Chapter 1 - Radar Fundamentals Flashcards

1
Q

What form might the transmitted energy be in primary radar?

A
  • CW
  • Pulses
  • Frequency modulated continuous wave
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2
Q

How does a secondary radar detect targets?

A

By means of a stimulate/response system

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3
Q

What are all secondary radar systems?

A

Pulse systems

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4
Q

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

A

1 GHz to 15 GHz

Centrimetric radar

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5
Q

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

A
  • Transmitter
  • Aerial system
  • Sensitive receiver
  • Indicator
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6
Q

What is pulse width (TD)?

A

Period that a transmitter is switched on

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7
Q

What is pulse recurrence period (PRP)?

A

Interval between the leading edges of the pulses

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8
Q

What is the pulse repetition frequency (PRF)?

A

Number of pulses occurring in one second

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9
Q

What does the transmitter generally consist of?

A
  • Master timing unit
  • Modulator
  • Transmitter oscillator
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10
Q

Where are the master timing unit outputs fed to?

A
  • Modulator to trigger the transmitter
  • Indicator to synchronise the time base
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11
Q

What does the master timing unit do?

A

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

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12
Q

What is the purpose of the modulator?

A

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

Switches the transmitter oscillator off and on

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13
Q

What turns the transmitter oscillator on and off?

A

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

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14
Q

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

A

200

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15
Q

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

A

PA=PmTDPRF

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16
Q

What frequencies do centrimetric radar equipments operate at?

A

3 GHz to 30 GHz

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17
Q

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

A
  • Parabolic reflector
  • Flat plate antenna
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18
Q

How is bearing information derived in airborne radar?

A

Scanning the aerial using a scanning motor

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19
Q

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

A

Synchro system

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20
Q

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

A

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

Connected the receiver to the aerial during the interpulse period

21
Q

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

A

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

22
Q

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

A

12.36 μs

23
Q

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

A

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

24
Q

What are the main external limiting factors affecting radar?

A
  • External noise
  • Reflections from unwanted objects
  • Target characteristics
25
Q

What is CLUTTER on a radar screen?

A

Unwanted echoes

26
Q

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

A
  • Material
  • Shape
  • Size
  • Distance from the aerial
27
Q

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

A
  • Transmitter power
  • Receiver sensitivity and noise factor
  • Receiver bandwidth
  • Frequency of operation
  • Pulse repetition frequency
  • Pulse duration
28
Q

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

A

Peak power has to increase by sixteen times

29
Q

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

A

1.19 times the range

30
Q

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

A

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

31
Q

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

A

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

32
Q

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

A

Greater than unity

33
Q

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

A

Keeping the bandwidth as narrow as possible

34
Q

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

A

The shorter the pulse duration, the wider the receiver bandwidth

but

A wide bandwidth receiver is a noisy receiver

35
Q

Why are high frequencies used in radar?

A
  • 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
36
Q

What factors affect PRF?

A
  • Maximum range required
  • Scanning speed
  • Mean power available
  • Displayed target definition
37
Q

What is second trace return?

A

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

38
Q

How is second trace return overcome?

A

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

39
Q

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

A

Directly proportional

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

40
Q

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

A
  • Minimum range
  • Target range discrimination
  • Frequency used
  • Mean power available
  • Receiver bandwidth
41
Q

What duration of pulses are required for short range working?

A

Short duration pulses

42
Q

What duration of pulses are required for range discrimination?

A

Short duration pulses

43
Q

How does the duration of the pulse affect the frequency?

A

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

44
Q

How does the pulse duration affect the bandwidth?

A

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

45
Q

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

A

Degree of asymmetry a waveform possesses

46
Q

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

A

10% and 90% of the amplitude of the pulse

47
Q

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

A

50% of the amplitude of the pulse

48
Q

What is the mean value of a rectangular waveform?

A

Average value of the waveform over one complete cycle

49
Q

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

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