References of "Mahfouz, Ahmed 50043866"
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See detailGNC of a cargo spacecraft for on-orbit servicing of Herschel at L2
Siena, Andrea; Mahfouz, Ahmed UL; Menzio, Davide et al

in IAC 2022 congress proceedings, 73rd International Astronautical Congress (IAC) (2022, September)

t is anticipated that space exploration will need to rely on In Situ Resource Utilization (ISRU) in order toextend spacecraft lifetime and/or reduce the missions cost and/or the cruise time. For the Moon ... [more ▼]

t is anticipated that space exploration will need to rely on In Situ Resource Utilization (ISRU) in order toextend spacecraft lifetime and/or reduce the missions cost and/or the cruise time. For the Moon and, inthe future, Mars colonization, relying on in-situ resources is a necessary step to become independent fromthe Earth. The possibility to produce resources on the Moon is advantageous for those missions that aretoo far from the Earth to be resupplied. At the SnT Research Centre, the Luxembourg Space Agency issupporting a feasibility study to assess the benefit of on-orbit servicing (OOS), exploiting lunar resourcesfor the Herschel Space Observatory. Herschel ended its operations in 2013 as a consequence of the depletionof its coolant and, in turn, of the capability of cooling down its instruments. To extend its operations,a resupply mission has been envisaged relying on an adapted cargo spacecraft employed in lunar gatewayoperations. This paper deals with the trajectory design and optimization of the cargo on its journey from theMoon to rendezvous with Herschel and with orbit and attitude control. Considering Herschel’s orientationon its orbit, with the sunshield in the direction of the Sun and the need to access to Herschel’s rear panelto perform the resupply operation, a final approach along the negative x-axis, where the primaries lie, isconsidered. A multiple shooting technique is used to perform a flanking manoeuvre. Moreover, an attitudecontrol algorithm is adopted to track the attitude trajectory provided by the guidance algorithms which, inturn, ensure minimum thrusting errors of the cargo spacecraft and a continuous visibility of Herschel. Infuture works, different perturbations will be considered and angular rates induced by the movement of therobotic arms will be compensated [less ▲]

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See detailReinforcement Learning for Attitude Control of a Spacecraft with Flexible Appendages
Mahfouz, Ahmed UL; Valiullin, Ayrat; Lukashevichus, Alexey et al

in IAC 2022 congress proceedings, 73rd International Astronautical Congress (IAC) (2022, September)

This study explores the reinforcement learning (RL) approach to constructing attitude control strategies for a LEOsatellite with flexible appendages. Attitude control system actuated by a set of three ... [more ▼]

This study explores the reinforcement learning (RL) approach to constructing attitude control strategies for a LEOsatellite with flexible appendages. Attitude control system actuated by a set of three reaction wheels is considered.The satellite is assumed to move in a circular low Earth orbit under the action of gravity-gradient torque, randomdisturbance torque, and oscillations excited in flexible appendages. The control policy for rest-to-rest slew maneuversis learned via the Proximal Policy Optimization (PPO) technique. The robustness of the obtained control policy isanalyzed and compared to that of conventional controllers. The first part of the study is focused on problem formulationin terms of Markov Decision Processes, analysis of different reward-shaping techniques, and finally training the RL-agent and comparing the obtained results with the state-of-the-art RL-controllers as well as with the performance ofa commonly used quaternion feedback regulator (Lyapunov-based PD controller). We then proceed to consider thesame spacecraft with flexible appendages added to its structure. Equations of excitable oscillations are appended tothe system and coupling terms are added describing the interactions between the main rigid body and the flexiblestructures. The dynamics of the rigid spacecraft thus becomes coupled with that of its flexible appendages and thecontrol strategy should change accordingly in order to prevent actions that entail excitation of oscillation modes.Again PPO is used to learn the control policy for rest-to-rest slew maneuvers in the extended system. All in all,the proposed reinforcement learning strategy is shown to converge to a policy that matches the performance of thequaternion feedback regulator for a rigid spacecraft. It is also shown that a policy can be trained to take into accountthe highly nonlinear dynamics caused by the presence of flexible elements that need to be brought to rest in the requiredattitude. We also discuss the advantages of the reinforcement learning approach such as robustness and ability of onlinelearning pertaining to the systems that require a high level of autonomy [less ▲]

