PDS_VERSION_ID = PDS3
LABEL_REVISION_NOTE = "Initiated JMAFI, 2004-03-01"
RECORD_TYPE = STREAM
OBJECT = INSTRUMENT_HOST
INSTRUMENT_HOST_ID = P11
OBJECT = INSTRUMENT_HOST_INFORMATION
INSTRUMENT_HOST_NAME = "PIONEER 11"
INSTRUMENT_HOST_TYPE = SPACECRAFT
INSTRUMENT_HOST_DESC = "
Instrument Host Overview
========================
The Pioneer 11 spacecraft was designed to fit within the 3-meter
diameter shroud of the Atlas-Centaur launch vehicle. To do so it had
to be stowed with its booms retracted, and with its antenna dish
facing forward (i.e. upward on the launch pad).
The spacecraft comprises several distinct subsystems: a general
structure, an attitude control and propulsion system, a
communications system, thermal control system, electrical power
system, navigation system, and, most important to the scientific
mission, a payload of 11 onboard instruments.
To communicate over long distances the spacecraft's dish-shaped
antenna has to be pointed toward Earth. A simple and inexpensive way
to do this is to spin stabilize the spacecraft and keep the spin
axis pointed to Earth. So the spacecraft is stabilized by rotation.
General Structure
-----------------
Each spacecraft is 2.9-meters long from its cone-shaped,
medium-gain antenna to the adapter ring which fastened the
spacecraft to stage three of the launch vehicle.
Spacecraft structure centers around a 36-cm deep, flat, equipment
compartment, the top and bottom of which are regular hexagons. Its
sides are each 71-cm long. One joins to a smaller 'squashed'
hexagonal compartment that carries most of the scientific
experiments.
The 2.74-meter diameter, 46-cm deep, parabolic, dish-shaped,
high-gain antenna of aluminum honeycomb sandwich material is
attached to the front of the equipment compartment. Its feed is
topped with a medium-gain antenna on three struts which project
about 1.2 meters forward. A low-gain, omni antenna extends about
0.76 meters behind the equipment compartment, mounted below the
high-gain dish.
Two three-rod trusses, 120 degrees apart, project from two sides
of the equipment compartment. At their ends, nuclear electric
power generators are held about 3 meters from the center of the
spacecraft. A third boom, 120 degrees from the other two, projects
from the experiment compartment and positions a magnetometer
sensor about 6.6 meters from the center of the spacecraft. All
three booms are extended after launch.
Attitude Control and Propulsion
-------------------------------
The spacecraft possesses a star sensor to provide a reference on
the bright southern star Canopus, and two sun sensors to provide a
reference to the Sun. Attitude position is calculated from the
reference direction to the Earth and the Sun, with the known
direction to Canopus provided as backup. Pioneer 11 's star sensor
gain and threshold settings were modified to improve performance
based on experience with this sensor on Pioneer 10.
Three pairs of rocket thrusters located near the rim of the
antenna dish are used to direct the spin axis of the spacecraft,
to keep it spinning at the desired rate of 4.8 revolutions per
minute, and to change the spacecraft's velocity. The system's six
thruster nozzles can be fired steadily or pulsed by command.
Each thruster develops its propulsive jet force from the
decomposition of liquid hydrazine by a catalyst in a small rocket
thrust chamber attached to the nozzle of the thrusters.
Attitude and velocity changes are made by two thruster pairs
mounted on opposite sides of the antenna dish rim. One thruster of
each pair points forward, the other, aft. To change attitude, the
spacecraft spin axis is rotated in the desired direction by firing
two thrusters, one on each side of the antenna dish. One thruster
is fired forward, one aft, in brief thrust pulses at a precise
position in the circle of spacecraft rotation. Each thrust pulse,
timed to the spacecraft's rotation, moves (precesses) the spin
axis a few tenths of a degree, until the desired attitude is
reached.
To change velocity, the spin axis is precessed until it points in
the desired direction, then two thruster nozzles, one on each side
of the antenna dish, are fired continuously, both in the same
direction, i.e., forward or aft to increase or decrease the flight
path velocity.
To adjust spin rate, two more pairs of thrusters, also set along
the rim of the antenna dish, are used. These thrusters are aligned
tangentially to the antenna rim, one pointing against the
direction of spin and the other with it. Thus to reduce spin rate,
two thrusters fire against spin direction.
Communications
--------------
The spacecraft carries two identical receivers. The omni and
medium-gain antennas operate together and as such are connected to
one receiver while the high-gain is connected to the other, though
the receivers do not operate together. The receivers can be
interchanged by command, or, should there be a period of
inactivity, automatically. Thus, should a receiver fail during the
mission, the other can automatically take over.
Two radio transmitters, coupled to two traveling-wave-tube power
amplifiers, each produce 8 watts of power in S-band.
The communication frequency uplink from Earth to the spacecraft is
at 2110 MHz, the downlink to Earth at 2292 MHz. The turnaround
ratio, downlink to uplink, is precisely controlled to be
compatible with the Deep Space Network.
