ITRF Combination: Theoretical and Practical Considerations and Lessons from ITRF2008
The current ITRF construction is based on a two-step approach, combining input data provided by space geodesy techniques (VLBI, SLR, GPS, DORIS) in the form of time series of station positions and Earth Orientation Parameters. In the first step, the indiv
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ITRF Combination: Theoretical and Practical Considerations and Lessons from ITRF2008 Z. Altamimi, X. Collilieux, and L. Me´tivier
Abstract
The current ITRF construction is based on a two-step approach, combining input data provided by space geodesy techniques (VLBI, SLR, GPS, DORIS) in the form of time series of station positions and Earth Orientation Parameters. In the first step, the individual technique time series are rigorously stacked (accumulated) yielding long-term secular solutions, while the second step forms the ITRF final combination of the four technique long-term solutions together with local ties at co-location sites. The combination model involves a 7- or 14-parameter similarity transformation formula, for time series stacking and multi-technique combination, respectively. Not all these parameters are necessarily estimated in the combination process, some or all of them could be eliminated from the constructed normal equation, depending on the combination purpose. The paper discusses the relevance of the combination model and its appropriateness for the ITRF combination activities, both from the theoretical and practical point of views, and in particular for the reference frame specifications (origin, scale, orientation and their time evolutions). Selected analysis tests of ITRF2008 input data and results are used to illustrate the discussion as well as to address lessons learned from ITRF2008 experience. Keywords
Reference systems Reference frames Time evolution ITRF
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Introduction
With the advent of space geodesy in the early 1980s, the importance of reference frames has became more and more important, as a function of technological and data analysis advances. Appropriate definition of a Terrestrial Reference System (TRS) and its precise materialization through a Terrestrial Reference Frame (TRF) are fundamental to many applications in geosciences. The main TRS and TRF specifications are the origin, scale, orientation and their time evolutions. Any defect on these parameters would
Z. Altamimi (*) X. Collilieux L. Me´tivier IGN/LAREG and GRGS, Universite´ paris Diderot, 35 rue, He´le´ne Brion 75013 Paris, France e-mail: [email protected]
have an impact on the results and interpretation of geodetic and geophysical applications that require the usage of a reference frame, such as: • Precise Orbit Determination, not only for Global Navigation Satellite Systems (GNSS), but also for other satellite missions dedicated to Altimetry, Oceanography, Gravity; • Earth sciences applications, such as tectonic motion and crustal deformation, sea level variations and Earth rotation (Collilieux and Altamimi 2012). Given the currently available reference frame products provided by space geodetic techniques, representations of terrestrial reference frames are divided in two categories: • “Quasi-instantaneous” reference frame which gives access to mean station positions at “short” interval, using space geodesy observations over, e.g. 1 or several hours, 1 day, and up to 1 week. Note that over 1
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