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Description
Recall that double-differenced phase observations are used in this processing.
Although data from all sites are processed simultaneously, conceptually the
basic data unit is a pair of sites forming a baseline. The run_survey script
will automatically construct a network of independent baselines from the sites
included. Here, independent baseline network means that only one path between
sites exists through the network, i.e. a minimal spanning tree between all
sites. There are no redundant connections; no closed baseline loops. The
automatic baselines selections are made to maximize the total amount of mutual
data. Some user control can be exercised through the manual selection of
"fixed" sites and "hub" sites. The appropriately named
fixsite.inp and hubsite.inp files
are used to store these lists.
Fixed sites are those sites whose coordinates will be constrained from
changing in the forthcoming solution, hence the generic name fixed sites. More
specifically, these sites' coordinates will be constrained to changes only at
the level of the uncertainties (sigmas) of those coordinates. Obviously, these
should have accurate coordinates such as permanent tracking sites or a temporary
site established at a benchmark. Other factors to consider are the reliability
of a site, and the timeliness and accessibility of its data. At least one fixed
site must be designated but more than one is allowed (and common).
Hub sites help define the structure of a network. The word hub as used here
is a bit of jargon but it's meaning is simple and straightforward: every site
must form a baseline with at least one hub site. As a result, hub sites tend to
be included in more baselines and, if these baselines were drawn on a map, hub
sites would appear like hubs of a spoked wheel. "Well behaved" sites
physically centered in the included group of sites are choices. At least one hub
site must be designated. Three common strategies are to:
- designate fixed sites as hub sites,
- select a single centrally located site as the hub site or
- place all sites in the hub sites list.
Note that a fixed site does not have to be a hub site.
Tutorial
At the end of the last tutorial, the current directory was
"~/tutorial/01_033/".
- The fixed and hub site lists are stored in the "files/"
directory. Change to that directory
- cd ../files
- Of the three sites used in this example, GODE, SOL1 and USNA, GODE is an
IGS global GPS tracking site with well determined coordinates in the
International Terrestrial Reference Frame (ITRF) and the most suitable to
act as a fixed site. In this tutorial we will only designate GODE as a fixed
site.
- echo "gode" > fixsite.inp
- The choice of hub sites is not critical in this case because of the small
size of the network, both in number of sites and physical expanse. Also
designate GODE as a hub site.
- echo "gode" > hubsite.inp
Notes
Unix is case sensitive. The four character site ID must match in all its uses.
In general, all characters will be lower case.
The directory "~/tutorial/files/" should now contain three files:
ls
ant_info.003 fixsite.inp hubsite.inp
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