Title: Transmission of EM Radiation through the Atmosphere
1Transmission of E-M Radiation through the
Atmosphere
2Attenuation 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.
3Radar 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. See extinction
coefficient.
4Atmospheric 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 - So be ba bs
5Absorption Transmission and Scattering
6Scattering
7Extinction be
- Function of wavelength
- While derived for homogeneous media does work
somewhat well for inhomogeneous media
8Extinction over Finite Path
- Fundamental relationship Beers Law
9Optical Thickness or Optical Depth t
- Note be can be variable over the range s1 to s2
10Transmittance
- The transmittance t is defined on the optical
thickness, - and
11Optical Depth vs Transmittance
- Total Optical Depth is sum of all of the
individual optical depths - Total Transmittance is product of the individual
transmittances
12Plane Parallel Approximation
13Transmission Spectrum of the Atmosphere
14Mass Extinction Coefficient
- Mass weighted extinction coefficient
- be r ke,
- where r is 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.
15Mass Extinction Coefficient
- If w is the mixing ratio of substance, then
- So the volume extinction coefficient is a
function of altitude
16Transmittance of the special atmosphere
17GOES Weighting Functions
18GOES Temperature Weighting Functions
19Weighting Functions for a Water Vapor Sounder
(Brazil)
20Future Satellite Sounder Channels
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