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Microsystems in Spacecraft Guidance, Navigation, and Control 211

Attitude Errors and Kalman Covariance
0.1
0.05
FoA Roll (deg) −0.05 0

−0.1
0 10 20
FIGURE 10.4 Typical ISC single-axis attitude error profile.

capabilities, resulting in a more robust attitude determination system than could be
achieved by integrating separate star tracker and gyro units.
The ISC technology, enabled by embedded MEMS gyroscopes, is a precursor
of things to come in the spacecraft avionics arena as much more highly integrated,
lower power, MFGS are developed in the future. There are a wide range of science
and exploration mission applications that would benefit from the infusion of the
compact, low-power ISC technology. Some envisioned applications include using the
ISC as a ‘‘single sensor’’ solution for attitude determination on medium performance
spacecraft, as a ‘‘bolt on’’ independent safehold sensor for any spacecraft, or as
an acquisition sensor for rendezvous applications. It has been estimated that
approximately 1.5 kg of mass and 26 W of power can be saved by employing a
single MEMS-based attitude sensor such as the ISC to replace the separate and
14
distinct star tracker and IRUs typically used on spacecraft. So in this case, MEMS
is an enhancing technology that serves to free up precious spacecraft resources.
For example, the mass savings afforded by using the MEMS-based ISC could
be allocated for additional propellant or, likewise, the power savings could poten-
tially be directly applied to the mission payload. Also worth noting is the fact
that the significantly low ISC power consumption will have a positive secondary
benefit of reducing the size and cost of the host spacecraft electrical power subsystem.
These are some of the advantages afforded by using MEMS technology for GN&C
applications.

10.3 MEMS ATTITUDE MEASUREMENT SENSORS

An attitude measurement is the measurement of any quantity sensitive to the
attitude of the spacecraft, for example, the magnetic field vector, the direction of
the Sun, a star, or some other body, the measurement of an angle such as the solar
aspect or the limb of a planetary body, or the measurement of integrated angular rates.
The latter is very different since it does not provide absolute attitude information. By
the resolution they provide, the attitude sensors can be divided into two groups, coarse
sensors such as magnetometers, sun sensors, and Earth horizon sensors, and fine
sensors such as fine sun sensors and star sensors. Attitude determination using the
global positioning systems (GPS) or similar systems will not be discussed here.

© 2006 by Taylor & Francis Group, LLC