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214 MEMS and Microstructures in Aerospace Applications

noise-equivalent temperature (NET) of 100 mK and smart processing to measure
the position of the horizon.

10.3.4 STAR TRACKERS
Star sensors are very similar to sun sensors. Star cameras are star sensors that sense
several stars at once. Recent developments in CCDs have reduced power require-
ments for these considerably, making them more practical. They are very accurate.
A system involving MEMS mirrors has been developed with the Air Force Research
Laboratory (AFRL) together with NASA Langley Research Center in form of the
intelligent star tracker (IntelliStar). 24 It uses several novel technologies including
silicon carbide housing, MEMS adaptive optics, smart active pixels, and algebraic
coding theory. In addition to being lightweight, it also offers advantages of speed,
size, power consumption, and radiation tolerance. The MEMS-adaptive optics,
utilizing MEMS mirrors developed at AFRL, and fabricated with Sandia’s SUM-
MiT V process (Chapter 3), compensate for geometrical aberrations and effects, and
allow the imager to match star patterns easier and faster. Research on miniature
MEMS star sensors has also been performed at JPL. 25,26

10.4 MEMS INERTIAL MEASUREMENT SENSORS
Gyroscopes (also commonly referred to as ‘‘gyros’’) and accelerometers are
the building blocks from which most spacecraft GN&C systems are built. They
are called inertial sensors since their operation takes advantage of an object’s
resistance to change momentum, or simply put, its inertia. Gyros have been
used in space mission applications for many decades and there is a rich body of
technical literature concerning the theory and practical operation of gyro instru-
27
mentation.
The technology of inertial sensors, first developed in the 1920s, has continually
evolved in response to the demands of the users. In the beginning the trend was
to maintain the same basic designs while pushing the technology for sensor-level
components (e.g., electronics, bearings, suspensions, motors, etc.) to achieve im-
provements in sensor performance and operational reliability. Significant increases
in inertial system accuracy and reliability accomplished over this time period
directly led to the successes in autonomous submarine navigation, the Apollo
missions, and the ubiquitous infusion of inertial navigation on commercial aircraft.
Since the 1970s or thereabout, performance plateaued and the emphasis shifted
from refining the technology to achieving equivalent high performance at reduced
cost. Over the past 20 years or so, MEMS technology breakthroughs have been
exploited to create innovative microsystem solution for applications not previously
considered feasible for inertial sensing. These emerging MEMS-based inertial
sensor technologies offer little performance improvement, but provide benefits of
low production and life-cycle costs, miniature size, low mass and power consump-
tion, and are enabling for microsatellites. 28

© 2006 by Taylor & Francis Group, LLC