Subject: [IGSMAIL-0965] Precise Point Positions
From: Michael B. Heflin
Date: Tue, 30 May 1995 11:20:28 PDT

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IGS Electronic Mail      Tue May 30 11:20:28 PDT 1995      Message Number 0965
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Author: Michael B. Heflin
Subject: Precise Point Positions

Time series for 82 sites derived with point positioning are now available,
including those from sites in Southern California.

URL:  http://sideshow.jpl.nasa.gov/mbh/point.html

These time series begin on April 2, 1995 and continue through the most
recently analyzed day, May 12, 1995.

We currently process data from 34 globally distributed sites to derive precise
orbits, clocks, and earth orientation parameters.  No sites are held fixed in
the global solution so all estimated parameters are in a free-network reference
frame.  We then fix the free-network orbits, clocks, and EOP to compute precise 
point positions for all remaining sites.  Point positioning takes about 2 CPU
minutes per site per day.  Use of the free-network orbits and clocks ensures
that coordinates derived with point positioning are in the same reference frame
as those derived in the global free-network solution.

Point positions computed with fiducial orbits have sudden jumps when the
reference frame changes from one year to the next or when an error is found
in the fiducial coordinates.  Free-network point positions avoid this problem.
Future refinements include ambiguity resolution and improved handling of
antenna height changes.

The strategy outlined above makes it possible for a thousand sites to be
analyzed in a consistent global reference frame using only two or three HP735
workstations.  Please let me know if you have any questions or comments.

Michael Heflin
5/30/95

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