Design of one gyro AOCS for the ERS-2 extended mission

The ESA Remote Sensing satellites ( ERS) successfully fulfil a mission of global measurement of Earth atmospheric properties using microwave techniques. ERS satellites are based on a platform built by MMS and belongs to the SPOT family. ERS-2 was launched on April 1995 and was designed for a minimum lifetime of two years. As all instruments still work properly in their measurement modes, ESA is interested in an extension of the ERS-2 lifetime. [GRAPHICS] On ERS-2, the redundancy scheme for the gyros is 3 out of 6. In the baseline design, all operational and back up modes use continuously three gyros. Only the safe mode uses one gyro. Today, ERS-2 has three failed gyrometers. A new gyro failure would jeopardise the extension of the mission, except if a back-up AOCS design is defined and validated sufficiently in advance. MMS has performed a feasibility study under ESA contract for a new AOCS design using only one gyrometer at a time in order to allow the mission to continue after new extra failures. This feasibility has been demonstrated. The paper describes the newly designed solutions for the different modes of the mission. In the first part of this paper, the operational modes are presented: the fine Pointing Mode and the Yaw Steering Mode: the feasibility is ensured with good performance : pointing error bellow 0.13 / 0.09 / 0.3 degree on pitch / roll / yaw axis respectively, rate stability performance from 0.0015 deg/sec to 0.004 deg/sec, there is a stability condition ( fulfilled for ERS-2) on the orientation of the selected gyro to ensure the good behaviour of the operational mode. In the second part, the Fine orbit Control Mode (FCM) and the large Orbit Control Mode (OCM) designs are detailed: a capability exists with a modified FCM using algorithms similar to the operational mode to perform small orbit correction with good performance, the use of a modified OCM is also possible in order to perform large in plane or out of plane orbit correction. The third part details the attitude re-acquisition (CAM, FAM1, FAM2) after a failure. A strategy is proposed based on which a modification of the current coarse acquisition logic enables to use it with no gyros, but with the constraint to perform a faster Earth acquisition. The conclusion of the paper gives the current development and validation status on the new design whose implementation has been decided.