Radar Imaging Wavelengths

Transcription

1 A Basic Introduction to Radar Remote Sensing ~~~~~~~~~~ Rev. Ronald J. Wasowski, C.S.C. Associate Professor of Environmental Science University of Portland Portland, Oregon 3 November 2015 Radar Imaging Wavelengths Remote sensing wavelength regions and bands Band name Wavelengths Notes Gamma rays < 0.03 nm X-rays 0.03 to 30 nm Ultraviolet (UV) 0.03 to 0.4 µm Photographic UV 0.3 to 0.4 µm Film Visible 0.4 to 0.7 µm Small! Infrared (IR) 0.7 to 100 µm Reflected (RIR) 0.7 to 3.0 µm Sunlight Thermal (TIR) 3.0 to 14.0 µm»?!?!? Radio Microwave 0.1 to 100 cm Passive Radar 0.1 to 100 cm Active Radio > 100 cm Passive Radar Imaging System Components Seven basic system components Pulse generator Generate a signal of specified frequency / wavelength Signal transmitter Amplify and send the outgoing signal Duplexer Two-way switch Alternate between outgoing & incoming radar pulse Radar antenna Broadcast the outgoing pulse & accept the return pulse Receiver Amplify the return pulse amplitude to an acceptable level Recorder Permanent record of the return pulse: Film or digital Image generator Conversion of return pulses into images Radar Imaging System Components Important Radar Terms Two basic types of radar imaging systems SLAR: Side-Looking Airborne Radar SAR: Synthetic Aperture Radar Azimuth direction Flight direction Look direction Perpendicular to the azimuth direction Range Near range Far range Depression angle Angle below horizontal to any feature of interest 0 : The horizon 90 : Nadir Important Radar Terms Illustrated

2 A Typical Radar Image: Columbia R. Radar Wavelengths & Frequencies Band Wavelength Frequency Designation (cm) (GHz) K 0.8 to to 12.5 X 2.4 to to 8.0 C 3.8 to to 4.0 S 7.5 to to 2.0 L 15.0 to to 1.0 P 30.0 to to 0.3 Depression & Incidence Angles Depression angle γ Incidence angle Θ Horizontal surface: γ + Θ = 90 Θ = 90 γ Depression & Incidence Angles Depression angle γ Incidence angle Θ Horizontal surface: γ + Θ = 90 Θ = 90 γ σ σ = Slope > 0 : Toward < 0 : Away Range Resolution of Radar Images Range resolution Increases from near range to far range Radar Image Azimuth Resolution Azimuth resolution Decreases from near- to far-range as beam widens

3 Radar Displacement & Layover Radar image displacement Pixel placement determined by straight-line distance Near range has more displacement than far range Slope effects Slopes facing toward the radar are smaller than actual Slopes facing away from the radar are larger than actual Brightness effects Radar image Slopes facing toward the radar are too bright Slopes facing away from the radar are too dark layover An extreme form of displacement An object s top is displaced past its bottom The object s top is closer than its bottom Radar Image Shadow EMR Polarization Radar Image Polarization Primary polarization Transmitted signal Horizontal Near range: Electric vector parallel to hor surfaces Far range: Electric vector parallel to hor surfaces Vertical Near range: Electric vector parallel to hor surfaces Far range: Electric vector perpendicular to hor surfaces Circular Secondary polarization Returned signal Horizontal HH: Non-depolarized return HV: Depolarized return Diagonal features Vertical VV: Non-depolarized return VH: Depolarized return Diagonal features Circular Radar System Properties Wavelength Short λ s Many surfaces are rough Insignificant feature penetration Long λ s Few surfaces are rough Significant feature penetration Still not forest cover or even grass cover!!! Depression angle Small γ s Relatively dark signatures Large γ s Relatively bright signatures Polarization Horizontal transmitted: Consistent with terrain Vertical transmitted Inconsistent with terrain Radar Terrain Properties Dielectric properties Electrical conductivity Dry rock / soil: 3 < Dielectric constant < 8 Dark Water: Dielectric constant = 80 Bright Geometry Micro- geometry: Surface texture = Surf. roughness Smooth Intermediate Rough Macro-geometry: Features parallel or perpendicular Specular reflectors One surface oriented nearly perpendicular to the look direction Two-sided reflectors Two perpendicular surfaces w/join line parallel to flightline Corner reflectors Three perpendicular surfaces open to the incident radar signal

4 A Typical Radar Image: Denver Shuttle Imaging Radar: San Francisco Radar Roughness: Smooth Texture L-band (23.5 cm) wavelength Radar-smooth surface: 0.0 cm < h < 1.0 cm Specular reflection Total forescatter Radar Roughness: Intermed. Texture L-band (23.5 cm) wavelength Radar-intermediate surface: 1.0 cm < h < 5.7 cm Composite specular/diffuse scattering Much forescatter Radar Roughness: Rough Texture L-band (23.5 cm) wavelength Radar-rough surface: 5.7 cm < h Diffuse scattering Uniform in all directions Return Intensity & Depression Angle A continuum Smooth surface: Approximately specular at nadir Rough surface: Approximately uniform at all γ s

5 Shuttle Imaging Radar: Los Angeles Radar Image Resolution Revisited Principal determining characteristics SLAR: Side Looking Airborne Radar Real aperture Directly proportional to λ Shorter wavelengths are better Rain may interfere Inversely proportional to antenna length Focus beam to preserve far range azimuth resolution Longer antennas are better Flexing may interfere SAR: Synthetic Aperture Radar Directly proportional to λ Shorter wavelengths are better Inversely proportional to antenna length Fake it by synthesizing a very long antenna» Use coherent radar signal Radar laser» Process Doppler shift data» Illuminate each target multiple times Live with image speckle A Typical Radar Image: Ice Floes A Typical Radar Image: Indonesia SIR L & C Radar Bands: Mt. Rainier A Typical Radar Image: Clearcutting

6 Radar Relief Map: Death Valley CA A Typical Radar Image: Taiwan Shuttle Imaging Radar: San Andreas Appalachians of Eastern Pennsylvania Part of the Appalachian Mountains Magellan Radar Mission to Venus