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Main Topics






Combining Data


Remote Sensing

Electromagnetic spectrum
Radiation Laws
with the Earth's atmosphere
windows and 
useful wave lengths


Remote sensing in it's broadest sense is simply defined as the observation of an object from some distance.  In the case of volcanic clouds this often means observing an eruptive event using imaging tools positioned on satellites.  To understand how these tools operate the first thing that must be understood is what the instruments are actually measuring.

Electromagnetic Spectrum

All objects at a temperature greater than absolute zero emit, absorb, and reflect electormagnetic energy.  This energy, measured in photons travels in the form of waves at a variety of wavelengths.  The complete array of wavelengths is known as the electromagnetic spectrum which is brocken down into regions that are defined by specific wavelenghts.

As this energy travels through the Earth's atmosphere some is reflected and some is absorbed and reemitted.  By measuring the energy reaching a satellite one can characterize the features producing the reponse recorded by the sensor.

Radiation Laws - having some problems making images of equations

Kirchhoff's Law states that for all blackbodies at the same temperature, the ratio of emitted radiation to absorbed radiation is the same.  Emissivity is then calculated as the ratio of the emittatnce of an object and the emittance of a blackbody at the same temperature.

Stefan-Boltzmann Law states that the energy per unit area that a blackbody emits increases as the temperature of the blackbody increases.  Total emitted radiation is calculated by:

Lastly, Wien's Displacement Law describes the relationship between the wavelength of emitted radiation and the temperature of the object.  This law shows that as the temperature of an object increases the wavelength of maximum emittance increaeses.

Interactions with the Earth's Atmosphere

All electromagnetic radiation must travel through the Earth's atmosphere and along the way several things can happen to the radiation that alter the radiation in some way either by redirection or a change in energy level.  The further away a sensor is from it's target, the the larger the atmospheric effects are upon the radiation.
  • Scattering - Scattering is the redirection of EM energy by particles suspended in the atmphere.  It is dependant upon the number of particles present in the atmosphere, the size of the particles, the wavemlength of incoming radiation and the depth of atmosphere that the radiation must travel through. 



    Particles that are small relative to the wavelength of incoming radiation create rayleigh scattering.  Rayleigh scattering is wave length dependant, favoring short wavelengths, ansd is responsible for our sky appearing blue.

Insert Rayleigh graph and or equation here
Mie scattering is produced by particles having diameter approzimately equal to the wavelength of the imcoming radiation.  Mie scattering is typically created by dust, smoke, haze and water droplets in the lower atmosphere.

The scattering produced by very large particles relative to the incoming radiation is nonselective.  As the name implies, nonselective scattering is not wavelength dependant and scatters all wavelengths equally.

  • Absorption - Absorption occurs when atmospheric particles do not allow EM radiation to be fully transmitted.  The amount of energy absorbed is dependant upon the absorber and the wavelength of incoming radiation.  Energy that is absorbed is then re-emitted at a longer wavelengths,  There are three gasses in the atmosphere that are responsible for most of the absorption in the earth's atmosphere. 



    CO2 - Carbon Dioxide is concentrated in the lower atmosphere at approximately .036% by volume of the the earth's atmosphere at sea level.  Interestingly, this amount is on the rise apprarently because of mankinds burning of fossil fuels.

    O3 - Ozone is concentrated in the upper stratoshpere and at ground levels with concentrations of .07 ppm and .1 to .2 ppm respectively.  Ozone is important because it is responsible for much of the absorption of the high energy, short wavelenght UV solar radiation from entering the Earth's lower atmosphere.  (ozone hole and importance of lower atmos O3)

    H2O - Water vapor varies dramatically throughout the Earth's atmosphere and therefore the role of H2O as an absorber of EM radiation varies with time and location.

    There are other items in the atmosphere that play an important role in the absorption of incoming solar radiation.  These include methane (CH4), Nitrous Oxice (NO), and clorofluorocarbons (CFCs).

  • Refraction - Refraction is the deflection of EM radiation as it passes from one medium with one refractive index to a medium with a different refractive index. Refractive index is defined as the ratio or the speed of light in a vacume to the speed of light in the medium and is calculated by:

  • In the Earth's atmosphere temperature, compostition and humidity all affect the density which affect the refractive index.  The angle that the radiation will be bent is defined by Snells Law:


    The direction that it is bent is determined by the density of the two mediums.  If the energy is passing into a more dense medium, then the energy is deflected toward the surface normal (a line drawn perpendicular to the surface separating the two mediums).

    Atmospheric Windows and Useful Wavelengths


    Resolution (?)


    Different Types of Sensors/Satellites(?)

    Polar orbiting
    Naidr viewing
    Side Scan


    http://www.geo.mtu.edu/volcanoes/vc_web/bachground/b_remotesensing.html -- Revised:  12 NOVEMBER 2000

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