PDS_VERSION_ID = PDS3
LABEL_REVISION_NOTE = "
J. MAFI (PPI), 1998-05-18;
R. SIMPSON, 2000-07-26"
RECORD_TYPE = FIXED_LENGTH
RECORD_BYTES = 78
OBJECT = INSTRUMENT_HOST
INSTRUMENT_HOST_ID = VG1
OBJECT = INSTRUMENT_HOST_INFORMATION
INSTRUMENT_HOST_NAME = "VOYAGER 1"
INSTRUMENT_HOST_TYPE = SPACECRAFT
INSTRUMENT_HOST_DESC = "
Instrument Host Overview
========================
For most Voyager 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 Earth. Radio Science experiments (such as
radio occultations) required that DSN hardware also participate
in data acquisition. The following sections provide an
overview first of the spacecraft and then of the DSN ground
system as both supported Voyager science activities.
Instrument Host Overview - Spacecraft
=====================================
The Voyager 1 and Voyager 2 spacecraft were identical and were
built by the Jet Propulsion Laboratory (JPL). With a mass of
815 kilograms, each carried its own power, propulsion, and
communications systems and its own science instruments.
Spacecraft electrical power was supplied by Radioisotope
Thermoelectric Generators (RTGs) that produced about 400 watts.
The Attitude and Articulation Control Subsystem (AACS),
Computer Command Subsystem (CCS), and Flight Data Subsystem
(FDS) managed spacecraft operations. Thrusters and gyros
provided physical propulsion and attitude control.
Communications between the spacecraft and Earth were carried
out via a high-gain radio antenna using both S-band and X-band
frequencies at data rates as high as 115.2 kilobits per second.
A Digital Tape Recorder (DTR) could save up to 500 million bits
when no Earth station was available for real-time data
transmission. Voyager control systems could record sets of
several thousand instructions, allowing autonomous operation
for days or weeks at a time. More information on the
spacecraft can be found in [MORRISON1982], [KOHLHASE1989], and
[JPLPD618-128].
The spacecraft itself was built around its 'bus' -- a decagonal
prism, which was about 2 meters in diameter and about 60 cm
deep. Each of the ten sides of the bus was associated with a
'bay' containing engineering systems or science instrument
electronics. Bay 1, for example, contained the radio
transmitter. The High-Gain Antenna (HGA) was mounted to the
end of the bus facing Earth. The bays were numbered 1 through
10 in a clockwise direction when viewed from Earth. Extending
away from the bus were three booms: a science boom and scan
platform to which most instruments were mounted, a magnetometer
boom, and a boom to which the RTGs were mounted.
Spacecraft Coordinate System
----------------------------
The centerline of the bus was the roll axis of the
spacecraft; it also served as the z-axis of the spacecraft
coordinate system with the high-gain antenna (HGA) boresight
defining the negative z-direction. The HGA boresight was
also defined as cone angle 0 degrees and as azimuth 180
degrees, elevation 7 degrees. The science boom, supporting
the scan platform, extended in the general direction of
positive y; this boom was also defined as being at cone angle
90 degrees, clock angle 215 degrees and at azimuth 180
degrees, elevation 97 degrees. A boom supporting the RTGs
was mounted on the bus in generally the negative y direction.
The positive y-axis (yaw axis) of the spacecraft coordinate
system passed through Bay 3; the negative y-axis passed
through Bay 8. The x-axis (pitch axis) was in a direction
which defined a right-handed rectangular coordinate system.
The positive x-axis was at cone angle 90 degrees, clock angle
305 degrees (azimuth 270 degrees, elevation 90 degrees).
Telecommunications Subsystem
----------------------------
The high-gain antenna was mounted to the spacecraft bus,
pointing in the negative z-direction. It was a parabolic
reflector 3.7 meters in diameter with a feed that permitted
simultaneous operation at both S-band (13 cm wavelength) and
X-band (3.6 cm). The half-power full-width of the antenna
beam was 0.6 degrees at X-band and 2.3 degrees at S-band.
