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OBJECT = INSTRUMENT_HOST
INSTRUMENT_HOST_ID = GO
OBJECT = INSTRUMENT_HOST_INFORMATION
INSTRUMENT_HOST_NAME = "GALILEO ORBITER"
INSTRUMENT_HOST_TYPE = SPACECRAFT
INSTRUMENT_HOST_DESC = "
Instrument Host Overview
For most Galileo Orbiter experiments, data were collected by
instruments on the spacecraft; those data were then relayed
via the telemetry system to stations of the NASA Deep Space
Network (DSN) on the ground. Radio Science also required that
DSN hardware participate in data acquisition on the ground.
The following sections provide an overview, first of the
Orbiter and then of the DSN ground system as both supported
Galileo Orbiter science activities.
Instrument Host Overview - Spacecraft
Launched 1989-10-18 by the Space Shuttle Atlantis, Galileo
was the first spacecraft to use a dual-spin attitude stabilization
system. The rotor (or spun section) turned at approximately three
revolutions per minute while the stator (or despun section)
maintained a fixed orientation in space. This design accommodated
the different requirements of remote sensing instruments (mounted on
the stator) and fields and particles instruments (mounted on the
rotor); spacecraft engineering subsystems were also mounted on the
rotor. The rotor and stator were connected by a spin bearing
assembly, which conducted power via slip rings and data signals via
There were eleven subsystems and nine scientific
instruments on the orbiter. The spacecraft power source was a pair
of radioisotope thermoelectric generators. Propulsion was provided
by a bipropellant system of twelve 10-newton thrusters and one 400
newton engine. The command and data subsubsystem consisted of
multiple microprocessors and a high-speed data bus. The
telecommunications subsystem was designed to transmit data to
Earth at rates ranging from 10 bps to a maximum of 134 kilobits per
second at S-band and X-band frequencies. The rotor had one 4.8 meter
high-gain antenna and two low-gain antennas, but the high-gain antenna
never deployed properly so data were returned from Jupiter at rates
far below the design maxima using the low-gain antennas. The stator
contained a radio relay antenna operating at L band for receiving data
from the atmospheric probe, which is described elsewhere.
Science instruments fell into two general categories. Remote
sensing instruments included:
PPR Photopolarimeter Radiometer
NIMS Near-Infrared Mapping Spectrometer
SSI Solid State Imaging Camera
UVS/EUV Ultraviolet Spectrometer/Extreme Ultraviolet Spectrometer
Instruments primarily designed for 'in situ' measurements
EPD Energetic Particles Detector
DDS Dust Detector Subsystem
HIC Heavy Ion Counter
PLS Plasma detector
PWS Plasma Wave Subsystem
Radio Science (RSS) experiments were conducted using equipment
on both the Orbiter and on the ground.
The mass of the Orbiter at launch was 2223 kg, of which 925 kg was
usable propellant. The Orbiter payload mass was 118 kg. Orbiter
height was 6.15 m.
Overall project management for Galileo was provided by the California
Institute of Technology's Jet Propulsion Laboratory in Pasadena,
California, which also built the orbiter. Ames Research Center in
Mountain View, California, was responsible for the development of the
probe, which was supplied by Hughes Aircraft Company and the General
Electric Company. The Federal Republic of Germany provided the
orbiter's main propulsion system, two complete scientific instruments
and major elements of others.
For more information see [GLL1985; SSR1992]
The Rotor was the spinning section of the Galileo Orbiter and
represented most of the spacecraft mass;
it carried the high-gain communications antenna, the propulsion
module, flight computers, and most support systems. Two booms
were attached to the Rotor; each was unfurled and extended
automatically after launch. The science boom extended to a
distance of three meters from the spacecraft centerline; to it were
mounted the EPD, DDS, HIC, and PLS instruments. The magnetometer
boom extended outward eleven meters from the centerline and was
attached to the science boom. It carried the PWS antenna and
two MAG sensors, one at the midpoint of the boom and the other
at its outboard end. The EUV spectrometer was mounted on the
Rotor bus. For more information see [GLL1985; SSR1992]
The Stator was the despun section of the Orbiter. It was
turned via an electric motor opposite to the rotation of the
Rotor, so that it maintained a stable orientation in space.
