Over recent years, there has been a growing interest in deep space missions involving small satellites. These missions have not only demonstrated their potential through remarkable achievements but have also spotlighted the critical role they will play in future space explorations. Simultaneously, satellite formations have gained popularity, opening up new possibilities for deep space exploration. Traditionally, these missions have relied heavily on ground-based radiometric tracking for navigation. However, ground-based operations pose several challenges, including limited tracking resources due to the increasing number of missions and high operational costs. In response to these challenges, autonomous operations, with minimal or no human intervention, emerge as a beneficial strategy, and navigation stands as a key area that could greatly benefit from autonomous operations.

In this context, various autonomous navigation strategies exist, and one of them stands out as a promising approach: Crosslink navigation, using existing systems to provide navigation solutions based on inter-satellite measurements, which primarily offers relative navigation solutions but can also facilitate absolute navigation solutions when integrated with ground-based tracking. However, absolute state knowledge, crucial for tasks such as station-keeping, often relies on ground-based commands, limiting autonomy. Alternatively, Satellite-to-Satellite Tracking (SST) data can be used for absolute state estimation, where an on-board navigation filter estimates spacecraft position and velocity, i.e. with respect to a fixed reference frame. Previous studies have shown that depending on the orbital dynamics, SST-data can provide absolute state estimation. However, this is not always straightforward, especially when inter-satellite measurements are not accurate or the observation geometry is not optimal. Since inter-satellite measurements cannot always be collected due to operational constraints, careful planning of optimal tracking windows is required. This planning can be challenging when considering possibly conflicting operational needs, such as commanding. Moreover, since radio frequency measurement techniques are used to derive navigation data, system performances must be investigated, considering varying systematic and random errors. It remained a significant challenge to determine which types of navigation data—range, range-rate, or angle—yield the most effective navigation solutions across different deep space scenarios. Since real-time navigation solutions may be needed, designing a robust on-board estimation filter can be challenging, including decisions on which parameters to be estimated or neglected.

Given these complexities, this research investigated SST-based autonomous orbit determination for satellite formations, consisting of small spacecraft, aiming to enhance current methodologies and explore new capabilities in both cislunar and deep space environments.@en

This manuscript aims to present and evaluate the applicability of combining optical line-of-sight (LoS) navigation with crosslink radiometric navigation for deep-space cruising distributed space systems. To do so, a set of four distributed space systems architectures is presented, and for each of those, the applicability of the combination is evaluated, comparing it to the baseline solutions, which are based on only optical navigation. The comparison is done by studying the performance in a circular heliocentric orbit in seven different time intervals (ranging from 2024 to 2032) and exploiting the observation of all the pairs of planets from Mercury to Saturn. The distance between spacecraft is kept around 200 km. Later, a NEA mission test case is generated in order to explore the applicability to a more realistic case. This analysis shows that the technique can also cope with a variable inter-satellite distance, and the best performance is obtained when the spacecraft get closer to each other.

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In recent years, there has been a growing interest in lunar missions, particularly with the growing role of small satellites facilitated by piggyback launch opportunities. Typically, ground-based radiometric tracking is the workhorse to establish the necessary navigation solution in these missions, however, this could be expensive, while small satellites development is expected to be at low cost. To address this challenge, autonomous navigation presents a potential solution: this study explores the satellite-to-satellite tracking-based autonomous on-board orbit determination method for a satellite formation in cislunar space. Several factors affect the performance of orbit determination, and one critical aspect is the timing of tracking windows. Basically, it is crucial to determine when to collect the most useful observations to optimize the outcome of the navigation filter. In some cases, there might be operational constraints such as inter-satellite distance due to the limited onboard power for ranging. This study investigates particle swarm optimization-based satellite-to-satellite tracking window planning. The findings of this work demonstrate that particle swarm optimization offers a near-optimal solution for tracking windows, taking into account constraints arising from the spacecraft itself or from other design choices. In summary, particle swarm optimization provides near-optimal tracking windows by minimizing the overall orbit determination error. The results presented have the potential to enhance the design of satellite formations performing autonomous on-board orbit determination and contribute to cost- effective mission planning solutions.@en

