Space avionics stellar-inertial subsystem

Draper Laboratory has developed a space avionics system for attitude determination and control of small low-power microsatellites. The goal is to determine microsatellite attitude to better than 0.1 deg with a minimum power expenditure. The revolutionary, enabling technologies that make this happen include an electron-bombarded Charge-Coupled Device (EBCCD) star camera that can detect dim stars (Magnitude=8) and a MEMS 3-axis inertial rate sensor module. The performance of the two enabling sensors (EBCCD and MEMS) depends on the early amplification and digital conversion of the optical and inertial signals, respectively, to a digital format. The amplification of the optical signal in the EBCCD star camera removes the effect of the CCD detector noise and the effect of the read-out noise. The digital conversion of the inertial sensor data at the preamplifier stage eliminates the subsequent effect of the noise and drift error terms. The immediate NASA application is microsatellites in LEO NASA science missions that often require 20 rpm spinning microsatellites. This paper describes some of the design features of the Draper Low-Power Avionics Sensor Suite (LoPASS) approach, including the digital processing of the EBCCD and MEMS data. Examples of the LoPASS sensor's digital processing include: use of the UTMC 69R000 microcontroller to implement the digital processing; measuring star centroids on a discrete CCD array with high accuracy; providing digital temperature compensation for the MEMS gyros; application of a simple Kalman filter to optimize the attitude solution; low-power operation with radhard sensors and processor. The result is a space avionics stellar-inertial subsystem that weighs approximately 1 kg and has an average power of approximately 2 W. The optical camera, inertial sensors, and the microcontroller are all radhard. The star camera is turned on every 5 min with a 3-second star sighting to update the MEMS gyro drift and achieve the 0.1-deg attitude knowledge requirement.