Steering multiple laser beams using spatial light modulators (SLMs) creates unwanted diffraction and reflections that are not modulated by the SLM, which can make beam tracking difficult. A novel, to the best of our knowledge, and simple beam steering methodology is proposed, which aims at reducing the influence of this clutter while maintaining tracking performance. The beam(s) are deliberately defocused before steering with a superposition of a phase ramp and Fresnel lens (PRFL) phase screen on the SLM. As a result, the non-modulated reflections and diffracted light are decreased in relative intensity to the steered beam, in turn allowing simple and standard peak intensity and center of gravity (CG) algorithms for tracking. Hardware demonstration shows tracking performance using the PRFL remained on-par with more complex filtering approaches while adding no additional hardware. This method has potential to improve the communication performance of multi-beam laser communication terminals.

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Humans are embarking on a new era of space exploration with the plan of sending crewed spacecraft to the Moon, Mars, and beyond. Extravehicular activities (EVAs) will be an essential part of the scientific activities to be carried out in these missions, and they will involve extensive geological fieldwork. These EVAs entail many challenges as real-time support from ground control cannot be provided to astronauts. Hence, new human–machine interfaces are urgently needed to enhance mission autonomy for astronauts and reduce ground communication dependability for real-time operations. This study introduces an Augmented Reality (AR) Internet of Things tool for astronauts to carry out geological activities. It proposes a theoretically-informed user-centred design method supported by expert feedback and an evaluation method. The tool was assessed via questionnaires and semi-structured interviews with European Space Agency (ESA) astronauts and geological field activities experts. Content analysis of the interviews revealed that user satisfaction was the first most mentioned (32% of 139 quotes) usability aspect. Key design factors identified were: displaying solely important information in the field of view while adjusting it to the user's visual acuity, easy usage, extensibility, and simplicity. User interaction was the second most mentioned (24% of 139 quotes) usability aspect, with voice seen as the most intuitive input. Finally, this research highlights important factors determining the usability and operational feasibility of an AR tool for analogue training missions and provides a foundation for future design iterations and an eventual integration of AR into the spacesuit's visor.

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This article focuses on the validation of a classical PID controller scheme for flexible spacecraft with regards to the effect of parameter uncertainty on system stability and pointing precision. A high-fidelity simulation environment with external disturbances was built in Simulink using a control-oriented model of an Earth-observing satellite with a flexible appendage and on-board microvibration sources in orbit around the planet. Then, a PID control loop was designed with sensor dynamics, time delay behaviour, and a smooth trajectory generator. After declaring the natural frequencies, damping ratio, and rotation angle of the appendage, as well as the propellant tank mass to be uncertain, two worst-case scenarios were identified. Comparing the response of worst-case systems with nominal settings, only a minor drop has been found in the phase margins, with little to no difference in the pointing errors (smaller than ±2 arcsec for both roll and pitch).

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Multiple CubeSat altimeters can work independently or corporately to form altimeter constellations. Different configurations of the constellations can acquire distinguished advantages: improved spatial/temporal sampling and high cross-track resolution, which will be helpful for observations of oceanic small-scale structures and weather forecasting. Compared to single conventional altimeters, CubeSat altimeter constellations may achieve better performances with lower costs. To fully understand these systems, this article focuses on the performance analysis and methodology for CubeSat altimeter constellations. Besides the typical analyses of the resolution, revisit, and absolute sea surface height (SSH) accuracy, the performance analysis was conducted by considering the characteristics of multiple measurements provided by CubeSat altimeter constellations. Local and global spatial sampling performances are investigated for various constellations and compared by sampling density and swath size. Moreover, relative SSH accuracy is introduced and evaluated based on the spatial structure functions of errors to effectively evaluate the measurement performance. Related system requirements on power, delta-v, etc., to achieve the performance are also discussed, which ensures that the analysis fits the boundary conditions of implementation. Finally, different concepts of the CubeSat altimeter constellations are compared, where their limitations and possible solutions are also discussed.

