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The stree program is used to pick the baselines for a session. The program does this by finding the "minimum spanning tree" that connects all the stations. The user specifies which stations to use as "hubs" (if any), which stations are unreliable and thus must be connected by a "spur" baseline, and which baselines are to be forced to be part of the spanning tree by the user. The input files for these choices are called hubsite.inp, spursite.inp, and fixline.inp, respectively. The minimum spanning tree that is chosen will always be a set of independent vectors that connects all the stations in the session. If the user wants to add a redundant, "dependent" vector then that can be done after the stree program (by adding the extra baseline to the aazz00.bls file).
The stree program reads in a control file called stree.inp which has the following five lines:
|aazz00||Database prefix for the multi-station mergedb run.|
|1||If this flag = 1, output the weighted baseline matrices.|
|80.0||Percentage (weight) for choosing baselines by minimum length.|
|20.0||Percentage (weight) for choosing baselines by maximum amount of data.|
|60||East Longitude line used as a starting point for sorting the final baselines by hub site and longitude.|
The stree program decides the number of common satellite observations between each pair of stations by reading in the elv.plt file. It determines the baseline lengths between each pair of stations by reading in the *hd.dat file from the multi-station mergedb run. Let's say the user were to specify hubsites, spur sites, and fixed baselines for a session, then the stree program (as a maximum) would output the following seven sets of independent baselines to the stree.sum file:
If the percentage weights are set at 100.0 and 0.0, then the unconstrained baselines will be chosen based only on length. If the weights are set at 0.0 and 100.0, then these baselines will be chosen based only on the amount of data for the baselines. To decide how best to set these weights, the user can run stree one time for the session and look at the first two sets of baselines. If they are significantly different from each other, then it may mean that some baselines have numerous missing epochs; thus the user might want to increase the weight that is based on the amount of data.
The stree program sorts the final set of baselines by hub site and by longitude. All baselines emanating from the same hub site are grouped together. The groups are sorted from west to east based on the longitude of the hub sites. Then within each group, the baselines are sorted from west to east based on the east longitude of the non-reference station. The final sorted baselines are output to the aazz00.bls file along with their 4- char database prefix (e.g.,acab, adab, etc.)
The stree.sum file shows all the input files found by the program and all the various set of baselines as mentioned above. The top of the stree.sum files also lists the satellites found in the *hd.dat file; a list of latitude, longitude, and height for each station; and the baseline lengths for all possible combinations of stations. The stree.sum file will also list error messages whenever a critical input file is missing. The critical input files are: stree.inp, aazz00hd.dat, and elv.plt.
The input and output files used in stree are summarized below:
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