The GOES-R magnetometer subsystem accuracy requirement is 1.7 nanoteslas (nT). During quiet times (100 nT), accuracy is defined as absolute mean plus 3 sigma error per axis. During storms (300 nT), accuracy is defined as absolute mean plus 2 sigma error per axis. Error comes both from outside the magnetometers, e.g. spacecraft fields and misalignments, as well as inside, e.g. zero offset and scale factor errors. Because zero offset and scale factor drift over time, it will be necessary to perform annual calibration maneuvers. To predict performance before launch, we have used Monte Carlo simulations and covariance analysis. With the proposed calibration regimen, both suggest that the magnetometer subsystem will meet its accuracy requirements.
Gimbal lock is a phenomenon which occurs in multi-gimbaled systems when two axes are driven into a coplanar
orientation, thereby resulting in the loss of one degree of rotational freedom. This paper presents a control
scheme which introduces a redundant fourth axis in conjunction with an algorithm that minimizes weighted
least-square gimbal rates, ultimately permitting the use of open inner and middle gimbals to achieve a wide
field of view. The control algorithm produces a singularity/gimbal lock measure which is derived using the inner
three gimbals, and used to adapt the weights and transition the table from three-axis operation to four-axis
operation at or near the three-axis gimbal lock orientation. Weight adaptation minimizes the peak gimbal rate
required to track a vehicle reference rate profile. The control strategy also minimizes tracking errors between
vehicle kinematic motion,which is obtained from the vehicle dynamics simulation, and kinematic motion induced
onto the table-mounted payload. The control algorithm accepts Euler angles and body rates, as defined in the
body fixed frame of reference, and generates four gimbal command sets. Each gimbal command set consists of
gimbal acceleration, rate, and angle. Mathematical analysis, simulation, and 3-D CAD multi-body dynamics
visualizations are included, illustrating differences between desired vehicle kinematic motion and that induced
by this control strategy onto the table mounted payload, including an examination of effects due to finite gimbal
control loop bandwidths.
This paper describes the efforts by Honeywell and MIT to develop a second generation hybrid D-StrutTM (patent pending). The D-Strut is a passive viscous damping and stiffening device used for structural control and isolation. The concept of using an active element synergistically with passive elements was presented in the 1994 SPIE Conference on Smart Structures and Materials paper titled `Actuator with Built-in Viscous Damping for Isolation and Structural Control.' A second generation hybrid D-Strut or active D-Strut with many applications to isolation of spacecraft hardware has been built and tested. The utilization of the device for several system applications is discussed. The primary advantage of the hybrid is that the passive elements are designed for small gravity sag and acceptable mount resonance and the active control is designed to reduce transmissibility over broad or narrow frequency ranges. The payload is then robustly isolated even in the unpowered (active stage off) state. Modeling, dynamic tests, and several single axis isolation experimental results are presented. Also, specific designs for six-axis implementations for microgravity isolation and optical payload isolation, pointing, and suppression are described.