Internal Research Fellow (PostDoc) in NGGM Project GNC Performance and Simulation

Our team and mission

Gravity is a well-established observable, and has been integral to Earth observation from space since the beginning, as shown, for example, by the measurement of Earth’s oblateness in 1958. Measuring the gravity field reveals Earth’s state of mass distribution and its dynamics. It also provides the geoid as a reference for sea level, global ocean circulation and height systems. In addition, the temporal variations of gravity and the geoid help to measure mass exchange processes in the Earth system. Knowledge about the time-varying gravity field contributes to all Earth observation science applications, including hydrology, cryosphere, solid Earth, oceanography, atmosphere, geodesy and climate research.

The measurement principle of the Next Generation Gravity Mission (NGGM) will rely on the high-accuracy measurement of the variation in inter-satellite distance due to the time-varying gravity field via a laser tracking instrument and accelerometers to measure the non-gravitational accelerations acting on each satellite, which, in the data processing, will be separated from those caused by the gravity signal. NGGM will make it possible to measure and monitor Earth’s time-varying gravity field and its trends at sub-weekly, monthly to seasonal, and long-term timescales in regions up to 70 North/South at a high resolution in time (down to 5 days) and space (down to 100 km).

NGGM is set to launch in 2032.

You will become an integral part of the NGGM Project System Section within the broader ESA NGGM Project Team, which consists of highly specialised engineers from various engineering disciplines.
The Section currently has nine members who work together closely and take pride in being part of an ambitious and motivated team. You will have the opportunity to contribute to a real project implementation, and your work will provide essential inputs for one of the most challenging and complex ESA Earth observation missions in the near future.

Specifically, your research will provide critical insight for NGGM performance assessment and predictions.

You are encouraged to visit the ESA website: https://www.esa.int/

Field(s) of activity/research for the traineeship

The Next Generation Gravity Mission consists of a pair of satellites providing two types of measurement to map Earth’s gravity field. The first measurement is the variation of inter-satellite distance, measured by a laser interferometer. The second is the non-gravitational acceleration at the centre of mass of each satellite, obtained from ultra-sensitive accelerometers on board each platform. This translates into tight control requirements of the fine relative pointing accuracy between the two satellites and the linear and angular accelerations sensed by the accelerometers. These objectives are achieved via a Drag-Free Attitude and Orbit Control System (DFAOCS) to provide each spacecraft with a drag-free environment and fine pointing capabilities. The control performance of the DFAOCS must then be assessed by means of a high-fidelity end-to-end simulator. The current design of the NGGM DFAOCS could benefit from the latest advances in control theory and the use of a robust control framework in which system design, parametric sensitivity and worse-case analysis can be rigorously performed using state of-the-art tools.

The goal of the proposed research is to explore the use of advanced control design and verification techniques for the design of the Drag-Free Attitude and Orbit Control System of the Next Generation Gravity Mission. To achieve this goal, four different objectives must be fulfilled:

• Set up a high-fidelity end-to-end simulator for the Drag-Free Attitude and Orbit Control System.
• Set up a linear modelling framework for frequency domain design and verification using linear fractional representations.
• Prototype and implement advanced control methodologies to meet the DFAOCS performance and stability objectives.
• Demonstrate the end-to-end stability and performance capabilities of the DFAOCS with parametric sensitivity and worst-case analysis.

To fulfil the above objectives, the proposed programme of work for the Research Fellowship is the following:

• Year 1: Detailed problem formulation, setup of the simulation environment and models, and preliminary control methodology.
  • Development of a high-fidelity simulation environment capturing the relevant dynamics, DFAOCS sensors and actuators and including all relevant payload elements used in the science-mode control loop;
  • From the high-fidelity simulator, perform the derivation of adequate analysis and synthesis models that capture model uncertainties using linear fractional representations;
  • Model identification, validation and uncertainty quantification;
  • Set up the performance and stability analysis frameworks to verify the DFAOCS performance and stability, using a combination of well-established and robust analysis tools (e.g. mu-analysis) and Monte-Carlo time domain non-linear simulations, including variations in all the uncertainties expected in the modelling of the systems involved in the DFAOCS;
  • Propose a baseline robust controller synthesis for drag compensation and fine pointing.

• Year 2: Detailed trade-off and implementation of selected control methodologies, advanced controller synthesis, performance and stability assessment.
  • Trade-off, prototyping and selection of advanced DFAOCS control methodologies: minimum set to be explored: model predictive control schemes, embedded-model control, data-driven approaches and robust controller synthesis;
  • Implementation of the selected control methodologies in the high-fidelity end-to-end DFAOCS simulator;
  • Model-in-the-loop performance and stability verification of the proposed design and control methodologies;
  • Design iterations.

Technical competencies

Behavioural competencies

Result Orientation
Operational Efficiency
Fostering Cooperation
Relationship Management
Continuous Improvement
Forward Thinking 

For more information, please refer to ESA Core Behavioural Competencies guidebook

Education

You should have recently completed, or be close to completion of a PhD in a related technical or scientific discipline. Preference  will  be  given  to applications submitted by candidates within  five  years of receiving their PhD. In particular for this position, the following is required:

PhD in control engineering, preferably applied to space systems AOCS/GNC.

Additional requirements

In addition to your CV and your motivation letter, please prepare a research proposal of no more than 5 pages. This proposal should be uploaded to the "additional documents" field of the "application information" section.

You should have:

• Experience with AOCS/GNC design and model-in-the-loop verification, preferably applied to LEO missions
• Experience in the implementation of high-fidelity non-linear simulators for complex systems, ideally in AOCS/GNC
• Expertise in frequency-domain control system design and analysis using advanced control techniques: robust controller synthesis, mu-analysis and model-predictive control
• Experience in model identification, validation and uncertainty quantification based on experimental results
• Expert proficiency in the use of MATLAB/Simulink
• Advanced proficiency in Python and C programming languages

You should also have good interpersonal and communication skills and should be able to work in a multi-cultural environment, both independently and as part of a team. Your motivation, overall professional perspective and career goals will also be explored during the later stages of the selection process.