alan watson - discovery sailingdiscoverysailing.com/newsletters/rya_radar_update.pdf · radar range...
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Alan Watson
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Analogue radar
Digital radar
Broadband radar
Pulse compressed radar
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Traditional pulse radar
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Pulsed Radar works by firing a short pulse of radio energy and then listening for
reflections coming back
It will only be aware of things that reflect radio waves
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Under the lid:Analogue pulse
TxSignal
processDisplay process
Single antenna used for tx/rx
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What determines performance? Beamwidth
Pulse duration
Power
Receive signal to noise
External: target characteristics, clutter etc.
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Radar range equation
PRX PTX G
2 2
(4)3 R4
PRX = Receive power which has to be above a threshold to show on the screen
PTX= transmitter powerG = antenna gain= wavelength (3cm or 10 cm for commercial radars)R = range = radar cross section of target
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Comparative performance
18 inch radome 4 foot open array
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Total energy determines target detection Energy= pulse length x pulse power
Long pulse, lots of energy but poor range discrimination
Short pulse, low energy but good range discrimination.
Energy is important factor when considering to safety.
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Under the lid: Digital step 1
TxSignal
ProcessLog amp
Display process
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Logarithmic amplifier
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Display progression Analogue, radial scan
Raster scan, first use of “digital”
Daylight viewing but limited by memory etc.
More memory, more processing
Correlation
Wakes (target history)
MARPA
Overlay
LCD screens, multicolour.
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Under the lid: Digital step 2
TxSignal
processDisplay process
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Transition to digital processing Analogue, logrithmic amplifiers.
Introduction of DSP chips.
Better S/N ratio so more sensitive
Relative level information retained
Colour palette on display can be used
Differentiates rain
Advanced signal processing “super HD”
Beam sharpening
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Coast showing in red, rain in blue, note two (red) targets in rain
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Wanted targets still visible in torrential rain
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What is it FMCW (frequency modulated continuous wave)
Very long pulse, so can be low power
Simultaneous receive and transmit
Two antennas
Solid state.
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Range calculated by measuring frequency difference between tx and rx.
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Solid state Tx
ReceiverMixer
TxAntenna
Rx antenna
Mixer produces difference between Tx and rx frequency which equates to range.
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Compared to pulse Better
Very short range performance
Range discrimination
Weight and reliability (solid state)
Worse
Won’t trigger Racons
Range performance beyond 15 miles
Performance in rain
Sensitivity to objects close to the scanner.
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Where next on broadband? Software improvements
Antenna isolation limiting factor
Increase in tx power but then receive problems
Best of both worlds will be the solid state pulse compressed radar.
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How it works Solid state low power
Transmits long swept frequency “chirp” pulse
Processed to simulate short high power pulse in the receiver.
Range resolution determined by “short” pulse
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Summing filter
Antenna
Swept frequency long pulse
Display
Long pulse effectively converted to short high power in the receiver.
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Compared to pulse. Better
Range resolution.
Short range performance
Sensitivity
Power consumption
Weight
Less sensitivity to nearby objects than FMCW
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Looking to the future Radars will go solid state, no magnetron
Emphasis will be on software not hardware.
Antenna size will still be important
Will still need training and experience.
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Horizontal and vertical beam widthsVertical
Horizontal18 inch scanner 5 degrees24 448 1.7572 1
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Range
Range depends on scanner and target heightBuoys to 5 milesShips to 12 milesCoast to 20 miles
Your radar can be detected further than you can detect
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Low RCS vessels
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