Commanding the Cassini Spacecraft
Technically, commanding the spacecraft is a little like commanding your TV set
with a remote control. We place command signals on the radio-frequency
uplink to the
spacecraft, which it receives, decodes, and acts upon. Your remote control
probably uses an
infrared signal with
its own specially coded commands. The TV set receives them, its circuitry
decodes them, and the TV acts upon them. The TV/VCR remote control depicted in
this sketch would be capable of sending thirty-five different commands to your
video cassette recorder and TV, such as volume up, volume down, play, stop.
Most remotes can probably send more commands than that. Each type of command
that it can send consists of a unique set of pulses, sent out as flashes of
infrared light through a dark plastic "window" in its front end, all controlled
by the micro-chip inside the hand-held unit, based on what buttons you press.
Cassini can recognize over a thousand different kinds of commands. They're
sent by computer-control which varies the phase of the uplink,
modulating
it with pulses, not too unlike the way the TV remote control works. (By the
way, we must ask you, please: never, never go outside and point your remote
control at Saturn! Heh-heh, just kidding.)
But sending the actual command signals to Cassini are the very last stage of
the uplink process. First, there have to be decisions about exactly what
commands to send. The broader definition of "uplink" embraces the whole
process of deciding what we want the spacecraft to do.
The Planning Part
If you just want to flip through the channels, you don't have to think very
much about what "commands" you want to send to your TV via the remote control.
But if you want to set up the VCR to record your favorite show tomorrow
evening, a bit of planning will be required. The situation becomes a little
more complex, say if the scientist in the family wants to record one show
tomorrow night, and the engineer in the family wants to record a different
show that night. Once you have negotiated your plan, you can then figure out
what buttons you will need to press on your VCR remote, and in what sequence.
You'll be sending your VCR a sequence of commands. The VCR receives and stores
the commands in its memory. Since the VCR has its own clock, it can sense when
to execute the commands, and will do so at the proper time.
Cassini's Uplink Process
In Cassini's uplink process, there are millions of possibilities to consider,
and many, many decisions to be made. For example, the process has to determine
what scientific
observations
Cassini is going to make, and when to make them; exactly when to fire its
rocket engine or thrusters, and how the spacecraft must be oriented when it
fires them; what kinds of measurements to send to Earth, and what data rates
to use. So Cassini's uplink process begins long before sequences of commands
are actually placed on the uplink.
The process starts with the scientists (also called investigators) associated
with Cassini and the
Huygens Probe, who are located all
over the world at universities, observatories, and aerospace companies. You
can view lists of them if you like.
They are supported by their own research assistants and grad students. These
teams were selected for participation in the Cassini Program through a process
of intense competition and negotiation. Generally speaking, each team designed
a scientific instrument which the
Cassini spacecraft carries, (or an
instrument on the Huygens Probe) to
answer their questions about the Saturnian system.
Things To Know First
To address the scientists' questions requires that Cassini operate these
instruments to make observations, and carry out experiments, within the
Saturnian system under just the right conditions, and at just the right time.
To match questions with exactly the right observations and experiments requires
a vast amount of information about the spacecraft and about the Saturnian
system. For example, exactly where will the spacecraft be at a particular
time, and what will be its orientation in space? When, and in what direction
must it turn, to capture a view of the various targets of interest? How long
must an instrument's shutter remain open to obtain the right exposure? What
other settings will the instrument need? It's far more complicated than
commanding your VCR!
Determining the location, or path (called trajectory) of the spacecraft is the
job of a team of navigators at JPL,
working as part of the Cassini Flight Systems Operations element. They obtain
their information from the intricate process of
tracking
the spacecraft. They determine where the spacecraft will be at any given time
in relation to objects in the Saturnian system. Of course, to do that, they
need to know where those objects are going to be. The predicted locations of
Saturn, its rings, its moons and so on, are data known as "ephemerides." These
are obtained, and maintained, by the worldwide
astronomical community. The JPL
navigators take the ephemerides and the spacecraft tracking data into account
in their processes, using highly developed computer programs. After much
number-crunching, they provide the predictions necessary for planning how and
when Cassini will be able to make observations. They also provide information
on what the spacecraft must occasionally be commanded to do, in order to make
small adjustments in its trajectory (called
Trajectory Correction Maneuvers,
or TCMs), so that it will be exactly where it needs to be at the proper time.
Places
Ok, we know where everything is. And, from the ephemeris and trajectory
information, we also know how things will be illuminated... how bright will
the sunlight be as it shines on Saturn, or its moons? At what angle (called
phase) will
the sunlight be striking the rings from Cassini's point of view at a certain
time? When will Cassini see the Sun set behind Saturn's cloudtops?
And we know what the spacecraft's
attitude
will be at any given time, based on the last sequence of commands sent to the
spacecraft. Normally, engineers command the spacecraft so that it will be
pointing its large communications antenna dish toward Earth. But since all the
scientific instruments are fixed to the spacecraft and immovable, new attitudes
will need to be commanded for making observations. The whole spacecraft must
rotate to point its instruments, store the collected measurements aboard its
solid state recorder,
and then finally rotate and point back toward Earth to beam these valuable
data down to the waiting scientists.
There's More To The Picture
This is a huge amount of information that needs to be considered for planning
Cassini's desired operations. But wait, there's lots more! We need to
consider many details about the spacecraft's on-board
subsystems,
too. How much power will be required to operate the desired instrument? Will
that power be available at the required time, or will we first need to turn
off something else? What other "consumables" will be affected? If we rotate
the spacecraft, will we lose communication with Earth? If so, for how long?
Can we afford to do without tracking data for that long at that point in time?
When the spacecraft rotates, is there any danger of sunlight entering a
sensitive instrument and causing damage? Exactly where in memory should the
data from this observation be stored? Do we need to do any on-board
"housekeeping" activities?
