Galileo - Glossary of Selected Terms
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Galileo - Glossary of Selected Terms
- 14, DSN
- 70M diameter Deep Space Network antenna at Goldstone, California
- 43, DSN
- 70M diameter Deep Space Network antenna at Canberra, Australia
- 63, DSN
- 70M diameter Deep Space Network antenna at Madrid, Spain
- Attitude and Articulation Control Subsystem. Navigates the spacecraft,
establishes and maintains its attitude (pointing in space; It tends to be
quite good-natured, otherwise), controls movements of booms, the scan platform,
the despun section, etc. The AACS has two "strings" for redundancy.
Elements of these strings can be swapped for additional robustness.
- More information on the AACS.
- Acetylene Map
- First time, unique observation that will determine the north-south
distribution of key components of Jupiter's atmosphere, specifically acetylene
and ammonia. These components lie in Jupiter's stratosphere and their distribution
will provide scientist's with information on Jupiter's stratospheric circulation
patterns (wind speeds, convection zones, density profiles, etc).
- AGD CAL
- Auto-Gyro Drift Calibration
Calibration of spacecraft gyroscopes; that is
performed entirely on-board the spacecraft with no intervention from the
ground other than to observe the results. This calibration is required to
keep the gyroscopes peforming within established limits.
- Alfven wing
- Electromagnetic waves that are generated when plasma flows past an
electrically conducting body such as Jupiter's moon Io.
- A glow from a planet's atmosphere produced by the impact and interation
of charged particles in a planet's magnetosphere with the atmospheric atoms
- buffer dump
- Taking data from the main computer memory buffer and recording them
on the DMS. Data in the buffer are usually transmitted immediately to Earth,
but if the data transmission rate is low, the buffer data may be overwritten
by newer data and lost forever. "Dumping" them to the DMS preserves
them for later transmission.
- 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.
- Data Memory Subsystem
The Data Memory Subsystem or tape recorder is a reel-to-reel four-track
digital data storage unit capable of storing approximately 9 x 10?8 bits
of data (a little more than a typical single density 3.5 in floppy disk).
Only about 80% of the storage space is currently used for storage due to
measures taken to protect the DMS from further damage after the Oct. '95
DMS anomaly. Data can be stored at one of five data rates: 806.4, 403.2, 115.2, 28.8 or 7.68 kilobits per second.
- DMS: x-y
- Action of switching the DMS from operating on track x to track y.
Motion of the tape on each track is alternately reversed, i.e. if track
1 motion is forward, track 2 is reverse, track 3 is forward and track 4
is reverse. Due to the Oct. '95
DMS anomaly where the actual
tape was stuck while the tape recorder driver mechanisms were trying to
move it in the reverse direction, tracking of these events has increased
in importance as they provide insight into whether on-board fault protection
has intervened and halted tape recorder activity or if recorder activity
is proceeding as planned.
- drk sky
- dark sky
Type of calibration in which the instrument is pointed at deep space or
dark sky. Usually performed by the NIMS instrument.
The seven days (roughly) surrounding the targeted encounter's closest
- Earth Received Time
- Time at which an event that has occured on the spacecraft will be
"seen" on Earth. Corresponds to Spacecraft Event Time (SCET)
plus One-Way Light Time (OWLT). Does not account for the delay in signal
processing once the signal is on the ground which could be as long as several
- F&P: 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.
- Part of the Attitude and Articulation Control Subsystem (AACS)
that is used to control where the spacecraft is pointing. The primary control
mechanism is provided by another part of AACS (the star scanner) that scans
the sky surrounding the spacecraft for visible stars. The gyroscopes are
used when the stars that are visible are not good enough to be used for
- 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
- New Telecommunications Strategy
- Ion cyclotron
- Ion cyclotron wave observation
- Ion cyclotron waves are electromagnetic waves that interact with ions
within the environment of the magnetospheric plasma. This interaction plays
a key part in understanding the dynamic processes that control the distribution
of ions within the magnetospheric plasma. In particular, ion cyclotron waves are one possible mechanism by which plasma generated by Io may fall into the Jovian atmosphere and cause the aurora.
