Four Satellites to Navigate the Moon’s South Pole: An Orbital Study

Abstract

New discovery of the ice on the Lunar South Pole has boosted several international initiatives to bring mankind back to the Moon and make our natural satellite the outpost to test technologies enabling deep space exploration mission (e.g. journey to Mars). The availability of a Lunar Navigation System is considered a key enabler for the design and the implementation of future lunar missions. The growing complexity of the missions around (and on) the lunar surface will require continuous and reliable positioning and timing services. Due to the high scientific interest in the South Pole area, it is reasonable that the near-future lunar exploration missions will star from there, and only in a second stage they may be extended to cover the entire lunar surface. This article aims to provide results of a design activity on the definition of the best orbital configuration for a constellation of four satellites. The proposed system is supposed to be enable for the provision of navigation (and timing) services to users operating on the Lunar South Pole (defined from 70°S to 90°S of latitude) and/or orbiting around our natural satellite. Different constellation orbital configurations have been evaluated through a comparison among well-known orbit families, already used for navigation on Earth, and new type of orbits more suited to exploration missions. These alternative configurations have been analysed with respect to the peculiar gravitational environment around the Moon, in which the third body effects of the Earth gravitational field are much more relevant than other orbital perturbation sources. The evaluated constellations are based on: • Walker orbits; • Elliptical Lunar Frozen orbit(ELFO) s; • Earth-Moon Libration Points orbits. The different orbital constellation have been optimized in order to maximize the service availability and continuity, as well as in order to minimize, the PDOP values on the South Pole region, considering a baseline constellation of four satellites. One of the most common issues when designing a constellation with a very reduced number of navigation satellites is indeed the presence of singularity values for PDOP, due to the very poor geometric configuration. Therefore, PDOP minimization with only 4 satellites is one of the main goals of this study as it would allow users to perform complete PVT even with a reduced, primitive constellation. The stability of the constellation (with the related impact on station keeping) has been also assessed together with the cost of constellation deployment (in terms of Delta-V) and the stability of the communication link with the Earth in terms of visibility with the Earth Ground Stations has also been taken in account. The main results of the trade-off analyses are presented and deeply discussed in this paper. Afterwards, a selection of the best candidate configuration solution (among the analysed ones), for a Lunar Navigation System of four satellites enabling services provision on the South Pole area, will be performed and its main performance described.