Radiation

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Radiation

• As It Applies to Remote Sensing

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Outline

• Electromagnetic Radiation
• Anatomy of a wave
• Electromagnetic spectrum
• Emission of Radiation
• Stefan-Boltzmann Law
• Wiens Law
• Reflectivity
• Absorption
• Basic Satellite Channels

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Electromagnetic Radiation

• What is it?
• Radiation is energy that is emitted by any object
  with a temperature gt 0 K
• Wave-like form no mass
• Has electrical and magnetic fields, normal to
  each other
• Travels at speed of light (in a vacuum)

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Wave Characteristics

• Waves can be defined by their amplitude and
  wavelength
• Wavelength is the distance from peak to peak or
  trough to trough. Usually measured in
  micrometers (10-6 m, or µm)
• Amplitude is the distance from origin to the peak
  or trough

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Wave Characteristics

• Frequency (in units of Hz, or 1/s) is the number
  of waves that pass a given point in a second
• Function of wavelength

where c speed of light (3 x 108 m/s and ?
wavelength (m)
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Electromagnetic Spectrum

• Range of all wavelengths of electromagnetic
  radiation

Low Energy High Energy
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Radiation Principles

• Every object emits (gives off) radiation if
  temperature gt 0 K
• Objects emit radiation in more than one
  wavelength
• Spectrum
• Temperature of object determines radiant
  characteristics
• High surface temperature objects emit high energy
  radiation at shorter wavelengths
• Cool objects emit lower energy radiation at
  longer wavelengths

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Radiation Principles

• Wiens Law
• The wavelength of maximum emission is
  proportional to the objects temperature

where T is in units of K
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Radiation Principles

• Stefan-Boltzmann Law
• Total energy radiated from an object per unit
  area (called the irradiance) is proportional to
  the 4th power of the objects temperature (K)

Emissivity (Ratio of amount of radiation emitted
to theoretical emission)
Stefan-Boltzmann constant 5.67 x 10-8 Wm-2K-4
where E irradiance () and T is in units of K
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Radiation Principles

• Suns Emission

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Fate of Radiation in Earth System

• 1367 Wm-2 of suns radiation intercepted at top
  of Earths atmosphere
• About 30 is reflected back to space
• Albedo
• About 19 is absorbed by atmosphere
• About 51 is transmitted to surface

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Albedo

• Ratio of the radiation reflected by an object to
  the amount coming in
• Dark objects have lower albedos than lighter
  colored objects
• Clouds (50-90), Water (5-15), Snow (40-90),
  Sand (35-45), Forest (5-20)

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Absorptive Gases in Atmosphere
The atmosphere is generally opaque to very short
wavelength radiation Most visible light is
transmitted Most infrared is absorbed
Total Atmosphere
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Satellites and Radiation

• Radiometers on board satellites sense radiation
  that escapes the atmosphere through atmospheric
  windows
• 8 12 µm, for example
• Infrared satellites measure thermal
  characteristics of Earths surface
• AVHRR
• Visible channel measures radiation escaping
  through the visible (0.4 0.7 µm) spectrum

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RADAR and Radiation

• RADARs create their own electromagnetic pulses of
  energy
• Wavelengths are configured so that energy
  backscatters off of particles in atmosphere such
  as raindrops
• Usually 0.8 10 cm