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Presentation at the 2nd optical wireless communication conference 28-9-2021@en

The purpose of this research was to investigate the suitability of the Fused Filament Fabrication (FFF) process for low pressure/vacuum environment. This included investigating the ability of an FFF printer to function in a vacuum and evaluation of the dimensional accuracy and mechanical properties of the manufactured components. For this purpose, a commercially available FFF printer using polycarbonate as raw material was placed in a vacuum environment of 10 mbar. Test components were then fabricated in vacuum with a control group fabricated in a normal atmosphere (1 bar). Test components were evaluated for dimensional and mass accuracy, quality and presence of defects. Flexural, tensile and compressive testing was carried out according to ASTM D790, D638 and D695 respectively. Dimensional analysis of components showed equivalent small deviation for both environments. Components fabricated in the vacuum environment had 5.4% higher tensile yield strength and 59% higher extension at break compared to components printed in a normal atmosphere indicating an increased strength and ductility. Components tested in compression had approximately 11.2% higher compressive strength when printed in a vacuum environment. No differences were observed during the flexural test. In space, due to the vacuum environment, polymers and organic material are susceptible to release molecules via an outgassing process. Assessment of the molecular organic contamination generate during the printing process in vacuum is low and seems to mostly originated from the components of the printer. The results provided demonstrated the possibility to use the FFF process in a vacuum environment to fabricate dimensionally accurate, high-quality polycarbonate components with a variety of geometries without loss of mechanical performance. This work provides a proof of concept that FFF can be used to develop out-of-earth manufacturing technologies (in orbit/in space/on planet) allowing part production for new maintenance and repair strategy or to potentially manufacture entire structure more efficiently overpassing launch constrain by using only raw material brought from earth.

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Free-space optical communication has proven to have many advantages over traditional radio communications. For instance, the hardware power efficiency and limited beam spread increase data rates for lower power consumption. Furthermore, the technology does not suffer from decreased bandwidth due to crowding. Free-space optical communication is usually done using laser communication terminals. In the past the focus has been on increasing the data rates for a single link and this has led to an exponential rise in the data rate performance. However, these modules have been limited to single beams and are hence capable of only one link. This decreases the number of users, networking, and relay capabilities of optical communication satellites. The advent of MEMS Micro-Mirror Arrays (MMA) and Spatial Light Modulators (SLM) have allowed for compact and lightweight control of wavefronts. These applications would also allow for scalable independent steering of multiple laser beams. The Delft University of Technology is conducting a study of a compact and scalable multi-beam terminal using these beam steering methods[1]. The terminal consists of a MEMS MMA for high frequency response in the aperture with a high resolution SLM for the beam steering and shaping. This terminal is designed for spacecraft-to-ground and spacecraft-to-spacecraft duplex communications. It is expected to support up to 10 or more duplex links in one terminal and, therefore, suitable for usage in small satellites and mega-constellations. The high resolution can also be used for sub-aperture wavefront correction in future. This paper discusses the design of the multi-beam terminal.@en

The tethered formation system has been widely studied due to its extensive use in aerospace engineering, such as Earth observation, orbital location, and deep space exploration. The deployment of such a multitethered system is a problem because of the oscillations and complex formation maintenance caused by the space tether's elasticity and flexibility. In this article, a triangle tethered formation system is modeled, and an exact stable condition for the system's maintaining is carefully analyzed, which is given as the desired trajectories; then, a new control scheme is designed for its spinning deployment and stable maintenance. In the proposed scheme, a novel second-order sliding mode controller is given with a designed nonsingular sliding-variable. Based on the theoretical proof, the addressed sliding variable from the arbitrary initial condition can converge to the manifold in finite time, and then sliding to the equilibrium in finite time as well. The simulation results show that compared with classic second sliding-mode control, the proposed scheme can speed up the convergence of the states and sliding variables.

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Laser satellite communications provides an attractive application for optical communications. It provides more bandwidth with improved security at potentially a lower cost due to reduced size weight and power (SWAP) and reduced license cost compared to radio frequencies (RF) communications. These advantages have been recognized for years and lead to the successful European data relay service (EDRS) [1]. This service provides the proof that optical communications can be used as a reliable means of satellite communications. In addition, the inherently save quantum key distribution (QKD) technology is demonstrated in a satellite to ground link in [2]...@en

