Vision Based Navigation for Autonomous Cooperative Docking of CubeSats

A realistic rendezvous and docking navigation solution applicable to CubeSats is investigated. The scalability analysis of the ESA Autonomous Transfer Vehicle Guidance, Navigation & Control (GNC) performances and the Russian docking system, shows that the docking of two CubeSats would require a lateral control performance of the order of 1 cm. Line of sight constraints and multipath effects affecting Global Navigation Satellite System (GNSS) measurements in close proximity prevent the use of this sensor for the final approach. This consideration and the high control accuracy requirement led to the use of vision sensors for the final 10 m of the rendezvous and docking sequence. A single monocular camera on the chaser satellite and various sets of Light-Emitting Diodes (LEDs) on the target vehicle ensure the observability of the system throughout the approach trajectory. The simple and novel formulation of the measurement equations allows differentiating unambiguously rotations from translations between the target and chaser docking port and allows a navigation performance better than 1 mm at docking. Furthermore, the non-linear measurement equations can be solved in order to provide an analytic navigation solution. This solution can be used to monitor the navigation filter solution and ensure its stability, adding an extra layer of robustness for autonomous rendezvous and docking. The navigation filter initialization is addressed in detail. The proposed method is able to differentiate LEDs signals from Sun reflections as demonstrated by experimental data. The navigation filter uses a comprehensive linearised coupled rotation/translation dynamics, describing the chaser to target docking port motion. The handover, between GNSS and vision sensor measurements, is assessed. The performances of the navigation function along the approach trajectory is discussed

Preparation of gem-difluorinated retrohydroxamic-fosmidomycin

International audienceFrom several decades, some organophosphorus compounds specifically designed to alterbiological systems were introduced on market as agrochemicals (ie glyphosate and glufosinate asherbicides). Nevertheless, it becomes necessary to find new compounds in order to counter plantresistances already observed with glyphosate. Fosmidomicyn and its N-acetyl analogues FR-900098 were perceived as starting points for elaboration of new herbicide candidates, targetingthe second enzyme of the non-mevalonate pathway in plants, the 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DOXP reductoisomerase or DXR). It is expected that theenhancement of bioactivity compared to the parent compounds, might be reached by insertion oftwo fluorine atoms close to the phosphonate function. Indeed, the presence of both fluorineatoms could improve the lipophilicity, affect the pKa of the phosphonic acid function and theninduce better activities. Herein, the synthesis of gem-difluorinated analogues of retrohydroxamicfosmidomycin and FR-900098-ester is reported using a radical addition mediated by acobaloxime comple

Guidance, Navigation and Control for Autonomous Cooperative Docking of CubeSats

Is it possible to dock CubeSats in Low Earth Orbit? The challenges are mainly associated with the level of miniaturisation. A docking mechanism was designed, built and tested in the laboratory. Results show that a relative precision better than 1 cm and 2 degrees is required for the docking. The docking mechanism and metrology system, composed of a monocular camera and sets of light- emitting diodes, are contained within 0.5U volume and can thus be used on nano-satellites. The chaser and target satellites have a complete 3-axis attitude pointing capability and are equipped with available CubeSats attitude sensors and actuators. The chaser is further equipped with a 6 degrees of freedom low-thrust cold gas propulsion system. Different robust control schemes have been investigated and their stability and performance assessed. Non-linear Monte Carlo simulations have been performed to assess the Guidance, Navigation and Control (GNC) performance and fuel consumption. Results show that the proposed GNC is robust to the various sources of uncertainties and that a lateral accuracy better than 5 mm is obtained at docking. Furthermore, it is not affected by the loss of the star trackers or by illumination conditions and can thus take place on a variety of orbits

CubeSpec, A Mission Overview

CubeSpec is an in-orbit demonstration CubeSat mission in the ESA technology programme, developed and funded in Belgium. The goal of the mission is to demonstrate high-spectral-resolution astronomical spectroscopy from a 6-unit CubeSat. The prime science demonstration case for the in-orbit demonstration mission is to unravel the interior of massive stars using asteroseismology by high-cadance monitoring of the variations in spectral line profiles during a few months. The technological challenges are numerous. The 10x20cm aperture telescope and echelle spectrometer have been designed to fit in a 10x10x20cm volume. Under low-Earth orbit thermal variations, maintaining the fast telescope focus and spectrometer alignment is achieved via an athermal design. Straylight rejection and thermal shielding from the Sun and Earth infrared flux is achieved via deploying Earth and Sunshades. The narrow spectrometer slit requires arcsecond-level pointing stability using a performant 3-axis wheel stabilised attitude control system with star tracker augmented with a fine beam steering mechanism controlled in closed loop with a guiding sensor. The high cadence, long-term monitoring requirement of the mission poses specific requirements on the orbit and operational scenarios to enable the required sky visibility. CubeSpec is starting the implementation phase, with a planned launch early 2024

