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1.Creating A Work Area
2.Downloading GPS Data
3.Downloading an Ephemeris
4.Selecting Sites To Hold Fixed
5.Creating The POM Files
6.Running run_survey
7.Manually Modifying The Processing


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.


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


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:

ant_info.003 fixsite.inp hubsite.inp
2009-02-15 18:41 UTC