Case4: Radiative Transfer - Model Uncertainties, up 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 calculation will be performed for given
atmospheric conditions, and instrumental characteristics but not fixed
assumptions for the other parameters (spectroscopic data, line shape,
pencil beam calculations, etc.). For a given set of viewing
directions, the spectra as recorded by the instrument will be
compared.

The instrument is assumed to have a perfect antenna, perfect single
sideband with. In other words, only the effect of the spectrometer
will be considered.

Spectrometer 
------------ 
The output of the spectrometer is a weighted mean of the signal
(monochromatic pencil beam) around some discrete frequencies (sensor
frequencies), channels, that together generate a spectrum. Each
channel is described by a response function. The response function is
assumed to have a Gaussian shape (FWHM =3MHz). All the channels are
treated the have the same response.    

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_Up.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_Up.txt -->> a text file which gives the set of the
   target species watched by the instrument.
 - Up_specifications.txt -->> the file gives the numerical 
   values for platform altitude and ground specifications (temperature, 
   altitude, emissivity). 
 - za_sensor_Up.aa -->> zenith angles for the sensor.
 - Freq_sensor_Up.aa -->> the frequencies observed by the 
   sensor (the middle points of the backend channels). 
 - channel_response_Up.aa -->> the file defining the backend 
   channel response. The response of all channels are assumed to be 
   identical. The channel file  has ARTS format, a 2 column matrix where 
   column 1 gives relative frequencies (with respect the middle points of the 
   backend channel) and column 2 gives response values.  

Output: 
======= 

 - the spectra seen by instrument (including the effect of the
   spectrometer). The file should have ARTS format. Each row of the data
   gives the spectra for discrete frequencies (sensor
   frequencies) corresponding to one viewing direction.