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Selecting A Site Or Sites To Hold Fixed

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 or 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. 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. Clearly named files are used to store this 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 be sites with well know coordinates such as a 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 accessability 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 action of these programs is the simplest definition: every site must connect to a hub site. As a result, hub sites tend to become have more connects and, if these connections were drawn, appear like hubs of a spoked wheel. "Well behaved" sites centered in a group are choices. At least one hubsite must be designated. Two common strategies are to:

  1. designate fixed sites as hub sites or
  2. 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/96_360.

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, CHL1, GODE and SOL1, GODE is an IGS global GPS tracking site with well determined coordinates in the International Terrestrial Reference Frame (ITRF) and CHL1 is CORS site also with well determined coordinates. Either or both of these two sites would be suitable as fixed sites. 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 four files:

ant_info.002 editdb.par fixsite.inp hubsite.inp

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bl_4.html
March 4, 1999
Steve Hilla