The ILuRS concept is that ILuRS co-rotates with the Moon, the choice of structure is an undistorted Moon, defined according to principle axis (PA) frame. The structure modelling is highly based on lunar ephemerides and Moon gravity field. The materialization of ILuRS, the International Lunar Reference frame (ILuRF) is done with the coordinates of the Lunar Laser Ranging Retro-Reflectors (LLRRR) relative to the Moon CoM. The extension of ILuRF will be based on additional LLRRR that should be installed in the coming years. A supplement delivery is the definition of the ME reference frame based on the Helmert transformation parameters from ILuRF PA to previously published ME.
The choice has been made to select a PA-based reference system in order to facilitate the updates of the system each time a modelisation break-through will be implemented in lunar ephemerides. Other choices were possible and transformation parameters from ILuRS realization in PA to these alternatives reference frames are part of the ILuRS realization with release of Helmert parameters as illustrated in this Figure.
The time-scale associated with ILuRS is TCL as defined by the BIPM. Its relativistic metric is the one proposed by the IAU in its 2024 resolution. The realization of TCL-TCB using ILuRF parameters (mainly its origin definition) is part of the ILuRS delivery and will be also available for users.
Finally, on this Figure, one can see the important role of the lunar ephemerides in the definition of ILuRS and its realization as they are required for defining the translation of the Moon center of mass relative to Earth center of mass (labelled translation of origin and the Lunar Orientation Parameters (LOP) for transforming the quasi inertial LCRS into the Rotating-with-the-Moon (Moon-fixed) ILuRS. This is why, to avoid relying for the definition of ILuRF over only one ephemeris provider, it has been decided to have an approach similar to the one proposed by \cite{Beutler1995} in 1995 regarding the combination of GNSS orbits, based on Variance Component Estimator (VCE) of the orbits. For ILuRF, we combine using VCE, state-of-the-art lunar ephemerides.