Lunar Gateway Autonomous Orbit Determination and Time Synchronization by the use of one or two small orbiters

In the last years the interest towards the Moon exploration has increased, and one of the key points towards the success of the future missions will be the lunar Gateway. The Gateway will be an outpost orbiting the Moon that will provide support and will enable the return of humans to the Moon. The Gateway will be deployed in a highly elliptical Near-Rectilinear Halo Orbit (NRHO) around the Moon and it will be essential to have an accurate localization of the Gateway that could be exploited as an anchor point for positioning. In this work it is proposed a solution that can enable an autonomous orbit determination of the Gateway exploiting other spacecraft deployed in the same orbit. The measurements considered for Orbit Determination can be simple scalar satellite-to-satellite tracking data, such as inter-satellite range measurements. In addition the Earth GNSS can be used to synchronize the Gateway with the Earth time scale: once the position of the Gateway has been estimated (up to a certain accuracy) thanks to the autonomous orbit determination, the time synchronization can take place collecting GNSS pseudorange and removing the previously aknowledged geopmetric component, by effectively overcoming the limitations due to the inversion of an ill-conditioned navigation problem or to the very high Dilution Of Precision usually experimented by a lunar GNSS user. Different simulations have been performed using a batch weighted least squares orbit determination algorithm in an unperturbed Circular Restricted Three-Body Problem exploiting one or two additional spacecraft, each assumed able to make one of two ISL connection. It was observed that there is no need for large angular separation between the orbiters and the Gateway and that sub-meter accuracy in the 3D orbit determination can be reached also with only one additional satellite. Finally, exploiting the GNSS signals the synchronization error is maintained below few nanoseconds.