![]() Disturbances induced by internal elements and propagating along the spacecraft large flexible appendages are the main contributor to micro-vibration pointing budget. Two main classes of spacecraft disturbance sources have been indeed identified in : external or natural events (micro-meteorids and debris impacts, atmospheric drag, Earth gravity field gradient, Earth magnetic field, solar flux and Earth albedo, eclipse entry and exit) and internal events (propulsion subsystem, avionics subsystem, electrical power subsystem, radio frequency/telemetry and telecommand subsystem, thermal control subsystem, structure subsystem). Micro-vibrations are defined in as low-level vibrations causing a distortion of the LOS during on-orbit operations of mobile or vibratory parts. Such instruments typically come with stringent pointing requirements and constraints on attitude and rate stability over an extended frequency range well beyond the attitude control system (ACS) bandwidth, by entailing micro-vibration mitigation down to the arcsecond (arcsec) level or less. This trend is leading to increased detector resolution and sensitivity, as well as longer integration time which directly drive pointing requirements to higher stability and lower line-of-sight (LOS) jitter. With the development of the next generation of Earth observation and science Space missions, there is an increasing trend towards highly performing payloads. In particular, it is shown how the proposed architecture is able to robustly guarantee an absolute performance error of 10 arcsec in face of system parametric uncertainties at low frequency (≈ 1 rad/s) with a progressive reduction of the jitter down to 40 marcsec for higher frequencies where micro-vibration sources act. While a classical Fast Steering Mirror in front of the camera can compensate for a large amount of microvibration, an innovative architecture with a set of six Proof-Mass Actuators installed at the payload isolator level can further improve the pointing performance. Thanks to a set of accelerometers placed at the isolated base of the payload and in correspondence of the mirrors with the largest size in a Space telescope (typically the primary and secondary ones), it is possible to estimate the line-of-sight error at the payload level by hybridizing them with the low-frequency measurements of the camera. In particular, a novel control architecture is proposed to reduce the microvibrations induced both by reaction wheel imbalances and Solar Array Drive Mechanism driving signal, by letting them work during the imaging phase. ![]() This approach opens the doors to modern robust control techniques that robustly guarantee the expected fine pointing requirements. This framework allows the authors to easily include all system dynamics with an analytical dependency on varying and uncertain mechanical parameters in a unique Linear Fractional Transformation (LFT) model. ![]() A multi-body framework, the Two Input Two Output Ports approach, is used to build all the elementary flexible bodies and mechanisms involved in a fine pointing mission. An extensive understanding of the system physics and its uncertainties is then necessary in order to push control design to the limits of performance and constrains the choice of the set of sensors and actuators. The increased need in pointing performance for Earth observation and science Space missions together with the use of lighter and flexible structures directly come with the need of a robust pointing performance budget from the very beginning of the mission design. ![]()
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