Pointing Stability Requirements Research Articles

GOES-R active vibration damping controller design, implementation, and on-orbit performance

GOES-R series spacecraft feature a number of flexible appendages with modal frequencies below 3.0 Hz which, if excited by spacecraft disturbances, can be sources of undesirable jitter perturbing spacecraft pointing. To meet GOES-R pointing stability requirements, the spacecraft flight software implements an Active Vibration Damping (AVD) rate control law which acts in parallel with the nadir point attitude control law. The AVD controller commands spacecraft reaction wheel actuators based upon Inertial Measurement Unit (IMU) inputs to provide additional damping for spacecraft structural modes below 3.0 Hz which vary with solar wing angle. A GOES-R spacecraft dynamics and attitude control system identified model is constructed from pseudo-random reaction wheel torque commands and IMU angular rate response measurements occurring over a single orbit during spacecraft post-deployment activities. The identified Fourier model is computed on the ground, uplinked to the spacecraft flight computer, and the AVD controller filter coefficients are periodically computed on-board from the Fourier model. Consequently, the AVD controller formulation is based not upon pre-launch simulation model estimates but upon on-orbit nadir point attitude control and time-varying spacecraft dynamics. GOES-R high-fidelity time domain simulation results herein demonstrate the accuracy of the AVD identified Fourier model relative to the pre-launch spacecraft dynamics and control truth model. The AVD controller on-board the GOES-16 spacecraft achieves more than a ten-fold increase in structural mode damping for the fundamental solar wing mode while maintaining controller stability margins and ensuring that the nadir point attitude control bandwidth does not fall below 0.02 Hz. On-orbit GOES-16 spacecraft appendage modal frequencies and damping ratios are quantified based upon the AVD system identification, and the increase in modal damping provided by the AVD controller for each structural mode is presented. The GOES-16 spacecraft AVD controller frequency domain stability margins and nadir point attitude control bandwidth are presented along with on-orbit time domain disturbance response performance.

Guidance, navigation, and control performance for the GOES-R spacecraft

The Geostationary Operational Environmental Satellite-R series (GOES-R) is the first of the next generation geostationary weather satellites. The series represents a dramatic increase in Earth observation capabilities, with 4 times the resolution, 5 times the observation rate, and 3 times the number of spectral bands. GOES-R also provides unprecedented availability, with less than 120 min per year of lost observation time. This paper presents the guidance navigation & control (GN&C) requirements necessary to realize the ambitious pointing, knowledge, and image navigation and registration (INR) objectives of GOES-R. Because the suite of instruments is sensitive to disturbances over a broad spectral range, a high-fidelity simulation of the vehicle has been created with modal content over 500 Hz to assess the pointing stability requirements. Simulation results are presented showing acceleration, shock response spectra, and line-of-sight (LOS) responses for various disturbances from 0 to 512 Hz. Simulation results demonstrate excellent performance relative to the pointing and pointing stability requirements, with LOS jitter for the isolated instrument platform of approximately 1 micro-rad. Attitude and attitude rate knowledge are provided directly to the instrument with an accuracy defined by the integrated rate error requirements. The data are used internally for motion compensation. The final piece of the INR performance is orbit knowledge, which GOES-R achieves with GPS navigation. Performance results are shown demonstrating compliance with the 50–75 m orbit position accuracy requirements. As presented in this paper, the GN&C performance supports the challenging mission objectives of GOES-R.

Spacecraft with Very High Pointing Stability: Experiences and Lessons Learned

Technical issues associated with spacecraft with very high pointing stability requirement for short to medium term (typically, sub-micro radians for 1-100 seconds) are reviewed. Specifically, microvibration and pointing jitter, that are induced by various internal disturbance sources, is a major issue for this class of satellite. The experiences and lessons learned through the development of several satellites are presented. In particular, some basic concepts on attitude and pointing, new evaluation techniques of pointing stability, influence of high frequency microvibration on pointing and attitude, and dynamic interface between payload and bus are described in this context.

ESA microgravity and microdynamics activities — An overview

The utilization of the low gravity environment of earth orbiting spacecraft provides new opportunities for experiment performance and applications in the field of fluid physics, material, life and biological sciences. However, the environment is not quiescent on-board of Earth orbiting spacecraft as well as scientific satellites with very high pointing stability requirements. It is affected by various acceleration sources and constituents that arise under typical orbital and operational conditions. Microvibrations affect microgravity experiments during their performance in space and they affect the line-of-sight stability of instruments. Such effects need to be taken into account during the spacecraft development, mission planning and operation, and postmission analysis. The paper provides an overview on the ESA microgravity and microdynamics activities in the field and applications of ongoing projects and programmes. A review is given on microgravity payload sensitivities, disturbance sources, micropointing stability requirements of satellites and scientific experiments. Microdynamics control approaches, plans and philosophies are presented which are employed in various project and programmes. Supporting technology tasks are presented which are performed in the framework of ESA R&D activities which shall provide the scientific projects and programmes with adequate solutions.

Ultrahigh-accuracy body-pointing system for the Large Space Telescope

The Large Space Telescope (LST) is a 3-m diffraction-limited telescope. Pointing stability requirements necessary to assure diffraction-limited images are plus or minus 0.005 arc-sec, over possible experiment observation times of several hours. In order to determine whether these stringent pointing requirements could be met, a complex simulation model was defined which consisted of detailed dynamic representations of control moment gyros (CMGs) and reaction wheels (RWs), including their noise characteristics, dynamic sensor representations with sensor noise, shock mounts for the CMG actuators, a detailed representation of an image motion compensation (IMC) system, and a detailed flexible body structural model with all significant vehicle and solar panel bending modes. On the basis of both stability and performance studies utilizing this model, it was determined that a body-pointing system will meet LST requirements in the presence of CMG vibrational disturbances and sensor noise. The recommended system consists of three orthogonally mounted RWs for primary short-term control, and a cluster of CMG actuators for continuous RW desaturation and vehicle maneuvering.