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See detailFormation design of an inter-satellite link demonstration mission
Menzio, Davide; Mahfouz, Ahmed UL; Dalla Vedova et al

Scientific Conference (2022, June)

The problem of secular growth of the inter-satellite induced by the J2 perturbations has been widely exploredin the literature. In this paper, we study the influence of altitude and initial mean anomaly ... [more ▼]

The problem of secular growth of the inter-satellite induced by the J2 perturbations has been widely exploredin the literature. In this paper, we study the influence of altitude and initial mean anomaly of the chief on theinter-satellite distance growth in a classical trailing formation. Mean elements are converted into osculatingones and then in Cartesian coordinates that are propagated together in the ECI frame. Preliminary resultsshows that zonal harmonics are responsible for a steep distance drift when the formation is inserted inthe vicinity of ±45 and ±135 degrees of mean anomaly in a polar orbit. Distance-invariant solutions aredetermined in term of the mean anomaly and characterized in terms of the difference between the averagerelative distance and the initial one and of the amplitude of the osculating distance. Finally, after the inter-satellite distance has grown to an unacceptable level, the electric thruster onboard of the deputy spacecraftis leveraged in order to reset the deputy’s orbit to the distance invariant mean elements. A finite-time LinearQuadratic Regulator (FTLQR) is designed and validated within a high-fidely dynamics framework [less ▲]

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See detailRelative State Estimation for LEO Formations with Large Inter-satellite Distances Using Single-Frequency GNSS Receivers
Mahfouz, Ahmed UL; Menzio, Davide; Dalla Vedova, Florio et al

Scientific Conference (2022, June)

Relative baseline estimation is an integral part of satellite formation flying missions. GNSS-based relativepositioning has been a dominating technology for formation missions in LEO, where very precise ... [more ▼]

Relative baseline estimation is an integral part of satellite formation flying missions. GNSS-based relativepositioning has been a dominating technology for formation missions in LEO, where very precise estimatescould be obtained for formations with small inter-satellite distances (1 − 10 km). Larger baselines betweenthe satellites (> 10 km) pose the problem of considerable differences in the ionospheric delays experiencedby the signals received by each receiver. This problem could be mitigated by using precise ionospheric-freecombinations that could only be obtained by dual-frequency receivers, which is not a cost-efficient optionfor modern low-cost miniature missions. In this paper, the problem of relative baseline vector estimation isaddressed for formation missions with large inter-satellite distances equipped with single-frequency receivers.The problem is approached using the space-proven relatively simple Extended Kalman Filter with anadvantageous setting for the observation vector [less ▲]

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Peer Reviewed
See detailAttitude Task Allocation and Control in a Swarm of Magnetically Controlled CubeSats
Mahfouz, Ahmed UL; Abdelrahman, Nourhan; Thepdawala, Salman Ali et al

in CDSR 2021 conference proceedings, 8th International Conference on Control, Dynamic Systems, and Robotics (CDSR'21) (2021)

The paper delineates the magnetic controller gains tuning procedure for an active magnetic attitude control system of a CubeSat in the ISS orbit. The optimization is conducted for arbitrary required ... [more ▼]

The paper delineates the magnetic controller gains tuning procedure for an active magnetic attitude control system of a CubeSat in the ISS orbit. The optimization is conducted for arbitrary required attitude given in the orbital reference frame of the spacecraft. The study has been conducted as a part of the Skoltech University project to deploy a swarm of 3U CubeSats for collective gamma-ray bursts detection, which requires the satellites' coordinated attitude control. We consider four identical CubeSats exhibiting swarm behaviour by optimal attitude task allocation on receipt of a command from the mission control center. The first part of this study shows how to obtain the controller gains via linearization of the spacecraft rotational dynamics in the vicinity of the required attitude regime and subsequent numerical optimization (carried out in terms of Floquet theory). The second part shows the collective attitude control scenario. The task of the swarm is to ensure maximum sky coverage around a principal direction uplinked to the spacecraft. The task extends to ensuring the maximum collective stability of the swarm through implementing the optimization algorithm. [less ▲]

