Radar

1
Radar

• The microwave atmospheric window is nearly 100
  clear
• Much longer wavelengths cm to m scale
• Primarily an active form of remote sensing
• Energy return is dominated by surface roughness
  and measured as a function of the travel time of
  the radar pulse
• Difference between Radar and other remote sensing
  are pulse generator, duplexer and antenna
  ,duplexer controls timing of pulse release and
  reception

2
Advantages

• All time / all weather capability
• Information on surface roughness at the human
  scale
• Centimeters rather than microns
• Penetration of soil function of the dielectric
  constant
• Rule of thumb is that for dry soils, penetration
  depth (cm) 10
• For hyper-arid environments, radar can penetrate
  3-5 meters

Disadvantages

• Very costly
•   Imagery is complex and typically hard to
  interpret
•   Little to no information on composition of the
  surface materials

3
Radar Bands

• Radar pulses are sent and received in discrete
  wavelength regions (designated with letters)
• Controlled by the federal government so as not to
  interfere with commercial broadcasting and
  emergency frequencies
• Most commonly used
• Ka-band 0.8 1.1 cm (1.0 cm)
• C-band 3.8 - 7.5 cm (5.3 cm) L-band 15.0 -
  30.0 cm (23.5 cm)
• X-band 2.4 3.8 cm (3.0 cm)
• S-band 12 cm
• P-band 30.0 - 100.0 cm (68 cm)

4
Two Radar Modes

• 1. Passive same principles as emitted energy in
  the Thermal IR however, energy is a function of
  the surface dielectric constant as well as the
  temperature the dielectric constant is greater
  for metals and soils with higher moisture content
   
• 2. Active most common form of radar remote
  sensing. 90 of all data collected
• known as SLAR (side-looking airborne radar) SLAR
  can be either real aperture radar (RAR) or
  synthetic aperture radar (SAR). Active remote
  sensing controls the source as well as the data
  collection
• Energy is transmitted and received by an antenna
  looking off at an angle to
• the surface typically mounted to the side of a
  planes fuselage for airborne systems
• Side-looking geometry affects how the signal
  interacts with the surface
• also causes unique geometric distortions that
  must be corrected
• Nadir-viewing radar systems are known as radar
  altimeters used for mapping topography

5
Terminology

•  
• 1. Ground range distance away from the nadir
  point (perpendicular to the flight direction)
• 2. Slant range distance along the beam path
• 3. Azimuth distance along the flight direction
• 4. Look angle angle from the vertical to the
  beam
• 5. Depression angle complement to the look angle
  
• 6. Swath width illuminated surface on the
  ground
• 7. Pulse duration time of the pulse

6
Radar in Operation

• Beam pulse transmitted to surface illuminates a
  narrow strip of land
• Returned energy (backscatter) is received by the
  antenna and the timing logged
• Energy only returned if the surface is rough
  compared to the wavelength
• Corner reflector is an object on the surface
  with a certain geometry with respect to the
  incident energy whereby all the energy is
  returned to the antenna
• Near-range is received first (shorter travel
  time) then the far-range  
• All backscatter within any given zone of the
  swath width perpendicular to the azimuth
  direction is received at the same time. there is
  no way to resolve features within this strip
  (azimuth resolution)

7
Radar Resolution

• The ability of the radar return to distinguish
  between two objects in the range direction only
  can happen if the received pulse from the object
  closest to the antenna ends before the returned
  pulse of the far-range object begins can be
  defined in terms of the pulse duration and the
  ground range distance
• Shorter pulse duration and smaller depression
  angles result in better range resolution  
• Common pulse duration 0.05 - 0.3 µseconds,
• Small depression angles produce large radar
  shadows
• Short pulse durations result in less return and
  more noise

R ct/2 cos 0  Where C speed of light T
pulse duration 0depression angle  
8
Azimuth Resolution

• Distance parallel to the azimuth (flight)
  direction
• Azimuth resolution equals the swath width
• Best resolution achieved with commercial systems
  15-60m

Ra GR B Where GR ground range B antenna
beamwidth
9
Synthetic Aperture Radar (SAR)

• way around the RAR limitation by using the
  motion of the plane (artificially enlarge the
  antenna length)
• Uses the principle of Doppler shift to track the
  motion of objects in the azimuth direction
  through successive pulses
• Objects in the near range are observed for
  shorter times than those in the far-range
• Synthesized beam is much narrower than the
  original swath width azimuth resolution can be
  decreased to 5-20m

10
Radar Polarization

• SLAR systems can commonly transmit and receive in
  different polarization planes (horizontal and
  vertical)
• results in image designations such as
• C-band horizontal send, vertical receive
• P-band horizontal send, horizontal receive
• Interaction with surface features can depolarize
  the beam
• The physical process of depolarization is not
  always well understood
• Horizontal send and receive is the strongest
  most objects on the surface have a vertical
  orientation therefore they scatter back most of
  the energy
• Depending on the surface properties of the
  surface under study, vertical send/receive may
  be important

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