A Proposal for Using Very Long Baseline Interferometer Observations to Improve Lunar Ephemerides

One result of future human activity on the lunar surface will be the installation of a number of radio transmitters there for communication with Earth. These transmitters present an opportunity to add a new observational data type, that of Very Long Baseline Interferometry (VLBI), to the construction of lunar ephemerides. Precision lunar ephemerides for the past fifty years have been constructed using a single data type, lunar laser ranging (LLR), because it is at least an order of magnitude more accurate than any other data type currently available. Modern LLR data has a one-way range uncertainty of less than a centimeter, and the soon-to-be deployed next generation of laser retroreflectors should reduce that uncertainty by about an order of magnitude. LLR provides only a single dimension, range. Reliance on a single dimension results in large covariance matrix values for the adjustable parameters of the ephemeris solution. VLBI has the ability to match LLR’s accuracy with a data type that offers three parameters complimentary to LLR’s range: X and Y plane-of-sky positions and radial velocity. The addition of VLBI to lunar ephemeris observational data types will potentially reduce covariance matrix values, and past proposals to establish lunar VLBI beacons were made for just this reason. The VLBI data reduction process for analyzing near-field narrow-band observations, such as those from radio transmitters on the lunar surface, is well developed, and in fact was used 50 years ago with the Apollo Lunar Surface Experiments Packages. The primary intent of this proposal is to investigate the viability of using the existing VLBI infrastructure with future lunar radio transmitters.