The Low-Gain Antenna (LGA) was mounted on the feed structure
of the HGA and radiated approximately uniformly over the
hemisphere into which the HGA pointed.
The Telecommunications Subsystem (TCS) electronics included a
redundant pair of transponders, meaning that a failed
functional unit in one transponder could be bypassed by
swapping to the redundant unit. The TCS could transmit
science data on the X-band link at rates between 4.8 and
115.2 kilobits per second and engineering data on the S-band
link at 40 bits per second. It could receive instructions
sent (uplinked) from ground stations at a rate of 16 bits per
second. Commands were extracted from the uplink signal by
the Command Detector Unit (CDU) and were then sent to the
Computer Command Subsystem (CCS).
Spacecraft receivers were designed to lock to the uplink
signal. Without locking, Doppler effects -- resulting from
relative motion of the spacecraft and ground station -- could
result in loss of the radio link as the frequency of the
received signal drifted. Unfortunately, a series of failures
in the Voyager 2 receivers left that transponder unable to
track the uplink signal. Beginning in April 1978, Doppler
shifts were predicted and the uplink carrier was tuned so
that Voyager 2 would see what appeared to be a signal at
constant frequency (to an accuracy of 100 Hz).
Attitude and Articulation Control Subsystem
-------------------------------------------
The Attitude and Articulation Control Subsystem (AACS)
provided three-axis-stabilized control so that the spacecraft
could maintain a fixed orientation in space. Attitude
control was accomplished using gyroscopes or by celestial
reference. The AACS also controlled motion of the scan
platform, upon which the four 'remote sensing' instruments
were mounted.
Gyro control was used in special situations (e.g., trajectory
corrections and solar conjunctions) for periods of up to
several days. The inertial reference unit operated with
tuned rotor gyros having an uncalibrated drift rate of less
than 0.5 degrees per hour and a calibrated drift rate of less
than 0.05 degrees per hour.
Celestial control was based on viewing the Sun (through a
sensor mounted on the high-gain antenna) and a single bright
star (through a second sensor named the Canopus Star Tracker,
after the star used most frequently as the reference). When
the spacecraft attitude drifted by more than a small amount
from the reference objects, the AACS fired small thrusters
which returned the spacecraft to the proper orientation. The
Sun sensor was an optical potentiometer with a cadmium
sulfide detector; its error was less than 0.01 degrees and
its limit cycle was +/-0.05 degrees. The Canopus Star
Tracker was an image dissector tube with a cesium detector,
an error of less than 0.01 degree, and a limit cycle of
+/-0.05 degrees.
Redundant (backup) sun sensors, star trackers, and computers
were also part of the AACS. The non-redundant portions of
the AACS were those controlling the pointing of the
instrument scan platform, which had two degrees of freedom --
elevation and azimuth (see below).
Propulsion Subsystem
--------------------
The propulsion system was part of the AACS and consisted of
16 hydrazine thrusters. These thrusters were also used to
control the three-axis stabilization of the spacecraft. Two
thrusters on opposite sides of the spacecraft were used to
perform positive roll turns around the +Z axis. Two
oppositely pointed thrusters were used to perform negative
roll turns. One thruster was used to perform positive yaw
turns (around the +Y axis) and one was used to perform
negative yaw turns. One thruster was used to perform
positive pitch turns (around the +X axis) and one was used to
perform negative pitch turns. A backup hydrazine system was
connected to a redundant set of eight thrusters.
Power Subsystem
---------------
Spacecraft power was provided by three Radioisotope
Thermoelectric Generators (RTGs) mounted on a boom in the
negative y-direction. At Launch the three RTGs converted
7000 watts of heat into 475 watts of electrical power. RTG
electrical output decreased by about 7 watts per year because
of decay of the plutonium dioxide fissionable material and
degradation of the silicon-germanium thermocouples. The
difference between available electrical power and the power
required to operate spacecraft subsystems was called the
'power margin.' Voyager Project guidelines required a power
margin of at least 12 watts to guard against electrical
transients and miscalculations; excess electrical power was
dissipated as heat in a shunt radiator.
