Case4: Radiative Transfer - Model Uncertainties, down looking geometry.

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The aim of this case is to check the differences in models when they use their 
standard inputs.

The radiative transfer calculations will be carried out for given
atmospheric conditions, and instrumental characteristics but not fixed
assumptions for the other parameters (spectroscopic data, line shape,
etc.). For a given set of viewing directions, the spectra as recorded
by the instrument will be compared. The refraction should be considered. 

To simulate the instrument characteristics, the AMSU-B, designed the
measure radiance in 5 channels, was chosen. 
The instrument is assumed to have a perfect antenna, and to be a perfect double
sideband. The spectrometer is assumed to have a perfect rectangular
response function with the width equal to bandwidth passband
corresponding to each channel. 

The AMSU-B channel characteristics are given below:

----------------------------------------------------------------------
  Center Frequency of Channel  Nr.of passbands	Bandwidth per passband
	  [GHz]						[MHz]
---------------------------------------------------------------------
	 89.0+/-0.9			2		  1000
	150.0+/-0.9			2	          1000
	183.31+/-0.9			2	           500
	183.31+/-3.0			2		  1000
	183.31+/-7.0			2		  2000
----------------------------------------------------------------------

For a given set of viewing directions the spectra as recorded by the 
instrument, for all 5 channels, will be compared.

Input - Output Files
--------------------
 Format:
 ========
  The files are ASCII, in ARTS format. 
  They can easily be recognized by the extension 'aa'.
  The file can start with an arbitrary number of comment lines.  
  These lines starts with the hash symbol (#) 
  The first row after the comment lines give the number of matrices 
  in the array. After this follows, for each matrix, a row giving 
  the matrix size followed by the data in row order.

Input:
======
 - atm_scenario_Down.aa -->> profiles of temperature,
   altitude, pressure, and trace gas volume mixing ratios. The data is
   display in ARTS format. See the header of the file for a more
   detailed description.
 - KeySpecies_Down.txt -->> a text file which gives the set of the
   target species watched by the instrument.
 - Down_specifications.txt -->> the file gives the numerical 
   values for platform altitude and ground specifications (temperature, 
   altitude, emissivity). 
 - za_sensor_Down.aa -->> zenith angles for the sensor.
 - Freq_sensor_Down.aa -->> the frequencies observed by the 
   sensor. 

Output:
=======
 - the spectra seen by instrument (including the effect of spectrometer).
  The output data should have ARTS format: each row gives the spectra
  corresponding to one viewing direction.