Attached to the Stator was a moveable scan platform which
contained the remote sensing instruments: PPR, NIMS, SSI, and
UVS. The Probe and the Probe relay antenna were also attached
to the Stator. For more information see [GLL1985; SSR1992].
The Rotor and Stator were connected by a spin bearing assembly
(SBA), which conducted power via slip rings and data signals
via rotary transformers.
The Telecommunications Subsystem was located in the Rotor section
of the Orbiter. It included elements for receiving uplink
command signals and for transmitting downlink telemetry. The
uplink portion of the system received radio signals with command
data at 2115 MHz and demodulated, detected, and routed those to
the Command and Data System (CDS). The downlink portion received
telemetry data from the CDS and was designed to modulate S-band
and X-band carriers at 2295 and 8415 MHz, respectively, at data
rates as high as 134.4 kilobits per second (kbps).
A 4.8 meter umbrella-like high-gain antenna (HGA) and two
low-gain antennas (LGAs) were mounted on the Rotor. The LGAs
operated only at S-band. One was mounted on a boom and was
included primarily to improve Galileo's telecommunications
during the flight to Venus (while the heat-sensitive HGA remained
furled). The other LGA was mounted at the top of the HGA. The
Stator contained a radio relay antenna operating at L-band for
receiving Probe data during its atmospheric entry.
On 1991-04-11 the HGA was commanded to unfurl; but telemetry
showed that the motors had stalled with the ribs only partly
deployed. Months of tests and simulations followed, but without
further progress in opening the antenna. Engineers deduced that
the problem most likely resulted from sticking of a few antenna ribs,
caused by friction between their standoff pins and sockets.
The excess friction resulted from etching of surfaces
after dry lubricant, bonded to the standoff pins during
manufacture, was shaken loose during pre-launch transport.
The mission was conducted using the LGA mounted on top of the
HGA (the boom-mounted LGA was stowed after its service en
route to Venus had been completed). Without adaptations,
the LGA data transmission rate at Jupiter would have been
limited to only 8-16 bits per second (bps), compared to the
HGA's 134.4 kbps. Onboard software changes, coupled with
hardware and software changes at Earth-based receiving stations,
increased the data rate from Jupiter by as much as 10 times,
to 160 bps.
'Lossless' data compression allows data to be recovered
exactly, once they have been received on the ground. 'Lossy'
data compression allows controlled corruption of the data
through mathematical approximations but with significant
increases in transmission rate. Lossy compression was used
with Galileo Orbiter imaging and plasma wave data to reduce
volumes to as little as 1/80th of their original volumes.
On the ground S-band communications capabilities were upgraded
at the Canberra DSN tracking station (because Jupiter was at
southern declinations during most of the Galileo tour, Canberra
received more data from the Orbiter than the other DSN
stations). 'Block V' receivers were installed at all stations;
these could operate without need for a residual carrier, meaning
all of the spacecraft radiated power could be assigned to carry
its modulation. Early in the tour, arraying of 34-m antennas
with the 70-m antenna at each site was implemented; arraying
of pairs of 70-m antennas and arraying with the 64-m CSIRO
antenna at Parkes (Australia) were also used to increase data
The TCS as designed would have provided a dual channel downlink.
The high-rate channel would have provided a convolutionally
coded, pulse-code modulated microwave channel, while a
low-rate channel data was uncoded. Downlink transmission of
telemetry data would have been possible at S-band and/or X-band
over a wide range of selectable data rates, including 134 and
115.2 kbps at Jupiter.
Approximately 160 W (33 percent of total available)
was provided for the combined S-band and X-band communications
function. Dual power level, traveling wave tube amplifier
transmitters were to provide maximum S-band cruise data return and
high-rate X-band data return from Jupiter while simultaneously
satisfying dual-frequency tracking and radio science requirements.
Several other features were incorporated in the
telecommunications area, mainly to enhance radio science and
navigation. A noncoherent tracking mode was available which
permitted the Orbiter to be commanded while the downlink
frequency source was controlled by an auxiliary oscillator or
an ultrastable oscillator -- providing short-term frequency
stability of better than 5 parts in 10^12. A differential
downlink-only ranging mode was also available using one S-band
and three X-band sine wave tones modulated onto the downlinks
to enhance navigational accuracy. A single X-band to S-band
down-converter receiver was available for receiving X-band
uplink signals to enhance radio science and the search for
gravity waves. These X-band capabilities were never used,
however, because X-band was only available through the high
gain antenna. The capability existed to completely remove
all telemetry modulation from the downlink carriers, thus
maximizing atmospheric penetration depth during Earth
The Galileo Retropropulsion Module (RPM system), located
on the Rotor platform of the Orbiter, was supplied by the
Federal Republic of Germany. It was based on earlier
bipropellant Symphonie designs.
