Dr Nick Shuley
Transcript of Dr Nick Shuley
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Dr Nick Shuley
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Radar in a nutshell?
• Primarily an active electromagnetic sensor (as compared to a passive sensor ‐ radiometer).
• Suffers much less attenuation compared to optical (so much larger range).
• Uses the known propagation velocity of light (3 x 108 msec‐1)• Can use sophisticated antennas (phased arrays to generate beams and process the return signals).
• Huge number of specialized radars.
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Some radar applications• Moving Target indicator) MTI.• Fire control radar• Rain radars• Wind Radar (Doppler)• Ground Probe Radar (GPR)• Tracking Radar (Monopulse)• Terrain avoidance Radar• Police radar (and LIDAR)• Air Traffic Control radars (ATC)• Insect radar• Collision Avoidance Radar
• Synthetic aperture radar (SAR)• Inverse Synthetic Aperture Radar
(ISAR) • Surveillance radar• Over The Horizon Radar (OTH).• Bistatic Radar• Airborne Early Warning (AEW)• Slope Stability Radar• Target acquisition and identification
radars
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Early beginnings
• Need for advance warning systems after WW I• Churchill leads the way – “Cursed hellish invention and development of war from the air”.
• Watson Watt at Met. Bureau reports that aircraft disturb receivers when flying close.
• Wilkens notes that at a wavelength of 50m (6MHz)a transmitter with a simple antenna could detect an echo from aircraft at 10 miles at 20,000 feet.
• “Detection of Aircraft by Radio Methods” Watson Watt, February 1935.
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Electromagnetic Wave
Electric and Magnetic fields
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The radar range equation
Transmitter Receiver
target
Incident wave Scattered wave
R1R2
RCS = (aspect and polarization dependent)
2 2
3 44
tr
G PPR Note the 4th power !
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Electronic Warfare
ES EA or ECM EP or ECCM
ElectronicSupport
ElectronicAttack
ElectronicProtection
Receiving only
Hostile radiations are: Searched for Intercepted Located Analysed Identified
Primarily, JammingDeceptionIFFDecoys
Low Probability of Intercept (LPI)
Low sidelobesMonopulsePRF agility
Defeating Radar
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Induced Currents on a radar target
Calculated using computational electromagnetic (CEM) software designed to numerically solve Maxwell’s equations.
Single frequency illumination from the LHS
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Stealth strategy for RCS
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Conventional Aircraft RCS Stealth Aircraft RCS
.
Low RCS overall, few narrow spikes in controlled directions
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Radar cross section reduction (RCSR)
Ogive shape Wedge shape
377ohms/
One method uses RAM (radar absorbing materials)
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LPI‐ Shaping is one strategy
Radar Cross Section Reduction (SR71 was an early effort).
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PAVE PAWS RADAR(Phased Array Warning System)
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Transmit/Receive (T/R) moduleA key component of digital beamforming technology.
64.5mm
13.5mm
Includes: A power amplifierA ferrite circulator (connects Tx and Rx)LimiterLow noise pre‐amplifierDown converter (mixer)
All in Gallium Arsenide (GaAs) semiconductor technology at RF.
Transmitter Receiver feed network to the antenna elements
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Most Radars are pulsed
• RADAR – RAdio Detection And Ranging
Tx
Rx
Duplexer
Rx Protect
T
R
time In pulsed radar the transmitted & receivedsignals are separatedin time.
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Radar range ambiguities
pulse #1 pulse #2echo of pulse #1
50T km
transit time 60kmapparent range 10 km
Did the echo come from Pulse #1 or Pulse #2?
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TransmitterHigh power magnetronoscillator which is “On” forthe duration of the pulse.
resonant cavitiescathode
anodeinteractionspace
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The total Picture
The larger the particles, the stronger the reflected signal.
Many Signals can enter the receiver
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Doppler effect
2‐way Doppler
0 0
2 2 td
vdRfdt
f0
vt
R A (small) frequency due to the motion of the target (or the radar). The shift is related to the velocity of the target and may be up (for a closing target) or down for an opening target.
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Radar Signal Processing
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One of the most important radar components: Matched Filter
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Rx Filter
Echoes from closely spaced targets A,B are merged on input but separate on output
A+B
Rx Filter
comp=1/B
peak power is increased by the compression
ratio /comp =B
So pulses are resolvable if they are separated in time by comp
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Pulse Compression (Chirp)Most Radars use some form of
Chirp!
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Pulse compression by numbers
Pulse compression allows huge increases in resolution.
The SEASAT satellite remote sensor without linear chirp pulse compression would have had a range resolution of 14.8km (angle dependent).
With pulse compression by a factor of 640, the ground resolution is increased down to 20.08m (at 20 look angle).
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Monopulse tracking radarMonopulse uses four squinted beams simultaneously to measure the targets angular position. This requires a special type of feed network so that the four beams are produced using a single pulse. Both phased arrays and reflector based systems are used.
A
BD
C
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A B
D C
Target echo signal on track
Target echo off tracking axis
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Monopulse feed system
(David Jenn)
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Synthetic aperture radar (SAR)The idea here is to use the forward motion of the vehicle to generate a large aperture. A very short antenna is used (wide beam) to increase illumination duration.
actual antenna length a
La
R0
With this arrangement, the azimuth resolution can be increased to ra = a/2
beamwidth of platform antenna
just encountering target
just loosing target
Important!(independent of range/height
and )
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Photo or SAR map? The SAR map was made from a range many times that of the photo.
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Increased roughness
coherent, specular (dark)
much brighter
Monitoring Climate
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Fully Polarized radars (POL SAR)
The decision on each pixel is unsupervised and based on a Wishart classifier
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Inverse SAR (ISAR)
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A 2D image is used as a basis for identification and recognizing the bulk carrier.
Photo
ISAR image
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Differences between ISAR and optical images
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Interferometric SAR
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Uses 2 or more SAR images to generate maps of surface information or digital elevation using phases of the waves returning to the platform.
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Topological MapsNASA has released SRTM (Shuttle Radar Topography Mission) data for Australia, and New Zealand comprising 1200 data cells each comprising 1 degree of latitude and longitude. See http://www2.jpl.nasa.gov/srtm/
Resolution of the data is 1arc second (30m)
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Subsidence in the city of Bologna, Italy at the rate of about 1cm per year using D‐InSAR techniques.
Differential Interferometric SAR (D‐InSAR)
33ESA Data processing by GAMMA
Can potentially measure millimetre changes in deformation over days or years.
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Underground mapping
Radar Image
Infrared Photo
Composite image of various polarizations (different colours) at both C and L band.
Image NASA (JPL)
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Ultrawideband microwave pulses for early stages stages of breast cancer detection.
Uses an array of small antennas around the breast and each antenna emits a very short pulse.
Signal processing then used to synthesise the image.
Imaging of the human body
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GroundProbe Pty Ltd
• Highwall Mining RadarWinner of ACARPResearch Prize
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THANK YOUQuestions?