The Earth-Moon system is constantly bombarded by meteoroids of different size and impact speed. Observation of the impacts on the Moon can enable thorough characterization of the Lunar meteoroid flux, which is similar to that of the Earth. While Earth-based Lunar observations are restricted by weather, geometric and illumination conditions, a Lunar-based observation campaign can improve the detection rate and, when observing the Lunar far side, complement in both space and time the observations taken from Earth. The Lunar Meteoroid Impact Observer (LUMIO), one of the two winning concepts of the ESA SysNova Lunar CubeSats for Exploration challenge, is a mission designed to observe, quantify, and characterize the micro-meteoroid impacts on the Lunar far side. It is based on a 12U CubeSat that carries the LUMIO-Cam, a custom-designed optical instrument capable of detecting light flashes in the visible spectrum. The spacecraft is placed on a halo orbit about the Earth–Moon L2 point, where permanent full-disk observation of the Lunar far side can be performed with excellent quality, given the absence of Earth background noise. After passing Phase 0 and an independent feasibility study in the ESA Concurrent Design Facility, the mission has successfully completed its Phase A in March 2021. Although the Phase 0 design of the LUMIO spacecraft was assessed as feasible by the ESA CDF study, a number of critical issues were identified, which have been tackled by the Phase A design. The paper presents the outcome of this Phase A design effort for the LUMIO spacecraft. Particularly relevant changes or updates in the spacecraft design include: a consolidated design of the LUMIO-Cam, with longer baffle for straylight protection; a set of ADCS sensors and actuators with increased redundancy; a combination of Direct-to-Earth communication and inter-satellite link with a mothership in Lunar orbit; use of Earth ranging to complement and validate the current innovative autonomous navigation strategy based on optical observations of the Moon by means of the LUMIO-Cam; re-assessment of the COTS components selection for the power and propulsion systems.

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This study provides a performance analysis of radiometric autonomous navigation for the lunar satellite network topologies formed by three spacecraft at various orbits. This work is built on the Linked Autonomous Interplanetary Satellite Orbit Navigation (LiAISON) method and uses mesh (distributed) and centralized (star) network topologies. The optimal interlink network topologies and Distributed Satellite Systems (DSS) geometry have been investigated based on the Circular-Restricted Three-Body problem (CRTBP) and the Extended Kalman Filter (EKF) for state estimation. The network topologies consisted of all the possible combinations of 16 spacecraft at various L1/L2 Halo, Lyapunov, and Lunar orbits. It has been shown that the autonomous navigation system provided better state estimation results for the mesh topology than for the centralized topology. Overall, the lunar satellite network topologies consisting of orbits with large inter-satellite link distances and short orbital periods would bene_t most from the radiometric autonomous navigation. @en

The Lunar Meteoroid Impact Observer (LUMIO) is a CubeSat mission at the Earth-Moon Lagrangian point 2 (L2) designed to observe, quantify, and characterize the meteoroid impacts by detecting their flashes on the Lunar farside. LUMIO can be deployed as one of the payloads in the NASA Commercial Lunar Payload System or from Artemis-2 mission to a low Lunar orbit and to demonstrate autonomous navigation capabilities to reach its operational orbit around the Earth-Moon L2. From there, its scientific mission to map and investigate the spatial and temporal characteristics of meteoroids impacting the Lunar surface will start and is expected to last for one year. LUMIO is a 12U CubeSat including a dedicated camera to monitor impact flashes in the visible and near-infrared spectrum, and also allows estimating the impact of temperature and energy. Optical navigation using the payload camera will also demonstrate increased on-board autonomy and drastically reduced mission costs. Navigation validation will be carried out using standard ground-based radiometric techniques enabled by a miniaturized X-band coherent transponder on-board. LUMIO can also use an inter-satellite link for telemetry and control via a commercial Lunar data relay system, providing a redundant communication system and lowering the need for high-gain ground stations for routine operations. The satellite bus derives from a commercial version designed for Low Earth Orbit and it will feature several improvements to operate in the Lunar environment, including a more advanced thermal control and radiation shielding. Commercial Off-The-Shelf systems will require a radiation screening and this will contribute to maintain the mission budget low and aim at a launch date in 2024.@en

In this study, the application of the Linked Autonomous Interplanetary Satellite Orbit Navigation (LiAISON) method for cislunar satellite formations is investigated considering range only and range-rate only measurements. The LiAISON method provides an autonomous orbit determination solution solely using satellite-to-satellite measurements such as range and/or range-rate. This paper presents a comparison between range only and range-rate only measurements in satellite formations at cislunar space including the Earth-Moon L!, L" and Lunar orbits by presenting the results of Monte Carlo simulations and observability analysis. The results show that range observations in general provide better state estimations than range-rate observations for cislunar satellite formations in the autonomous navigation applications. However, range-rate only measurements could be an alternative to range-only measurements if range measurements are not precise and high precise range-rate measurements could be collected on-board. It has been found that range only measurements could be good enough to meet the orbit determination requirements for certain small satellite missions and allow to simplify the communication system design and reduce power usage. @en

