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Microsystems in Spacecraft Guidance, Navigation, and Control 215

In certain highly maneuverable spacecraft, or propulsive upper stage applica-
tions, a three-axis gyro sensor complement for rotational sensing is combined with
a three-axis set of accelerometers for translational sensing to implement a full six
degree-of-freedom (6-DOF) inertial measurement unit (IMU). In navigation and
flight-control systems, an IMU is used to measure angular rates and translational
accelerations about three orthogonal axes of the spacecraft: the roll, pitch, and yaw.
Depending on the mission applications, IMU’s may have 4-for-3 gyro and acceler-
ometer redundancy. In other attitude control systems a three-axis gyro sensor
configuration alone, forming an inertial reference unit (IRU), is employed on
spacecraft.
The technologies commonly used in today’s IRUs include high-performance
mechanical (spinning mass) gyros such as those used on the Hubble Space Tele-
scope, Ring Laser Gyros (RLGs), Fiber Optic Gyros (FOGs) and HRGs. Three-axis
IRU packages based upon these gyro technologies are the mainstay of spacecraft
GN&C systems. One such IRU that has been used on a wide range of LEO, GEO
and deep-space mission applications has a mass of approximately 4.5 kg and
typically requires over 20 W of power to operate. Another representative IRU
used on a large space platform had mass of 5 kg and consumed about 18 W of
power. Consequently, these types of conventional IRU will not be amenable to
microsatellite (and other mission) applications where mass and power are at a
premium.
MEMS inertial sensors are therefore an attractive technology option to pursue
for future microsatellite missions, and other science or exploration applications
such as probes, rovers, robots and the like, where available mass and power
resources are severely constrained. Microsatellite designers and developers can
leverage the considerable R&D funding that has already been invested by the
Department of Defense (DoD) in the development of MEMS inertial sensor tech-
nologies. The primary mission applications of these investments have been preci-
sion-guided munitions (PGMs) and unmanned robotic vehicles. In both these
military applications, the MEMS-based IMUs have supplanted competing techno-
logies (e.g., RLGs or FOGs) by virtue of their miniature size, cost, and mechanical
robustness. In the case of the extended range guided munition (ERGM) the MEMS-
based IMU is coupled with a GPS receiver to create a highly compact, very
accurate, and jamming-resistant GPS/INS navigation system for a 5-in. artillery
shell. 29
As mentioned above, the overwhelming majority of the technology investment
to date has been focused on consumer class and tactical class MEMS gyros, not
MEMS gyros intended for space applications. This legacy of nonspace MEMS gyro
R&D work has been extensively reviewed and reported on in the literature and will
not be discussed in detail here. 30,31 While there has been a considerable R&D
investment in MEMS gyros for military and commercial applications since the
1980s, it is only recently that the development of navigation class MEMS
gyros (with bias stability performance in the range of 0.002 to 0.018/h)
specifically designed for space mission applications has grown at a number of
R&D organizations.

© 2006 by Taylor & Francis Group, LLC