Transmission of EM Radiation through the Atmosphere

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. See extinction
  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
• So be 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 Beers 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
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,
• 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.

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
21
(No Transcript)
22
(No Transcript)