Moon Transfer Orbit Navigation Using Signals Coming from the Moon

Abstract

The moon transfer orbit (MTO) is a path that a spacecraft going to the moon actually flies and the spacecraft navigation on the MTO is becoming highly important as space agencies are planning moon exploration projects such as the US Artemis. The spacecraft navigation is a real-time spacecraft orbit determination, which is performed onboard the spacecraft, and for the earth orbiting satellites, the navigation signals emitted from the GPS satellites are processed in real-time using a filtering technique such as the extended Kalman filter. In our previous research (Murata et al., 2021), the GPS navigation accuracy on the MTO reached about 200 meters at the moon altitude by using the GPS signals coming from the far side of the earth. Considering that the GPS navigation accuracy on the low earth orbit (LEO) is currently a few meters, a few hundred meter accuracy is not high enough, although the moon is far away from the earth. The accuracy deterioration was primarily due to the poor geometry of the GPS satellites that became visible from the MTO and the challenge of the MTO GPS navigation became clear. Our aim in this paper is achieving the accuracy less than 100 meters by using the other navigation sources: the lunar navigation satellite system (LNSS) to be deployed in the moon orbits (Murata et al., 2022). Following this background, we have been conducting the simulation evaluation for the MTO navigation accuracy using signals coming from the moon as well. We assume that these signals will be emitted from beacons on the moon surface or the LNSS satellites to be orbiting around the moon (Murata et al., 2022). The signals of these LNSS satellites are those coming from the far side of the moon, similar to the signals of GPS satellites coming from the opposite side of the earth, because these LNSS satellites will be pointed towards the moon to provide the PNT service on the moon surface, especially at the lunar South Pole region. Although there are currently no such signals coming from the moon, as the moon exploration projects proceed, these signals will become acquirable onboard the spacecraft flying to the moon in the cis-lunar space and the use of these signals together with the GPS weak signals will contribute to significantly improving the poor-geometry problem of the GPS satellites mentioned above. In this paper, we provide the simulation evaluation results for the MTO GPS+LNSS navigation accuracy and discuss about the possibility of achieving the navigation accuracy less than 100 meters. As for the LNSS satellites, the constellation shown in Murata et al. (2022) was used.