Remote Sensing How we know what we know A Brief Tour

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Remote SensingHow we know what we knowA Brief
Tour
Dr. Erik Richard Dr. Jerald Harder LASP
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Remote Sensing

• The measurement of physical variables (usually
  light or sound) from outside of a medium to infer
  properties (other physical variables) of the
  medium.
• Electro-magnetic radiation which is reflected or
  emitted from (or absorbed by) an object is the
  usual source of remote sensing data. However any
  media such as gravity or magnetic fields can be
  utilized in remote sensing.

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Measurement Fundamentals

• Key Instrument Components
• Sensing device, or sensor
• Transducer
• Translates a sensed quantity (i.e. photons,
  acoustic waves, etc.) into a measurable quantity
  (e.g. voltage, current, displacement etc.)
• Readout device

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Everyday example Digital camera
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Functional Classes of Sensors
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Element of optical sensors characteristics
Sensor
Spectral bandwidth (?) Resolution (??) Out of
band rejection Polarization sensitivity Scattered
light
Detection accuracy Signal to noise Dynamic
range Quantization level Flat fielding Linearity
of sensitivity Noise equivalent power
Field of view Instan. Field of view Spectral band
registration Alignments MTFs Optical distortion
Spectral Characteristics
Radiometric Characteristics
Geometric Characteristics
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Resolving Power
Na spectral lines
Na D-lines
D1589.6 nm D2589.0 nm
Instrument Detector
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Schematic Wave of Radiation
Electromagnetic (EM) energy at a particular
wavelength l (in vacuum) has an associated
frequency f and photon energy E. Thus, the EM
spectrum may be expressed equally well in terms
of any of these three quantities
Visible Spectrum
0.4
0.5
0.6
0.7
Wavelength (µm)
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The electromagnetic spectrum

• Remote sensing uses the radiant energy that is
  reflected and emitted from Earth at various
  wavelengths of the electromagnetic spectrum
• Our eyes are only sensitive to the visible
  light portion of the EM spectrum
• Why do we use nonvisible wavelengths?

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Passive or Active?

• Passive sensor
• energy leading to radiation received comes from
  an external source
• e.g., direct Sun, reflected Sun, thermal emission
  etc.
• Active sensor
• Energy generated from within the sensor system,
  beamed outward, and the fraction returned is
  measured.
• e.g. laser LIDAR, microwaves, RADAR, SONAR, etc.

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Operational Classes of Sensors
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Scanning or Non-scanning?

• Scanning mode
• Motion across the scene over a time interval
  (think of your video recorder)
• Non-scanning
• Holding the sensor fixed on the scene or target
  of interest as it is sensed in a brief moment
  (think of your digital camera)

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Scanning Types
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Multi or Hyper-spectral?

• Multidimensional data cube
• Spatial information
• Spectral information
• Full spectrum
• Hyperspectral
• Partial spectrum
• Multispectral

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EM derived information
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Spectral Reflectance

• Spectral reflectance is assumed to be different
  with respect to the type of land cover. This is
  the principle that in many cases allows the
  identification of land covers with remote sensing
  by observing the spectral reflectance (or
  spectral radiance) from a distance far removed
  from the surface.

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Spectral Reflectance

• Shown below are three curves of spectral
  reflectance for typical land covers vegetation,
  soil and water. As seen in the figure, vegetation
  has a very high reflectance in the near infrared
  region, though there are three low minima due to
  absorption. Soil has rather higher values for
  almost all spectral regions. Water has almost no
  reflectance in the infrared region.

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Earths Albedo

• Albedo is defined as the reflectance using the
  incident light source from the Sun

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MODIS

• MODIS MODerate-resolution Imaging
  Spectroradiometer
• NASA Terra Aqua satellites
• Launched 1999, 2002
• 705 km polar orbits, descending (1030 am)
  ascending (130 pm)
• Sensor Characteristics
• 36 spectral bands ranging from 0.41 to 14.385 ?m
• Cross-track scan mirror with 2330 km swath width
• Spatial resolutions
• 250 m (bands 1-2)
• 500 m (bands 3-7)
• 1000 m (bands 8-36)
• 2 reflectance calibration accuracy
• movie

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Black Body Radiation

• An object radiates unique spectral radiant flux
  depending on the temperature and emissivity of
  the object. This radiation is called thermal
  radiation because it mainly depends on
  temperature. Thermal radiation can be expressed
  in terms of black body theory.
• Black body radiation is defined as thermal
  radiation of a black body, and can be given by
  Planck's law as a function of temperature T and
  wavelength

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Blackbody Radiation Curves
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The Suns spectrum
Radiometric definitions Irradiance Radiant
power incident per unit area upon a surface
(W/m2) Spectral Irradiance Irradiance per unit
wavelength interval (W/m2/nm)
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The Suns spectrum
with Planck distributions at different
temperatures
M. Planck
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Black body radiation

• Planck distributions

2 key points
Hot objects emit A LOT more radiation than cool
objects
I (W/m2) ????x T4
The hotter the object, the shorter the peak
wavelength
T x ?max constant
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Spectral Characteristics of Energy Sourcesand
Sensing Systems
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Emissivity

