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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).

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 and molecules.

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 approach.

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 minutes.

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 this purpose.

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

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 solar wind.


Continuous survey of Fields and Particles data through at least one complete orbit.


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 maneuver (OTM).

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.

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 impact crater.

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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