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