Data Storage Subsystem
----------------------
The Digital Tape Recorder (DTR) was used to store data when
real-time communications with Earth were either not possible
or not scheduled. The DTR recorded data on eight tracks;
rates were 115.2 kilobits per second (record only), 21.6
kilobits per second (playback only), and 7.2 kilobits per
second (record and playback). Capacity of each track was 12
images or equivalent.
Computer Command Subsystem
--------------------------
The Computer Command Subsystem (CCS) consisted of two
identical computer processors, their software algorithms, and
associated electronic hardware. The CCS was the central
controller of the spacecraft. During most of the Voyager
mission the two CCS computers on each spacecraft were used
non-redundantly to increase the command and processing
capability of the spacecraft.
Flight Data Subsystem
---------------------
The Flight Data Subsystem (FDS) consisted of two
reprogrammable digital computers and associated encoding
hardware. The FDS collected and formatted science and
engineering telemetry data for transmission to Earth.
Convolutional coding was imposed on all data transmitted from
the spacecraft. Additionally, both Golay encoding and Reed-
Solomon encoding were available for use on spacecraft data.
Data compression was also performed within the FDS.
Science Boom
------------
The Voyager science instrument boom carried the plasma
detector, cosmic ray detector and the low energy charged
particle detector. The scan platform was mounted on the
science boom.
Scan Platform
-------------
Four instruments (Imaging, PhotoPolarimeter, Infra-Red
Interferometric Spectrometer, and Ultra Violet Spectrometer)
were mounted on the scan platform, which could be slewed by
motors and gears (called actuators). Elevation of the scan
platform was measured with respect to a plane slightly offset
(by approximately 7 degrees) from the spacecraft x-z plane;
the spacecraft positive y-axis was at 97 degrees elevation
(see Spacecraft Coordinate System above). The scan platform
azimuth reference was defined by the y-z plane, with zero
azimuth being in the negative z-direction. Drive actuators
were controlled by fine feedback potentiometers; the error of
each was less than 0.03 degrees, and the final pointing error
of the scan platform was nominally +/-0.1 degrees (2-sigma
per axis). Subsequent analysis by the Navigation and
Ancillary Information Facility (NAIF) at JPL has shown larger
errors during at least the Jupiter and Saturn encounters.
High rate slews of 1 deg/sec were discontinued after the
azimuth drive mechanism on Voyager 2 temporarily froze a
short time after Saturn closest approach. The medium slew
rate was 0.33 deg/sec, and the low slew rate was 0.08
deg/sec.
Magnetometer Boom
-----------------
Two low-field magnetometers were mounted on a 13-meter-long
boom that was unfurled and extended automatically after
Launch. One low-field magnetometer was mounted at the end of
the boom and a second was mounted about 3 meters from the
end. Two high-field magnetometers were mounted at the base
of the boom.
Science Sensors
---------------
Each Voyager spacecraft carried instrumentation to support
eleven science investigations. Target body (or remote
sensing) instruments included:
(1) Imaging Science Subsystem (ISS)
(2) Photopolarimeter Subsystem (PPS)
(3) Infrared Radiometer Interferometer Spectrometer (IRIS)
(4) Ultraviolet Spectrometer (UVS)
Fields, waves, and particles (or in situ) sensors included:
(1) Plasma Subsystem (PLS)
(2) Low-Energy Charged Particle (LECP)
(3) Cosmic-Ray Subsystem (CRS)
(4) Magnetic Fields (MAG)
(5) Plasma Wave Subsystem (PWS)
(6) Planetary Radio Astronomy (PRA)
The Radio Science (RSS) investigation was carried out using
the on-board and ground elements of the Telecommunications
Subsystem (TCS). More information on instrumentation for
each of the science investigations can be found elsewhere.
