T is threshold voltage

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Radar and Navigational Aids Unit I Prepared by R. Rajesh, PEC

Ψ 0 is the mean square value of the noise voltage.

Probability of detection
The probability of detecting the signal is the probability that the envelope R will exceed the
threshold V T .

13. Define integration. What are the types of integration?

 The process of summing all the radar echo pulses for the purpose of improving detection is
called integration.
 The main purpose of Pulse Integration in radar is to improve signal-to-noise ratio.

Types of integration
There are two types of integrations

1. predetection, or coherent integration

2. postdetection, or noncoherent integration

14. What is integration efficiency of post detection integration?

The integration efficiency for postdetection integration is defined as

( S/ N )1
Ei ( n )=
n ( S/ N )n
where
n = number of pulses integrated
( S/ N )1= single pulse SNR
( S/ N )n= SNR per pulse

15. What is integration improvement factor?

The improvement in the signal-to-noise ratio when n pulses are integrated postdetection is called
the integration improvement factor. The integration improvement factor is given by

I i ( n )=n Ei (n)
where
Ei (n) = integration efficiency for postdetection integration

PART B
1. Derive the simple form of radar range equation.
2. With neat block diagram explain the operation of pulse radar.
3. Derive the expression for range of radar in terms of noise figure and minimum detectable SNR
( SNR)min .
4. A radar operates at 10GHZ and peak power of 500KW. Its minimum receivable power is 0.1 pW.
Its antenna has effective (capture) area of 0.5sqm and radar cross section of target is 20sqm. Find
maximum range of radar.

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Radar and Navigational Aids Unit I Prepared by R. Rajesh, PEC

5. A radar operates at 10GHZ and peak power of 500KW. The receiver of the radar has noise figure
6dB and IF bandwidth of the receiver is 3MHz. The antenna has effective (capture) area of
0.5sqm and radar cross section of target is 20sqm. Find maximum range of radar.
6. Derive the expression for probability of false alarm in terms of false alarm time. Derive the
expression for probability of detection.
7. Define integration? Why integration of radar pulses is necessary? Explain the types of
integration.Give the expression for integration efficiency for post integration. Define integration
improvement factor and integration loss. Express the range equation in terms of integration
efficiency.
8. What is radar cross section of target? Explain it in detail.
9. Explain in detail about various system losses.
10. Explain about transmitted power and pulse repetition frequency