Optical solid Corner Cube Retro-reflectors (CCR) are often implemented to allow laser tracking of satellites from ground. In order to achieve a measureable return
signal, such corner cubes are typically rather large and, as conventional corner cubes are made of glass, also high in mass. A hollow CCR, made of three orthogonal flat
mirror surfaces, offers theoretically superior performance and a mass reduction compared to solid CCR’s.

However, technologically there are still too many disadvantages and uncertainties for hollow cubes to be considered a viable option. Currently the mass advantage that
hollow cubes could potentially allow is lost by the need for a stable opto-mechanical support structure, and the tolerance accuracy achievable does not match that of
solid cubes.

Despite this, there is still a clear interest in qualifying hollow cube technology for satellite PNT. It will possibly provide added value in the form of mass reduction, and
improved ranging precision (which enables better independent precision orbit determination, as well as geodesy applications). Additionally, it could be deployed on the
Moon for Earth-Moon distance measurements, which is strongly desired by the scientific community.

Hollow CCRs have the potential to deliver the same functionality as solid CCRs, and with improved performance and lower mass. However, an advance in technology is
required to achieve long-term mechanical stability and accurate control of the corner cube angles.

A very accurate control of the corner cube angles, which include a correction for velocity aberrations, is critical as this defines the return direction of the reflected beam.
Different manufacturing techniques can be investigated that could achieve this. In case bonding of the mirrors is required, this could for example be done using a glue
or molecular bonding. In this respect, a monolithic hollow CCR design would be attractive, however, the implementation of the mirror surfaces is not obvious.