Lec 13 - Introduction to Radar Remote Sensing Flashcards
Uses natural energy, either reflected sunlight or emitted thermal or microwave radiation.
Passive Remote Sensing
Sensor creates its own energy transmitted toward Earth and interacts with atmosphere and/or surface, then reflects back toward sensor (backscatter)
Active Remote Sensing
Examples of Active Sensors (2)
(1) SAR - Synthetic Aperture Radar
(2) LIDAR - Light Detection And Ranging
Widely used Active Remote Sensing System (3)
(1) RADAR
(2) LIDAR
(3) SONAR
It has long-wavelength microwaves (1-100cm), recording the amount of energy back-scattered from the terrain
RADAR
It has short-wavelength laser light (e.g., 0.90 μm), recording the light back-scattered from the terrain or atmosphere
LIDAR
Sound waves pass through a water column, recording the amount of energy back-scattered from the water column or the bottom
SONAR
RADAR
The acronym for Radio Detection And Ranging
It consists of a transmitter, a receiver, an antenna, and an electronics system to process and record the data.
RADAR
Primary Functions of RADAR (3)
(1) transmits microwave (radio)
(2) signals towards a scene
receives the portion of the transmitted energy backscattered from the scene
(3) observes the strength (detection) and the time delay (ranging) of the return signals.
It provides its own energy source (1-100cm) and therefore, can operate both day or night, through cloud cover and partially penetrate canopy, soil and snow.
RADAR
Advantages of RADAR (5)
(1) Controllable source of illumination (own source)
(2) All weather, day or night
(3) Sensitive to water content and surface roughness: ice, ocean waves, soil moisture,
vegetation mass, man-made objects (buildings),
geological structure
(4) Atmosphere is essentially transparent at radar
wavelengths (longer than ~2 cm)
(5) Penetrates clouds, vegetation, dry soil, dry snow
Disadvantages of RADAR (3)
(1) Penetrates clouds, vegetation, dry soil, dry snow
(2) Sensitive to water content, surface roughness
(3) Sensitive to polarization and frequency
What RADAR measures (4)
(1) Radar has side-viewing geometry
(2) Platform illuminates terrain and receives scattered signal
(3) Characteristics of radar signal are carefully controlled
(4) Compare transmitted with scattered signal and deduce target characteristics
Types of Scattering from a Tree Stand (3)
(1) Surface scattering from the top of the canopy
(2) Volume scattering
(3) Surface and volume scattering from the ground
Observations for Forests (4)
(1) C-band (cm-tens of cm) – low penetration depth, leaves/needles/ wigs
(2) L-band – leaves / branches
(3) P-band – can propagate through canopy to branches, trunk and ground
(4) C-band quickly saturates (even at relatively low biomass, it only sees canopy); P-band maintains sensitivity to higher biomass as it “sees” trunks, branches, etc.
(1) the direction in which the terrain is illuminated is the range or look direction
(2) the terrain illuminated nearest
the aircraft is the near-range
(3) the farthest point of terrain
illuminated is the far-range
Aircraft or satellite travels in this straight line.
The azimuth direction
This edge is closest to NADIR (the points directly below the radar)
Near range edge
This edge is farthest to RADAR
Far range edge
The across-track dimension
Range
The along-track dimension
Azimuth
In a radar system, this is defined for both the range and
azimuth directions.
Resolution
This is dependent on the length of the processed pulse; shorter pulses result in “higher” resolution.
Range resolution
For RAR, this is determined by the angular beam width of the terrain strip illuminated by the radar beam.
Azimuth resolution
Resolving two objects
They must be separated in the azimuth direction by a distance greater than the beam width on the ground.
This uses signal processing to refine the azimuth resolution to be shorter than the antenna length.
SAR
This can be achieved by using a shorter pulse length.
Finer range resolution
This can be achieved by increasing the antenna length
(limited).
Finer azimuth resolution
Increasing antenna length (2)
(1) airborne radars: one to two meters
(2) satellites: 10 to 15 meters
To overcome this size limitation, the forward motion of the platform and special recording and processing of the backscattered echoes are used to simulate a very long antenna and thus increase this.
Azimuth resolution
It refers to the orientation of the electric field.
Polarization
How RADARs transmit microwave radiation (2)
(1) horizontally polarized (H)
(2) vertically polarized (V).
Horizontally or vertically polarized backscattered energy (4)
(1) HH - for horizontal transmit and horizontal receive,
(2) VV - for vertical transmit and vertical receive,
(3) HV - for horizontal transmit and vertical receive, and
(4) VH - for vertical transmit and horizontal receive.
Choice of Polarization (3)
(1) Basic or operational SARs usually have only one polarization for economy
(2) Research systems tend to have multiple polarizations
(3) Multiple polarizations to help distinguish the physical structure of the scattering surfaces
Vertically polarized light is reflected from the surface. At this view angle, horizontally polarized light is not, so horizontal filter allows us to see the bottom
Polarization with visible light