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See detailMulti-Agent Attitude Task Allocation and Control in a Swarm of Magnetically Controlled CubeSats
Mahfouz, Ahmed UL; Thepdawala, Salman Ali; Abdelrahman, Nourhan et al

in IAC 2021 congress proceedings, 72nd International Astronautical Congress (IAC) (2021)

The collective behavior of four magnetically controlled CubeSats in LEO is considered. The spacecraft flyas a group and when commanded, use intersatellite link to negotiate and determine the most ... [more ▼]

The collective behavior of four magnetically controlled CubeSats in LEO is considered. The spacecraft flyas a group and when commanded, use intersatellite link to negotiate and determine the most favorablearrangement to provide maximum sky coverage around a specified reference direction by onboard sensors.The most favorable arrangement is attained through attitude task allocation consensus and arrived at by afully magnetic controller implemented onboard of each spacecraft [less ▲]

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See detailHybrid Attitude Control for Nano-Spacecraft: Reaction Wheel Failure and Singularity Handling
Mahfouz, Ahmed UL; Pritykin, Dmitry; Biggs, James

in Journal of Guidance Control and Dynamics (2020), 44(3), 548-558

This paper addresses the attitude control of nano-spacecraft with three reaction wheels and three magnetorquers in low Earth orbits, and develops algorithms to maintain control when any one of the ... [more ▼]

This paper addresses the attitude control of nano-spacecraft with three reaction wheels and three magnetorquers in low Earth orbits, and develops algorithms to maintain control when any one of the reaction wheels completely fails. In the event of a reaction wheel failure, it is shown that the key challenge of maintaining accurate autonomous three-axis tracking is a problem of singularity avoidance in control allocation. In particular, control law singularities occur depending on the type of orbit, the environment, and the attitude, where the system is no longer controllable. Two algorithmic singularity avoidance techniques based on singularity prediction and artificial potential functions are proposed. In addition, an optimized reaction wheel configuration is proposed to enhance singularity avoidance capability. To illustrate the effectiveness of the singularity avoidance control laws, they are tested in simulations that include the effects of perturbing environmental torques, reaction wheel jitter, and actuator misalignment. [less ▲]

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See detailAttitude control algorithms in a swarm of cubesats: Kriging interpolation and coordinated data exchange
Afanasev, A.; Ivanov, A.; Mahfouz, Ahmed UL et al

in Advances in the Astronautical Sciences (2020), 173

This study is a part of the Skoltech University project to deploy a swarm of four identical 3U CubeSats in LEO. The CubeSats are to be equipped with gamma-ray sensors and their collective behavior will be ... [more ▼]

This study is a part of the Skoltech University project to deploy a swarm of four identical 3U CubeSats in LEO. The CubeSats are to be equipped with gamma-ray sensors and their collective behavior will be exhibited in detecting gamma-ray bursts and coordinated attitude control. We consider a fully magnetic attitude control system, comprising a magnetometer as a part of attitude determination routine and three orthogonal magnetorquers as actuators. Having implemented and tested the conventional three-axis magnetic attitude determination and control algorithms, we proceed to study how the performance of such ADCS may be enhanced by using measurements and state vectors exchange. We interpolate the exchanged data, using the Kriging algorithm in conjunction with Extended Kalman filter and Lyapunov-based controller, since it provides the auto correlation and variance information about the environment of the magnetic field, which is of utmost importance for heterogeneous and noisy fields. In our simulations we compare the performance of the controller for a single satellite to that of the satellite in the swarm of CubeSats, which maintains the form of a regular tetrahedron and carries out distributed measurements with interpolation. Improved attitude stabilization for the latter scenario is demonstrated by mean squared errors. [less ▲]

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