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Oil reaching shore. Oil spill off NW coast of Spain IKONOS image. Radar image of oil spill off NW coast of Spain (Black areas = oil). Active Remote Sensing. Radio Detection and Ranging = Radar Light Detection and Ranging = Lidar. Passive vs . Active Remote Sensing.

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Oil spill off nw coast of spain ikonos image

Oil reaching shore

Oil spill off NW coast of Spain

IKONOS image


Oil spill off nw coast of spain ikonos image

Radar image of oil spill off NW coast of Spain

(Black areas = oil)


Active remote sensing

Active Remote Sensing

Radio Detection and Ranging = Radar

Light Detection and Ranging = Lidar


Passive vs active remote sensing

Passive vs. Active Remote Sensing

  • Passive remote sensing uses the energy from the sun

  • Active remote sensing sends out its own energy and records how much bounces back

    • Imaging Radar uses microwave wavelengths

    • Lidar uses visible wavelength (laser) or sometimes NIR


Side looking radar

Side-looking Radar

  • Most radar systems do look off to the side (off-nadir)

    • For military applications allows planes to fly over friendly territory and look into enemy territory

    • More importantly, gives us more info about surface characteristics

      • Radar is sensitive to surface roughness


Radar terminology

Radar Terminology

  • Swath width = range

  • Direction of flight = azimuth

  • Backscatter = reflectance

  • Angle of view = depression angle

  • Etc.—unique terminology


Oil spill off nw coast of spain ikonos image

Radar Geometry

Depression Angle


Radar advantages

Radar Advantages

  • Can penetrate clouds

  • Active, so can use day or night

  • Less of a radiance vs. reflectance problem since you know exactly how much energy you send out and can measure what you get back—and atmosphere not a problem

  • Can penetrate dry soil and get subsurface characteristics (e.g., archaeology)


Radar disadvantages

Radar Disadvantages

  • Developed by military, less civilian experience so far than passive remote sensing

  • Difficult to interpret—complicated causes of reflectance

  • Geometric distortions caused by side looking geometry


Oil spill off nw coast of spain ikonos image

Radar bands were originally code names assigned by the military


Oil spill off nw coast of spain ikonos image

Radar penetration increases with wavelength


Polarization

Polarization

  • Electromagnetic radiation can be polarized

    • the plane of the traveling electric fields can be all horizontal or all vertical

  • Radar uses polarization information to record info about the surface

    • Rough surfaces tend to depolarize microwave radiation

    • Buried objects (subsurface inhomogeneity) can also depolarize microwaves

From Wikipedia Commons


Polarization cont

Polarization (cont)

  • TransmitReceiveTogether

    V H VH

    H V HV

    V V VV

    H H HH


Polarization cont1

Polarization (cont.)

  • By recording an HH image and an HV image you can measure the amount of cross-polarization (radiation that changes from horizontally to vertically polarized when it interacts with a surface)

    • Highlights areas on the ground that tend to depolarize radiation (e.g., rough surfaces, structurally complex materials like plants with leaves oriented various ways)


Interpretation of radar data

Interpretation of Radar Data

  • Surface “smoothness” or “roughness” with respect to radar depends on wavelength and incident angle

    • A smooth surface reflects in one direction (specular)

    • A rough surface scatters radiation in all directions (Lambertian or diffuse)

    • Rough surfaces tend to depolarize radiation


Oil spill off nw coast of spain ikonos image

Surface Roughness

Shorter wavelength radar can resolve roughness more finely


Type of backscatter reflection

Type of Backscatter (Reflection)

  • Diffuse

    • Vegetation canopies

    • Cross polarization (use HH and HV)

  • Specular

    • Microwaves reflected away (not scattered) (e.g., oil slick)

  • Corner reflectors

    • Tree trunks and buildings

    • can make objects very “bright” or very “dark” depending on orientation


Oil spill off nw coast of spain ikonos image

Surface roughness is described as a function of wavelength and the angle of incidence of the incoming radiation


Interpreting radar data

Interpreting Radar Data

  • Longer wavelength bands (P and L bands)

    • Penetrate canopy and reflects off of standing tree trunks

    • Can detect amount of wood in a forest

    • Estimate forest biomass

  • Shorter wavelengths (C and X bands)

    • X band can detect leaves

    • C (and L) band can detect twigs


Resolving objects with radar

Resolving Objects with Radar

  • Radar resolution determined by…

    • Size of area “illuminated” by microwaves at one time

      • Depends on antenna length (longer antenna = better spatial resolution)

        • But…difficult physically for airplane or satellite to carry a very large antenna.

      • Depends on length of radar pulse (longer pulse = lower spatial resolution)


Oil spill off nw coast of spain ikonos image

Pulse length in part determines radar resolution


Real aperture vs synthetic aperture radar sar

Real Aperture vs. Synthetic Aperture Radar (SAR)

  • Real aperture radar actually uses a single antenna of a given length – resolution limited to what plane or satellite can carry.

  • Synthetic Aperture Radar (SAR) can simulate a large antenna by taking advantage of the Doppler effect

    • Doppler shift allows sensor to identify electromagnetic waves from ahead and behind the platform and therefore track an object for longer than it otherwise could, as if the antenna were longer.


Oil spill off nw coast of spain ikonos image

Synthetic Aperture Radar – Doppler Effect


Radar sensors

Radar Sensors

  • There are many imaging radar sensors available, both airborne and on satellites

    • Most aircraft use SAR

    • All satellites use SAR (to achieve reasonable spatial resolution)


Radar applications

Radar Applications

  • Forest inventory

  • Oceanography

  • Archaeology

  • Sea ice studies

  • Digital Elevation Models (DEMs)

  • Urban mapping

  • Wildfire studies


Oil spill off nw coast of spain ikonos image

Radar Image – Angkor, Cambodia. Purple spots are possible buried ruins


Oil spill off nw coast of spain ikonos image

Radar image of oil slicks


Oil spill off nw coast of spain ikonos image

Radar derived elevation with TM draped over it


Oil spill off nw coast of spain ikonos image

Kamchatka Peninsula – Shuttle Radar Topography Mission (SRTM)

(Mission generated detailed topographic data for 80% of earth’s land surface)


Lidar remote sensing

Lidar Remote Sensing

  • Like Radar but sends laser pulses instead of microwave/radio pulses

  • Can collect extremely accurate elevation data quickly (vs. ground survey)

  • Typically flown on aircraft


High resolution lidar topography

High-resolution LIDAR topography


Lidar vs dems from topo sheets

Lidarvs. DEMs from topo sheets


Oil spill off nw coast of spain ikonos image

landslides

Southern Bainbridge Island


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