Lecture 14 Flashcards
Definition of active sensors
- Active - they transmit their own microwave energy (pulses) at a particular wavelength (single frequency) for a particular duration of time - known as pulsed coherent radar
- They receive the reflected (back- scattered) signal of these transmitted energy pulses
They resolve the Earth’s surface obliquely (side-looking)
two types of imaging radar
(1) side-looking airborne radar (SLAR), and
(2) synthetic aperture radar (SAR), has high resolution refinement of the SLAR
RADAR: Radio Detection and Ranging
SLAR: Side Looking Airborne Radar
SAR: Synthetic Aperture Radar
LIDAR: Light Detection and Ranging
What do active sensors measure?
- – The returned backscatter signal (intensity)
- – The change that has occurred in the wave form as a result of interacting with the Earth’s surface (phase)
- – The slant range distance to the feature(s) resolved
Two defining characteristics of RADAR
looking from side
measuring slant range distance
Imaging from RADAR
- Radar is extremely sensitive to the surface roughness of the area being imaged
- Radar does not detect the visible color of the surface, but detects the moisture (or lack of it) and electromagnetic properties of the surface
- Radar systems record the phase and polarization (orientation of EM) of the reflected pulse
- Radar produces images with speckle due to coherent nature of the system
- Radar produces images with certain geometric distortions, such as slant range geometry, image layover and shadowing
Two types of scattering (SLAR SAR)
specular–smooth surfaces–deflected AWAY from angled sensor
diffuse–rough surfaces–some reflection returns to sensor
depends on the wavelengths
Two types of penetration in RADAR
Atomphere and surface (ground)
What can the bands do in AM, due to different penetration abilities?
- Only Ka-band (0.8-1.1 cm) radar has some cloud mapping capability
- C-, L-, and P- bands are defined as all weather
- X- band radar does not penetrate heavy precipitation
- The depth of active microwave penetration into a surface or medium is strongly dependent upon wavelength and the complex dielectric constant (
Two factors affecting radar
surface roughness and moisture content (diaelectric constant)
Why does moisture affect dielectric constant?
Because water has very high dielectric constant
Conditions for subsurface radar imaging
- Cover material must be extremely dry (<1%) moisture content
- Cover material must be fine grained (r=1/10wavelength)
- Cover material must be free of clay minerals (water-bearing minerals)
- Subsurface must be rough enough to generate backscatter
• 10% of the Earth’s surface is amenable to subsurface radar imaging
Active microwave distortions: Slant range distortion
Slant-range distortion is the compression, to varying degrees, of an image in the direction of the sensor. It results from the timing and the spherical (circular) nature of the propagation of radar waves
This type of distortion can easily be corrected by calculating the corresponding ground range
More compressed in near range = distortion
Active microwave distortions: Foreshortening
The appearance of an object or surface as compressed when seen from a particular viewpoint.
Leaning toward sensor.
It is dependent on viewing angle and surface aspect and slope
Active microwave distortions: Layover
The slope AB appears steepened and the slope BC flattened. The general effect is that the hill ABC appears to lean towards the radar
Active microwave distortions: Shadowing
The Point C is not visible to the radar, being obscured by the point B. Because of the steepness of the slope AB the corresponding points have been imaged in the wrong order
Of benefit, because can see topography.
Disadvantage–lose some information in the shadows.
