SPACE TIE · Unifying the three pillars of Geodesy using space ties

Terrestrial Reference Frames (TRFs) are the basis to which all positions on the Earth’s surface and all satellite orbits in the near Earth space have to refer to. The changes in the Earth's shape, rotation, and gravity field, the so-called ""three pillars"" of geodesy, provide the conceptual and observational basis for the TRFs. For today’s TRF realizations, four space geodetic techniques are combined and linked by co-location sites on the Earth’s surface (“Earth’s shape”) and by common Earth orientation parameters (“Earth rotation”). The third pillar (“Earth’s gravity field”) is today only contributing to the TRF determination via its associated center-of-mass. In SPACE TIE we will pave the way to unify the “three pillars” of Geodesy in future TRF realizations. We propose to use two satellite geodetic techniques, namely Global Navigation Satellite Systems (GNSS) and Satellite Laser Ranging (SLR), to connect them by co-location sites in space. These so-called space ties shall be realized on satellites of the currently existing space infrastructure, as well as on satellites due for launch in the near future. This includes the Medium Earth Orbits (MEO) of the GNSS satellites and, in particular, all satellites in Low Earth Orbits (LEO) with GNSS and SLR co-located on-board. To maximize the sensitivity to the Earth’s gravity field, the ultra-precise inter-satellite ranging between LEO satellites of dedicated gravity missions shall be added as a third satellite geodetic technique. One and the same state-of-the-art space geodetic software package will be used to ensure that standards, background models, and processing strategies are consistently applied across all co-location satellites and measurement techniques. The outcome of SPACE TIE will allow it to assess the geometric and gravimetric impact of mass transport in the atmosphere, oceans, and ice caps in a most consistent way to globally quantify the mass exchange between the different components of the system Earth.""