Multimedia Mania/abacus-multimedia-mania.iso/dp/0115/01157.txt

Text File | 1993-07-27 | 18KB | 301 lines

$Unique_ID{bob01157}
$Pretitle{}
$Title{Pioneer
Chapter 2: Part 2 - The Pioneer Jupiter/Saturn Mission}
$Subtitle{}
$Author{Fimmel, Richard O.;Allen, James Van;Burgess, Eric}
$Affiliation{Ames Research Center;University Of Iowa;Science Writer}
$Subject{spacecraft
pioneer
earth
command
jupiter
commands
data
saturn
mission
signals
see
pictures
see
figures
}
$Date{1980}
$Log{See Mission Phases*0115701.scf
}
Title:       Pioneer
Book:        Pioneer: First To Jupiter, Saturn, And Beyond
Author:      Fimmel, Richard O.;Allen, James Van;Burgess, Eric
Affiliation: Ames Research Center;University Of Iowa;Science Writer
Date:        1980

Chapter 2: Part 2 - The Pioneer Jupiter/Saturn Mission

Command, Control, and Communications

     Mission Phases - Five distinct phases of command and control
characterized the Pioneer mission to Jupiter, Saturn, and beyond.  Each phase
required different approaches and techniques.  Two phases - Earth launch and
planetary encounter - were critical from the standpoint of controllers having
to quickly make corrections if any problems arose.  For the other three
interplanetary phases - travel from Earth to Jupiter, from Jupiter to Saturn,
and beyond planetary encounters - time was not so critical.

[See Mission Phases: The Pioneer mission to Jupiter and Saturn consisted of
several phases of spacecraft operations newar launch, interplanetary,
encounter, and postencounter. When the first Pioneer survived its encounter
with Jupiter and achieved all its scientific objectives, the second spacecraft
was directed into a path that, after encounter with Jupiter, would allow it to
rise high above the ecliptic plane and fly across the Solar System toward
Saturn. Both spacecraft reached sufficiently high velocities from their
planetary encounters to fly completely out of the Solar System.]

     During pre-launch and launch at John F. Kennedy Space Center, launch
teams from Ames Research Center and Lewis Research Center controlled the
spacecraft and launch vehicle, respectively.  Shortly after the spacecraft
separated from the launch vehicle and entered the transfer orbit to Jupiter,
control of the spacecraft was transferred to the Ames flight operations team
at the Jet Propulsion Laboratory.  Simultaneously, control of the scientific
instruments within the spacecraft was transferred to the Pioneer Mission
Operations Center (PMOC) at Ames Research Center.  This period of split
control between engineering at the Jet Propulsion Laboratory and science at
the Pioneer Mission Operations Center was arranged to take advantage of the
multiple consoles and backup computers at the Jet Propulsion Laboratory for
the critical first days of Pioneer's epoch-making flights to the outer Solar
System.  Engineer specialists were thereby able to simultaneously monitor all
subsystems, such as telemetry, power, thermal, attitude control, data
handling, and command.

     As Pioneer 10 moved away from Earth, passing the orbit of the Moon less
than 11 hr after liftoff (compared with 3 days for the Apollo spacecraft to
reach the Moon), the monitoring activities changed from assessing the "health"
of the spacecraft and its scientific instruments to readying Pioneer for its
momentous voyage to Jupiter and beyond.  Several days after liftoff and after
midcourse maneuvers, with all equipment and scientific instruments performing
well, the mission crews left the Jet Propulsion Laboratory and John F. Kennedy
Space Center and returned to Ames Research Center - now the control center for
the entire mission.

     As the spacecraft settled into the interplanetary mode, the task for
those monitoring the Pioneer became one of watching and waiting, and becoming
familiar with the unavoidable and increasing delays for signals to travel to
the spacecraft and from the spacecraft to Earth.  During the interplanetary
"cruise" phase, a team of five to seven scientists with supporting personnel
at the Pioneer Mission Operations Center monitored the spacecraft.  During
this phase, and in the subsequent flight of Pioneer 11 from Jupiter to Saturn
and beyond the planetary encounters for both spacecraft, all data received
from the spacecraft were continually monitored by computers and by personnel
to alert ground control to any malfunction at the earliest possible moment -
an important consideration if corrective action were required.  A computer at
Ames Research Center monitored telemetry signals on critical aspects of both
spacecraft and their payloads.  If a voltage or temperature were to rise or
fall too much, or if the status of an instrument were to change without being
commanded to do so, the computer sounded an alarm and printed out a message.
Day or night, whenever the situation required it, the duty operator
immediately notified the cognizant engineer or scientist, who then resolved
the problem.  The mission controllers were given specific procedures to cover
any emergency and they were advised where to obtain specialized technical help
if it were needed.

     During the long voyage through interplanetary space, data from each
scientific instrument were sampled periodically to assess how well the
instrument was functioning, in both a scientific as well as an engineering
sense.  Controllers, engineers, and 36 scientists watched for any need to
change bias voltages, to adjust the range or sensitivity of instruments, or to
switch modes of operations.

