Glossary - Autonomous and Autonomic Systems - page 246

in positions leading and following the one major body on its orbital path
around the other major body. An example of clumping of small natu-
ral objects at off-axis Lagrange points is the Trojan Asteroids (here, the
two large gravitational bodies are the Sun and Jupiter). See Fig. 3.2 for
Lagrange Points illustrations.
Lagrange point orbit The complex, nonplanar motion of a spacecraft
when in orbit, the so-called halo orbit , near one of the unstable Lagrange
Points, L1, L2, or L3. The halo orbit in general cannot be maintained
without station-keeping.
Libration point See Lagrange point.
Limit checking A validation procedure in which a data point value is
checked against a threshold (upper, lower, or both).
Low Earth orbit An orbit whose altitude above the Earth's surface
is between about 160 km and about 2,000 km. Atmospheric drag in-
creases dramatically at lower altitudes. A popular LEO altitude range is
500-600 km.
Magnetic coil A wire wrapped about a cylinder in a series of loops; a mag-
netic field results when the wire carries an electrical current. When the
cylindrical space contains a ferromagnetic core, the configuration is called
a magnetic torquer bar (MTB).
Magnetic disturbance torque A spacecraft torque generated by the in-
teraction of the spacecraft's residual magnetic dipole as the spacecraft
moves along its orbital path through the external magnetic field.
Magnetic torque Torque on the spacecraft arising from the interaction be-
tween fields generated by the spacecraft's magnetic coils and the external
magnetic field. If the torque arises from an interaction between a space-
craft residual dipole moment and the external magnetic field, the torque
is called the magnetic disturbance torque .
Magnetic torquer bar See magnetic coil.
Magnetometer An attitude sensor measuring the strength and direction of
the magnetic field external to the spacecraft. Note that this measurement
will include not only the geomagnetic field (for spacecraft in Earth orbit),
but also contributions from MTBs and spacecraft residual dipole moment.
Processing of the raw magnetometer measurements must remove these
other field sources to obtain accurate geomagnetic field measurements.
Managed component A component that is protected by the autonomic
manager.
MAPE Monitor, analyze, plan and execute components within autonomic
manager.
Measured attitude The current best estimate of the spacecraft attitude.
Molniya orbit An orbit designed to support Russian communications satel-
lites. It freezes the orbit's perigee over the Southern hemisphere, tilting the
orbit to place the apogee over the Northern latitudes, ensuring maximum
communication opportunities between spacecraft and ground stations.