ESA GNC Conference Papers Repository

Microscope mission: drag-free and attitude control system expertise activities toward the scientific team
S.D. Delavault, P.P. Prieur, T.L. Liénart, A.R. Robert, P.Y.G. Guidotti
Presented at:
Salzburg 2017
Full paper:

Microscope is a CNES-ESA-ONERA-OCA mission whose main objective is to progress in fundamental physics by testing the Equivalence Principle (EP), which is one of the postulates at the basis of general relativity, with an expected accuracy of 10-15. The scientific instrument is a differential electrostatic accelerometer developed by ONERA. The more than 300kg drag-free microsatellite, based on Myriade line of product, was launched on April 25th 2016 into a 710km dawndusk sun-synchronous orbit for a 2-year mission. The Drag Free and Attitude Control System (DFACS) uses the scientific instrument itself in the control loop for sensing linear and angular accelerations. A set of 8 cold gas proportional thrusters perform the 6-axis actuation. In mission mode, the propulsion subsystem continuously overcomes the non-gravitational forces and torques (air drag, solar pressure, etc.) in such a way that the satellite follows the test masses in their pure gravitational motion. Several attitude guidances (inertial, rotating) are available to choose the frequency of scientific interest Fep of the session (Fep, from 0.17 to 3.11mHz). Once in mission mode, within the CNES drag-free expertise group (CECT) the DFACS team provides several services to the system and to the scientific mission center (CMS): cold gas monitoring and management, ‘Attitude' ancillary data, DFACS expertise ancillary data The paper will be divided into three main sections: After a brief description of the Microscope mission, we will present the organization of the CECT and its central role within the mission: the CECT stands between the ground control center (which insures the realization of the mission plan and provides all the telemetry from the satellite), and the CMS (which generates the N1 and N2 data and performs the final evaluation of the EP violation signal). In this section, we will also define the mission scenario and the different scientific session planned to test the EP. In the next parts, the activities of the DFACS team within the CECT will be detailed. In the second part, we will focus on the cold gas consumption monitoring. In addition to the short term monitoring, the estimation of the consumption for each type of guidance is used to foresee the evolution of the gas capacity depending on the mission scenario, which is weekly updated. We will see how the attitude guidance impacts the perturbation torques and finally the gas consumption, typically from 1 gram per orbit to 6 grams per orbit: these observations led to take into account carefully the gas capacity prediction in the scenario update process. The estimation method, proposed and qualified by the propulsion expert, will be briefly described and some results will be presented. In the third part, we will present the DFACS expertise products provided to the CMS for every scientific session in the mission scenario: As well as the orbit ephemeris, the CMS needs the ‘Attitude' ancillary data to perform the exploitation of the session. This ancillary data is composed of the precise attitude, but also of the angular rate and acceleration. These data are estimated with a special algorithm focusing on the frequency of scientific interest Fep of the session. DFACS expertise ancillary data are also provided, necessary to the CMS to evaluate the overall session performance. Analyses of both linear and angular data are performed. First of all, the EP testing is directly linked to the drag-free performance through the linear acceleration measurements: DFACS expertise demonstrated that the performance on Microscope is now by far the finest ever achieved on low Earth orbit: <10-12 m/s2 @Fep, three axis for up to 8 days. In addition, the angular performance (assessed through the fine attitude estimation) is necessary to evaluate the error budget of EP testing. Furthermore a detailed analysis of the behavior of the DFACS over the session is also performed. The specificities of the algorithms will be discussed and some typical results will be presented. The 10th International ESA Conference on Guidance, Navigation and Control Systems will take place one year after the Microscope launch, the paper and the presentation will provide the first description of the DFACS expertise activities and results provided to the scientific team.