Active Sensors

1
Active Sensors

• Recall that active sensors actually emit EMR in
  certain wavelengths and then detect the returning
  signal.
• Active sensors are much more complex than passive
  sensors, both in their technology and the
  interpretation of the signal.
• LIDAR (Light Detection and Ranging) measures the
  time for a laser pulse (usually visible or near
  IR) to return to generate distance measurements
• RADAR (Radio Detection and Ranging) use pulses of
  long wave EMR in the radio spectrum ( 1 mm. 1
  m.) to illuminate the terrain to determine the
  distance and angular position of objects.

2
SLAR
SLAR Real Aperture Side-looking Airbone Radar
3
Outgoing Transmission

• The transmitter send a pulse of EMR at a specific
  wavelength and duration (pulse length).
• Wavelengths are up in the cm. range, 0.75 to 100
  cm.
• Remember that wavelength changes as a function of
  the density of the material but frequency does
  not, and c??, so engineers will refer to the
  frequency of the radar pulse and not the
  wavelength

4
RADAR Principles
Slant range (SR) is distance between transmitter
and object SR ct/2 c speed of light t time
between transmission and echo reception 2 is
there because the pulse has to go to and from the
object By calculating SR, we can then transform
the return signal into an image!
(Distance)
5
Outgoing Pulse Parameters

• Azimuth direction the direction the
  airplane/satellite is moving
• Range direction direction of radar illumination
  and is perpendicular to the azimuth direction
• Depression angle (?) the angle between the
  horizontal plane formed by azimuth and range, and
  a the direction of the EMR pulse to a point on
  the ground. The angle can vary between the
  near-range ? and far-range ?, with an average ?
  between the two.
• Incident angle (?) the angle between the EMR
  pulse and a line perpendicular to the surface.
  For a flat surface, ? 90 - ?.
• Polarization

6
Polarization

• EMR vibrates in all directions, but polarized EMR
  vibrates in only one plane.
• RADAR systems transmit and receive polarized EMR,
  typically either vertical (V) and/or horizontal
  (H).
• Polarization send/receive designations can be VV,
  HH, HV or VH.

7
Slant-Range vs. Ground-Range

• Slant-range (SR) distance is the straight-line
  distance between the transmitter and the object.
• Ground-range (GR) distance is the horizontal
  distance between the transmitter and the object.
• We can easily convert between the two

8
Spatial Resolution of SLAR

• Range resolution (across-track)
• Proportional to pulse length, shorter pulses have
  higher range resolution (but less energy is
  reflected, so signal-to-noise decreases).
• Rr range resolution (scaled to ground-range)
• tduration of transmission
• cspeed of light
• ?depression angle

9
Spatial Resolution of SLAR

• Rr varies by depression angle, so closer objects
  are harder to resolve than distant objects

10
Spatial Resolution of SLAR

• Azimuth resolution (across-track)
• Width of the terrain strip the radar beam
  illuminates
• The width is proportional to the wavelength of
  the EMR and inversely proportional to the antenna
  length
• Ra azimuth resolution
• Sslant-range distance to the point of interest
• ?wavelength of EMR
• Lantenna length
• We can also set it as a function of the height of
  the sensor and the depression angle
• Hheight of the sensor above the ground
• ?depression angle

11
Geometric Distortions in RADAR

• Foreshortening slopes angled towards the sensor
  will appear compressed vs. angles away from
  sensor.
• Higher objects are closer to the sensor (H is
  smaller), so will appear to have a smaller GR.
• Larger depression angles cause more
  foreshortening.
• Nearer objects will have more foreshortening
• Layover extreme case of foreshortening, when top
  of object is detected before bottom of object.
• Shadowing objects may block radar from hitting
  objects behind them.
• Shadowed areas are totally black (no EMR hits
  them)
• Closer objects have less potential shadowing than
  farther objects
• Shadows only occur in cross-track dimension

12
Geometric Distortions in RADAR
13
Geometric Distortions in RADAR

• Speckle EMR waves are randomly constructive and
  destructive, causing bright and dark pixels
• Process different portions of the image and
  recombine the portions ( of portions combined
  a look)
• Lose resolution with more looks

14
Synthetic Aperture RADAR (SAR)
A longer antenna is synthesized electronically
by using the same antenna but moving it. Recall
that the azimuth resolution gets better with
longer antennas in SLAR systems.
15
Synthetic Aperture RADAR (SAR)
If we include the ability to detect wavelengths,
we can see the Doppler shift (lower frequency
behind the sensor, higher ahead).
16
SAR vs. SLAR

• By using the Doppler shift, we know the azimuthal
  position of each radar return.
• The azimuthal resolution of SAR is
• SARaL/2
• The azimuthal resolution of SLAR is
• SAR resolution is independent of height and
  decreases with smaller antennas!

17
RADAR Limitations

• Why not just make a tiny antenna?
• Power that the antenna receives is a function of
• The power of the radar pulse
• The width of the pulse which spreads as a portion
  of a sphere
• The larger the sphere (the higher the sensor),
  the less power per unit area
• The area the beam illuminates which then
  reradiates back in a spherical fashion (if
  perfectly Lambertian)
• Again, the larger the sphere, the less power per
  unit area will hit the antenna
• The area of the receiving antenna

18
LIDAR Principles

• Light travels at a constant speed through air (c
  3 x 108 m/s).
• LIDAR emits a pulse of laser light (narrow
  spatial and spectral light) and record the amount
  of time it takes for the laser light to return
  (t)
• distance c t
• Several factors must be known to accurately
  determine the geographical position of each laser
  return LIDAR calculated distance, position of
  the sensor, angle of laser pulse, atmospheric
  effects on c, attitude of the platform (pitch,
  roll, heading).

19
LIDAR Principles
20
GPS

• Remember GPS (Global Positioning System) is
  not a remote sensor, rather is a tool often
  used for RS research
• GPS has one purpose to determine the geographic
  x,y,z and time of a GPS detector. Every point on
  the planet has a unique geographic location, and
  can be measured at a specific time.

21
GPS Principles

• Recall that light travels at a constant velocity
  through a medium.
• If a time signal is encoded in a light wave, and
  a receiver has a very accurate clock, the
  distance to the emitter can be determined.
• If the position of the emitter (a GPS satellite)
  is known, the receiver knows s/he is somewhere on
  the surface of a sphere given by the calculated
  distance.
• If the position along FOUR spheres (distance from
  4 different known positions) is known, the x,y
  and z position can be determined using the
  principles of triangulation.