Effect of IGS Reference Frame Changes
From time to time, the IGS has changed the terrestrial reference frame
that is used to express its products, usually related to updates of the
International Terrestrial Reference
Frame (ITRF). See this chronology of the
IGS frame changes for details.
Regional solutions are sensitive to reference frame changes of the orbits
because most regional solutions are normally processed keeping the IGS
orbits fixed. This implies that the reference frame changes of the orbits
will show up in the coordinate estimates of the regional solutions.
There are two ways to mitigate this problem. The first approach is to
transform the IGS orbits into the required reference frame prior to the
data processing. The effects of the reference frame changes on the orbits
have been fairly well documented and distributed using the IGSMAIL. The IGS
provides a special program for this task to transform the precise orbits.
This approach, however, will not always be a feasible option because it
implies usually a reprocessing of older data. Especially for permanent
networks, reprocessing is not always a realistic option. For campaign type
of solutions, reprocessing is probably the best option. In addition, the
transformation approach is only approximately valid, especially frame
changes early in the IGS history. Note that the effects of any residual
errors in the orbits scale directly with increasing baseline length.
The second solution is to transform the solved-for coordinate results (e.g.,
a set of normal equations) into the desired reference frame using the known
reference frame differences. The problem here is that the (ITRF) reference
frame transformations may not be strictly applicable to the specific region
of interest. This may be problematic especially for the early periods when
the IGS used only 13 reference stations (before March 1998). This problem
may be overcome by estimating the transformation parameters between the
solutions based on different reference frames.
The orbit transformation program may be obtained from NRCan at:
The program name is "trnfsp3n.f". Please read the documentation provided
with the program.
08 February 2011, originator J. Ray