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To support the increasing number of planned lunar missions, a collaborative international initiative is underway to conceptualise and establish a lunar satellite constellation for communication and navigation. In this context, the goal of the current paper is to analyse what the obtainable performance is for a lunar lander that executes state estimation employing one-way ranging signals from such a Lunar Navigation Service (LNS). In particular, a small-sized optimised navigation constellation is considered as the main source of measurements, which, coupled with an accelerometer and an altimeter, is used to estimate the lander absolute trajectory during the main braking phase. The guidance is extracted on board by interpolation of a ground-optimised trajectory, followed by a reference-tracking regulator. Two alternative control tuning cases are presented, one targeting high performance, the other targeting low propellant mass. Nominal performance and associated sensitivity analyses assessed the feasibility of supporting such a critical phase with a reduced LNS constellation, reaching final control errors below 500 m , with the better performing one going down to 56 m. Among the two proposed alternatives, the one targeting low fuel expenditure has proven, however, to also be more robust against time and state uncertainty, providing much larger success rates. [ABSTRACT FROM AUTHOR]

Recent studies demonstrate the possibility of navigating in proximity of uncooperative space resident objects by using only monocular images. Despite the results achieved, the development and testing of new algorithms are strongly constrained by the availability of spaceborne image datasets. To overcome this, a new algorithm embedded in a tool to generate synthetic high-fidelity spaceborne image datasets is presented here. The architecture developed can be tailored to a wide range of scenarios and it is based on an open-source ray-tracing software. All assumptions and simplifications adopted are discussed in detail for the different models considered, including a trade-off between accuracy and rendering time. The new method described is subsequently adapted to a baseline scenario where the optical properties of a reference spacecraft model are tuned. Both qualitative and quantitative validations are detailed and successfully carried out for the baseline case, demonstrating the high photo-realism achievable with the proposed method. As a consequence of this main outcome, the paper details the generation of labeled spaceborne image datasets publicly available and reports the analyses that confirm the high level of representativeness, making them suitable for training and testing image-based navigation algorithms. As another outcome, the most comprehensive multi-purpose labeled dataset of validated spaceborne synthetic images currently publicly available is presented. • New methodology to generate spaceborne synthetic images via open-source ray-tracer. • Detailed analysis of atmosphere and topology modeling for celestial bodies. • Definition of a qualitative and quantitative validation step via image comparison. • Successful validation of the proposed approach and generation of validated datasets. • Detail of the largest multi-purpose labeled spaceborne image dataset available. [ABSTRACT FROM AUTHOR]

The exploration of celestial bodies such as the Moon, Mars, or even smaller ones such as comets and asteroids, is the next frontier of space exploration. One of the most interesting and attractive purposes from the scientific point of view in this field, is the capability for a spacecraft to land on such bodies. Monocular cameras are widely adopted to perform this task due to their low cost and system complexity. Nevertheless, image-based algorithms for motion estimation range across different scales of complexities and computational loads. In this paper, a method to perform relative (or local) terrain navigation using frame-to-frame features correspondences and altimeter measurements is presented. The proposed image-based approach relies on the implementation of the implicit extended Kalman filter, which works using nonlinear dynamic models and corrections from measurements that are implicit functions of the state variables. In particular, here, the epipolar constraint, which is a geometric relationship between the feature point position vectors and the camera translation vector, is employed as the implicit measurement fused with altimeter updates. In realistic applications, the image processing routines require a certain amount of time to be executed. For this reason, the presented navigation system entails a fast cycle using altimeter measurements and a slow cycle with image-based updates. Moreover, the intrinsic delay of the feature matching execution is taken into account using a modified extrapolation method. [ABSTRACT FROM AUTHOR]

Formation flying represents a promising opportunity for next space missions, due to its benefits in cost saving, redundancy and fault tolerance given by multiple small and cheap space segments exploitation, with comparable performance of a single, large monolithic spacecraft. That is even more relevant in asteroid exploration, as the harsh environment poses great risks to the probes survival. The low, irregular gravity field, the poor knowledge of shape and composition, and the possible presence of floating particles in the surroundings suggest adopting a low-risk strategy for the exploration of these bodies, delegating proximity operations to multiple nanosatellites, while keeping the main spacecraft at safe distance. As a downside, multiple cooperating spacecraft imply advanced capabilities in accurately reconstructing the relative positions and displacements and in fixing reconfiguration manoeuvres. Moreover, whenever relative distance is very close, agents' guidance and control shall be autonomously computed by the spacecraft, as promptness of commands is not ensured relying on ground segment only. The partially unknown environment exacerbates this issue, demanding navigation and control schemes capable of withstanding potentially large uncertainties in the dynamics. If a binary system is considered, the multiple gravitational sources complicate even more the setup of the formation, requiring an accurate search and selection of operative orbits. This study will be thus defined by three different steps. First the search and development of suitable trajectories to host a reconfigurable satellite formation is carried out. Then the design of a Model Predictive on-board guidance and control law is performed to assess the capability of the spacecraft to execute successful reconfiguration transfers relying on a simplified dynamical model (namely the Circular Restricted Three-Body Problem). At the end the navigation error requirements are derived, in order to ensure the feasibility of the guidance and control scheme. Inspired by the Hera mission, this study explores the aforementioned aspects of the design of a formation for the exploration of the Didymos binary asteroid system, which comprises many complications but also challenges for many other future missions to fly in unexplored environments. [ABSTRACT FROM AUTHOR]

