Chapter 1 - Radar Fundamentals

Question 1

What form might the transmitted energy be in primary radar?

Answer 1

• CW
• Pulses
• Frequency modulated continuous wave

Question 2

How does a secondary radar detect targets?

Answer 2

By means of a stimulate/response system

Question 3

What are all secondary radar systems?

Answer 3

Pulse systems

Question 4

What is the frequency range of airborne radar equipment and what is this known as?

Answer 4

1 GHz to 15 GHz

Centrimetric radar

Question 5

In order to detect targets, and discern their range, bearing and height, what components do pulse-modulated radar units require?

Answer 5

• Transmitter
• Aerial system
• Sensitive receiver
• Indicator

Question 6

What is pulse width (T_D)?

Answer 6

Period that a transmitter is switched on

Question 7

What is pulse recurrence period (PRP)?

Answer 7

Interval between the leading edges of the pulses

Question 8

What is the pulse repetition frequency (PRF)?

Answer 8

Number of pulses occurring in one second

Question 9

What does the transmitter generally consist of?

Answer 9

• Master timing unit
• Modulator
• Transmitter oscillator

Question 10

Where are the master timing unit outputs fed to?

Answer 10

• Modulator to trigger the transmitter
• Indicator to synchronise the time base

Question 11

What does the master timing unit do?

Answer 11

Produces timing pulses which recur at precise and regular time intervals and determines the PRF of the equipment

Question 12

What is the purpose of the modulator?

Answer 12

Electronic high powered switch triggered by pulses from the master timing unit

Switches the transmitter oscillator off and on

Question 13

What turns the transmitter oscillator on and off?

Answer 13

On - leading edge of the modulator output pulse
Off - trailing edge of the modulator output pulse

Question 14

What is the minimum number of cycles which must be included in each pulse?

Answer 14

200

Question 15

What is the relationship between average power, maximum power, PRF, and T_D?

Answer 15

P_A=P_mT_DPRF

Question 16

What frequencies do centrimetric radar equipments operate at?

Answer 16

3 GHz to 30 GHz

Question 17

What are the two aerial arrangements commonly used in airborne radar equipments?

Answer 17

• Parabolic reflector
• Flat plate antenna

Question 18

How is bearing information derived in airborne radar?

Answer 18

Scanning the aerial using a scanning motor

Question 19

What kind of remote position indicator might be used in a system which would indicate azimuth and tilt angle of the aerial during scanning?

Answer 19

Synchro system

Question 20

What is the purpose of the duplexer or T/R switch?

Answer 20

Connected the transmitter to the aerial for the duration of each transmitted pulse

Connected the receiver to the aerial during the interpulse period

Question 21

Why is a very sensitive super heterodyne receiver required as the receiver of radar equipment?

Answer 21

In order to amplify weak returns from long range targets while maintaining a good signal to noise ratio

Question 22

How long does it take a reflected radar pulse to travel 1nm and back?

Answer 22

12.36 μs

Question 23

What is the most common method by which to display a partial PPI on a CRT display?

Answer 23

Radial trace (originating from the bottom centre of the screen) that moves from left to right synchronous with the aerial

Question 24

What are the main external limiting factors affecting radar?

Answer 24

• External noise
• Reflections from unwanted objects
• Target characteristics

Question 25

What is CLUTTER on a radar screen?

Answer 25

Unwanted echoes

Question 26

What does the proportion of incident energy reflected from an object depend on?

Answer 26

• Material
• Shape
• Size
• Distance from the aerial

Question 27

What are the main factors of a radar which can be controlled by design?

Answer 27

• Transmitter power
• Receiver sensitivity and noise factor
• Receiver bandwidth
• Frequency of operation
• Pulse repetition frequency
• Pulse duration

Question 28

If double the range is required of a radar transmitter, what effect does this have on the peak power?

Answer 28

Peak power has to increase by sixteen times

Question 29

If the peak power is doubled, what effect does this have on the range?

Answer 29

1.19 times the range

Question 30

How is noise factor calculated as a unitless quantity in a radar receiver?

Answer 30

(Signal/noise ratio at input)/(signal/noise ratio at out)

Question 31

How is noise factor calculated in decibels in a radar receiver?

Answer 31

(Signal/noise ratio at input) - (signal/noise ratio at output)

Question 32

What is the noise factor in a radar receiver always, practically speaking?

Answer 32

Greater than unity

Question 33

How can the receiver noise be reduced as far as possible?

Answer 33

Keeping the bandwidth as narrow as possible

Question 34

What is the relationship between bandwidth, receiver sensitivity and pulse duration?

Answer 34

The shorter the pulse duration, the wider the receiver bandwidth

but

A wide bandwidth receiver is a noisy receiver

Question 35

Why are high frequencies used in radar?

Answer 35

• To obtain a good echo, the radar wavelength must be less than four times the size of the target
• A very narrow beam is required, which can be achieved much easier at higher frequencies, to obtain good discrimination, accurate bearing indication and adequate concentration of radiated energy
• There requires to be at least 200 cycles per pulse, so high frequencies can ensure short pulse widths

Question 36

What factors affect PRF?

Answer 36

• Maximum range required
• Scanning speed
• Mean power available
• Displayed target definition

Question 37

What is second trace return?

Answer 37

When a target is outside the maximum required range and the first pulse interval, the echo will show up during the second pulse interval

Question 38

How is second trace return overcome?

Answer 38

Pulse spacing is made much longer than the minimum value of 1075 μs

Question 39

What is the relationship between PRF and aerial scanning speed and why?

Answer 39

Directly proportional

Targets may be missed because the aerial will have turned through part of its ‘scanning’ angle during the pulse interval

Question 40

The pulse duration selected for a particular radar depends on what factors?

Answer 40

• Minimum range
• Target range discrimination
• Frequency used
• Mean power available
• Receiver bandwidth

Question 41

What duration of pulses are required for short range working?

Answer 41

Short duration pulses

Question 42

What duration of pulses are required for range discrimination?

Answer 42

Short duration pulses

Question 43

How does the duration of the pulse affect the frequency?

Answer 43

Shorter the pulse, the higher the frequency needs to be to ensure at least 200 cycles in each pulse

Question 44

How does the pulse duration affect the bandwidth?

Answer 44

The shorter the pulse duration, the greater the range of frequencies within that pulse

Question 45

What is the Mark-Space ratio a useful indicator of?

Answer 45

Degree of asymmetry a waveform possesses

Question 46

At what levels of the pulse are the decay time and rise time taken to begin and end?

Answer 46

10% and 90% of the amplitude of the pulse

Question 47

What is the pulse duration (T_D) always measured at?

Answer 47

50% of the amplitude of the pulse

Question 48

What is the mean value of a rectangular waveform?

Answer 48

Average value of the waveform over one complete cycle

Question 49

What is the mean value of a waveform also known as?

Answer 49

• Mean level
• Average value
• DC component
• DC level