Computation of atmospheric opacity and brighntess temperature
in microwave range 1 to 360 GHz using spd_client library.


I Overview
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  Computation of atmospheric opacity and brighntess temperature
for reduction of radioastronomy data is performed by three procedures. 
The first procedure, data acquisition is performed by the designated 
server. The server downloads the numerical weather model 
GEOS-IT (Rienecker et al. 2008) developed and maintained by the Goddard 
Modeling and Assimilation Office. The model has resolution 
0.625 deg x 0.5 deg x 72 layers x 3 hours, runs from 2000.01.01 through 
presents, updated 4 times a day and has latency 10-15 hours. The output of 
the numerical weather model among other parameters contains thickness of 
atmospheric layers, air temperature, and specific humidity at each grid 
point. These parameters are used for computing refractivity in moist air
in a range of 1 to 360 GHz. The refractivity accounts for both dry air 
and water vapor. The data acquisition procedure stores for each epoch 
3D refractivity field at a regular grid with the height range from -1000, 
to 80000 km above the reference ellipsoid. The data acquisition procedure 
runs every hour and check whether the new output from the GEOS-IT model 
become available.

  The second procedure computes opacity and slant path delay for a designated 
set of stations. The path delay are computed on a regular, but non-uniform grid
over azimuth and elevation. For each station, each epoch, each direction
the trajectory of the wavefront is computed by solving a system of non-liner 
differential equations of the 4th order that are the solution of the 
variational problem of wave propagation in accordance with the Fermat 
principle. Then the path delay is computed by integrating the refractivity
along the trajectory from the receiver (i.e. observing station) to the 
top of the atmosphere defined at the height of 80 km. In addition 
opacitiy is computed for a set of frequencies for a set of heights. Then 
brightness tempera\ture is cmpppputed by solving radiative transfer equaitons.

   All slant path deay, atmosphere opacity and atmosphere brightness temperature
for all directions, all stations for a given epochs are stored in 
an output file. Path delays for designated "continuous" stations are computed
immediately after completion of the data acquisition procedure.


II. Routines:
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  The package spd_client provides two routines that are supposed 
to be called from a radioasronomy data reduction software. 
The first routine, SPD_RES_INTRP,

...  not completed ...
