Galileo Terms

AACS - 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.


CDS  - 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.


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


RJOI - Relay/Jupiter Orbit Insertion. The acronym describing the entire
       Jupiter arrival activity. "Relay" signifies the original plan for
       the data from the atmospheric entry probe to be relayed to Earth
       in real-time, with backup storage of data on the on-board tape
       recorder. With the failure of the HGA to unfurl, the tape
       recorder has become the primary recipient of the Probe's data. As
       a backup, the CDS software has been modified to strip extraneous
       data from the data-stream 
       and store only the
       symbols containing science data into extra RAM in its memory.
       This is still referred to as "Probe Data Relay".

       Jupiter Orbit Insertion is the firing of the 400N (90lbs) main
       engine for the third of four planned firings. The first two
       firings were for 2 seconds and 5 minutes in the "Wakeup Burn" and
       "Orbiter Deflection Maneuver" to change Galileo's trajectory
       following the Probe Release. It occurs about an hour after the
       Probe Data Relay is complete, and lasts about 49 minutes (the
       final planned burn of the 400N engine is the "Perijove Raise
       Maneuver, several months after JOI).

       RJOI is under the control of a "sequence," which is a program
       running in the CDS to control the spacecraft. RJOI is designed to
       require little intervention from the ground, and none during the
       critical period. A listing of highlights of the RJOI sequence is
       available in several different forms here. Look for the "RJOI..."
       filename.



RRH  - Radio Relay Hardware.  The antenna and electronics designed to
       track and receive data from the atmospheric probe during its
       short operational life descending into Jupiter's atmosphere. 
       The probe data will be stored on the orbiter and relayed back
       to Earth.

       More information on the RRH.


Critical Mode/Critical Sequence -

       RJOI is a "Critical Sequence," only the second such to be run on
       the Galileo Orbiter. Critical sequences are for activities for
       which timing is critical, and for which "there is no second
       chance." The CDS is placed into "Critical Mode" (pretty much
       defined as "able to run a Critical Sequence") and a sequence
       containing "marks" is loaded. When the CDS encounters the first
       "6MARK" command in the sequence, the sequence becomes a "Critical
       Sequence". This means that the sequence cannot be stopped.

       Typically, when in non-critical mode and some sort of fault occurs, 
       for instance a reset of
       one of the CDS's microprocessors resulting from noise or
       radiation, the affected string goes "down" and stops processing
       commands and telemetry. The other string goes into "safing" the
       spacecraft, cancelling any running sequences and reconfiguring
       the spacecraft to a state where it can safely wait out ground
       analysis of the situation and commands to restore it back to full
       function. This includes turning the spacecraft to point at the
       Earth, turning on certain heaters and others off, putting some
       science instruments into safe states (including OFF), etc.

       During Relay/JOI, unlike most spacecraft activities, the timing
       of Relay and JOI is critical. The Probe Release and ODM could
       have been delayed for weeks with no more cost than the propellant
       required for a delayed ODM. However, the laws of physics determine
       when the Probe will arrive at Jupiter and begin transmitting its
       data, and when the engine firing _must_ take place for the
       Orbiter to end up in the required orbit. Light (and radio waves)
       will take around 50 minutes to traverse the gulf between Earth
       and Jupiter on December 7th. Since response to any anomaly would
       require a minimum of 110 minutes, plus any analysis and decision
       time, it is not feasible to follow the standard path to respond.
       Instead, the CDS, once in critical mode and running a critical
       sequence, will do what it can to restore itself to function and
       proceed with the critical sequence from the last mark.



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.



OPNAV - Optical Navigation is as simple as it sounds: Galileo's camera,
       the Solid State Imaging instrument (SSI), takes a picture of the
       starfield (and any moons, planets, or other helpful markers)
       which is used to determine the exact position of the spacecraft.