Principles of Satellite Remote Sensing

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Principles of Satellite Remote Sensing
Sundar A. Christopher Department of Atmospheric
Science UAHuntsville, AL
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Definition

• Remote sensing is the
• measurement of an
• object by a device that is
• not in physical contact
• with the object.

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Types of Remote Sensing

• Passive Sensors Satellites
• Active Sensors
• Radar

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Electromagnetic energy

• The frequency of EM radiation is directly
  proportional to the speed of light and Inversely
  proportional to the wavelength
• Commonly used satellite sensors sense EM
  radiation from visible to infrared part of the
  spectrum

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Why Satellite Remote Sensing?

• Advantages Repeated reliable measurements
• Disadvantages Expensive and need expertise to
  convert measurements to geophysical values such
  as temperature.
• Numerous satellites are now in orbit

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Common types of orbits
Geostationary orbit An orbit that has the same
Earths rotational period Appears fixed above
earth Satellite on equator at 36,000km
Polar orbiting orbit fixed circular orbit above
the earth, 1000km in sun synchronous orbit with
orbital pass at about same local time each day

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Example of satellite imagery

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Shortwave and Longwave Radiation

• Shortwave radiation from the sun is reflected
  from earth atmosphere that is seen by satellite
  (lt 4 micron)
• Longwave radiation is earth emitted radiation (gt
  4 um)

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Fate of EM Radiation
Incident solar energy can either be reflected,
transmitted or absorbed

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Satellite imagery

• In visible imagery water is dark because it
  absorbs most of the energy.
• Clouds are white because most of the incoming
  energy is reflected
• Pollution is hazy depending upon its absorptive
  properties

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Atmospheric Windows
Portions of EM spectrum where absorption and
scattering is minimal is called Atmospheric
Windows

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Remote Sensing - Resolutions

• 4 major resolutions
• Spatial resolution
• Spectral resolution
• Temporal resolution
• Radiometric resolution

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Spatial Resolutions
IFOV
FOV

• Spatial Resolution A simple definition is the
  pixel size that satellite images cover.
• Satellite images are organized in rows and column
  called raster imagery and each pixel has a
  certain spatial resolution.

Satellite height
Nadirpixel size

Off-nadirpixel size
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Spectral Resolution

• The number of bands is sometimes referred to as
  spectral resolution
• Although a better definition is the width is the
  spectral band

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Temporal Resolution

• How often is data obtained for the same area
• Twice daily for polar orbiting satellites
• Hourly or sub hourly for geostationary satellites

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Radiometric Resolution

• The ability to separate small differences in
  energy striking a sensor
• For example 8bits per pixel means the image is
  quantized as 256 gray levels.
• Examples, AVHRR 10 bit, MODIS 12 bit

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Trade Offs

• A sensor cannot extremely high spectral, spatial
  and radiometric resolutions.
• We discuss this in the Critical Thinking module.

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Spectral Signatures
The unique signature of various classes (e.g.
Vegetation, Water, bare Soil) in the exam above
allows multi-spectral satellite imagery for
identification

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What does satellite see?

• Visible spectrum

-Satellite measure radiance and not geophysical
quantities such as temperature these radiance
values must be converted to parameters of
interest.

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Radiance - Definition
Radiance I flux per unit area per unit solid
angle normal to the direction of propagation
Wm-2sr-1
The concept of Stera-radian (sr)

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From pretty pictures to numbers

• Radiance is converted to reflectance and
  temperature
• Multi-spectral Image must be separated into
  various features (clouds, aerosols, ocean, land
  etc.)
• This must now be converted to geophysical
  parameter
• Examples include Cloud top temperature

• For a known satellite
• Measured radiance the
• Cloud top temperature
• Can be calculated if the
• Wavelength is given.
• H, c, and k are all constants