In order to assist the ever growing number of missions planned to visit the Moon, an international effort is being made to design and commission a communication and navigation lunar constellation of satellites. To this purpose, the goal of this study is to provide first a strategy to help the design of a small-sized lunar constellation, entailing different objectives and users to assist. Then, various optimised constellations are tested including the orbital users' filtering architectures, extracting insights on the state estimation achievable and its capability to support critical GNC tasks. Given the early stage design phase in which this study is carried out, the constellation design does not entail strong and clearly defined requirements yet. For such reason, a multi-objective optimisation (MOO) has been employed, which proved to be extremely beneficial, ensuring a correct amount of versatility to provide a large set of Pareto-optimal solutions. Different objectives associated to both various services and users have been put in place, exploring a search space based on classical Keplerian elements of the constellation servicers. To compare the resulting constellations and validate correctly the performance from a navigation perspective the users navigation scheme has been considered to be based on a GNSS/INS tightly coupled formulation. The GNSS receiver measurements are fused with accelerometer outputs through a navigation filter, providing thus a strategy where the absence of GNSS observables is partially covered by an inertial propagation of the on-board state. Attention is here focused on polar orbiters operational in Low Lunar Orbit (LLO) environment and the obtained results showed navigation capabilities good enough to enable correct execution of missions in the low lunar orbital region. [ABSTRACT FROM AUTHOR]

• Phisically-based thermal infrared images rendering chain for artificial satellite targets. • Assessment and comparison of most relevant pixel-level VIS-TIR image fusion techniques. • Fused noisy image processing within the context of uncooperative space-craft relative navigation. State-of-the-art techniques for vision-based relative navigation rely on images acquired in the visible spectral band. Consequently, the accuracy and robustness of the navigation is strongly influenced by the illumination conditions. The exploitation of thermal-infrared images for navigation purposes is studied in the present paper, as a possible solution to improve navigation in close proximity with a target spacecraft. Thermal-infrared images depend on the thermal radiation emitted by the target, hence, they are independent from light conditions; however, they suffer from a poorer texture and a lower contrast with respect to visible ones. This paper proposes pixel-level image fusion to overcome the limitations of the two types of images. The two source images are merged into a more informative one, retaining the strengths of the distinguished sensing modalities. The contribution of this work is twofold: firstly, a realistic thermal infrared images rendering tool for artificial targets is implemented; secondly, different pixel-level visible-thermal infrared images fusion techniques are assessed through qualitative and quantitative performance metrics to ease and improve the subsequent image processing step. The work presents a comprehensive evaluation of the best fusion techniques for on-board implementation, paving the way to the development of a multispectral end-to-end navigation chain. [ABSTRACT FROM AUTHOR]

Complex robotics missions require complex GNC and Robotics algorithms, often using vision sensors. The problem of vision-in-the-loop GNC is addressed using photorealistic simulated images and simple computer vision algorithms, coupled with relative estimation and control of a servicing satellite in close proximity operations with a customer satellite. In the near future it will be required to simulate the behavior of automated servicing missions comprehending also vision data, hence the request for vision-in-the-loop simulations. In this article is proposed a GNC and Robotics scheme for proximity operations between a servicer and a customer satellite through the use of adaptive control and computer vision. The scheme is then put to test through simulation of orbital robot dynamics, sensors and camera inputs, and computer vision algorithm in the loop. • Orbital space robotics require good control capability and relative sensors. • Proposed Joint planning and filtering increase controllability of the robotic arm. • Vision based navigation and adaptive control increase controllability of the servicer. • Testing done with a space robotics simulator and photorealistic images in the loop. [ABSTRACT FROM AUTHOR]

Abstract Nowadays, one of the most active research fields in space engineering is autonomous relative navigation around uncooperative objects. A common approach used to tackle this problem is through vision-based pose determination techniques. This paper investigates the possibility of using non-linear filtering techniques to improve the attitude estimation performance of vision-based methods. Furthermore, a simulation study is presented to compare the proposed nonlinear techniques with the multiplicative extended Kalman filter for attitude estimation. First-order and second-order nonlinear filters are adapted, implemented and tested for relative attitude estimation. Finally, the consequences of uncertainty in the knowledge of the target inertia matrix are investigated. [ABSTRACT FROM AUTHOR]

Abstract: The paper investigates the problem of nonlinear filtering applied to spacecraft navigation. Differential algebraic (DA) techniques are proposed as a valuable tool to implement the higher-order numerical and analytic extended Kalman filters. Working in the DA framework allows us to consistently reduce the required computational effort without losing accuracy. The performance of the proposed filters is assessed on different orbit determination problems with realistic orbit uncertainties. The case of nonlinear measurements is also considered. Numerical simulations show the good performance of the filter in case of both complex dynamics and highly nonlinear measurement problems. [Copyright &y& Elsevier]

Recent studies highlighted the benefits of a support infrastructure located in Cislunar environment, which would ease the design of forthcoming space missions with a favorable access from and to the lunar surface, the Earth and many interplanetary destinations. Multi-body orbits rose a peculiar interest and were selected to stage a human-robotic exploration outpost; the family of Near Rectilinear Halo Orbits (NRHO), in particular, appears specifically suitable in these regards. Among the different capabilities that such outpost will tend to, the docking with other crewed or autonomous vehicles is a key feature that shall be present. Although low Earth orbit (LEO) rendezvous and docking is well assessed, no mission has performed such task in a multi-body gravitational environment. The paper presents a guidance, navigation and control (GNC) framework for 6 degrees of freedom (6DOF) coupled Cislunar rendezvous and docking. A feasible operational rendezvous scenario is detailed and exploited to define open-loop and closed-loop GNC functions for far-range and close-range. Then, the final approach is analyzed, proposing a closed-loop GNC that encompasses coupled translational-rotational dynamics. Vision-based only relative navigation techniques are applied to Cislunar multi-body dynamics to guarantee a coupled state estimation with a simple suite of sensors and a broad applicability range, ranging from passively cooperative to non-cooperative or unknown target spacecraft. [ABSTRACT FROM AUTHOR]