Command and Data Subsystem. The "Master Computer" of the
spacecraft. The CDS provides the interface between the ground and
the spacecraft. All commands are either directed to the CDS,
passed by the CDS to other subsystems (eg AACS), or control
relays directly. The CDS also collects data from all the
subsystems and science instruments and formats them for
transmission to the ground.
The CDS has two strings also. In addition, complete copies of the
flight software are in the normal and extended (spare) memory on
each string, to enable rapid switching in case of a memory
failure on one string.
More information on the CDS.
Fields and Particles instruments
Complement of instruments designed to
provide data on the structure and dynamical variations of the Jovian magnetosphere.
This complement is made up of the Dust Detector, Energetic
Particles Detector, Heavy Ion Counter, Magnetometer, Plasma and Plasma Wave
Subsystems.
HGA
High-Gain Antenna. Galileo's original design called for a
deployable antenna to unfurl, providing approximately 34dB of
gain at X-band (10GHz) for a 134kbps downlink of science and
priority engineering data. When it did not unfurl following the
Earth fly-by in 1992, the spacecraft was reconfigured to utilize
the S-band (2.8GHz) omnidirectional antenna for downlink at much
lower data rates, from 8-16 bps through Jupiter Orbit Insertion.
This 8dB gain low-gain antenna
(LGA) was originally supposed to "trickle" down low-rate
engineering data, and to be utilized in case a fault resulted in
the spacecraft "safing" and shifting to a Sun-pointed attitude,
resulting in loss of signal from the HGA. Enhancements to the
Deep Space Network and reprogramming the flight computers on Galileo
will increase the telemetry bit rate to 8-160 bps which will be
used starting in the spring of 1996.
New Telecommunications Strategy
magnetosphere
The volume of space in which a planet's magnetic field dominates
that of the solar wind.
magnetotail
The portion of a planetary magnetosphere pulled downstream
by the solar wind.
mini-tour
Continuous survey of Fields and Particles data through at least one complete orbit.
occultation
Period of time when the view to one celestial body is blocked
by the body of another, e.g. when the spacecraft's view of the Earth or
Sun is blocked by Jupiter.
OPNAV
SSI image taken for optical naviation; image typically
consists of the limb and terminator of one main
body (Jupiter or a satellite) and stars. The image is
used to determine the position of the imaged body against a star
background which enables the navigators to better estimate the
relative position of the spacecraft and the imaged body.
palimpsest
A roughly circular spot on icy satellites, thought to identify
a former impact crater.
phase angle
The angle between the Sun, an object, and an observer. 0 degrees
phase means the Sun is behind the observer.
plasma
A highly ionized gas, consisting of almost equal numbers of free
electrons and positive ions.
plasma sheet
Low energy plasma, largely concentrated within a few planetary
radii of the equatorial plane, distributed throughout the magnetosphere
throughout which concentrated electric currents flow.
satellites, Galilean
Io, Europa, Ganymede, and Callisto; four largest
satellites of Jupiter discovered by Galileo in 1610.
satellite wake
Region created in front of the Galilean satellites as the
charged particles that corotate with the Jovian magnetosphere sweep past
the satellites.
solar conjunction
Period of time during which the Sun is in or near the
spacecraft-Earth communications path, thus corrupting the communications
signals.
thruster flush
The term "thruster flush" may sound mysterious, but it's really
a simple form of preventive maintenance. Galileo's thrusters
operate by the controlled burning of fuel (MonoMethyl Hydrazine)
and oxidizer (Nitrogen Tetroxide). The oxidizer is especially
reactive. Even though the thruster valves are made from special
steels, the liquid still dissolves minute amounts of iron. The
erosion is too small to harm the valves, but the dissolved iron
compounds eventually condense and cling to the fine mesh filters
used to keep debris from the valve seats. Unchecked, these
deposits may clog the filters and hold back oxidizer to the
thrusters, a process known as "flow decay". Periodic short burns
of all the thrusters (about one second) clears out propellant
resting in the valves, flushing the iron solution before it
settles. A flush every three weeks protects the thrusters from
flow decay.
torus, Io
Donut-shaped cloud of neutral and ionized gases (plasma) along
Io's orbit believed to be supplied by the volcanic eruptions on Io.