This paper presents a new approach for autonomous reconfiguration of distributed space systems, which ensures safe guidance of spacecraft formations towards the desired patterns while optimizing the total propellant consumption. The orbital transfer is reduced to the form of a convex optimization problem to guarantee rapid computation of control laws. Hence, tasks are iteratively assigned to the component platforms to detect the best reconfiguration strategy. The path-planning is entrusted to a reference satellite of the cluster, that coordinates the remaining ones by means of a procedure based on genetic algorithms. Two methods are proposed, depending on the organizational architecture of the spacecraft formation. In the first one, the maneuver is completely planned by the reference satellite, that determines final tasks and control actions for the whole cluster. As an alternative to such a fully-centralized approach, a distributed version of the algorithm is proposed: tasks are sorted by the reference satellite and transfer orbits are computed by exploiting the computational resources of the whole cluster. Whatever the considered framework, both the planners ensure a safe transition of the formation towards the target geometry. Simulation results show that, when relative distances are of the order of hundreds of meters, a mean delta-v per satellite of the order of 0.1 m/s is required to reconfigure LEO clusters within one orbital period.

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This paper presents innovative approaches for guidance of spacecraft formations during reconfiguration, developed under the paradigms of cluster autonomy and safe maneuverability. A task-assignment algorithm identifies those satellites that need to be rearranged in order to minimize the total propellant consumption. The decision process relies on convex optimization procedures which estimate control laws and maneuvering trajectories, accounting for thruster technology constraints and collision avoidance. The path-planning has a twofold implementation. In the centralized approach, the reconfiguration strategy is elaborated by a single unit: the maneuvering trajectories are generated considering solely collision avoidance constraints at expenses of computational costs. The alternative method is conceived such that each satellite autonomously determines its tasks. This de-centralized architecture allows the algorithm to run faster, but entails a higher traffic on the intersatellite communication link. In addition, collision avoidance is approximated to make each optimization independent from the other. For preliminary validation of the developed routines, a specific mission scenario is considered, with a distributed Synthetic Aperture Radar in a low Earth orbit that has to be reconfigured for tomographic applications. Simulation results show that, once remote-sensing requirements are specified within the limits set by standard products, the developed algorithms can manage autonomously the maneuver towards the corresponding operative pattern. In details, the centralized approach allows the reconfiguration to be handled with mean delta-v savings around 30%. On the contrary, strategies are elaborated by the decentralized planners about 7 times faster.

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Orbiting Low Frequency Array for Radio Astronomy (OLFAR) is a radio astronomy mission that has been studied since 2010 by several Dutch universities and research institutes. The mission aims at producing sky maps by collecting cosmic signals at ultra-low wavelengths regimes, below the 30 MHz frequency band. A satellite swarm comprising more than 10 satellites equipped with passive antennas will be deployed in space where the radio frequency interference can be minimized, e.g., on the far side of the moon. So far, several studies have been devoted to design the space segment, which consists of the payload and platform elements. However, the ground segment, particularly the mission planning system has not yet been designed in detail. In this paper, a systematical mission planning method for OLFAR is presented after a mission planning problem is formulated based on the current satellite design. @en

Employing a satellite swarm for radio astronomy has been continuously addressed in the Orbiting Low Frequency ARray (OLFAR) project. A 100 km diameter of aperture array constructed by distributed satellites will be able to provide sky maps of better than 1 arc-minute spatial resolution at 10 MHz. However, an orbit design strategy for the swarm satellites that ensures safe intersatellite distances and relative orbit stability has not yet been developed. In this paper, a new method for OLFAR orbit design is proposed. A deterministic solution is presented based on three algebraic constraints derived here, which represent three orbit design requirements: collision avoidance, maximum baseline rate, and uvw-space coverage. In addition, an idea for observation planning over the mission lifetime is presented.

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This paper introduces a strategy for an autonomous reconfiguration of a fractionated synthetic aperture radar (SAR) spacecraft system. The radar antenna is distributed on a small-satellite formation that can be reconfigured on orbit depending on the mission requirements. Once the acquisition geometry is specified in terms of formation type and the desired requirements are defined, the information is transmitted to the cluster. Hence, each satellite determines its own final state and elaborates the necessary trajectory for maneuvering. The reconfiguration algorithm is decentralized and exists in a distributed computational architecture. Therefore, the spacecraft platforms are assumed to be equal and able to communicate among each other. To demonstrate the viability of the proposed approach, a specific scenario is considered, with a distributed SAR operating at X-band that has to be reconfigured for interferometric applications. Simulation results show that once remote sensing requirements are specified, the developed algorithm can manage autonomously the spacecraft reconfiguration toward the corresponding operative pattern.

