Dependence of IGS Products on the ITRF Datum

Abstract

Throughout its nearly two decades, the International GNSS (Global Navigation Satellite Systems) Service (IGS) has sought to align its products closely to successive realizations of the International Terrestrial Reference Frame (ITRF). This has been disruptive for IGS users at times, especially during the 1990s when some radical ITRF datum choices were adopted. During the past decade, IGS impacts due to ITRF updates have been smaller and mostly caused by errors in the results from the contributing space geodetic techniques.

Frame orientations (rotations) are purely conventional, so the IGS relies on the ITRF via a subset of reliable, globally distributed stations. Except for the period when ITRF93 was used, this procedure has worked well. The IGS origin in principle could be self-reliant or contributory to ITRF by direct observation of a frame origin aligned to the long-term center of mass of the entire Earth system. In practice, however, GNSS-based results have been less reliable than those from satellite laser ranging (SLR). So the ITRF origin, based on SLR only, has been adopted historically. Until the transition from ITRF2005 to ITRF2008, there have sometimes been significant origin shifts as SLR results have evolved. However, the present stability of the ITRF origin may finally have reached the few-mm level.

In many respects, the IGS dependence on the ITRF scale is most subtle and problematic. In addition to an overall Helmert alignment of the IGS frame to match the ITRF scale (and other datum parameters), since 2006 the IGS calibration values for the GNSS satellite antenna z-offsets depend directly on the same ITRF scale (due to high correlations if the IGS frame scale is not fixed). We therefore face a non-linear situation to maintain full consistency between all IGS products and the ITRF scale: each IGS frame contribution to ITRF based on one set of antenna calibrations must be used, together with frames from other techniques, to determine an updated ITRF and new antenna calibrations, which are then no longer strictly consistent with the starting IGS frame. One can hope that the process will iteratively converge eventually. But large shifts in the ITRF scale, such as the −1ppb change from ITRF2005 to ITRF2008, are highly disturbing, much more so than the associated rotational or translational shifts.

Only SLR and very long baseline interferometry (VLBI) have been considered reliable and accurate enough to be used for the ITRF scale. But experience and theoretical studies have shown that neither is accurate to better than about 1ppb. Note in particular that a 1ppb uncertainty in the GM constant fundamentally limits the possible scale agreement between SLR and VLBI to no better. Consequently, the authors strongly urge that the ITRF scale hereafter be fixed conventionally to the ITRF2008 scale indefinitely until it is convincingly shown that VLBI and/or SLR can determine the ITRF scale within 0.5ppb. If this is not done, the IGS might maintain its own ITRF2008 scaled frame to minimize future operational dislocations.