Impact of the Network Effect on the Origin and Scale: Case Study of Satellite Laser Ranging

The study of station non linear motions requires being able to discriminate between the local motion of every station and their global motion (e.g. geocenter motion). A common used approach consists in estimating Helmert parameters with respect to a secular reference frame. But if the network is small or not well distributed, the global parameters, and mainly the translation and scale parameters, will not be rigorously decorrelated from the station displacements. The distribution of the currently operating Satellite Laser Ranging (SLR) stations is one example of such a network. The understanding of the network effect needs an analysis of the station distribution over time. It appears that SLR station distribution on the X positive hemisphere of the Earth varies seasonally. We suggest here that using GPS results to constrain the estimation of SLR station displacements may improve the estimation of the Helmert parameters. These additional constraints as well as stochastic constraints are numerically tested here in a first attempt to limit the impact of the network effect. We show that the X component of the translation and the scale are the two parameters that are the most sensitive to any change in the configuration of the added constraints.