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Transmission of E-M Radiation through the Atmosphere

Transmission of E-M Radiation through the Atmosphere. Attenuation of EM Radiation (AMS). 1. Attenuation, also called extinction , especially in reference to optical frequencies, is a general term used to denote a decrease in signal strength in transmission from one point to another.

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Transmission of E-M Radiation through the Atmosphere

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  1. Transmission of E-M Radiation through the Atmosphere

  2. Attenuation of EM Radiation (AMS) • 1. Attenuation, also called extinction, especially in reference to optical frequencies, is a general term used to denote a decrease in signal strength in transmission from one point to another. • For the propagation of electromagnetic waves through a medium, attenuation is caused by absorption and scattering. The volume attenuation coefficient (m−1) of such a medium is the fractional reduction of radiance per unit pathlength. • 2. The scattering and absorption of radiant energy by clouds or the atmosphere that decreases the radiation received by satellite sensors.

  3. Radar Usage (AMS) • In radar usage, the specific attenuation is the fractional reduction in power density per unit path length as plane-wave radiation propagates through a medium, usually expressed in decibels per kilometer. Attenuation ordinarily does not refer to the inverse-square falloff of irradiance or power density with range that is simply a consequence of beam divergence. Seeextinction coefficient.

  4. Atmospheric Transmission • From 2.5 of Petty, the Intensity, Il(x)= Iloexp (-bax), where ba is the total absorption coefficient. • Since the extinction is accomplished by two process, absorption and scattering, we need more betas, be and bs • Sobe=ba +bs

  5. Absorption Transmission and Scattering • From Petty

  6. Scattering

  7. Extinction be • Function of wavelength • While derived for homogeneous media does work somewhat well for inhomogeneous media

  8. Extinction over Finite Path • Fundamental relationship – Beer’s Law

  9. Optical Thickness or Optical Depth t • Note be can be variable over the range s1 to s2

  10. Transmittance • The transmittance t is defined on the optical thickness, • and

  11. Optical Depth vs Transmittance • Total Optical Depth is sum of all of the individual optical depths • Total Transmittance is product of the individual transmittances

  12. dz ds q ds = dz sec q Plane Parallel Approximation Sec q = 1/Cos q

  13. Transmission Spectrum of the Atmosphere

  14. Mass Extinction Coefficient • Mass weighted extinction coefficient be= r ke, whereris the mass per unit volume of the absorbing material • Now, suppose we have an attenuating atmosphere where the density decays with height, to a scale height H of ~8 km.

  15. Mass Extinction Coefficient • If w is the mixing ratio of substance, then • So the volume extinction coefficient is a function of altitude

  16. Transmittance of the special atmosphere

  17. GOES Weighting Functions

  18. GOES Temperature Weighting Functions

  19. Weighting Functions for a Water Vapor Sounder (Brazil)

  20. Future Satellite Sounder Channels

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