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Operational spacecraft are facing a risk of collision with space debris objects. The net capturing method has been proposed to mitigate this risk on spacecraft. The mass-spring model is usually applied for net modeling by discretizing a cable into one or several mass-spring-damper elements in simulation. The absolute nodal coordinates formulation (ANCF) has also been applied to model the net, and this model is able to describe the flexibility of a net using less elements. However, the influence on the net behavior in simulation by the flexibility modeling of a net is not well understood and barely discussed. In this paper, flexibility models of a net are established based on the mass-spring model and the ANCF model,respectively. The influence on the net behavior by the flexibility modeling is, for the first time, analyzed via simulations. Two case studies of capturing a ball and a cube shaped targets are performed. It is found that the flexibility modeling has little influence on the net dynamics in simulation. Finally, the characteristics and benefits of the ANCF model are described and analyzed. A drawback of the ANCF model was found to be its inferior computational performance.

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In order to satisfy the requirements of micro-satellite attitude determination and control system, northwestern polytechnical university and delft university of technology have jointly developed a low-cost micro wireless automatic sun sensor (μWASS). μWASS chooses a four-quadrant detector as its sensing part, uses the low-power 51 microcontroller unit Kernel with integrated ZigBee communication protocol as its processor, and utilizes two triple-junction GaAs solar panels as its energy generating part. And a super-capacitor is creatively be used as its energy storage part. In the meantime, the space environment radiation and high and low temperature variation have been considered during the system design, and the additional protective circuit has been added. The field-of-view of μWASS is 120°×120°, and the accuracy is better than 0.5° (1σ). The average power consumption of μWASS is 30 mW. Compared with the existing analog sun sensors, μWASS is a plug-and-play component. Its measurement data can be transmitted through wireless communication and does not need any special connectors or external A/D conversion circuit, which could not only reduce the installation difficulty but also improve the system reliability of the satellite. μWASS has achieved the low cost, miniaturization and wireless of sun sensor, and it can be widely used in various micro-satellites in the future.

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Contact dynamic models for active debris capturing using a net are presented and analyzed using numerical simulations. The contact dynamics are based on two methods: the penalty-based method and the impulse-based method. Both methods apply contact detection algorithms based on the Axis-Aligned Bounding Box. The impulse-based method is, for the first time, being used in a net capturing scenario. Strengths and weaknesses of both models are compared and discussed. Moreover, the results from numerical simulations of target capturing are presented and analyzed to evaluate the effectiveness of the contact dynamic models. It is found that the average difference of bullet trajectories obtained by two models can be restricted within 6% when it compares with the dimension of the net.

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Bus saturation is deemed as one of the primary causes of delays in the data propagation between spacecraft components. However, the conditions under which bus congestion can affect measurements variability were not well characterized before. This paper presents a bus saturation model and a set of experiments to characterize the bus performance of satellite missions for different traffic load, data rate, and synchronization periods. The results showed an increase of measurements variance of up to 18% caused by bus saturation. Additionally, an algorithm was proposed to reduce the data delay by controlling the saturation on the communication channel at the operational level.

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This work proposes PageRank as a tool to evaluate and optimize the global performance of a swarm based on the analysis of the local behavior of a single robot. PageRank is a graph centrality measure that assesses the importance of nodes based on how likely they are to be reached when traversing a graph. We relate this, using a microscopic model, to a random robot in a swarm that transitions through local states by executing local actions. The PageRank centrality then becomes a measure of how likely it is, given a local policy, for a robot in the swarm to visit each local state. This is used to optimize a stochastic policy such that the robot is most likely to reach the local states that are “desirable,” based on the swarm’s global goal. The optimization is performed by an evolutionary algorithm, whereby the fitness function maximizes the PageRank score of these local states. The calculation of the PageRank score only scales with the size of the local state space and demands much less computation than swarm simulations would. The approach is applied to a consensus task, a pattern formation task, and an aggregation task. For each task, when all robots in the swarm execute the evolved policy, the swarm significantly outperforms a swarm that uses the baseline policy. When compared to globally optimized policies, the final performance achieved by the swarm is also shown to be comparable. As this new approach is based on a local model, it natively produces controllers that are flexible and robust to global parameters such as the number of robots in the swarm, the environment, and the initial conditions. Furthermore, as the wall-clock time to evaluate the fitness function does not scale with the size of the swarm, it is possible to optimize for larger swarms at no additional computational expense.

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