Case 4: Radiative Transfer - Model Uncertainties, limb looking geometry.

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

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

A perfect single sideband instrument will be assumed.
Two components of the sensor part will be considered:

Antenna
-------
The purpose of the antenna is to maximize the sensitivity of the radiation in
a narrow angular direction while suppressing radiation from the other
directions. This sensitivity is described by the antenna pattern. The
simulated antenna is  assumed the have a Gaussian function, with a width of 
the main beam (FWHM) of 0.07 degrees.

Spectrometer
---------- 
The output of spectrometer is a weighed mean of the signal around some discrete
frequencies (sensor frequencies), channels, that together generate a spectrum.
Each channel is described by a response function. In our simulations all the 
channels were assumed to have the same Gaussian shape (FWHM = 3MHz). 
All the channels are assumed to have the same response.

Input - Output Files
--------------------
 Format:
 ========
  The 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:
======
 - atm_scenario_Limb.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_Limb.txt -->> a text file which gives the set of the
   target species watched by the instrument.
 - Limb_specifications.txt -->> the file gives the numerical 
   values for the platform altitude and ground specifications (altitude, 
   temperature, emissivity). 
 - za_sensor_Limb.aa -->> zenith angles as seen by the sensor.
 - Antenna_Limb.aa -->>  file in ARTS format giving the antenna pattern:
   column 1 gives the relative zenith angles in degrees, column 2 gives 
   response function. The response function is not normalized.
 - Freq_sensor_Limb.aa -->> the frequencies observed by the 
   sensor (the middle points of the backend channels). 
 - channel_response_Limb.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 is (relative) frequencies and column 2 response values.  

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