The Propulsion Subsystem provided all directed impulse for
attitude control, trajectory correction, and Jupiter orbit
insertion. The propulsion functions consisted of spin rate
control, fine turning to point the HGA to Earth, and
orientation of the spacecraft for propulsive or science
The RPM included four propellant tanks (two
fuel tanks containing monomethylhydrazine and two oxidizer
tanks containing nitrogen tetroxide), two helium pressurant
tanks, twelve 10-N thrusters (six each mounted on separate
cantilevered booms), one 400-N engine, and necessary
isolation and control elements. At launch, the system
was fully loaded with 932 kg of usable propellant and
weighed about 1145 kg. Four of the 10-N thrusters were mounted
in a direction to provide a functional backup for the 400-N
engine. The thrusters were mechanized on two separate branches
providing redundancy for spin control, HGA pointing, and
trajectory correction. The 400-N engine was used three times --
all subsequent to Probe separation.
Control of propellant to the 10-N thrusters and the 400-N
engine was accomplished by opening and closing fuel and
oxidizer solenoid latch valves via electrical signals from
the attitude control system propulsion drive electronics.
The propulsion drive electronics also provided the control
signals for opening and closing the thruster and 400-N engine
Command, Telemetry, and Data Handling Subsystem
Primary command, control, and data handling was performed
by the actively redundant Command and Data Subsystem (CDS).
Its major functions included receiving and processing real-time
commands from Earth and forwarding them to appropriate spacecraft
subsystems, executing sequences of stored commands
(either as part of a normal preplanned flight activity or
in response to the actuation of various fault recovery
routines), controlling and selecting data modes, and
collecting and formatting science and engineering data for
downlink transmission. The CDS architecture
used multiple microprocessors and a high-speed data bus for
both internal and user communication.
A majority of the CDS electronics were located on the Orbiter
Rotor platform in proximity to the data storage, science, and
telecommunications equipment. CDS Stator elements were limited
to those necessary to support the Probe and relay radio
hardware equipment, the remote sensing instruments mounted on
the scan platform, the launch vehicle, and sequence operations.
Six 1802 microprocessors, memory units, and the data bus comprised
the 'heart' of the CDS. Four of the microprocessors (two high-level
modules and two low-level modules) and four memory units contained
a total of 144000 words of random access memory (RAM) and were
located on the Rotor platform along with supporting electronics.
The low-level modules of the remaining two microprocessors, each
with 16K RAM, were located on the Stator platform. The data bus
comprised three dedicated busses. The bus interface was used by
all data systems -- that is, Orbiter science, the attitude and
articulation control subsystem, and relay radio hardware receivers.
Interfacing between Rotor and Stator portions of the CDS was
accomplished via slip rings and rotary transformers mounted
on the spin bearing assembly. Efficient and effective
communication among data systems was accomplished using a
specifically defined protocol structure and real-time interrupt
time slicing. The protocol addressing schemes provided for
either a relatively simple bus adapter that relied on direct
memory access by the user's processor or a more complex bus
adapter with direct memory access capability independent of
Attitude and Articulation Control Subsystem
The Attitude and Articulation Control Subsystem (AACS)
was responsible for maintaining spin rate of the spacecraft;
orienting the spin vector; controlling propulsion isolation
valves, heaters, 10-N thruster firing, and 400-N engine
firing; and controlling the science platform containing
the remote sensing instruments on the Stator platform.
Design of the AACS was profoundly influenced by
science requirements and the various spacecraft operational
configurations that had to be accommodated. Configurations
included the basic cruise dual spin configuration (Orbiter
with Probe), dual spin without the Probe (for orbital operations)
and 'all spin' configurations with and without the Probe for
trajectory corrections at spin rates from 3 to 10 rpm.
