## 1 Introduction
In recent years, a study introduced the Linked, Autonomous, Interplanetary Satellite Orbit Navigation (LiAISON) architecture as an alternative way for tracking spacecraft deployed in orbits around libration points, such as the Halo orbits. LiAISON is a concept of OD that uses relative inter-satellite measurements to estimate the absolute states of two or more spacecraft simultaneously [7]. An example of the use of this technique is the Cisluar Autonomous Positioning System Technology Operations and Navigation Experiment (CAPSTONE) mission [8]. It has started its operational activities, and it will serve as a pathfinder for navigation and operations in the same NRHO that will be utilized by NASA's Lunar Gateway. In addition to this objective, CAPSTONE contains a dedicated flight board for the Cisluar Autonomous Positioning System (CAPS), a framework for autonomous or near-autonomous navigation in cisluar space. The navigation architecture is based on inter-satellite radiometric data transferred between the CAPSTONE spacecraft, which is deployed in the NRHO orbit, and the Lunar Reconnaissance Orbiter (LRO), which is deployed in a low lunar orbit [9]. The combination of an Earth-Moon L2 Halo orbit and a low-lunar orbit has been shown to yield enough geometric and dynamic asymmetry such that the states of both spacecraft are observable. The basic working principle proposed by the LiAISON study is based on the assumption that the Halo orbit has a unique size and shape because of the strong asymmetry of the three-body force field. This means that a spacecraft on these types of orbits can track a second spacecraft using inter-satellite measurements and determine their absolute positions and velocities without any Earth-based tracking or any other mathematical constraints. In this work, we propose and evaluate, for a given case study, a method for Orbit Determination and Time Synchronization (OD&T) of spacecraft deployed in the same Halo orbit exploiting an inter-satellite link (ISL) for dual one-way measurements of the range and range rate between spacecraft. The proposed approach not only gives the possibility to have a fully autonomous orbit determination but also a total independent time synchronization of the spacecraft, avoiding the use of the DSN, which can be used only for periodic checks and calibrations. For example, in the case of a single spacecraft mission, such as the JWST or any other scientific observation mission, exploiting only one spacecraft, additional small and low-costorbiters (called \"floating anchors\" hereafter) can be deployed from the same vector in the same orbit to perform the inter-satellite measurements to be used for autonomous OD. The mission's main spacecraft, exploiting the measurements (to and from the floating anchors), will be able to solve the OD problem autonomously and without any operation from the ground. Moreover, for other types of missions, such as for navigation systems like the ones proposed in [10; 11] or telecommunication systems in which, typically, more spacecraft are needed to cover a given region, the additional spacecraft can also be used to provide these services, being telecommunication relays to/from the Earth or other celestial bodies, or providing ranging signals for navigation purposes to other spacecraft or rovers. Finally, they can also be equipped, if needed, with instruments for remote sensing such as SAR, laser altimeters, or others, exploiting the precise orbit of the spacecraft. After the description of the proposed OD and Timing procedure, the possible performances are evaluated for application in a NAV/COM constellation. In particular, in the case study, the possibility of adding two floating anchors to the Lunar Gateway, and to exploit this system of three spacecraft to deliver navigation and telecommunication signals to the Moon's South Pole, is investigated. The paper is arranged as follows: in Section 2, the system concept and the inter-satellite measurements approach are introduced.