ATTITUDE CONTROL on GRACE FOLLOW-ON Experiences from the first years in orbit

The two satellites for the GRACE* Follow-on mission were successfully launched in May 2018 on a polar orbit at an altitude of 491 km. The mission continues the measurements of the gravity field of the Earth (with emphasis on the time variability) and also delivers radio occultation measurements. The twin satellites are kept at a relative distance of 170 to 270 km and act as probes in the gravity field of the Earth. The inter-satellite distance is measured by a microwave tracking system to an accuracy of 1 μm. A laser ranging interferometer is added as a technology demonstration. Stable and accurate relative pointing, as well as the minimization of disturbance torques, is required in order to optimize scientific results. This poses stringent demands upon attitude control. The performance of the GRACE Follow-on attitude control system will be presented, as well as the special actions and changes that became necessary as the mission evolved. A short description of the sensors and actuators used for attitude control is given and improvements with respect to GRACE are discussed in some more detail. The operational modes are described with a focus on the so-called nominal fine-pointing mode, in which the front-ends point towards each other in order to enable microwaveand laser-ranging. The third Section opens with a description of special tasks, such as the fine-tuning of the control and monitoring parameters and the complex determination of the satellite’s center of gravity. A comparison is made with a tracking model based upon the fuel expenditure from the two tanks that can be determined independently. Several series of involved tests with manual thruster firings were performed in order to characterize the response of the accelerometers to thruster actuations. A description of the design of the tests, their execution and results is presented. A switch to the redundant instrument was made five months after launch on one of the satellites. The consequences for attitude control are discussed in Section 4. A method that was developed to cope with a situation where also the redundant GPS receiver would become unavailable is discussed in detail. Conclusions and an outlook for the upcoming years of operations are presented in the last Section