Instrument Host Overview - DSN
==============================
Voyager Radio Science investigations utilized instrumentation
with elements on both the spacecraft and at ground stations of
the NASA Deep Space Network (DSN). Much of this was shared
equipment, used for routine telecommunications as well as for
Radio Science.
The DSN is a telecommunications facility managed by the Jet
Propulsion Laboratory of the California Institute of Technology
for NASA. The primary function of the DSN is to provide
two-way communications between the Earth and spacecraft
exploring the solar system. To carry out this function the DSN
is equipped with high-power transmitters, low-noise amplifiers
and receivers, and appropriate monitoring and control systems.
During the Voyager era 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 are located in the northern
hemisphere while the third is in the southern hemisphere.
The network comprised several subnets, each of which included
one antenna at each complex. The subnets were defined
according to the properties of their respective antennas. Over
the course of the Voyager Mission, those antennas were expanded
and improved. Nominal dimensions at the end (and beginning) of
the Voyager Mission were: 70-m diameter (initially 64-m),
standard 34-m diameter (initially 26-m), and high-efficiency
34-m diameter (did not exist at beginning of Voyager).
Additional ground equipment was provided by the Commonwealth
Scientific and Industrial Research Organization (CSIRO) in
Australia, the Institute of Space and Astronautical Science
(ISAS) in Japan, and the National Radio Astronomy Observatory
(NRAO) in the United States. For the Voyager 2 encounters with
Uranus and Neptune, the CSIRO 64-m diameter radio astronomy
antenna near Parkes (Australia) was included in the receiving
system for both telemetry and Radio Science. For the Voyager 2
encounter with Neptune, the ISAS 64-m diameter antenna near
Usuda (Japan) was added for Radio Science and the NRAO Very
Large Array (VLA) near Socorro (New Mexico) was added for
telemetry. The VLA consisted of 27 25-m antennas. Parkes,
Usuda, and the VLA were integrated with the permanent stations
at Goldstone, Robledo, and Tidbinbilla by DSN personnel.
Acronyms and Abbreviations
==========================
AACS Attitude and Articulation Control Subsystem
CCS Computer Command Subsystem
CDU Command Detector Unit
CRS Cosmic Ray (investigation) Subsystem
CSIRO Commonwealth Scientific and Industrial Research
Organization
DSN Deep Space Network
DTR Digital Tape Recorder
FDS Flight Data Subsystem
HGA High-Gain Antenna
IRIS Infra-Red Interferometric Spectrometer
ISAS Institute for Space and Astronautical Science
ISS Imaging Science Subsystem
JPL Jet Propulsion Laboratory
kbps kilobits per second
LECP Low-Energy Charged Particle (investigation subsystem)
LGA Low-Gain Antenna
MAG Magnetometer (subsystem)
NAIF Navigation and Ancillary Information Facility
NASA National Aeronautics and Space Administration
NRAO National Radio Astronomy Observatory
PLS Plasma (science investigation) Subsystem
PPS PhotoPolarimeter Subsystem
PRA Planetary Radio Astronomy (investigation subsystem)
PWS Plasma Wave (investigation) Subsystem
RSS Radio Science Subsystem
RTG Radioisotopic Thermoelectric Generator
TCS TeleCommunications Subsystem
UVS Ultra-Violet Spectrometer
VLA Very Large Array "
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OBJECT = INSTRUMENT_HOST_REFERENCE_INFO
REFERENCE_KEY_ID = "MORRISON1982"
END_OBJECT = INSTRUMENT_HOST_REFERENCE_INFO
OBJECT = INSTRUMENT_HOST_REFERENCE_INFO
REFERENCE_KEY_ID = "KOHLHASE1989"
END_OBJECT = INSTRUMENT_HOST_REFERENCE_INFO
OBJECT = INSTRUMENT_HOST_REFERENCE_INFO
REFERENCE_KEY_ID = "JPLPD618-128"
END_OBJECT = INSTRUMENT_HOST_REFERENCE_INFO
END_OBJECT = INSTRUMENT_HOST
END
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