• In remote sensing, a correction for emissivity
  should be made because normal observed objects
  are not black bodies. Emissivity can be defined
  by the following formula-

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Atmospheric Absorption in the WavelengthRange
from 1 to 15 ?m
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Atmospheric Observation Modes
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Transmittance of the Atmosphere

• Transmission of solar radiation through the
  atmosphere is affected by
• Absorption
• Scattering
• The reduction of radiation intensity is called
  extinction (expressed as extinction coefficient,
  ?ext)

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Optical thickness

• The optical thickness of the atmosphere (?t) is
  the integrated value ?ext with altitude

Total attenuation in a vertical path from the top
of the atmosphere down to the surface
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Atmospheric absorption of solar radiation
99 penetrates to the troposphere
Altitude (km)
lt 2 RE
stratosphere
troposphere
Altitude contour for attenuation by a factor of
1/e
I(km) 37 x Io
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Global Ozone Monitoring

• The Total Ozone Mapping Spectrometer (TOMS)
  samples backscatter UV at six wavelengths and
  provides a contiguous mapping of total column
  ozone.

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Composition of atmospheric transmission
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Atmospheric Scattering

• Factors influencing atmospheric transmittance
• Atmospheric molecules (size ltlt ?)
• CO2, O3, N2, etc.
• Aerosols (size gt?)
• Water drops (fog haze), smog, dust, etc.

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Scattering

• Rayleigh scattering
• Scattering by atmospheric molecules with size ltlt
  ?
• Scattering coefficient ?s

The strong wavelength dependence of the
scattering (?-4) means that blue light is
scattered much more than red light.
Scattering by aerosols with larger size than the
wavelength is called Mie scattering (think of a
movie projector with dust)
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Radiometry

• Radiant energy
• Energy carried by EM radiation (J)
• Radiant flux
• Radiant energy transmitted per unit time (W)
• Radiant intensity
• Radiant flux from a point source per unit solid
  angle in a radial direction (W sr-1)

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Radiometry cont

• Irradiance
• Radiant flux incident upon a surface per unit
  area (Wm-2)
• Radiant emittance
• Radiant flux radiated from a surface per unit
  area (Wm-2)
• Radiance
• Radiant intensity per unit projected area in a
  radial direction (Wm-2sr-1)

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Understanding the Earths Energy Budget
Solar radiation is the Earths only incoming
energy source. The balance between the Earths
incoming and outgoing energy controls daily
weather as well as longterm weather patterns
(i.e. climate). Since we are dealing only with
electromagnetic radiation as a heat transfer
mechanism, we can start by applying the basic
laws of radiation physics to begin to understand
the Earth-Sun system and the Earths energy budget
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Radiation Balance
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Radiation Balance
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Radiation Balance
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Earths Energy Balance
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So, just how bright is the Sun?
If T 5780 K _at_ Suns surface
Then the Suns emission from the photosphere is
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It is ridiculous to try to measure variations in
a constant - Dove Maury (ca.
1890) famous oceanographers
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SORCE
Solar Radiation and Climate Experiment
http//lasp.colorado.edu/sorce/
A Mission of Solar Irradiance for Climate Research
Launched January 25, 2003
Daily measurements of Total Solar Irradiance
(TSI) Solar Spectral Irradiance (SSI) 0.1
nm-27nm 115 - 2400 nm
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Total Irradiance Monitor (TIM)
Detector Head Board
Heat Sink
Vacuum Door
Shutter
Vacuum Shell
Light Baffles
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1360 W/m2
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30 year TSI record from space
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?T of 1.5 C on Sun
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Clouds and the Earths RadiantEnergy System
(CERES)

• NASA, TRMM, Terra Aqua
• launches 1997, 1999, 2002
• 350 km orbit (35 inclination), 705 km polar
  orbits, descending (1030 a.m.) ascending (130
  p.m.)
• Sensor Characteristics
• 3 spectral bands
• Shortwave (0.3-5.0 µm)
• Window (8-12 µm)
• Total (0.3-gt200 µm)
• Spatial resolution
• 20 km
• 78 cross-track scan and 360 azimuth biaxial
  scan
• 0.5 calibration accuracy
• onboard blackbodies solar diffuser

CERES Swath Movie
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CERES Results

• Longwave (thermal) radiation
• Longwave (thermal) simultaneous Shortwave
  (reflected) radiation

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If the Sun had no magnetic field it would be as
boring as most astronomers seem to believe it
is - R. Leighton Astrophysicist,
CalTech
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The Suns magnetism is ultimately responsible for
all manifestations of solar activity
Erupting prominences
CMEs
Sunspots
Coronal loops
Flares
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The Suns spectrum
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Magnetic Fields and Sunspots
P. Zeeman
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The formation of sunspots
Animation
Hale provided the first proof that sunspots are
the seats of strong magnetic fields
TRACE image
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The Suns Magnetic Cycle
Hales polarity Law (1919)
Well-organized large scale magnetic
field Changes polarity approximately every 11
years (22 year magnetic cycle)
S
N
N
S
t 0
t 3 yrs
t 9 yrs
t 11 yrs
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Seeing the Suns magnetic fields
SOHO MDI Magnetograms