5.1. Combining h-files

There are several reasons why you may want to combine the h-files from your daily GAMIT processing with other h-files before generating a times series or velocity solution. If you have more than ~ 50 sites in a regional network, it is more efficient and just as accurate to process these in GAMIT networks of 30–50 sites and then combine them in GLOBK than to use a single large network (the GAMIT limit is 99 sites). Further, if you are processing a regional network and want to tie it rigorously to a larger regional or global reference frame, you can do so by combining your h-file(s) with those generated by MIT or SOPAC from their IGS processing or another analysis center’s processing of regional continuous networks. Finally, to obtain more useful long-term statistics from your time series, to strengthen the reference frame for survey-mode observations, or to reduce the computational time for velocity solutions, you may wish to first combine the h-files from 5–30 days into a single h-file to be used in subsequent solutions. We discuss the pros and cons of these strategies in Section 5.3 and Section 5.4, but their mechanical implementation is straightforward. For processing of a single day, sh_glred combines the GAMIT processing from multiple GNSS or multiple subnets using the -netext option and includes independently processed regional or global networks using the LA, LB or LC options. Running sh_glred for a single day with -globk_prefit COMB specified will direct globk to save a combined h-file for further processing. For multiple days, this option together with -ncomb will aggregate the data over several days.

sh_glred allows you to automatically download and/or link in h-files from an external source. If the F option is specified, h-files implied by the -net option will be downloaded from MIT or SOPAC into the primary h-file directory (specified by glfpth in process.defaults and nominally glbf/). Alternatively, you can collect the h-files in advance in the directory specified by hfnd in process.defaults and then set the LA, LB, and/or LC options to link these files into glbf/. LA refers to ascii h-files (requiring htoglb translation) of the form h<net><expt>?.<yy><ddd> (e.g. SOPAC network files), LB to binary h-files of the form h<yy><dd><mm>????_<net><expt>.gl? (with .glx given precedence over .glr), and LC to binary h-files of the form H<yy><mm><dd>_<net><expt>.GLX (e.g., MIT combined h-files). For example, to use local processing in combination with MIT global h-files previously downloaded onto your system, the command to generate a 30-day time series for experiment “emed” would be

$ sh_glred -expt emed -s 2009 121 2009 150 -net MIT -opt H LC G T

Since sh_glred will look for all binary h-files in the glbf/ directory, created previously when you generated the time series, you can leave out the -net, H, and LA options when combining the files. This feature is particularly useful if the days you want to aggregate are not continuous (e.g. from a survey) and you have used the -local option in the sh_glred command that generated the time series.

If you are combining data over a span that is long enough that the error in the a priori velocity of any of your sites is large enough to cause an error in position, you should estimate velocities in the combination; however, it’s generally better to make the span of the combination short enough that this is not an issue.