**Geocentre Motion from Multi-Satellite SLR Data Integration**
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Spatar, Ciprian Bogdan; Moore, Philip; Clarke, Peter
Newcastle University, UNITED KINGDOM
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The centre of mass of the Earth system (CM), including the atmosphere, oceans, groundwater and the cryosphere is conventionally chosen as the geocentre, the origin of terrestrial reference frames (TRFs). Mass redistribution within and between the Earth system’s components induces spatial and temporal variations of the geocentre referred to as geocentre motion. These variations can be modelled in principle within the precise orbit determination process and thus geocentre coordinates can be estimated as global parameters. The amplitude spectra of geocentre motion time series are dominated by annual and semi-annual signals varying from millimetres to a few centimetres. Despite lacking a sufficiently dense and well distributed network of ground stations, the space geodetic technique of Satellite Laser Ranging (SLR) provides the least noisy time series of geocentre variations. Hence only SLR measurements to the LAGEOS-1 and -2 satellites are used for defining the International Terrestrial Reference Frame (ITRF) origin. The similarly shaped Etalon satellites are placed in higher GNSS-like orbits, therefore exceeding the ranging capabilities of many laser ranging systems. On the contrary, non-gravitational forces acting on low Earth orbit (LEO) geodetic satellites pose orbit modelling challenges and affect the accuracy of estimated parameters. Observations to LEO satellites such as Starlette, Stella and Ajisai are however routinely used to estimate the long-wavelength time-variable gravity field, for which SLR is considered the prime technique. The degree-one spherical harmonic coefficients are proportional to the geocentre coordinates but generally equated to zero. We combine data from seven spherical geodetic satellites in order to determine geocentre motion, defined in our study as the temporal variation of the vector offset between the centre of surface figure (CF) and CM. Improved results over single-satellite analyses are expected. To validate our solution and assess its quality we compare it to a LAGEOS only solution and independent results from other studies.