From Spectra to Product

1
From Spectra to Product
Some steps along the way from satellite
observations to useful geophysical content.
Aviris Spectra
MODIS Band 4
MODIS Aerosol Optical Depth
MODIS Cloud Fraction
2
The signal reaching any space borne sensor is a
complex mixture of surface and atmospheric
components.
One of the advantages of MODIS is its broad
spectral range. The wider the spectral range the
more information content we have when we observe
the Earth - Atmosphere system.
Lets begin simply by examining some sample
spectra of different surfaces.
3
Leaves top - Forest bottom
Pine Leaves
Spruce Leaves
Spruce Forest
Pine Forest
4
Conifer Forest Meadow
Pine Forest
Fescue and Wheatgrass
Cheatgrass
5
Rangeland
Concrete
Melting snow 25 vegetation
Melting snow 50 vegetation
6
Gaseous Absorption
Atmospheric gases - CO2, O2, and H2O absorb the
solar radiation at specific locations in the EM
spectra. In most cases the absorption
bands limit our ability to obtain useful
information There are some cases where we
can exploit the absorption bands to
obtain additional information.
7
Primary Atmospheric Windows in the Visible and
near-IR Spectra
From E.Vermote et. al. 6s Manual
8
From Spectra to Product
Some steps along the way from satellite
observations to useful geophysical content.
Aviris Spectra
MODIS Band 4
MODIS Aerosol Optical Depth
MODIS Cloud Fraction
9

• We have examined spectra in the visible to
  near IR range for a few surfaces going from
  relatively uniform to mixed surface types.
• Now lets look at one wavelength at a time for
  an entire MODIS scene.
• As the film plays notice how various features
  become more or less easy to distinguish from each
  other as we change from band to band.
• The film includes the thermal bands which we
  have
• not yet discussed.

10
(No Transcript)
11
Complications

• Two broadly grouped difficulties with determining
• Aerosols and clouds are
• Surface based problems
• 2) Satellite based problems

12
Observe the following true color images.
See how many surface issues, satellite issues or
combinations of the two you can find from the
following three images.
09 April 2004
12 June 2004
19 December 2004
13
What are some complications as we add more
aerosol?
14
In an ideal situation with no atmosphere all of
the incoming radiation would reach the surface.
A portion of the photons would be absorbed at the
surface. The remaining photons reflect back up
into space.
Measured radiance Directly depends on surface
properties
NO ATMOSPHERE
From E. Vermote et. al, 6S manual
15
The real atmosphere complicates the signal. Only
a fraction of the photons reach the sensor so
that the target seems less reflecting.
Real Atmosphere

• Photons lost due to
• Atmospheric absorption
• Scattering

From E. Vermote et. al, 6S manual
16
What does the satellite see?
1) Backscattered photons which never reach the
surface.
Signal or Noise?
and for whom?
17
Diffuse solar radiation. 2)
Scattered photons which illuminate the ground.
Signal or Noise?
3) Photons reflected by the surface and
then scattered by the atmosphere.
18
Multiple scattering events.
This is usually ignored after one or two
interactions.
19
Geometric issues of the illumination and the
measurement
Very important for surface and atmospheric signal
Solar zenith angle
Sensor zenith angle
Sensor view angle
Solar view angle