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11/6/15
1
Radar Remote Sensing
Introduc6on to Radar Review • Wavelengths • Sensor basics • Geometry • Polariza6on • Aperture
• BackscaEer • Speckle • Interferometry • Sensors
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What is Radar? Radio Detec6on And Ranging
What is Radar? Radio Detec6on And Ranging
• Ac6ve sensor (provides its own source of energy) – Transmits pulses and receives returns (backscaEer)
• Passive Radar (detects emiEed radar waves) – Satellite FOV must be large to detect emiEed signals
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Polariza6on of Radar
Polariza6on of Radar
TransmiEed
Received
TransmiEed Received
Horizontal Vertical
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Radar Aperture: Real vs. Synthe6c • Real aperture systems require a long antenna to focus energy to a small area on the ground – If the area illuminated is too large, reflec6ons from mul6ple targets are averaged
– Smaller area illuminated preserves features
Radar Aperture: Real vs. Synthe6c • Synthe6c aperture radar (SAR) systems allow smaller antennas while maintaining resolu6on – The area is illuminated by the sensor over a 6me interval as the system moves along a flight path
– Reflected signals are averaged – SAR is more common on satellite plaXorms
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Synthe6c Aperture Radar (SAR)
Radar Range Equa6on
PR = (PTG2λ2LTσ)/((4π)3R4)
PR = Power received PT = Power transmiEed G = Antennae gain λ = Wavelength L = Losses R = Range σ = BackscaEer coefficient
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Radar BackscaEer • BackscaEer value is dependent on:
– System variables: wavelength, polariza6on, incidence angle
– Landscape parameters: surface roughness, soil moisture, vegeta6on, and local topography
Radar BackscaEer
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Radar Speckle • Salt-‐and-‐pepper effect from mul6ple scaEering mechanisms in the return signal
• Use speckle reduc6on filtering – Common filter is an averaging technique using a moving window across the image (ex. 3 x 3 pixel)
Radar Interferometry • Using two images with different view angles, the phase difference can explain differences in height (i.e. plaXorms with two sensors, or images from different dates)
• Applica6on examples: earthquakes and glaciers
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Check Your Radar Understanding • What are some of the advantages of radar over op6cal? Disadvantages?
• What are some factors that determine the pixel value of an object in a radar image?
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MythBusters "One of the most feared weapons in a police officer's arsenal is the dreaded radar gun. By firing at passing cars with microwaves that then bounce back at different frequencies, the devices detect whether drivers are cruising over the speed limit. MythBusters Tory Belleci, Kari Byron and Scome Chapman tried out a lineup of supposed radar gun-‐dodging tricks, including jingling keys at the radar gun; hanging disco balls and CDs from rearview mirrors; installing LED light license plates; and covering hubcaps and an en6re car in aluminum foil. The team also assembled a microwave gun and aluminum foil-‐shoo6ng gun meant to scramble the police radar gadgets. As a last resort, the MythBusters even covered their test Cadillac in maEe black paint in hopes of sneaking it past the radar undetected. But as the MythBusters suspected, none of these stealth tac6cs helped them give the radar gun the slip. Let the busted myth be a lesson to lead-‐footed folks everywhere — the only surefire way to pass by police speed guns without gemng caught is to drive the speed limit."