Chapter 1 - Radar Fundamentals Flashcards
What form might the transmitted energy be in primary radar?
- CW
- Pulses
- Frequency modulated continuous wave
How does a secondary radar detect targets?
By means of a stimulate/response system
What are all secondary radar systems?
Pulse systems
What is the frequency range of airborne radar equipment and what is this known as?
1 GHz to 15 GHz
Centrimetric radar
In order to detect targets, and discern their range, bearing and height, what components do pulse-modulated radar units require?
- Transmitter
- Aerial system
- Sensitive receiver
- Indicator
What is pulse width (TD)?
Period that a transmitter is switched on
What is pulse recurrence period (PRP)?
Interval between the leading edges of the pulses
What is the pulse repetition frequency (PRF)?
Number of pulses occurring in one second
What does the transmitter generally consist of?
- Master timing unit
- Modulator
- Transmitter oscillator
Where are the master timing unit outputs fed to?
- Modulator to trigger the transmitter
- Indicator to synchronise the time base
What does the master timing unit do?
Produces timing pulses which recur at precise and regular time intervals and determines the PRF of the equipment
What is the purpose of the modulator?
Electronic high powered switch triggered by pulses from the master timing unit
Switches the transmitter oscillator off and on
What turns the transmitter oscillator on and off?
On - leading edge of the modulator output pulse
Off - trailing edge of the modulator output pulse
What is the minimum number of cycles which must be included in each pulse?
200
What is the relationship between average power, maximum power, PRF, and TD?
PA=PmTDPRF
What frequencies do centrimetric radar equipments operate at?
3 GHz to 30 GHz
What are the two aerial arrangements commonly used in airborne radar equipments?
- Parabolic reflector
- Flat plate antenna
How is bearing information derived in airborne radar?
Scanning the aerial using a scanning motor
What kind of remote position indicator might be used in a system which would indicate azimuth and tilt angle of the aerial during scanning?
Synchro system
What is the purpose of the duplexer or T/R switch?
Connected the transmitter to the aerial for the duration of each transmitted pulse
Connected the receiver to the aerial during the interpulse period
Why is a very sensitive super heterodyne receiver required as the receiver of radar equipment?
In order to amplify weak returns from long range targets while maintaining a good signal to noise ratio
How long does it take a reflected radar pulse to travel 1nm and back?
12.36 μs
What is the most common method by which to display a partial PPI on a CRT display?
Radial trace (originating from the bottom centre of the screen) that moves from left to right synchronous with the aerial
What are the main external limiting factors affecting radar?
- External noise
- Reflections from unwanted objects
- Target characteristics
What is CLUTTER on a radar screen?
Unwanted echoes
What does the proportion of incident energy reflected from an object depend on?
- Material
- Shape
- Size
- Distance from the aerial
What are the main factors of a radar which can be controlled by design?
- Transmitter power
- Receiver sensitivity and noise factor
- Receiver bandwidth
- Frequency of operation
- Pulse repetition frequency
- Pulse duration
If double the range is required of a radar transmitter, what effect does this have on the peak power?
Peak power has to increase by sixteen times
If the peak power is doubled, what effect does this have on the range?
1.19 times the range
How is noise factor calculated as a unitless quantity in a radar receiver?
(Signal/noise ratio at input)/(signal/noise ratio at out)
How is noise factor calculated in decibels in a radar receiver?
(Signal/noise ratio at input) - (signal/noise ratio at output)
What is the noise factor in a radar receiver always, practically speaking?
Greater than unity
How can the receiver noise be reduced as far as possible?
Keeping the bandwidth as narrow as possible
What is the relationship between bandwidth, receiver sensitivity and pulse duration?
The shorter the pulse duration, the wider the receiver bandwidth
but
A wide bandwidth receiver is a noisy receiver
Why are high frequencies used in radar?
- 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
What factors affect PRF?
- Maximum range required
- Scanning speed
- Mean power available
- Displayed target definition
What is second trace return?
When a target is outside the maximum required range and the first pulse interval, the echo will show up during the second pulse interval
How is second trace return overcome?
Pulse spacing is made much longer than the minimum value of 1075 μs
What is the relationship between PRF and aerial scanning speed and why?
Directly proportional
Targets may be missed because the aerial will have turned through part of its ‘scanning’ angle during the pulse interval
The pulse duration selected for a particular radar depends on what factors?
- Minimum range
- Target range discrimination
- Frequency used
- Mean power available
- Receiver bandwidth
What duration of pulses are required for short range working?
Short duration pulses
What duration of pulses are required for range discrimination?
Short duration pulses
How does the duration of the pulse affect the frequency?
Shorter the pulse, the higher the frequency needs to be to ensure at least 200 cycles in each pulse
How does the pulse duration affect the bandwidth?
The shorter the pulse duration, the greater the range of frequencies within that pulse
What is the Mark-Space ratio a useful indicator of?
Degree of asymmetry a waveform possesses
At what levels of the pulse are the decay time and rise time taken to begin and end?
10% and 90% of the amplitude of the pulse
What is the pulse duration (TD) always measured at?
50% of the amplitude of the pulse
What is the mean value of a rectangular waveform?
Average value of the waveform over one complete cycle
What is the mean value of a waveform also known as?
- Mean level
- Average value
- DC component
- DC level