The AACS incorporated many functional elements to meet
the demanding performance, lifetime, and reliability
requirements of the mission. The majority of the AACS
functional elements were block redundant and located on
the Rotor platform. Stator elements included those necessary for
controlling the pointing and slewing of the scan platform,
pointing the relay antenna, and interfacing with the
Rotor section electronics.
The central element of the AACS was the attitude control
electronics (ACE) package that controlled the AACS
configuration; monitored its health; performed executive,
telemetry, command, and processing functions; provided spin
position data to other subsystems; and provided AACS fault
recovery. The 'heart' of the ACE was a high-speed 2900
ATAC-16 processor and memory containing 31K words of
16-bit RAM and 1K words of 16-bit read-only memory (ROM).
ROM storage was used only for those functions required
to safeguard the science instruments, switch to the
low-gain antenna, and Sun point the Orbiter to permit
ground commanding. Activation of the ROM sequences
occurred only when a loss of RAM was detected.
The ACE also contained electronics necessary to interface
with AACS peripheral elements in the Rotor section, the Stator
electronics, and the CDS. Interfacing between Rotor and
Stator AACS elements was accomplished via rotary
transformers located on the Spin Bearing Assembly (SBA).
Other major AACS functional elements included:
- a radiation hardened star scanner employing
photomultiplier tubes for star field identification
during in-flight attitude determination
- linear actuators for raising or lowering the RTG booms
to reduce wobble and maintain stability
- acquisition sensors for attitude determination, spin
rate sensing during launch, and Sun acquisition
- propulsion drive electronics to control the RPM latch
valve, thrusters, and 400-N engine valves
- a spin bearing assembly to provide the mechanical and
electrical interface between Rotor and Stator sections
of the Orbiter as well as to provide despun orientation
- gyros mounted on the Stator scan platform to control
platform articulation and stabilization.
- accelerometers mounted on the Stator platform diametrically
opposite to each other and aligned parallel to the
Orbiter spin axis to measure velocity changes during
- a scan actuator subassembly to provide scan platform
cone actuation and positioning information.
After launch vehicle separation and RPM pressurization, the
spacecraft assumed the 'all-spin' configuration. This was used
frequently during the mission and for all propulsive
maneuvers to provide stabilization. In all-spin configuration
for 10-N thruster burns, the entire Orbiter would spin at
roughly 3 rpm; for 400-N engine burns, the Orbiter would
spin at 10 rpm. This configuration was also used during science
calibration target observations by the remote sensing
For most of the mission, the AACS operated in the cruise
mode, in which the Orbiter operated in the dual-spin
configuration with the Rotor platform inertially fixed.
Major AACS functions performed in this mode were wobble
control, high-gain antenna pointing, attitude determination,
and spin rate control.
The final AACS mode was the inertial mode. Transition to
this mode was from the cruise mode with gyros active.
While in this mode the AACS performed functions such as
closed-loop commanded turns using the RPM thrusters,
accurate pointing and slewing of the scan platform,
and closed-loop control for wobble angle compensation.
Electric Power Subsystem
Electrical power was provided to Galileo's equipment by two
radioisotope thermoelectric generators. Heat produced by
natural radioactive decay of plutonium 238 dioxide was
converted to electricity (570 watts at launch, 485 watts at
the end of the mission) to operate the Orbiter equipment for
its eight-year baseline mission. This was the same type of
power source used by the two Voyager spacecraft missions to
the outer planets, the Pioneer Jupiter spacecraft, and the
twin Viking Mars landers.
Spacecraft Coordinate Systems
The Rotor coordinate system consisted of three mutually
perpendicular axes: Xr, Yr, and Zr. The Zr axis was nominally
parallel to the spin bearing assembly (SBA) axis and passed
through the center of the Rotor with +Zr directed opposite
to the HGA boresight direction. +Yr was normal to Zr and was
directed toward the science boom. +Xr was normal to both Yr
and Zr and formed a right-handed system. The angular
momentum vector for the spinning spacecraft was in the +Zr
\ / HGA
\ /\ /
| ROTOR |-------------------\ Science and MAG
| |-------------------/ Boom
| ---❯ +Yr
The Stator coordinate system consisted of three mutually
perpendicular axes: Xs, Ys, and Zs. The Zs axis was nominally
parallel to the SBA axis and passed through the center of the
Stator with +Zs directed opposite to the HGA boresight
direction (+Zs was parallel to +Zr). +Ys was normal to Zs and
was directed opposite to the scan platform direction. +Xs was
normal to both Ys and Zs and formed a right-handed system.
