Glossary - Autonomous and Autonomic Systems - page 250

Stellar-pointer A spacecraft that conducts its science by pointing a science
instrument (or instruments) at targets on the celestial sphere, i.e., stars,
galaxies, etc. Stellar-pointers are slewed to a commanded attitude, acquire
their target(s), conduct their science, and then slew to the next target(s).
Sun sensor An attitude sensor that detects the presence of the Sun and,
in the case of analog and digital Sun sensors, also outputs measurements
from which the Sun vector or Sun angle can be computed.
Sun-pointer A spacecraft whose attitude is maintained so as to keep its
science instrument(s) oriented toward the Sun.
Sun-synchronous orbit A spacecraft orbit whose plane rotates at the
same rate as the Earth's orbital rate about the Sun. The spacecraft orbital
plane motion then matches the apparent motion of the Sun (relative to
the Earth). Sun-synchronous spacecraft imaging the Earth will, therefore,
(for example) always image points on the Earth's equator at the same
local time.
Swarm A large group of autonomous individuals each having simple ca-
pabilities, cooperative actions, and no global knowledge of the group's
objective.
Tachometer A device that measures the rotation rate of a reaction wheel
or momentum wheel.
Three-axis attitude A specification of the orientation of a spacecraft's
body axes with respect to a reference frame, typically expressed as a
transformation from the reference frame to the body frame.
Three-axis magnetometer An attitude sensor that measures the magni-
tude and direction of the magnetic field in which the spacecraft is im-
mersed.
Three-axis stabilized A type of spacecraft the orientations of all of whose
body axes are controlled; distinct from a spin-stabilized spacecraft, which
only controls the orientation of its spin axis.
Thruster Spacecraft hardware that generates thrust by expelling propellant.
Torque The vector rate of change of the angular momentum. A rigid body
experiences a torque if a net force is applied to the body along a line that
does not pass through the body's center of mass. Such forces introduce
rotations about the object's center of mass.
Tracking and data relay satellite A communications relay satellite in
NASA's TDRSS. TDRS satellites are geostationary spacecraft.
Tracking and data relay satellite system The collection of NASA's
TDRS satellites, their ground stations, and their control systems.
Uplink An RF communications channel for data flow from the ground to
the spacecraft.
Virtual mission A mission that consists of both virtual and real spacecraft
assets to meet a mission objective that would otherwise require one or
more real spacecraft and their instruments to be designed, constructed,
launched, and operated.