NASA Institute for Advanced Concepts

To meet the asymmetric (sub micro-radian towards Earth, tens of micro-radians elsewhere) pointingrequirements of the RIP mission, a hybrid approach to optical attitude determination is suggested. Aredundant pair of Coarse Star Scanners (CSS) have line-of-sight normal to the spin axis. If attitude updaterates are not too high, a straightforward extension of presently available technology is expected to producescanners that meet mass and power budgets, and 3-axis attitude accuracy of < 50 μrads 3σ. If the spacecraft tumbles, signal processing is more complex but operation is still possible. CSS provides aninitial attitude estimate to the Fine Attitude Sensor (FAS), at least one area array focal plane integratedinto and sharing aperture with the Earth communications subsystem. The possibility of an earth beacon,either man-made or natural is suggested. At different mission times and mission phases, FAS views stars,Sun, and partially illuminated planets including Earth. Some pointing difficulties are avoided byintentionally defocussing the telescope with the same mechanism used to steer beams. Obtaining lowscattering and diffraction limited performance is critical. Radiation damage assessment is also critical, sincebetter pointing performance is required late in life when damage has most accumulated.