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Creating The POM Files

Description

Initial coordinates for a site are required. There are two sources for these values: the POM files and the site information files. Initially, coordinates and other information from the POM (Position / Offset / Met) file are used to set up processing. The values in this file do not need to be "perfect" but the more accurate this information is the better. Later in the processing, if a site appears in a site info file, values from that file will be substituted. The site info files are designed to be a detailed, accurate history of the coordinates, setup and history of a site. This info is often complex and cumbersome for the user to input during processing and so the simpler POM file is used.

A POM file contains basic pieces of information on four lines. They are, in this order:

  1. the Earth-centered, fixed (XYZ) coordinates of the monument or reference point at a site in meters,
  2. the north / east / up offsets from the reference point to the antenna's L1 phase center and the L1-L2 offset in height, all in meters,
  3. surface temparature, pressure and humidity, in degrees C, millibar and percent saturation respectively to enhance the tropo model,
  4. the antenna type using a convetional antenna ID

One POM file for each site, named with the four character site ID plus the extension .pom, is required. All POM files in the files directory, and because the info slowly or rarely changes, can be reused for other processing. The POM files are simple ASCII; therefore, they can be created and modified with an editor.

Tutorial

At the end of the last tutorial, the current directory was ~/tutorial/files.

GODE is an IGS tracking site; CHL1 a CORS tracking site. site info files for these networks were included in the distribution. Copy these files into the files directory. As before, this tutorial assumes the distribution directories were reconstituted in the users home directory. Make the appropriate substitutions as required.
cp /ngslib/data/GPS/site_info.igs .
cp /ngslib/data/GPS/site_info.cors .
Use the site_info.igs information to create a GODE pom file using the xsites program.
xsites -s gode -f2 -p site_info.igs > gode.pom
Use the site_info.cors information to create a CHL1 pom file .
xsites -s chl1 -f2 -p site_info.cors > chl1.pom
SOL1 does not appear in the distributed site info files, so the pertinent values must be taken from other sources such as previous solutions and knowledge of the site. For the example below, the coordinates were taken from a previous solution; this site uses a Dorne Margolin with T type choke rings so standard offsets from published documentation are used; and a reasonable guess for Maryland winter weather. Normally, the sol1.pom file would be created or modified with an editor. For convenience, we will use the following commands.
echo "1173608.885 -4871160.839 3933263.081" > sol1.pom
echo "0.0000 0.0000 0.1100 -0.0180" >> sol1.pom
echo "5.0 1010.0 50.0" >> sol1.pom
echo "JPL D/M+CRT" >> sol1.pom

Notes

Descriptions of antennas are available from the IGS. Offsets appropriate to the antenna patterns stored in that file ant_info.002. Antenna ID's can also be taken from the file of antenna patterns or from the list given in this documentation.

xsites is an X application include in this distribution. Its purpose is to view and maintain the site information files. xsites documentation is included elsewhere but for completeness, the command-line options used here are:

The directory ~/tutorial/files should now contain nine files:

ant_info.002 chl1.pom editdb.par fixsite.inp gode.pom hubsite.inp site_info.cors site_info.igs sol1.pom

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March 18, 1999
Steve Hilla