Operational Implementation of Mars Express Orbit and Attitude Control

On June 2 2003 the Mars Express spacecraft was put onto Earth escape orbit by a Soyuz rocket. Following ascend trajectory and separation from the rocket ESOC took control of the S/C. Initial attitude acquisition and solar array deployment performed nominally, leading to convergence in Sun pointing mode. The S/C established 3-axis stabilisation based on star tracker measurements and performed successfully a series of autonomous slews to direct the high gain antenna towards the Earth, thus reaching the default attitude guidance for cruise. However star tracker measurements, which had been nominally interrupted during the slews, were not recovered. Soon after, a surveillance on the inertial measurement package triggered a transition to safe mode. A challenging trip towards Mars had started. During the cruise of Mars Express the ground segment and in particular the Flight Dynamics team had to master several problems. Some of those were originated by S/C anomalies. In other occasions, as for the October solar flare event, the problem was of natural cause. Payload commissioning activities (e.g. MELACOM antenna beam characterisation, instrument calibration) involving highly tailored attitude profiles were supported during cruise. Also platform characterisation (e.g. High Gain Antenna pointing calibration, Main Engine calibration) and S/C troubleshooting (e.g. star tracker sky scans) required attitude control strategies of high complexity. Several orbit manoeuvres were executed during cruise. The spacecraft was put in collision course 50 days prior to arrival to Mars. On December 16 an accurate manoeuvre was performed to fine tune the trajectory for the release of the landing probe Beagle-2, which took place on Dec 19. The day after lander ejection, a deflection manoeuvre put the S/C onto its final Mars arrival hyperbola. On December 25 Mars Express performed a perfect insertion manoeuvre with the 400N main engine that led to capture into Mars orbit. Eleven further orbit manoeuvres were executed to change the orbital plane and reduce the apocentre height. Operational orbit was reached on January 28. Since then, intensive science operations are being conducted. The Mars Express Flight Dynamics command generation team has been in charge of providing command support for the Attitude and Orbit Control and Measurement Subsystem (AOCMS) of the spacecraft. This paper deals with the operational implementation of this support. Activities related to operations that required special support are highlighted. Innovative methods for the implementation of attitude control are described. The approach for delta-V manoeuvre implementation is shown. 1. THE EARLY OPERATIONS Table 1 gives a summary of the main Mars Express (MEX) events which took place during the Launch and Early Orbit Phase (LEOP). Table 1:LEOP main events T (Jun03) Event 02T17.45 Lift-off 02T19.17 S/C separation 02T19.44 AOS New Norcia 02T19.51 SA deployment 02T22.15 Star Tracker (STR) on 02T23:22 Transition to Safe Hold Mode (SHM) STR does not recover measurements 03T02:47 HGA switch-on to verify pointing 03T06:30 Safe mode (SAFE) triggered by gyro surveillance 03T20:00 Reaction wheels (RW) switched on 03T21:21 S/C in Normal Mode (NM) (without STR) 04T03:30 Slew around +Y to bring Sun away from STRs. STR recovers measurements. 04T05:45 Slew back. STR measurements lost again. 04T08:30 Series of slews at steps of 5 deg to characterise STR straylight 04T13:45 Test RW off-loading (WOL) 04T15:15 S/C inertia tensor calibration 04T20:31 Test Orbit Control Mode (1m/s) 05T02:35 STR patched. Series of slews to test. 05T20:10 Launcher dispersion correction (5.84 m/s) The main challenge of the early operations of MEX was the problem generated by the unavailability of STR measurements. The S/C had gone through Sun Acquisition Mode SAM, for which the STR provided good measurements. After this the S/C performed an autonomous slew based on gyros to the default Earth pointing. STR measurements were not recovered. One immediate concern of this situation was that since the attitude measurement on-board was restricted to gyros, the S/C could be significantly drifting away from the default guidance, and attitude constraints might be violated. The second concern was that the S/C was still being controlled by thrusters, meaning fuel penalty and orbital disturbances. The STRs had been able to provide measurements in the acquisition phase. Now an obvious difference for the Earth pointing attitude was that the angular distance from the Sun direction to the STR FoV was much smaller than for the acquisition attitude (although within the assumed operational range of the STR). Soon it was understood that the problem was due to straylight. Different methods were analysed to perform attitude slews to bring the STR away from the Sun. The basic problem was that the S/C was in a mode (under thruster control) which does not allow commanded attitude slews. It was proposed to command to the S/C a fake Earth direction, such that when pointing to it the STR would go away from the Sun. Finally the approach taken was to switch on the RW wheels and to force the S/C to go to Normal mode and from that point to command attitude slews to recover STR measurements. The transition to NM would normally not be authorised with a failed STR, but this was overwritten by the ground. Once in normal mode a slew was performed to bring the sun away from the STR FoV. STR measurements were recovered. At the slew back to Earth pointing measurements were lost again. A large number of experiments was performed, to characterise the problem, involving many attitude slews. Based on the results of these experiments the STR manufacturer developed a S/W patch that managed to resolve the acquisition problem of the STR. Apart from the STR problems and initial triggering of a SAFE by a gyro surveillance, the performance of the S/C was outstanding. The delta-V manoeuvres were executed with high accuracy and all functionalities of the Attitude and Orbit Control and Measurement System of the S/C performed as expected.