| STATOR |-------------------\ Scan
| |-------------------/ Platform
+Ys ❮--- |
| _.--_/\/' -
__---- /|||\----__ _/\/' +Yr,-Ys
_--\ __----------__ /--_ _/\/' /
/ _--\ __|___ /--_ \/\/' /
\-/ __-\- | /-- \/\/' /
,_`-`---| |___| |__/\/' /
,--/---===_/||\ -`---(o) /
,/--/ ,-, ,--('||))|---|)\|\
,/--/ |]]=\== \_|/ |___]-)\|\,--
/--/: '-' `__-------_=]= \|[[[
[=[=/! : [_-------_\== \[[[
' //_-- --_[=-- [-_ ---------- +Xr, -Xs
-Xr,+Xs ------- ---`\ /[_]' \/_\_
/'|`\[|`\_ //' [ ]=
`-[-'_] - [___]=]
Figure - Perspective view of Galileo Orbiter spacecraft
(Should be viewed in a mono-spaced font such as Courier)
The scan platform coordinate system consisted of three mutually
perpendicular axes: L, M, and N. The platform had a primary
mounting plane which was established by three mounting points
on the platform. Two reference pins (Pin 1 and Pin 2) were
installed on the primary mounting plane to establish platform
alignment. The origin of the coordinate system was at the
intersection of the center line of Pins 1 and 2 and the primary
mounting plane. The coordinate axis L, defining look direction,
was parallel to the SSI instrument and passed through the
center line of Pins 1 and 2. Coordinate axis M was in the
primary mounting plane, perpendicular to L, and passing through
the origin. Axis N was mutually perpendicular to both L and M
such that L = M x N. Individual instruments were assigned
subscripted Li, Mi, Ni coordinate systems such that an instrument
pointing vector was specified by direction cosines of its
coordinate axes Li, Mi, Ni with respect to the platform
coordinates L, M, N.
Instrument Host Overview - DSN
Galileo Radio Science investigations utilized
instrumentation with elements both on the spacecraft and at
the NASA Deep Space Network (DSN). Much of this was shared
equipment, being used for routine telecommunications as
well as for Radio Science.
The Deep Space Network was a telecommunications facility managed
by the Jet Propulsion Laboratory of the California Institute of
Technology for the U.S. National Aeronautics and Space
The primary function of the DSN was to provide two-way
communications between the Earth and spacecraft exploring the
solar system. To carry out this function the DSN was equipped
with high-power transmitters, low-noise amplifiers and receivers,
and appropriate monitoring and control systems.
The DSN consisted of three complexes situated at approximately
equally spaced longitudinal intervals around the globe at
Goldstone (near Barstow, California), Robledo (near Madrid,
Spain), and Tidbinbilla (near Canberra, Australia). Two of the
complexes were located in the northern hemisphere while the third
was in the southern hemisphere.
The network comprised four subnets, each of which included one
antenna at each complex. The four subnets were defined according
to the properties of their respective antennas: 70-m diameter,
standard 34-m diameter, high-efficiency 34-m diameter, and 26-m
These DSN complexes, in conjunction with telecommunications
subsystems onboard planetary spacecraft, constituted the major
elements of instrumentation for radio science investigations.
For more information see [ASMAR&RENZETTI1993]. "
END_OBJECT = INSTRUMENT_HOST_INFORMATION
OBJECT = INSTRUMENT_HOST_REFERENCE_INFO
REFERENCE_KEY_ID = "ASMAR&RENZETTI1993"
END_OBJECT = INSTRUMENT_HOST_REFERENCE_INFO
OBJECT = INSTRUMENT_HOST_REFERENCE_INFO
REFERENCE_KEY_ID = "GLL1985"
END_OBJECT = INSTRUMENT_HOST_REFERENCE_INFO
OBJECT = INSTRUMENT_HOST_REFERENCE_INFO
REFERENCE_KEY_ID = "SSR1992"
END_OBJECT = INSTRUMENT_HOST_REFERENCE_INFO
END_OBJECT = INSTRUMENT_HOST