Case 3: Check of the Radiative Transfer Implementation, down looking geometry.

***************************************************************************
The aim of this case is to check the implementation of the
radiative transfer algorithm, for a down looking instrument. 

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 perfect rectangular
response function with a 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
----------------------------------------------------------------------

Pre-calculated absorption coefficients will be used as input. 
For specified calculation procedure and instrumental characteristics, the
participants shall provide both, pencil beam monochromatic spectra and
spectra as recorded by the instrument. No refraction is considered.


Input - Output Files
--------------------
 Format:
 ========
  The input files are ASCII files, 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:
======
 - Freq_mono_Down.aa -->> the frequency vector for which the absorption 
   coefficients were calculated.
 - p_z_abs_Down.aa -->> the pressure (altitude) levels corresponding to 
   pre-calculated absorption coefficients. The file is in ARTS format,
   a 2 columns matrix: column 1 gives the levels in pressure units
   [Pa], while column 2 gives the corresponding levels in geometric altitude 
   units [m]
 - Down.abs.aa -->> the file gives the pre-calculated  absorption 
   coefficients, in ARTS format. 
   Each row data gives the absorption coefficients calculated for the
   frequencies given in Freq_mono_Down.aa and for one atmospheric
   level (given p_z_abs_Down.aa), 
 - Down_specifications.txt -->> the file gives the numerical 
   values for platform altitude and ground specifications (temperature, 
   altitude, emissivity). 
 - za_pencil_Down.aa -->> zenith angles for the pencil beam calculation.
 - Freq_sensor_Down.aa -->> discrete frequencies for which the spectra
   should be provided.

Output:
=======      
 - the pencil beam monochromatic spectra (brightness temperature) for the 
   viewing direction specified in za_pencil_Down.aa.
 - the spectra seen by instrument (including the effect of
   spectrometer) for discrete frequencies given in Freq_sensor_Down.aa.
  The output data should have ARTS format: each row gives the spectra
  corresponding to one viewing direction.