3. Automatic Processing with GAMIT and GLOBK

GAMIT is composed of distinct programs which perform the functions of preparing the data for processing (makexp and makex), generating reference orbit and rotation values for the satellites (arc, yawtab), interpolating time- and location-specific values of atmospheric and loading models (grdtab), computing residual observations (“\(\text{o} - \text{c}\)”s) and partial derivatives from a geometrical model (model), detecting outliers or breaks in the data (autcln), and performing a least squares analysis (solve). Although the modules can be run individually, they are tied together through the data flow, particularly file-naming conventions, in such a way that most processing is best done with shell scripts and a sequence of batch files set up by a driver module (fixdrv) for modeling, editing, and estimation. Though the data editing is almost always performed automatically, the solution residuals can be displayed or plotted so that problematic data can be identified (cview).

Likewise, GLOBK operates through distinct programs, which can be invoked with a single command or run separately. The primary functions are to combine quasi-observations—either GAMIT/GLOBK “h-files” or the internationally accepted SINEX format—from multiple networks and/or epochs (glred or globk), and to impose on this solution a reference frame appropriate to the scientific objective (glorg). Note that globk and glred are the same program, just called in different modes: glred to read data from one day at a time for generating time series; globk for stacking multiple epochs to obtain a mean position and/or velocity.

The full sequence of steps to take you from phase data to time series is accomplished with two shell scripts:

sh_gamit looks for RINEX data over a range of days and invokes the GAMIT programs to produce constrained and loose estimates of coordinates together with sky plots of phase data as a record of the processing;

sh_glred uses the GAMIT results to produce time series of day-to-day repeatability or a combined h-file that may be further combined with those from other epochs to estimate station velocities.

The only preparation required is assembling the meta-data from station logs; setting up the control files, most of which are common to all analyses of a particular era; and assembling RINEX files not available from public archive in one or more directories on your system.

Note that all of the scripts and programs that use command-line control are self-documenting: to see the input commands, just type the script or program name with no arguments.