In December 2014 experts from 14 different agencies and departments attended the joint GSICS – CEOS/IVOS Lunar Calibration Workshop meeting organised by EUMETSAT in collaboration with USGS, CNES and NASA. Altogether, this represents potentially more than 25 instruments capable of observing the Moon. The main objectives of the workshop were i) to work across agencies with the GSICS Implementation of the ROLO model (GIRO) - a common and validated implementation of the USGS lunar radiometric reference, ii) to share knowledge and expertise on lunar calibration and iii) to generate for the first time a reference dataset that could be used for validation and comparisons. This lunar calibration community endorsed the GIRO to be the established publicly available reference for lunar calibration, directly traceable to the USGS ROLO model. However, further effort is required to reach inter-calibration between instruments, in particular for each instrument team to accurately estimate the over-sampling factor for their images of the Moon. A way to develop a cross-calibration algorithm and GSICS inter-calibration products is proposed. This includes key issues of fixing the GIRO calibration to an absolute scale, addressing spectral differences between instruments, and improving the existing calibration reference, which translates into future updates of the GIRO. The availability of extensive Moon observation datasets will help to further improve this reference and is expected to grow with the availability of additional lunar observations from past, current and future missions. All participants agreed on EUMETSAT pursuing its efforts in developing and maintaining the GIRO in collaboration with USGS to ensure traceability to the reference ROLO model.