2.4. Using the GNSS Product Files

The survey and continuous GNSS time series are provided in the GAGE’s comprehensive “.pos” format, which is a plain text file. As such, no specialized software is required to read the time series files, only simple commands to read the appropriate fields. The format has a header section (37 records) and a data section (all lines beginning with a whitespace character) consisting of two types of date strings (calendar date and the sequential modified Julian date) and three systems of coordinates (geocentric, geodetic and local east/north/up relative to the nominal reference coordinates in the header). All length units are meters quoted to five decimal places and geodetic coordinates to ten decimal places, approximately the same level of precision (hundredths of a mm). Most users will want to read the local (e,n,u) fields, which are columns 16 (north), 17 (east) and 18 (up), with associated uncertainties (sigmas) in columns 19 (north), 20 (east) and 21 (up), and inter-component correlation coefficients in columns 22 (north-east), 23 (north-up) and 24 (east-up). These fields can be read with a simple awk command, e.g. to read calendar date in ISO format and the north displacement and associated sigma in mm (one line; ignore wrapping):

$ awk '{if ($0 ~ /^ /) printf "%4d-%02d-%02dT%02d:%02d:%02d %.2f %.2f\n",substr($1,1,4),substr($1,5,2),substr($1,7,2),substr($2,1,2),substr($2,3,2),substr($2,5,2),$16*1e3,$19*1e3}' <input .pos file>

For those who plan to calculate transformations, such as rotating the time series into a chosen reference frame, and perform other manipulations, these are easier done in Earth-centered Earth-fixed (ECEF) coordinates, which are columns 4 (geocentric X), 5 (Y) and 6 (Z), with associated sigmas in columns 7 (X), 8 (Y) and 9 (Z), and inter-component correlation coefficients in columns 10 (X-Y), 11 (X-Z) and 12 (Y-Z).

The “Soln” column at the end of each line shows the number of contributing solutions from source analysis centers that were used to calculate the weighted average, e.g. “avg05” means all five analysis centers had data available for that time series point.

A simple script is provided to help read the .pos files and read them out into a preferred format. Currently, only the .pos files are readable and may be written out to the following formats: NGL’s .txyz2 format, a free-format GeoCSV (e.g. http://geows.ds.iris.edu/documents/GeoCSV.pdf) and GMT’s “psxy” format; NGL’s .tenv3 format will soon be added.