In recent years, there is a growing interest in small satellites for deep space exploration. The current approach for planetary navigation is based on ground-based radiometric tracking. A new era of low-cost small satellites for space exploration will require autonomous deep space navigation. This will decrease the reliance on ground-based tracking and provide a substantial reduction in operational costs because of crowded communication networks. In addition, it will be an enabler for future missions currently impossible. This review investigates available deep space navigation methods from an autonomy perspective, considering trends in proposed deep space small satellite missions. Autonomous crosslink radiometric navigation, which is one of the best methods for small satellites due to its simplicity and the use of existing technologies, is studied, including available measurement methods, enabling technologies, and applicability to the currently proposed missions. The main objective of this study is to fill the gap in the scientific literature on the autonomous deep space navigation methods, deeply for crosslink radiometric navigation and to aim at showing the potential advantages that this technique could offer to the missions being analyzed. In this study, a total of 64 proposed deep space small satellite missions have been analyzed found from a variety of sources including journal papers, conference proceedings, and mission websites. In those missions, the most popular destinations are found to be cislunar space and small bodies with the purpose of surface mapping and characterization. Even though various autonomous navigation methods have been proposed for those missions, most of them have planned to use the traditional ground-based radiometric tracking for navigation purposes. This study also shows that more than half of the missions can benefit from the crosslink radiometric navigation through the inter-satellite link.

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Recent advances in space technology provide an opportunity for small satellites to be launched in cislunar space. However, tracking these small satellites still depends on ground-based operations. Autonomous navigation could be a possible solution considering the challenges presented by costly ground operations and limited onboard power available for small satellites. There have been various studies on autonomous navigation methods for cislunar missions. One of them, LiAISON, provides an autonomous orbit determination solution solely using inter-satellite measurements. This study aims at providing a detailed performance analysis of crosslink radiometric measurements based on autonomous orbit determination for cislunar small satellite formations considering the effects of measurement type, measurement accuracy, bias, formation geometry, and network topology. This study shows that range observations provide better state estimation performance than range-rate observations for the autonomous navigation system in cislunar space. Line-of-sight angle measurements derived from radiometric measurements do not improve the overall system performance. In addition, less precise crosslink measurement methods could be an option for formations in highly observable orbital configurations. It was found that measurement biases and measurements with high intervals reduce the overall system performance. In case there are more than two spacecraft in the formation, the navigation system in the mesh topology provides a better overall state estimation than the centralized topology.

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This study presents an autonomous orbit determination system based on crosslink radiometric measurements applied to a future lunar CubeSat mission to clearly highlight its advantages with respect to existing ground-based navigation strategies. This work is based on the Linked Autonomous Interplanetary Satellite Orbit Navigation (LiAISON) method which provides an autonomous navigation solution solely using satellite-to-satellite measurements, such as range and/or range-rate, to estimate absolute spacecraft states when at least one of the involved spacecraft has an orbit with a unique size, shape, and orientation. The lunar vicinity is a perfect candidate for this type of application due to the asymmetrical gravity field: the selected lunar mission, an Earth-Moon L2 (EML2) Halo orbiter, has an inter-satellite link between a lunar elliptical frozen orbiter. Simulation results show that, even in case of high-measurement errors (in the order of 100 m, 1σ), the navigation filter estimates the true states of spacecraft at EML2 with an error in the order of 500 m for position, and 2 mm/s for velocity, respectively and the elliptical lunar frozen orbiter states can be estimated in the order of 100 m for position and 1 cm/s for velocity, respectively. This study shows that range-only measurements provide better state estimation than range-rate-only measurements for this specific situation. Different bias handling strategies are also investigated. It has been found that even a less accurate ranging method, such as data-aided ranging, provides a sufficient orbit determination solution. This would simplify the communication system design for the selected CubeSat mission. The most observable states are found to be position states of the lunar orbiter via the observability analysis. In addition, the best tracking windows are also investigated for the selected mission scenario.@en

