Glossary - Autonomous and Autonomic Systems - page 244

Commanded attitude Desired spacecraft orientation with respect to an
inertial reference frame.
Constellation Two or more spacecraft engaged in coordinated operations
with the objective of meeting a set of mission requirements.
Control loop Closed loop of feedback control.
Control torque Torque generated by spacecraft actuators for the purpose
of controlling the spacecraft's attitude.
Digital Sun sensor A device that measures output from a series of
photocells to determine Sun angle. If a photocell's output is greater than
a threshold, it is considered to be “on.” The pattern of “on” photocells is
directly associated with the Sun angle. Note that this scheme provides a
digital representation of the Sun angle.
Distributed satellite systems Multiple networked spacecraft, analogous
to distributed client-server, or networked, computing systems of to-
day, as opposed to the traditional “monolithic” centralized computing
environment of the past.
Distributed science mission Multiple networked space assets, analogous
to distributed client-server, or networked, computing systems of today, as
opposed to the traditional “monolithic” centralized computing environ-
ment of the past. The assets could include rovers, stationary instrument
packages, and spacecraft.
Downlink A point-to-point RF communications channel carrying data from
the spacecraft to the ground.
Earth-pointer A spacecraft whose fine-pointing science instruments are ori-
ented along the nadir vector toward the Earth.
Effector In the context of autonomic management, a defined means to bring
about a change to a part of the managed component.
Environment awareness A capability of a system through which the sys-
tem is continually able to perceive its external operating conditions in
relation to its knowledge of its abilities. From this perspective, environ-
ment awareness may be considered a part of self-awareness - the ability
to know one's place in the environment. In another view of environment
awareness, the environment is aware of the individuals themselves - for
instance, through their heartbeat or pulse.
Ephemeris In the context of space missions, a table of positions (of either
celestial objects or spacecraft) at a given time (or at given times) in a
given coordinate system. Also, colloquially referred to as an empirical
(as opposed to algorithmic) specification of a spacecraft's orbital position
(either historical or predicted).
Fine Sun sensor A high precision digital Sun sensor (DSS).
Formation flying A flight concept in which multiple spacecraft perform
their science operations while keeping a fixed position relative to one
another.
Geomagnetic field The magnetic field of the Earth.