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This article provides a basic overview of the principles and components of radar remote sensing, including radar wavelengths, system components, important terms, and image properties.
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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 mm • Photographic UV 0.3 to 0.4 mm Film • Visible 0.4 to 0.7 mm Small ! • Infrared (IR) 0.7 to 100 mm • Reflected (RIR) 0.7 to 3.0 mm Sunlight • Thermal (TIR) 3.0 to 14.0 mm • ? ! ? ! ? • Radio • Microwave 0.1 to 100 cmPassive • Radar 0.1 to 100 cmActive • 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
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
Radar Wavelengths & Frequencies Band Wavelength Frequency Designation (cm) (GHz) K 0.8 to 2.4 40.0 to 12.5 X 2.4 to 3.8 12.5 to 8.0 C 3.8 to 7.5 8.0 to 4.0 S 7.5 to 15.0 4.0 to 2.0 L 15.0 to 30.0 2.0 to 1.0 P 30.0 to 100.0 1.0 to 0.3
Depression & Incidence Angles • Depression angle gIncidence angle Q • Horizontal surface: g + Q = 90° Q = 90° – g
Depression & Incidence Angles • Depression angle gIncidence angle Q • Horizontal surface: g + Q = 90° Q = 90° – g– s s = 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
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 arelargerthan actual • Brightness effects • Slopes facingtowardthe radar are too bright • Slopes facing away from the radar are too dark • Radar image 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 Polarization • Primary polarization Transmittedsignal • Horizontal • Near range: Electric vector parallel to horsurfaces • Far range: Electric vector parallel tohor surfaces • Vertical • Near range: Electric vector parallel tohor surfaces • Far range: Electric vector perpendicular tohor 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 l’s • Many surfaces are rough • Insignificant feature penetration • Long l’s • Few surfaces are rough • Significant feature penetration • Still not forest cover or even grass cover ! ! ! • Depression angle • Small g’s • Relatively dark signatures • Large g’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
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 g’s
Radar Image Resolution Revisited • Principal determining characteristics • SLAR: Side Looking Airborne Radar Real aperture • Directly proportional to l • 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 l • 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
