ESA GNC Conference Papers Repository

Modern observation satellite missions always aim for better performances in high resolution real time imagery and video products. The needs are several: automated moving target identification, detection, reconnaissance and identification capabilities augmentation, borders and assets surveillance, disaster monitoring, search and rescue). Line-of-Sight (LOS) satellite jitter caused by micro-vibrations deteriorates these resolution performances. The main sources of micro-disturbances are found in AOCS components (especially control wheels), cry-coolers (when present), Solar Array Drive Mechanism (SADM), antenna trimming mechanism or payload mechanisms. In this paper jitter-estimation caused by SADM will be addressed by considering the impact of different stepper-motor drive solutions: full-step and micro-step. Current solutions to solve the micro-disturbances problem rely frequently on fixed solar arrays configurations without any steerable capabilities. However this solution presents two main drawbacks: necessity to oversize the solar panels in order to guarantee full payload power generation and reduction of imaging time window caused by more complicated yaw-steering like orbital maneuvers. In this paper a micro-step drive mechanism for oriented solar arrays will be studied in order to potentially reverse this current design trend. The final objective of our work is to show that solar arrays driven by micro-step mechanisms associated with an active control by Fast Steering Mirror (FSM) can lead to the same pointing performance as fixed solar arrays while relaxing the mass penalties, thus fulfilling two design constraints: need of low level of micro-disturbances and avoidance of solar arrays oversizing. This paper concerns the first step of this work. We focus on the design of the micro-step mechanisms and on the analysis of the disturbances caused by the SADM. This result will be used later on for the design of the FSM and of the active control law. For the design of the micro-step mechanism and the analysis of the disturbances, this paper provides tools to model the spacecraft dynamic coupled with a SADM model. The two main contributions are: (i) the extension of the Satellite Dynamics Toolbox (SDT) [1] to take into account flexible appendages composed of kinematic chains of flexible bodies (like a solar array composed of several plates) and varying appendage tilt angles; (ii) the analysis of spacecraft dynamics with steerable solar arrays driven by micro-step mechanisms. The former version of the SDT computed a linear model of a satellite with flexible appendages in a given angular configuration. Such a model was used in [3] to derive a model parameterized according to the tilt of the solar array thanks to the minimal Linear Fractional Representation (LFR) of the rotation matrix (between the hub and the appendage) proposed in [2]. The new version presented in this article allows the user to request a Linear Fractional Transformation (LFT) model of a satellite with varying tilt angle of each appendage and uncertainties on all dynamics parameters (asymmetries in resonant frequencies or inertia of a two solar panels configuration, uncertain spacecraft inertia, misalignment of solar panels attachment points). This result is then applied in order to reach the second objective: a simulator that includes a SADM dynamic model with different step-driving mechanisms in order to estimate the induced micro-vibration jitter. The possibility to easily introduce any parameter uncertainties in the SDT model permits the user to perform a robust analysis of stability and a study of jitter induced by different SADM configurations. This analysis can be used for co-designing the motor parameters (stator teeth number, pairs of poles, micro-steps number, phase current and required holding torque constant) in order to assess their impact on the spacecraft LOS jitter. Simulations will prove the advantages provided by the micro-step solution in order to limit vibrations and resonance problems (coupling with solar panels natural frequencies) and to increase positioning accuracy and resolution.