QUESTION:
How does MGS track stars by rotating in a cone-shape?

ANSWER from Charles Whetsel on July 18, 1997:
After getting a "rough" (.5 degrees) idea of where it is pointed in space
based on Sun-sensors, MGS uses a combination of gyroscopes and a star
"scanner" to maintain precision pointing in space.  The spacecraft slowly
rolls (either about the line from the spacecraft back to the earth during
cruise, or about the orbit-plane normal pole once in mapping) about an axis
all of the time.  As it does so, the gyroscopes continuously tell the
spacecraft where they think the spacecraft (and the star scanner) is
pointed.  Also, a catalog of stars is uploaded to the spacecraft.  The
software onboard the spacecraft knows how fast the gyros say the spacecraft
is rolling, and they also know how much time should elapse between the
transits of each successive star in the catalog.

If the star scanner generates its star-detection "pulse" a little earlier
than the gyros were predicting, then the software knows that the gyros were
reporting a roll rate that was slightly slower than the actual roll rate.
The opposite correction is applied to the gyro estimation if the star
transit is reported later.  By placing the star scanner at a slight "cock"
or tilt (causing the scanner to "sweep out" a cone of stars in the sky), the
spacecraft software is able to estimate the drift rate of two different
gyros at the same time.

Charles Whetsel
MGS Chief Project Engineer