GOCE Flight Dynamics System Validation at ESOC

Paper number

IAC-09.C1.9.7

Author

Mr. Stefano Pessina, Terma GmbH at ESA/ESOC, Germany

Coauthor

Mr. Michael Flegel, Terma GmbH at ESA/ESOC, Germany

Coauthor

Mr. Livio Tucci, Terma GmbH at ESA/ESOC, Germany

Coauthor

Mrs. Susanne Kasten-Coors, ESA/ESOC, Germany

Year

2009

Abstract

The ESA satellite GOCE, scheduled for launch in March 2009, is dedicated to measuring Earth's gravity field and modelling the geoid with unprecedented accuracy and spatial resolution. It will fly on a Sun-synchronous (dusk-dawn), near-circular low-Earth orbit, at very low altitude (down to 268 km). In measurement phases, the spacecraft operates in drag-free mode, with air drag compensated by using ion propulsion. GOCE is designed to keep aerodynamic drag force and torque to a minimum by employing a slender body with winglets, for additional aerodynamic stability. Its dynamics are therefore characterised by a strong cross-coupling of orbital and attitude aspects.

The GOCE launch and early orbit phase (LEOP) foresees several activities where Flight Dynamics (FD) support is essential:
\begin{itemize}\item ground-stations antenna prediction and orbit-determination over the first orbit revolutions;\end{itemize}
\begin{itemize}\item magnetometers calibration (for on-board compensation of residual, soft-component, and magnetorquers’ magnetic fields); \end{itemize}
\begin{itemize}\item on-board orbit propagator performance assessment; \end{itemize}
\begin{itemize}\item attitude sensors cross-coherency check (star-trackers, digital Sun sensors, Coarse Earth\&Sun Sensor); \end{itemize}
\begin{itemize}\item on-ground orbit determination based on satellite-to-satellite GPS telemetry data, and comparison with radiometric-measurement based orbit solutions. \end{itemize}
Additionally, FD involvement is crucial for various activities during commissioning and routine operations:
\begin{itemize}\item coalignment calibration of star-trackers and digital Sun sensors ; \end{itemize}
\begin{itemize}\item estimation of altitude decay/raise rate, while transitioning to the measurement/hibernation phases; \end{itemize}
\begin{itemize}\item nitrogen/xenon gauging from tank sensor readings, xenon bookkeeping from integration of on-board measured mass-flows; \end{itemize}
\begin{itemize}\item drag-free-mode bias determination to compensate low-frequency residual along-track acceleration. \end{itemize}

The FD Test and Validation group (T\&V) at ESOC is in charge of the FD system verification, from individual software components to system level validation and team training.

To validate the GOCE FD implementation, T\&V has developed dedicated test-tools along with a high precision test-data generator . The latter is based on a high-fidelity representation of sensors and actuators, integration of the actual on-board flight software and accurate modelling of spacecraft dynamics and perturbations, with run-time performance optimisation to quickly produce test-data over long simulated time intervals (e.g. months of telemetry data). 
Thanks to its accuracy and flexibility, this approach has proved to be very valuable also for additional spacecraft performance verification, such as attitude stability and drag evolution related analyses, based on variable solar flux forecasts.

This paper will focus on the FD system validation and team training performed for the activities mentioned above, both in nominal and contingency scenarios, reporting on the exercised test cases for LEOP and routine-phase operations preparation.

Abstract document

IAC-09.C1.9.7.pdf

Manuscript document

(absent)