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Session B3: Lunar Navigation and Time

Enhanced GNSS-based Positioning in Space Exploiting Inter-Spacecraft Cooperation
Anaïs Delépaut, Alex Minetto, Fabio Dovis, Department of Electronics and Telecommunications (DET) at Politecnico di Torino, Italy; Floor Melman, Pietro Giordano, Directorate of Technology, Engineering and Quality (D/TEC) at ESA-ESTEC, Netherlands; Javier Ventura-Traveset, Directorate of Navigation (D/NAV) at ESA-Toulouse, France
Location: Beacon B

Peer Reviewed

Peer Reviewed

For the following decade, dozens of missions are planned towards the Moon to pave the way for Mars exploration [1-4]. For many years, space missions have been consistently relying on a terrestrial network of deep-space antennas to navigate their way to their respective destination. Although this has proven successful in terms of performance requirements, the rising number of commercial missions on top of governmental ones is likely to congest such a facility [5].
Among the possible solutions, relevant studies address the use of Satellite Radio Navigation Signals ubiquitously in the cislunar volume [6]. As a matter of fact, the Magnetospheric Multiscale (MMS) mission has already proven the use of Global Navigation Satellite System (GNSS) signals practical and highly beneficial up to half the Earth-to-Moon distance [7]. Leveraging on this success, additional work has been performed both in governmental and commercial entities and in 2024, the ESA-SSTL Lunar Pathfinder will be the very first mission to navigate our Moon autonomously solely based on Galileo and GPS signals [8]. This will be possible thanks to the use of a high-sensitivity receiver supplied by a high-gain antenna pointing to Earth [9-15]. Despite this considerable progress related to the use of GNSS signals in Space, the high Dilution of Precision (DOP) resulting from the fact that the signals all come from a similar direction still presents a significant challenge [10,14,15].
As part of the ESA Lunar Radio Navigation Roadmap within the ESA Moonlight initiative, ESA aims to provide a radio navigation service in the cislunar volume, called Lunar Communication and Navigation System (LCNS), to supplement Earth-GNSS signals [16,17]. Indeed, this is necessary to meet the requirements of the more demanding missions as well as those that cannot afford good visibility of GNSS signals. Moon limb obstruction or lower budget allocated to the GNSS navigation subsystem are some of the potential reasons for poor GNSS signals visibility [18].
However, deployment of a constellation (even if limited to a few satellites) will require time and effort and the number of satellites within LCNS will remain limited. In parallel, the use of GNSS in cis-lunar space has clear limitations and in order to cope with these challenges, we propose in this paper the use of a GNSS Cooperative Positioning (GNSS-CP) paradigm, which was already proven to be beneficial for specific terrestrial scenarios. Indeed, such a solution was investigated in the fields of cooperative vehicular networks and mobile users [22]. It allows GNSS users within a network temporarily suffering from a bad satellite visibility and high DOP to be supported by other connected GNSS users in order to improve their position performance [19]. The paradigm has proven successful even with the availability of a single surrounding GNSS user [19-28]. The basic idea of GNSS-CP is to obtain a relative distance measurement between GNSS users by exchanging information on their respective pseudoranges measured w.r.t. the trackable GNSS satellites. This relative distance is then used in the estimation of the PVT solution. Even if noisy and correlated to the other measurements, this additional range measurement is beneficial in the case of low-visibility and high Dilution Of Precision (DOP) scenarios [19,29]. The estimation of this range between users (whose position is affected by errors) relies on the extension of differential GNSS techniques to compute the baseline vector. More specifically, a Weighted Least Square is applied to Double Differences between pseudorange observables, thus leveraging the removal of both satellites and receiver clock bias [27,28].
Thus, considering the number of spacecrafts expected to be found in the cislunar volume in the next years, and with the aim of relaxing the requirements related to the design of a dedicated lunar navigation system, this contribution looks at the benefit of revisiting the concept of Cooperative Positioning to support Space applications in the Moon environment. Technically, a spacecraft with poor GNSS visibility could share GNSS raw measurements through a communication link to another spacecraft, the aiding agent, in order to estimate their baseline vector and, subsequently, to determine an enhanced estimation of its own position [22]. This assumes that the pair of spacecrafts would have a common set of visible GNSS satellites. Resorting to such a technique in the context of GNSS in Space is singularly significant given the well-known challenge that high DOP represents for GNSS users at lunar altitudes. Indeed, using navigation assistance from another spacecraft located away from the aided agent-to-Earth line could substantially improve the spatial geometry of the GNSS assets by actively contributing in the state estimation. Furthermore, when compared to other direct radio-ranging techniques such as the Cislunar Autonomous Positioning System (CAPS) from NASA [29], the GNSS-CP process stands out in that it does not require any dedicated equipment as it can rely on pre-existent data link present aboard the spacecraft, hence lower in power consumption and implementation cost [19]. As a result, it also does not require the collaborating spacecrafts to be in Line-of-Sight, enabling missions located in different zones of the Moon to contribute as well for improved geometrical distribution of the ranging sources.
In this paper, we present the preliminary results obtained during the study of the use of GNSS-CP for Lunar missions. To assess the quality of the algorithm on space applications, a realistic scenario, expected to be in Space in 2024, is chosen: the Volatile and Mineralogy Mapping Orbiter (VMMO) mission in its Low Lunar Orbit represents the to-be-aided agent while the Lunar Pathfinder (LPF) in its Elliptical Lunar Frozen Orbit acts as the aiding agent, given its more favourable visibility of the Earth GNSS constellation [8,30-32]. Both spacecrafts receive navigation signals from Earth-GNSS Galileo and GPS constellations as well as one LCNS satellite. Additionally, as it will be the case in practice, a data link in S-Band between both missions is available [32]. The algorithm to retrieve the inter-agent distance between VMMO and LPF is defined and the accuracy of the baseline estimates is assessed. Contextually, the improvement of the Dilution of Precision resulting from the collaboration with the LPF is evaluated. Results show that collaboration between lunar missions could support GNSS-based positioning in the cislunar volume. Indeed, the hybridization of GNSS measurements with the estimates of inter-agents baseline vectors can increase the performance of standalone GNSS positioning by improving the spatial distribution of the assets contributing to the navigation solution estimation. This works especially well when the aiding spacecrafts are located on or near the plane including the aided agent and perpendicular to the aided agent-to-earth line. This results in a great benefit for the final GNSS user position accuracy. This research is particularly significant as it could dramatically support the LCNS while being a non-invasive solution. Eventually, it could potentially lower the budget needed for meeting the various Lunar missions navigation requirements.
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