Layover Layover occurs when the incidence angle (  ) is smaller than the foreslope (  + ) i.e.,  <  +. i.e.,  <  +. This distortion cannot be corrected!

Radar Shadowing Radar shadowing can be useful for interpreting geomorphological features

Radar Backscatter Power received = Power per unit area at target x Effective scattering area of the target x Spreading loss of reradiated signal x Effective receiving area of antenna

Radar Backscatter Coefficient The efficiency the terrain to reflect the radar pulse is termed the “radar cross-section”,  The radar cross-section per unit area, (A) is called the “radar backscatter coefficient” (  ˚) and is computed as : The radar backscatter coefficient determines the percentage of electro- magnetic energy reflected back to the radar from within a radar pixel This is similar to the reflectance in optical remote sensing

Radar Backscattering

Depends on the properties of the target: –roughness –dielectric constant Depends on characteristics of the radar: –depression angle –frequency/wavelength –polarization

Electrical (E) and magnetic field (B) are orthogonal to each other Direction of each field is perpendicular to the direction of wave propagation.

PolarizationPolarization

Polarization Plane polarized light can be either –vertically polarized (E 0 is perpendicular to the plane of incidence) –horizontally polarized (E 0 is parallel to the plane of incidence) Solar radiation is unpolarized (random) but can become polarized by reflection, scattering, etc. Lasers and radars produce polarized radiation

Radar Polarization Cinder cone and basalt lava flow in north-central Arizona. Strong return in the HH polarized image and weak HV polarization indicates that the lava is not depolarizing the radar pulse (it is composed of large blocks with smooth faces)

Rayleigh Criterion for Roughness A surface is considered smooth at or below a height, h, if: [ cm ] h = the vertical relief (average height of surface irregularities) = the radar wavelength (measured in cm)  = the depression angle

Nile River Sudan SIR-C Color Composite: Red = C-band HV Green = L-band HV Blue = L-band HH Space Shuttle Color- Infrared Photograph C-band,  = 6cm L-band,  = 24cm

Radar and the Dielectric Constant Dielectric constant depends on the type of material as well as its moisture state –it is analogous to the refractive index of the material –it is primarily a function of moisture content –also depends on chemical properties such as salinity Dielectric constant is the ratio of the capacitance of a material to that of a vacuum. Also known as the “relative permittivity”

Dielectric Constant dielectric constant of liquid water is 80; dry soil is 2-4.

Radar frequency and backscatter Depth of radar penetration through the vegetation canopy varies directly with

Types of Active Microwave Surface and Volume Scattering that Take Place in a Hypothetical Pine Forest Stand

Response of A Pine Forest Stand to X-, C- and L-band Microwave Energy

SIR-C/X-SAR Images of a Portion of Rondonia, Brazil, Obtained on April 10, 1994

RADAR records both the backscattered intensity as well as the phase of the wave when it interacted with the terrain Interferometric Synthetic Aperture Radar: InSAR

Waves of microwave energy can have constructive interference or destructive interference depending on how they match up constructive interference destructive interference

With SAR interferometry, radar images of the same area are acquired at two different times (“multiple pass”) or from two side-by-side synthetic aperture radars (“single pass”) The interference pattern between the two phase images gives information on height and displacement It’s analogous to stereo-photogrammetry except that it uses the phase information

Geometric Relationship Between Two SAR Systems Used for Interferometry to Extract Topographic Information

Range differences produce meter-scale accuracy but phase differences produce millimeter-scale accuracy The phase shift between the two images corresponds to relative difference in range between the two radars

Shuttle Radar Topography Mission (SRTM) Used C and X-band interferometry to produce the first near-global topographic map of the Earth. February 2000 (10-day mission) Space Shuttle Endeavor: 2 radar antennas, 60 m apart Covered 80% of Earth’s surface (60 o N to 54 o S)

InSAR and velocity If you have exact repeat overpass then you can use phase shifts from one time to another to indicate ground movement This has been used to map glacier velocities, movement along faults, volcanic uplift

Lambert Glacier, Antarctica Interferometric Synthetic Aperture Radar (InSAR) uses phase difference between two images to determine glacier speed and direction Image courtesy CSA, NASA, Ohio State Univ., JPL, ASF

Radar Sensors Geosat Seasat Shuttle Imaging Radar (SIR-A, SIR-B, SIR-C) ERS-1, ERS-2 JERS-1 Radarsat JPL AirSAR TRMM SRTM