     When each Pioneer reached the edge of the Jovian system, quick action
again became the mode of operation - but action quite different from that
during the Earth launch phase.  Since the Pioneer spacecraft were then over
800 million kilometers (500 million miles) from Earth, radio signals took 92
min for the round trip to the spacecraft and back.  When Pioneer 11 reached
Saturn, the round-trip time was 173 min.  All commands to the spacecraft had
to be planned well in advance because of the delay in communication.

     The most critical piece of equipment in this respect was the imaging
photopolarimeter (IPP) (described in detail in the Appendix).  It required
long sequences of commands during the planetary encounters to best utilize the
time when the spacecraft passed by the planets and their satellites.  A
sequence of contingency commands was designed to reconfigure the Pioneer
spacecraft and their instruments should spurious commands be generated by the
accumulation of electrical charges or by intense radiation during close
approach to Jupiter.

     The fourth phase of command and control, which applied to Pioneer 11
during its voyage from Jupiter to Saturn, was a quiet period as the spacecraft
flew high across the Solar System above the plane of the ecliptic.  During
this period, project personnel were busy planning how the spacecraft should
penetrate Saturn's ring plane and were executing maneuvers of the spacecraft
to allow a close look at Titan, Saturn's largest satellite, while ensuring an
encounter with Saturn on the far side of the Sun from Earth early enough to
prevent too much interference from the Sun itself.

     The Pioneers entered the final phase of command and control as they
passed beyond planetary encounter.  As the Pioneers continue to move farther
away, their received signals become fainter and fainter and take longer and
longer to reach Earth.  Ultimately, somewhere beyond the orbit of Neptune,
contact with these tiny emissaries from Earth will be lost as both spacecraft
continue to move out of our Solar System and into interstellar space.

     Tracking and Data Acquisition Support - The NASA Communications Network
operated by Goddard Space Flight Center provided worldwide ground
communications circuits and facilities to link Earth terminals that receive
signals from the spacecraft with control centers on the west coast of the
United States.

     Extending around the world, the Deep Space Network (operated for NASA by
the Jet Propulsion Laboratory) provided deep space tracking, telemetry data
acquisition, and commanding capabilities through the 26-m (85-ft) and 64-m
(210-ft) diameter antennas at Goldstone, California, and in Spain, South
Africa (until July 1, 1974), and Australia.  During the later phases of the
mission, some of the smaller antennas were enlarged to 34-m (112-ft) diameter
dishes; then each of the three stations had one of each size of antenna.  As
Earth turned on its axis, the controllers maintained contact with the
spacecraft using stations in Goldstone, Australia, Spain, or, for part of the
Pioneer 10 mission, South Africa, operating in turn each day.  As each Pioneer
spacecraft began to set at one station, the next station acquired it, with
periods of overlapping coverage.  The encounter with Saturn was timed so that
signals from Pioneer were received at both the Madrid and Goldstone stations
during the most critical period of the enccounter to reduce the chance that
vital data might be lost.

     Telecommunications - Communications over the vast distances to Jupiter
and Saturn and beyond presented problems never before encountered in our space
program.  Transmitters and antennas onboard the spacecraft had to be designed
to conserve power and to be as lightweight as possible.  Communications with
Earth relied heavily on the extremely sensitive 64-m (210-ft) antennas of the
Deep Space Network and their advanced receiving systems.  During the long
interplanetary cruises, the spacecraft used the 26-m (85-ft) diameter antennas
when the larger antennas were required for other space missions, but with the
smaller antennas, information had to be transferred from the spacecraft to
Earth at a lower rate.

     When used to transmit commands to the spacecraft, the 64-m (210-ft)
antennas were so precise in directing their radio beams and provided such a
high radiated power (up to 400,000 W at Gold-stone) that these commands could
be received by the spacecraft at greater distances (to several hundred times
the distance between Earth and the Sun) than those at which messages from the
spacecraft could be received at Earth (perhaps 40 times the distance between
Earth and the Sun).

     The spacecraft carried three antennas to receive and to send signals -
low gain (omnidirectional), medium gain, and high gain (beamed).  It also
carried two redundant receivers (for commands from Earth) and two redundant
transmitters.  The redundancy provided a back-up in the event of a failure
during the journeys of the two Pioneers which lasted more than two decades.

     The amount of energy received at Earth from the spacecraft via radio
links from Jupiter's distance is incredibly small: from the distance of Saturn
the amount is smaller still by more than two-thirds.  A 26-m (85-ft) diameter
antenna collecting this energy from the distance of Jupiter would require 17
million years to gather enough energy to light a 7.5-W night lamp for a mere
one thousandth of a second.  From Saturn it would require nearly 56 million
years.  Only the sophisticated data coding and signal modulation techniques,
coupled with the large antennas and the advanced, ultra-cold receiving devices
attached to them made it possible to receive and record these faint signals
from the two Pioneer spacecraft.  All the pictures of Jupiter and Saturn
reproduced in this volume, all the information from space to beyond the orbit
of Uranus, all the information about the environment of the two planets, all
the engineering data about the spacecraft and their many scientific
instruments, all the tracking of the spacecraft to 2 billion miles from Earth
derived from these incredibly weak radio signals.  The communications system
of the Pioneer spacecraft and the Deep Space Network are truly great
technological achievements.

