Full-Sky Astrometric Mapping Explorer Solar Precession

Introduction: The Full-Sky Astrometric Mapping Explorer (FAME) mission is designed to collect astrometric and photometric data at a geosynchronous orbit to map the position, parallax, and brightness of over 40 million stars at an unprecedented level of accuracy.¹ To collect the data, the optical FAME instrument requires smooth, stable scanning motion covering the full sky multiple times during the 5-year mission duration. The scanning motion is produced by spinning the vehicle at a nominal 40-min period and precessing the spin axis about the Sun-spacecraft line at a nominal 20-day period (Fig. 8). The spin axis is maintained at a nominal 35-degree angle with respect to the Sun. Required torque to precess the spin axis is obtained from the solar radiation pressure on its Sun shield, which is designed primarily to provide power and shade for the instrument. Solar radiation pressure balancing devices called trim tabs and trim areas have been developed to accommodate changes in vehicle mass and Sun shield optical properties by updating their positions periodically. During science operation, the trim devices remain passive to ensure high-quality data collection free from jitter.

FIGURE 8
FAME observation concept driven by solar precession.

Solar Radiation Torque Estimation: The FAME Sun shield, shown in Fig. 9, consists mainly of an annular solar array and a truncated cone covered with thermal blankets. When the photons from the Sun strike its surface, a momentum exchange occurs. This produces a reaction force that is determined by the optical property, geometry, and orientation of the surface. ² The reaction force has three components corresponding to absorption, specular reflection, and diffuse reflection. Thermal radiation emitted from the surface also contributes to the reaction force. The Sun shield may contain geometric variations stemming from the fabrication, assembly, and launch process. The optical properties are expected to vary gradually for the mission duration due to the exposure of the Sun shield to the space radiation environment. Solar irradiation typically goes through seasonal changes. Furthermore, the vehicle center of mass migrates as onboard propellant is consumed for station keeping. To maintain the required solar precession under these potential error sources, solar radiation torque balancing devices are developed.

Solar Radiation Torque Balancing: As shown in Fig. 9, three rectangular trim tabs are located 120 degrees apart from each other, attached along the circumference of the electronics deck below the solar array. To maximize the effectiveness, trim tabs are coated with silver Teflon (Ag FEP). Trim tabs are sized to accommodate the expected changes in solar radiation torque with sufficient margin. Figure 10(a) shows how the deflection of the trim tabs affects precession rate. Positive deflection moves the trim tabs toward the propulsion deck, and vise versa. At the beginning-of-life, the tabs are deflected +12.5 degrees to increase solar precession torque and -23 degrees at the end-of-life to reduce the increased solar radiation torque of the Sun shield, mainly due to optical property degradation. Three pie-shaped trim areas are also placed under the electronics deck between the trim tabs. The trim areas swing out to control the offset between the vehicle center of mass (CM) and the effective center of pressure (CP) of the Sun shield. The CM-CP offset produces undesirable variations in precession and spin rates.³ Figure 10(b) shows how the 20-mm CM-CP offset along the Y-axis is compensated by rotating out the trim areas 1 and 2 simultaneously.

(a) Precession rate control by trim tabs.


(b) Spin and precession rate variation control by trim areas.

Summary: Fundamental principles of solar precession for spinning satellites are developed in this research as well as the devices to maintain spin and precession rates, called trim tabs and trim areas, to accommodate changes in surface optical properties, center of mass, and environmental disturbances.

Acknowledgments: The authors acknowledge contributions from the NRL FAME team for the design of trim tabs and areas.

[Sponsored by NASA]