JWG1.3: Strategies for epoch reference frames

Terms of Reference:

The International Terrestrial Reference Frame (ITRF) is based on a mathematical model, which describes the station movement as a piece-wise linear function of time. This motion is mainly driven by lithosphere plate motion and crustal deformations, which are assumed to be linear and constant over long time periods. Thus, the ITRF provides a very high long-term stability. Even if the linear component dominates the station motion, most of the station positions show additional non-linear effects. The most important causing reasons are atmospheric and hydrospheric mass load changes, which lead to seasonal and long-period position variations, and seismic events causing co-seismic abrupt and post-seismic exponentially decreasing movements of the stations. The approximation of non-linear seismic station motions within the ITRF is performed by estimating offsets and piece-wise constant velocities. Loading effects are not considered as they are very difficult to model or parameterize. One possible solution to overcome this problem would be to compute time series of epoch reference frames, which would provide the station positions at consecutive epochs. The mentioned types of non-linear station motion would be captured very well by such time series of epoch reference frames. A second advantage of epoch reference frame would be that a new solution could be available with a short time delay after a seismic event, when updated coordinates are requested.

Today, GPS-based weekly reference frames are a standard product of the IGS and serve as a near real-time reference frame. The datum of these frames is realized by an alignment to ITRF, which depends on the set of stations used for the alignment - due to station non-linear motions and which does not account for a motion of the centre of figure w.r.t. the centre of mass. Thus, there is a need for epoch reference frames based on the combination of the four different techniques contributing to the ITRF in order to ensure consistency for the reference frame products.

The national reference frames consist of station positions at a certain epoch or are fixed to a moving plate and do often not consider regional station motions. Station positions resulting from todays measurments are derived in the actual ITRF or epoch reference frames and need to be transformed to the officially defined national reference frames. Especially, in the case of seismic active areas, the network geometry changes significantly over time and cannot be well represented by the current ITRF with linear station motions. Strategies must be developed, which allow for a transformation with a minimal reduction of accuracy.

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