Updating the Lunar Reference Frame

Introduction: The WGCCRE has made recommendations regarding the lunar reference frame (LRF) [1]. Over the last 2 years both the Artemis III SDT report [2] and the LEAG-MAPSIT LCDP SAT report [3] have included recommendations for an updated lunar reference frame. Park et al. [4] have published new Solar System ephemeris results that include a new lunar laser ranging (LLR) solution and lunar orientation ephemerides. The latter includes the DE440 ephemeris in the Mean Earth/polar axis (ME) frame, which is compatible with their earlier DE421 ME frame recommended for use by the WGCCRE.

Given the recent activities and interest on the LRF, and the expected increase in lunar missions by the various nations, it is appropriate for the WGCCRE to consider updating the recommendations on a LRF. We are soliciting input on such a recommendation.

Issues to consider: The Moon is one of few bodies in the Solar System without a specific longitude defining feature. It may be timely to use an LLR solution to define the LRF, following long-standing IAU and WGCCRE recommendations [1, p. 7]. Currently, a particular such LLR solution is already the underlying basis for the DE421 ME frame. Such a solution and similar future improved solutions could instead serve to directly define the frame in the ME system, and in practice would match in a no-net rotation sense the existing recommended DE421 ME frame.

Separately, the lunar orientation model could now be specified by using the JPL DE440 ephemeris in the ME frame. The new JPL solutions use substantially more available data, and improved modeling compared to the previous (2008) DE421 solution. Differences from the previous model are less than 1 meter during the period 1900–2050. Differences in the underlying LLR solutions are < 1.5 meters. Such differences are not so significant as to be noticeable in the positioning of data products except at the highest current levels of accuracy. This update would nevertheless help to prepare for the best future accuracy by removing one source of error.

We will present the benefits of updating the LRF and weigh them against the burden of changing the established definition.

Request for input: The WGCCRE is requesting feedback from the lunar community on these issues. Is using (the current new JPL) LLR solution to define the LRF appropriate? Is using the DE440 ephemeris in the DE421 ME frame appropriate as a new lunar orientation model? Are there other LLR and lunar ephemeris solutions that could be considered for use in this process? Feedback to the lead author is welcome, preferably by the time of or at the EGU meeting. We hope to complete the next version of our main WGCCRE report this year and possibly include an update for a recommended LRF definition.

References: [1] Archinal et al. (2018) CMDA 130:22. [2] NASA (2020) NASA/SP-20205009602. [3] LEAG-MAPSIT Special Action Team (2021), see MAPSIT website. [4] Park et al. (2021) The JPL Planetary and Lunar Ephemerides DE440 and DE441, Astron. J. 161(3), 105.