     The rate at which information is passed over a radio link is expressed in
bits/second, where a bit is defined as a unit of information analogous to the
dots and dashes of the Morse code.  Onboard each spacecraft, a data-handling
system converted science and engineering information into an organized stream
of data bits for transmission to Earth.  Just as a street lamp shining at
night appears fainter and fainter with increasing distance, radio signals from
a spacecraft also become fainter with distance.  Also, natural background
radio signals create interference, and even the components of the electronic
apparatus generate radio noise by the movement of electrons within them.  As
signals become fainter with distance they tend to be drowned out by this
background of noise.  Therefore, sophisticated techniques and equipment had to
be developed to receive information from these extreme distances.

     As the Pioneer spacecraft moved farther into our Solar System, their
signals became weaker and weaker at Earth.  The telemetry system adjusted to
these weaker signals by commanding a change in the rate at which information
was transmitted to Earth.  Power per unit of information depends on the rate
at which the information is sent - the bit rate.  To extract information from
a radio signal, the energy level of the signal must exceed the energy of the
background noise.  As the spacecraft moved farther and farther away, the bit
rate was reduced so that less information was sent per second.  Each bit of
information lasted longer and thereby possessed more energy, so that it could
be detected above the radio noise.

     By reducing the bit rate, controllers compensated for the fainter signals
received from the Pioneer spacecraft.  When each spacecraft was on its way to
Jupiter, the communication system could pass a maximum of 2048 bits of
information to Earth every second, using the 26-m (85-ft) antennas.  But at
Jupiter, because of the increased distance, the maximum rate was only 1024
bits/sec, using the 64-m (210-ft) antennas.  Near Saturn the maximum rate was
maintained at 1024 bits/sec with the larger antennas because their sensitivity
had been improved during the five years the Pioneer spacecraft took to travel
from Jupiter to Saturn.  Because of the increased sensitivity of the Deep
Space Network, communication between Earth and the two Pioneers far exceeded
original expectations.  At Saturn encounter, however, proximity to superior
conjunction with the Sun forced a reduction in the transmitting rate from 1024
to 512 bits/sec for part of each station's view period.

     A digital telemetry unit onboard each spacecraft prepared the data for
transmission in one of 13 data formats at one of 8 bit rates from 16 to 2,048
bits/sec.  An onboard data storage unit was able to store 49,152 data bits for
later transmission to Earth.  This storage capability allowed data to be
gathered by the spacecraft during important parts of the mission faster than
the data could be sent to Earth.  It also stored data when the data could not
be transmitted at all, for example, when the spacecraft was passing behind
Jupiter or Saturn.  The data were later transmitted in response to ground
command.

     Command and Control - At Pioneer Mission Operations Control, 222
different commands were used to operate the Pioneer spacecraft.  The command
system consisted of two command decoders and a command distribution unit
within each spacecraft.  Commands were transmitted from Earth at 1 bit/sec.
Since each command message consisted of 22 bits, a command required 22 sec to
transmit.

     Each spacecraft also carried a small command memory that could store five
commands.  When a series of up to five commands had to be executed in less
time than was needed to transmit them from Earth at the command rate, that is,
22 sec for each command, this memory was used.  The command memory with time
delay was also used to command the spacecraft when it was behind Jupiter and
Saturn and out of touch with Earth.

     A command distribution unit in each spacecraft routed the commands within
the spacecraft:  73 commands to operate experiments and 149 to control
subsystems of the spacecraft.  Science commands, for example, included those
to calibrate instruments, change modes, move the photopolarimeter telescope,
and change instrument data types.  Spacecraft commands included firing the
rocket thrusters and changing from one component to another redundant
component, selecting different antennas, and changing the modes of the
data-handling subsystems.

     Any command not properly verified by the decoder in the spacecraft was
not acted on by the command distribution unit.  Thus, precautions were taken
against the spacecraft accepting wrong commands.  Commands were also verified
on Earth by a computer and by controllers before the commands were
transmitted.  A Pioneer Encounter Planning Team, headed by the Project Science
Chief, considered many possible contingencies that might arise during the
weeks when each Pioneer spacecraft was passing through the systems of the
giant planets, and they developed command sequences to meet such
contingencies.

     The decision early in the planning stages to "fly" the Pioneer spacecraft
by command required constant scrutiny and diligence on the part of the
controllers well in advance of any commanded events taking place on the
spacecraft.  Indeed, two years of careful planning preceded the first
encounter with Jupiter.  All commands (e.g., more than 16,000 total for the
first Pioneer) were meticulously sequenced, checked, and stored in a
ground-based computer in 8-hr-long files suitable for transmission during the
time that a ground station would be in contact with the distant spacecraft.
Most of these commands were transmitted to the spacecraft in a four-week
period.  Personnel at the Pioneer Mission Operations Center, the Ground Data
System, and the Deep Space Network met all the demands of the mission -
sending commands on time, with the high level of reliability the mission
demanded.  This performance was repeated for the encounter of the second
Pioneer with Jupiter and, five years later, with Saturn.