Glossary - Autonomous and Autonomic Systems - page 245

Geostationary orbit An orbit around the Earth that maintains a space-
craft directly above the same point on the Earth's surface at all times. This
is necessarily an equatorial orbit (with orbit inclination to the equatorial
plane equal to zero degrees) where the orbit altitude is precisely chosen
so that its associated period is 24 h, matching the Earth's rotational rate.
Geosynchronous orbit An orbit having the same altitude as a geostation-
ary orbit, but not necessarily maintaining the spacecraft above the same
point on the Earth at all times. Geosynchronous orbits may have nonzero
inclinations.
Gimbal A rotatable or pivotable hardware contrivance whereby an attached
element of a spacecraft, for example a dish antenna or solar array, may
be reoriented (in two degrees of freedom) relative to the body of the
spacecraft.
Global positioning system A constellation of low Earth orbiting satel-
lites continually broadcasting radio signals supporting realtime, onboard
determination of position by spacecraft with compatible receivers.
Goddard trajectory determination system The primary ground soft-
ware system at NASA Goddard Space Flight Center for computing defini-
tive and predictive spacecraft ephemerides.
Gyro Originally, a mechanical sensor containing a spinning mass that ex-
ploits conservation of angular momentum to measure changes in atti-
tude. Recently, spacecraft have employed gyros that utilize nonmechanical
structures (for example, hemispheric resonating gyros (HRGs) and laser
ring gyros).
Gyro drift A ramping error (varying with time) in a gyro's output.
Gyro scale factor and alignment calibration An operational process
consisting of a series of large slews executed to generate gyro data that
the ground system uses to determine the gyro scale factors and gyro
alignments relative to absolute attitude sensors (such as star trackers).
Gyroscope See Gyro.
HBM Heart-beat monitor
Inertial measurement unit See Gyro.
Inertial reference unit See Gyro.
Kalman filter A sequential estimator with a fading memory commonly
used for realtime spacecraft attitude estimation. Implemented in the ACS
flight software of most GSFC spacecraft flying gyros. Enables onboard
estimation of both attitude error and gyro drift bias.
Lagrange point Positions of stable or pseudo-stable gravitational equilib-
rium within a three-body system consisting of two major bodies and one
body of negligible mass relative to the other two. In a gravitational three-
body system, there are, relative to one of the two major bodies, always
exactly five such points, referred to as L1, L2, L3, L4, and L5, all in
the plane of the orbit of the one major body about the other. The three
pseudo-stable points (L1, L2, and L3) lie along an axis between the one
major body and the other. The two stable points (L4 and L5) are off-axis,