Mission Control is the heart of Flight Operations. You may have visited, or have seen images of the Mission Control areas at JPL where the flight operations of unpiloted, robotic spacecraft like Voyager, Magellan, or Galileo are controlled. Cassini's flight will be controlled from a small area, visible from the Public Viewing Gallery, within the Space Flight Operations Facility at JPL. This can be seen in this photo. The team responsible for Cassini Mission Control is the Cassini Realtime Operations Element (RTO). RTO, as the name suggests, operates in the "right now" time-frame, as opposed to long-range future planning, or downstream data analysis, which are performed by other teams. RTO normally assigns one person per shift as the "Mission Controller" who communicates with other teams by "voice-net" or electronic mail, as well as via the regular telephone. Computer workstations show the mission controller displays of the spacecraft's health and displays of realtime status of the Deep Space Network (DSN) antennas and systems which are linked together to provide two-way communications with the spacecraft.
Ace is the call-sign used on the voice-net for the Mission Controller.
The letters in Ace do not stand for anything. The name simply
implies a single point of contact. And (not a totally inappropriate
connotation) in the aviation world, Ace also means an expert flyer!
The Ace's main responsibility is to monitor the ground data system
and the spacecraft during the periods when the DSN is tracking the
spacecraft. Usually, everything runs like clockwork. Hundreds of computers,
programs, data files, and mechanical systems operate perfectly together. But
the second anything amiss is detected, the Ace initiates
corrective actions. For the most part, corrective actions are quick,
and loss of data due to problems in the spacecraft or Ground Data
System is rare.
The voice net is a system of hot-line telephone voice links which are used to communicate quickly with people working on different aspects of flight operations. The user only has to press a button or two, and talk. It isn't necessary to dial phone numbers. The Voice-net units, which you'll see everywhere when you view the Mission Control area, are these "big white boxes:"
Typically, many people from different parts of the Cassini project are
listening to the voice nets. To reach someone, you press the talk button
and say the call-sign of the person you are calling, followed by your own
call-sign. For example, the Mission Controller calling a
navigator would
press the button and say, "Nav, Ace" to get the navigator's attention. Once
"Nav" responds, the Ace would then go ahead with his or her message.
Call-signs, which briefly indicate someone's functional position, are used
rather than actual names, because various individuals may be on shift at
different times. Its very much like the radio communications used in
aircraft flight operations.
Additional information is displayed for the Ace, or given by voice-net, concerning the status of other components in the ground data system. The entire ground data system includes the DSN, the Ground Communications Facility which transports data and voice between JPL and the DSN sites worldwide, and the computer systems located at JPL. Once the spacecraft's Telemetry is received and propagated through the ground data system, selected measurements from it are displayed on the Ace's workstation so s/he can verify that the spacecraft is healthy and has received its latest set of commands. Telemetry is the type of data that carries information about status, voltages, pressures, temperatures, etc. within the spacecraft's subsystems. It also carries the bulk of the scientific measurements, such as images and spectra, produced by the spacecraft's instruments.
The Ace also verifies that the ground data system is obtaining the data needed by the navigation team to determine the spacecraft's speed and trajectory. This data type is known as Tracking data.
The Ace is the one who sends commands to the
spacecraft, but the determination of what commands to send is made in
processes involving other teams long before the commands are actually
sent. Typically, the Ace sends the spacecraft one quick command to
make sure the two-way communications link is operating properly.
When the spacecraft receives this command, it resets a software timer on
board. If this timer, called the Command Loss Timer, ever reaches a
predetermined value, it will cause the spacecraft to make the assumption
that something is wrong since it hasn't heard from Earth for a long
time, and to begin taking certain corrective actions such as swapping
radio receivers. We told you it's a smart spacecraft!
Typically, commands that the Ace sends have been arranged into files known as command sequences. It is the Ace's job to make sure these sequences are radioed to the spacecraft, and to make sure the spacecraft has received literally every bit of the commands in perfect order. In the rare instances when the spacecraft misses a command or a part of a command, the Ace will re-send the missing part(s) and then verify that the whole sequence is properly installed aboard the spacecraft.
In short, the Ace makes sure that all the various types of data flow properly in real time to the right places: that commands are properly installed on the spacecraft; that valid tracking data are captured for the navigators; and that telemetry data are delivered to the spacecraft engineers and the scientists, and stored in large on-line databases and archives for future reference. This is the role of Mission Control in the life of Cassini flight operations.
