VTD a priori files management.

   Computation of VLBI time delay depends on values of a priori 
parameters. They are gathered in a priori files. A priori files 
belong to three categories:

1) Constants -- never changed

2) Infrequently updated, 2-3 times a year

3) Regularly updated on a daily basis. 

This document contains suggestions how the files with a priori parameters
should be managed.


I. Constant files.
==================


1) DE_EPHEMERIDES.

      The files supplied with VTD distribution is valid for the 
      range of dates [1950, 2050]

2) STATION_ECCENTRICITIES  

   Should updated for new stations. You can edit this file yourself.

3) AEM_FILE 

4) ANTENNA_DEFORMATIONS_FILE

   May be updated for new stations.

5) HYDROSTATIC_MAPPING_FUNCTION/WET_MAPPING_FUNCTION: (MMF) 

6) Ocean loading 

   May be updated for new stations.

7) Loading due to atmospheric stations.

   May be updated for new stations.

8) SLANTED_PATH_DELAY_BIAS_FILE 

   May be updated for new stations.


II. Infrequently updated files:
===============================


1) LEAP_SECOND

   You check http://tycho.usno.navy.mil/leapsec.html
   and when a new leap second epoch is announced, edit
   the file accordingly.

2) STATION_DESCRIPTION 

   You can edit this file and add a new station.
   You can ask for an apriori value of antenna axis
   offset. Antenna axis offset length can be found from
   VLBI solutions at http://astrogeo.org/gsol/ or
   http://astrogeo.org/rfc

3) SOURCE_COORDINATES 

   The best source of source coordinates of known calibrators
   is http://astrogeo.org/rfc, which is updated 2-3 times 
   a year. You can download the apriori file from there and 
   then to add your sources to the bottom. NB: if a source
   is defined more than once in the file, the last definition
   will take effect.

4) STATION_COORDINATES  

   You can download the latest solution, updated 2-3 times 
   a year from http://astrogeo.org/rfc


5) STATION_VELOCITIES 

   You can download the latest solution, updated 2-3 times 
   a year from http://astrogeo.org/rfc


III. Regularly updated files: 
=============================


1) EOP_SERIES 

   This file is updated daily. The Earth orientation parameters 
   are derived from analysis of dedicated VLBI observations.
   There may many ways to generate a priori EOP file. The file
   provided with VTD distribution is generated at NASA GSFC by
   processing USNO EOP data file and applying rotations that
   make it consistent with a priori station and source catalogues.

   The file can be downloaded from http://gemini.gsfc.nasa.gov/500/oper/solve_save_files/last.erp

   Bash-program for updating EOP files is supplied in 
   support/update_eop_series.sh

2) Atmospheric pressure loading

   These files is updated daily. 

   Visit http://astrogeo.org/aplo for further information where
   to find data files.
   For example, here: http://lacerta.gsfc.nasa.gov/aplo/aplo_bds.tar.bz2
   
   Bash-program for updating atmospheric pressure loading files 
   is supplied in support/update_aplo_bds.sh

   Size of aplo_bds data is 320Mb


4) IONOSPHERE_DATA_FILE

   These files is updated daily.

   There are several data centers that update ionosphere data in 
   the ionex format. You need to download them and then transform
   from ionex format to viono format. This is done by an auxiliary
   program gti_update. 

   
   Bash-program for updating ionosphere data files
   is supplied in support/update_viono.sh

   Date size: data in ionex format: 720 Mb, data in viono format: 320 Mb

5) EXTERNAL_DELAY_DIR

   In 2010 this files were updated once a month.
   These file may be updated daily in the future.

   The data can be downloaded from http://astrogeo.org/spm

   Size of data in SPD_3D format:     114 Gb
   Size of data in SPD binary format:  14 Gb
