Michael Fehringer is responsible for overseeing the system design of the GOCE mission, which means ensuring that the satellite and related data-processing facilities all meet the scientific and technical requirements that were originally laid down when the mission was selected.
Michael Fehringer, an Austrian national, has been working for the GOCE Project at ESA's European Space Research and Technology Centre (ESTEC) in the Netherlands since 2004.
Fehringer gained his Masters and PhD in Physics from the Technical University of Vienna, Austria. Prior to his role as System Manager, which he took up in October 2007, he worked as a System Engineer for the GOCE Project where he was responsible for the electric propulsion system and mission analysis.
ESA: Would you explain the role of a System Manager?
The System Manager has to make sure that the satellite and related data-processing facilities meet the scientific and technical requirements that were set out at the beginning of the project and form that basis on which the mission was originally selected for implementation. The term 'system' indicates that the emphasis is on making everything work together, from the individual spacecraft subsystems, payloads, ground processing and – not to forget – making sure the satellite flies in the correct orbit. The System Manager is supported by a team of engineers who each take responsibility for the procurement of individual subsystems.
ESA: GOCE is an unusual satellite in a number of ways, one being that it orbits the Earth in a very low orbit, why is this necessary?
GOCE orbits the Earth at a low altitude to achieve the best possible data. The gravitational force between two bodies decays with the square of their distance. So, for example, doubling the distance between two bodies reduces the attraction by a factor of four. The main GOCE instrument, the gradiometer, determines the second derivative of the gravitational potential, and the mathematical method to get to the final gravity map is further benefited from a low orbit.
ESA: What are the technical challenges to overcome designing a satellite for this low orbit?
Form follows function not only in the world of fashion! To fly low and avoid air drag the best shape for the satellite to be is long, slender and absolutely symmetrical along the direction of flight. However, the challenge was not so much the design itself, but rather how to ensure that GOCE would not 'sink' from this low orbit, which would result in permanently having to counteract air drag and this would interfere with the science measurements. The trick is to use the main instrument, the gradiometer, to also measure drag and feed the result into an electric propulsion system that, in turn, counteracts the drag exactly. The air drag that GOCE experiences is roughly equivalent to what one would feel holding a mass of a bit less than 1g in your hand.
ESA: The term 'Sun-synchronous orbit' is often mentioned but what does this mean?
In our case Sun synchronous means the satellite is 'always' in sunlight. As the Earth moves around the Sun throughout the year, GOCE’s orbital plane turns with it such that it always faces the Sun. We exploit this to permanently have solar power and a stable thermal environment. However, 'always' is not exactly true. GOCE will fly at altitudes just above 260 km and the proper inclination of the orbit for Sun synchronicity for the mission is 96.7º. This means the plane is inclined by 6.7º in respect to a true polar orbit. The drawback is that, every spring we will experience an eclipse period of 135 days where we will be in the Earth's shadow for up to 28 minutes per orbit.
ESA: What has been the most rewarding aspect of being involved with the GOCE mission?
Given what the workload has been for quite some time and a summer passed without much holiday – indeed there has to be something special about the mission. Being part of the Project seeing an incredibly demanding satellite being built in all its glory has to be a boy's dream come true. Still, the personal experiences are the most rewarding ones and I've had the privilege to work with some great people throughout the project.
ESA: Where will you be at launch?
I will be at ESA's Space Operations Centre (ESOC) in Darmstadt, Germany. The Launch and Early Operations Phase, the LEOP, is considered the most critical of a mission and I will be there to support the team. The operations are run by the ESOC Flight Operations Team conducted by the Spacecraft Operations Manager headed by the Operations Director. I will be with the ESOC team in the main control room and my role is to liaise with and coordinate support from industry and ESA colleagues.
This is one in a series of interviews with a few of the key people that are involved in the GOCE mission. Please check back as the list will be added to over the coming weeks.