The spacecraft data system turns science and engineering
information into a specially coded stream of data bits for radio
transmission to Earth. A convolutional encoder rearranges the data
in a form that allows detection and correction of most errors by a
ground computer at the receiving site of the Deep Space Network.
There are 11 data formats divided into science and engineering
data groups. Some science formats are optimized for interplanetary
data, others for the Jovian encounter. Engineering data formats
specialize in data handling, electrical, communications,
orientation, and propulsion data. All formats are selected by
ground command.
Thermal Control
---------------
Temperature on the spacecraft is controlled at between -23 and 38
degrees C inside the scientific instrument compartment, and at
various other levels elsewhere for satisfactory operation of the
onboard equipment.
The temperature control system coped with gradually decreasing
heating as the spacecraft moved away from the Sun, and with two
frigid periods - one when Pioneer 10 passed through Earth's shadow
at launch; the other at the time of passage through Jupiter's
shadow during flyby. The system also controlled the effects of
heat from the third-stage engine, atmosphere friction, spacecraft
nuclear electric power generators, and other equipment.
The equipment compartments are insulated by multi-layered blankets
of aluminized plastic. Temperature-responsive louvers at the
bottom of the equipment compartment, opened by bi-metallic
springs, allow controlled escape of excess heat. Other equipment
has individual thermal insulation and is warmed by electric
heaters and 12 one-watt radioisotope heaters, fueled with
plutonium-238.
Electrical Power
----------------
Nuclear-fueled electric power for Pioneer Jupiter comes from the
four SNAP-19 type Radioisotope Thermoelectric Generators (RTGs),
developed by the Atomic Energy Commission (AEC), similar to those
used to power the Nimbus-3 meteorological satellite. These units
turn heat from plutonium-238 into electricity.
The RTGs are on the opposite side of the spacecraft from the
scientific instrument compartment to reduce the effects of their
neutron radiation on the instruments. Mounted two each on the end
of each boom, these four RTGs developed about 155 watts of
electrical power at launch, which decayed to approximately 140
watts by the time the spacecraft reached Jupiter. One hundred
watts output is expected five years after launch. The depletion of
power is not from the nuclear source itself but from deterioration
of the junctions of the thermocouples which convert heat into
electricity. The RTGs supply adequate power for the mission since
the spacecraft needs only 100 watts to operate all systems, of
which 26 watts are for the science instruments. Excess power from
the RTGs over that needed by the spacecraft is radiated to space
thermally through a shunt radiator, or charges a battery which
automatically supplies additional power needed for short periods
when the spacecraft demands more power than the output of the
RTGs.
Navigation
----------
The axis of the high-gain antenna dish is slightly offset from,
but parallel to, the spin axis of the spacecraft within close
tolerances throughout the mission. Except during initial stages of
the flight near Earth and for periods when alignment must be
changed to suit course correction, the spin axis of the spacecraft
is always pointed toward Earth within a tolerance of one degree to
provide best communication.
Analysts use the shift in frequency of the Pioneer radio signal
and angle tracking by the antennas of the Deep Space Net to
calculate the speed, distance, and direction of the spacecraft
from Earth. Motion of the spacecraft away from Earth causes the
frequency of the spacecraft's radio signals to drop and their
wavelength to increase. Known as a Doppler shift, this effect
allows the speed of the spacecraft to be calculated from
measurement of the frequency change in the signal received at
Earth.
The radio beam is offset one degree from the spin axis. As a
result, when the spin axis is not directed exactly towards Earth,
uplink signals received at the spacecraft from Earth vary in
intensity synchronously with rotation of the spacecraft. A system
on the spacecraft, known as Conical Scan (CONSCAN), was originally
intended to be automatically used to change the spacecraft's
attitude in a direction to reduce these variations in signal
strength, thereby returning the spin axis to the precise Earth
point to within the threshold of 0.3 degrees. However, flight
operations personnel developed and used a direct command technique
that results in conserving the spacecraft's gas supply.
Scientific Payload
------------------
Investigation of interplanetary space on the way to and beyond
Jupiter aimed to resolve a number of unknowns about the magnetic
field in interplanetary space; cosmic rays, fast moving parts of
atoms from both the Sun and the Galaxy; the solar wind, a flow of
charged particles from the Sun, and its relationships with the
interplanetary magnetic field and cosmic rays; and interplanetary
dust concentrations, if any, in the asteroid belt.
[INSTRUMENT_HOST_DESC was adapted from FIMMELETAL pp. 39-45.]"
END_OBJECT = INSTRUMENT_HOST_INFORMATION
OBJECT = INSTRUMENT_HOST_REFERENCE_INFO
REFERENCE_KEY_ID = "FIMMELETAL1977"
END_OBJECT = INSTRUMENT_HOST_REFERENCE_INFO
END_OBJECT = INSTRUMENT_HOST
END
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