H∞ and μ-Synthesis for Nanosatellites Rendezvous and Docking

In this brief, the nanosatellite rendezvous and docking problem is tackled. It was never attempted for small spacecraft, as critical technologies, such as six-degree-of-freedom (DoF) micropropulsion systems, have only recently become available due to advances in MEMS. The typical level of noise in nanosatellites' sensors and actuators combined with the dynamics uncertainties, low actuation capabilities, and reliability requirements makes the use of robust control appropriate. The system is described by a linearized rotation/translation, six DoFs, and coupled dynamics, including fuel sloshing. An H∞ controller is first designed, in which robust stability and performance are assessed using structured singular values. The controller robustness is then improved using μ-synthesis. Nonlinear Monte Carlo simulations for both controllers, including realistic sensors and actuators models, are provided allowing a thorough assessment of the complete guidance, navigation and control (GNC). The sought GNC schemes are shown to be robust to the modeled uncertainties and to satisfy the docking requirements

X-ray pulsar-based GNC system for formation flying in high Earth orbits

In this paper, comprehensive pulsar-based Guidance, Navigation and Control (GNC) system is designed and applied to satellites formation flying. The complete autonomy of the X-ray pulsar navigation technology provides both absolute and relative positioning information for spacecraft in or even beyond the solar system, and provides an interesting alternative solution to classical navigation techniques such as Global Navigation Satellite System (GNSS). The navigation measurements are studied using a relative time of arrival (RTOA) estimating method, which takes advantage of the noise-filtered pulsar profiles in the cross-correlation estimator to achieve robustness. An adaptive Kalman filter is exploited to minimise the effect of processing noises caused by the primary satellite. A linear Quadratic Gaussian regulator is then applied to control the formation. Two different configurations are studied in high Earth orbits (HEOs). Simulation results show that the designed GNC algorithms can fulfill the required accuracy (10% of the baseline) of the Magnetospheric Multiscale Mission, with the position root-mean-square error of similar to 1.6 km.. The excellent robustness of the designed GNC system on the positioning errors of the primary satellite suggests a role for X-ray pulsar-based formation flying technique in HEOs and the solar system

Hydrophosphonylation of alkenes or nitriles by double radical transfer mediated by titanocene/propylene oxide

International audienceHydrophosphonylation reactions have emerged as efficient processes for the functionalization of alkenes or alkynes. Synthesis of alkylphosphonates was achieved by an original double radical transfer mediated by titanocene and propylene oxide. By the same way, nitriles which are considered as inert functions in radical process lead to aminobisphosphonates

Oxaphospholene and oxaphosphinene heterocycles via RCM using unsymmetrical phosphonates or functional phosphinates

International audienceNew phosphorus heterocycles were synthesized using RCM reaction. They were prepared from unsymmetrical or polyfunctional insaturated precursor in 50 to 87% yields solving the problem of possible competitive side reactions. In parallel hydroxyphosphinate scaffolds represent a versatile starting material and could be of great interest for the synthesis of phosphosugar libraries

Revisited Synthesis of Aryl-H-phosphinates

International audienceA systematic study of the reaction conditions for the preparation of pure aryl-H-phosphinate esters, originally developed by Sander and optimized by Petneházy, is reported. The influence of the reaction concentration has been investigated for the formation of phosphonite intermediates via direct addition of triethyl phosphite to the appropriate Grignard reagent. Subsequent hydrolysis of the phosphonites under acidic conditions gives various aryl-Hphosphinates in high yields and purities

Mission Design and GNC for In-Orbit Demonstration of Active Debris Removal Technologies with CubeSats

Within the frame of the ESA’s General Support Technology Programme, EPFL studied two In-Orbit Demonstration (IOD) missions using CubeSat technologies. These IODs aim at alleviating the technical risk inherent to new technologies required for Active Debris Removal (ADR) of large space objects, by using small and low-cost CubeSat systems. Rendezvous and docking with uncooperative debris was only partially demonstrated and still raises technological issues. To test technologies such as the navigation and Rendezvous (RV) sensors or capture systems, our studies show that CubeSat missions are appropriate. Already, Guidance, Navigation & Control (GNC), communications and power technologies have successfully been miniaturized and the corresponding equipments are now available to the CubeSat community. This fact extends the range of feasible CubeSat missions from the initial flight of simple sensors to more complex systems. This paper presents two CubeSat ADR experiments and demonstrates how mission design and GNC can serve the verification of navigation sensors performances as well as the validation of uncooperative debris capture using a net. Each mission is composed of a Chaser and a Target. The former being an 8 Units (8U) CubeSat and the latter a 4U, launched together in a 12U deployer. Both satellites are 3-axis attitude controlled. The Chaser has in addition 3 Degrees of Freedom (DoF) translation capability using 1mN cold gas thrusters. Both CubeSats will carry GNSS receivers to assist in the determination of range and relative velocity. The global positioning and attitude data of the Target will be transmitted to the Chaser using an inter-satellite link having the additional capacity to measure the corresponding range. This system provides a reference validation for the RV sensors. The relative position and velocity to be controlled are fully observable. Thus a linear quadratic regulator is appropriate to ensure robust and optimal control. Based on the mission design, various close inspection configurations are demonstrated. To emphasize the feasibility of such missions, a system approach will be briefly addressed leading to a complete description of both CubeSats including mass, power, volume and data budgets. Both missions are analyzed using a 6 DoF simulator. The performances and absolute errors of the GNC as well as fuel consumption are provided. Power consumption, telecom capability and thermal aspects will be shown for sake of completeness. Current issues and limitations of the current GNC will be discussed, as well as conclusions regarding the feasibility of such missions