The Lunar Meteoroid Impacts Observer (LUMIO) is a CubeSat mission to observe, quantify, and characterize the meteoroid impacts on the lunar farside by detecting their flashes. This complements the knowledge gathered by Earth-based observations of the lunar nearside, thus synthesizing global information on the lunar meteoroid environment and contributing to the lunar situational awareness. The goal of LUMIO is to advance our current knowledge of meteoroid models in the solar system. In this work, we present the methodology devised to predict the scientific contribution of LUMIO. Our approach relies on combined modeling and simulation of payload, orbit, and environment. The analyses carried out have been used to drive the design of the LUMIO mission and its payload, the LUMIO-Cam. A payload radiometric model is derived and exploited to assess the quality of the scientific measurements. A dedicated study about straylight rejection is carried out to assess how straylight noise affects LUMIO-Cam measurements. Our results indicate that a 150 mm baffle grants good performance when the Sun angle is between 20° and 90°. Furthermore, the present-day LUMIO mission has the potential to detect more than 6000 impact flashes during the activity peak of the Geminids in 2024 in the range of the equivalent impact kinetic energy at Earth of [10 ⁻⁶,10 ⁻¹]kton TNT Equivalent. Compared to previous programmes, LUMIO could refine information and fill the knowledge gap about the meteoroid population in the ranges of the equivalent impact kinetic energy at Earth of [10 ⁻⁶,10 ⁻⁴]kton TNT Equivalent and [10 ⁻⁴,10 ⁻¹]kton TNT Equivalent, respectively.

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This study investigates the application of the Linked Autonomous Interplanetary Satellite Orbit Navigation (LiAI-SON) technique for multiple small spacecraft in cislunar orbits considering high inter-satellite range measurement errors. The LiAISON method provides an autonomous orbit determination solution using crosslink measurements such as range, and/or range-rate. Inter-satellite ranging can be done via conventional tone or code based methods. Considering the limited on-board transmission power available on small satellites, ranging and data transfer, required to cope with the limited contact time, introduce further observable degradation and limiting performance. For such cases, and to increase the supported data rates, telemetry ranging and time-derived ranging architectures can be used. Unfortunately, in time-derived methods, measurements are not as accurate as using other methods, limiting the ap-plicability of such technique only to few missions. This paper presents a simulation based analysis to understand the limits of LiAISON for a multi-spacecraft mission at the Earth-Moon L1, L2 Halo and Lunar orbits considering high inter-satellite measurements errors due to time-derived and telemetry-based ranging methods and without Doppler measurements. This is specifically targeted at small satellites with limited power budgets and radio links lacking coherent Doppler tracking. The simulation results show that the LiAISON-based autonomous orbit determination works well for configurations of cislunar orbits, having the link between Lagrangian and Lunar orbits, even with high crosslink ranging errors.@en

The Lunar Meteoroid Impact Observer (LUMIO) is a mission designed to observe, quantify, and characterize the meteoroid impacts by detecting their flashes on the lunar far side. Earth-based lunar observations are restricted by weather, geometric and illumination conditions, while a lunar orbiter can improve the detection rate of lunar meteoroid impact flashes, as it would allow for longer monitoring periods. This paper will focus on the communications and radio navigation system of the mission, designed for the ESA roadmap for lunar exploration. LUMIO has been designed to operate autonomously after deployment from a lunar mother spacecraft in a low inclination lunar orbit and to reach without human intervention his final destination orbit close to the Earth-Moon L2 point, where science can be carried out. Being the destination orbit always in view from Earth (despite a distance of 460000 - 480000 km), Direct-to-Earth communication was added to the mission as a mean to reduce risk and allow independent verification of several of the innovative technologies that would be demonstrated, first of all autonomous navigation. A detailed link budget analysis will be presented for all mission phases for both the link with the mother spacecraft in low lunar orbit and the link with Earth. Beside defining the achievable data transfer, we will focus also on evaluating the available ground stations to better evaluate mission cost with respect to science return. Radio-navigation performances will also be evaluated to estimate the position and relative velocity accuracy, given also the limited performances available for the on-board navigation transponder. This will help also better defining the on-board autonomous navigation system, constraining the total error budget. Further strategies, such as beacon tones, will be evaluated to lower the overall operational cost by employing continuous monitoring with a low performances ground station and, only when needed, perform high speed downlink using a deep-space class ground station. This strategy is considered of extreme importance, especially for small missions, to allow opportunistic operations on high gain antennas, given their very busy schedule. Keywords: LUMIO, CubeSat, Lunar, Radio, link @en