A bit of serious planning has to be done. Much of the planning for the
achievement of the Cassini Program's major goals, such as reaching Saturn via
gravity assist,
arriving and entering into orbit at
Saturn, were accomplished many years before launch, and have been incorporated
into the spacecraft's design, choice of launch vehicle, and the original design
of the overall mission.
Plan As You Go
But all the planning can't be done at once; it has to be an ongoing process.
The people responsible for ongoing planning come together from teams within the
Cassini organization, to form two "virtual" teams: the Mission Planning virtual
team, and the Sequence virtual team. They are called "virtual" teams because
they draw upon people who have other assignments within the Cassini Program,
who convene only long enough to accomplish the planning needed for one clearly
defined period of time in Cassini's future, a few weeks for example. Once this
task has been completed, the members return to their regular jobs on Cassini,
and other, different virtual team members are then convened to plan for the
next period of time.
Mission Planning Virtual Team
The Mission Planning virtual team concerns itself with a time frame farther in
the future than does the Sequence team, and they prepare sets of guidelines and
constraints that govern how resources will be used to achieve the highest
possible return of scientific data, keeping in mind the mission's long-term
goals. Members of this team include mission designers, engineers responsible
for the spacecraft (from the Flight System Operations element), representatives
responsible for the scientific instruments (from the Science Office), and
schedulers of Earth-based tracking coverage (from the Realtime Operations
element). The resources to be considered include opportunities for making
observations, time available from the
Deep Space Network
tracking system (DSN), human workforce availability, spacecraft consumables
such as electrical power and propellant, and so on. It is not uncommon for
the scientific investigators to desire conflicting observations, nor is it
uncommon for more than one spacecraft to desire conflicting use of the DSN's
precious time. The DSN must serve other projects as well, including
Voyager 1,
Voyager 2, Galileo,
Mars Global Surveyor,
Mars Pathfinder, and many others. Conflicts are
resolved in high-level meetings with the principal people concerned.
Sequence Virtual Team
Once the Mission Planning virtual team has done its job, all the details of its
plan of action for a given period are passed to the Sequence virtual team.
These people flesh out the plans into the properly formulated commands which
can be sent to the spacecraft, to make it carry out the carefully determined
plans. To do this, the Sequence virtual team relies on highly advanced
computer programs to help them do their task. This includes selecting and
time-tagging the proper command data, placing them in the correct time-order,
checking that no operating constraints are violated, and making sure all of the
instructions to the spacecraft will fit into the spacecraft's available
computer memory.
The command data produced for a particular time period are called a
"sequence," or a "command load."
The sequence is then passed to the Realtime Operations element, who finalizes
plans for actually sending it to the spacecraft. There may be other commands,
usually called "realtime" commands, which are required to be uplinked during
the normal course of flight operations. These are typically shorter than the
large command loads delivered by the Sequence virtual team, and are sent on
short notice. They may come from scientists desiring to make quick adjustments
to their instruments' states, for example.
Preparing to Uplink
The Ace, a member
of the Realtime Operations element, in coordination with the other teams,
determines the proper time for transmitting the command loads and realtime
commands to the spacecraft during an appropriate DSN tracking period. First,
the command data are formatted for transmission and sent electronically, using
the Ground Communications Facility (GCF), to the proper site in the DSN, where
they are loaded onto disk in the remote command computer. The GCF uses a
dedicated combination of communications satellites and conventional surface
and undersea cables to electronically link JPL with the remote DSN sites.
There are three remote DSN sites, each called a Deep Space Communications
Complex (DSCC). Their locations were chosen at widely separated longitudes in
order to provide continuous tracking of any interplanetary spacecraft as the
Earth rotates. Each DSCC has similar equipment, including several large radio
telescope dishes and control equipment. The Goldstone DSCC is located near
Goldstone Dry Lake in the heart of the Mojave desert in California, and is
operated by JPL for NASA. The Canberra DSCC is located in the semi-arid
rolling hills of New South Wales at Tidbinbilla, not far from the Australian
Capital Territory of Canberra. The Canberra DSCC is operated for NASA by the
Australian Department of Science. The Madrid DSCC is located in the foothills
of Robledo, Spain, near the capital city of Madrid. It is operated for NASA
by the Spanish National Institute for Aerospace Techniques. Voice links,
basically continuously open telephone calls, are maintained between all three
DSCCs and the Network Operations Control Center at JPL.
Pressing the Buttons
Meanwhile, back at the ranch, we have a load of command data sitting on the
command computer disk at one of the DSCCs, finally ready to go to the
spacecraft after its long process of plans and preparations. The Ace makes
sure the DSN's transmitter is on, radiating a carrier signal to the spacecraft.
The uplink radio signal is then made ready for modulation with command data.
Boy, that was a mouthful. That just means the transmitter is getting ready to
make subtle changes in its signal, sort of like the changes your vocal cords
make when you speak. The Earthly transmitter is going to speak to Cassini.
The Ace manipulates the command computer under remote control from JPL, causing
the command data to be modulated (vocalized!) onto the uplink: the commands
begin their journey to the spacecraft. At the speed of light, it takes about
an hour for the uplinked commands to travel the great distance to Saturn.
(For comparison, signals sent to the Moon get there in less than a second.)
The spacecraft reports, via
telemetry,
the fact that the command data have been received and are properly stored on
board (it takes another hour for this report to travel back to Earth. Once
properly stored in the spacecraft's memory, the on-board clock will cause each
of the timed commands to execute at the proper instant... not unlike the way
your VCR does.
The Ace and others will watch the downlink over the period of time covered by
the command sequence, making sure all is going according to plan. That's the
command uplink process in a nutshell.