- Kilometric Radio Emisssions
- First observed by the Voyager spacecraft in April '78 while at a distance
of 2 AU from Jupiter, these are highly polarized radio emissions
with wavelengths in the kilometer-long range. They are thought to originate
above the Io torus and appear to be periodic. The
narrowband component drifts relative to Jupiter's
rotation, lagging by about 3%. The broadband component
is periodic at the Jovian rotation rate. Both are typically
very impulsive and resemble Earth's kilometric radiation.
- L-shell, Satellite
- A region defined by where magnetic field lines cross Jupiter's magnetic
equator. For example: An L-shell equal to 5 roughly represents the set
of field lines that cross the magnetic equator at 5 Jovian radii from Jupiter.
A satellite's L-shell represents the set of field lines that cross the
equator near the satellite's orbit. Energetic particles from the magneto-
sphere can be lost as they impact with the satellite. As a
result, there are generally fewer particles in the regions
surrounding a satellite's L-shell.
- The volume of space in which a planet's magnetic field dominates that
of the solar wind.
- The portion of a planetary magnetosphere pulled downstream by the
- Continuous survey of Fields and Particles data through at least one
- Near Infrared Mapping Spectrometer
- 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.
- orbit determination
- Process of calculating the current orbit of the spacecraft. Typically
orbit determination is performed a shortly before designing an orbit trim
- Optical Navigation
- 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.
- OPT CAL
- Optical Calibration
- Type of calibration in which the NIMS dispersion grating is calibrated
by pointing it at a known (incandescent tungsten filament) light source.
The dispersion grating is part of the NIMS instrument that splits light
into its wavelength components, not unlike the way a prism splits white
light into different colors.
- orbit trim maneuver
- Using on-board thrusters to keep the spacecraft on the desired path.
- One-Way Light Time
- Amount of time it takes for a communications radio signal to travel
from the spacecraft to Earth (or vice-versa) at the speed of light (3 x
10?8 kilometers per second or 186,000 miles per second).
- A roughly circular spot on icy satellites, thought to identify a former
- PCT CAL
- Photocalibration Target Calibration
- Type of calibration in which an instrument is calibrated by pointing
it at the Photocalibration Target. The calibration is generally instrument
specific. For example, the NIMS instrument uses the Photocalibration Target
to calibrate its optical detectors. These optical detectors are sensitive
to wavelengths in the 0.7 to 2.5 micron range.
- Pacific Daylight Time
- phase angle
- The angle between the Sun, an object, and an observer. 0 degrees phase
means the Sun is behind the observer.
- 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.
- Visible ejecta from volcanic or geyser-like activity.
- Photopolarimeter Radiometer
- 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.
- Solid-State Imaging (camera)
- observation type which involves scanning the target from North to
South or East to West.
- 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.
- tape increment new count
- Unit of length used with the Data Memory Subsystem (DMS - the tape recorder). Each tinc is approximately 3.15
inches long. Previously tic (tape increment count), the name was change
after the Oct. '95 DMS anomaly to aid sequence
planners in separating the usable sections of the DMS from those that were
declared to be off-limits to protect the DMS from possible further damage.
- Transmission Time
- Time at which a transmission is sent from Earth to the spacecraft.
The spacecraft will receive the transmission at a Spacecraft Event Time
(SCET) which is equal to TRM plus One-Way Light Time (OWLT).
- Process of making (and uplinking) slight changes to commands that
are already on-board the spacecraft. Usually performed because new knowledge
is gained late in the planning process and there is not sufficient time,
prior to execution, to reliably replace all of the commands that are on-board.
- Transmission of a set of commands from the Earth to the spacecraft
(similarly, downlink - D/L - means to send a transmission from the spacecraft
to Earth). Also refers to the actual set of commands that are uplinked.
- Ultraviolet Spectrometer
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