SPACE FLIGHT OPERATIONS

Cruise Phase

Objectives:

Upon completion of this chapter, you will be able to list the major factors involved in spacecraft checkout and characterization, and preparation for encounter. You will be able to characterize typical daily flight operations.

Cruise phase is bounded by launch phase at the beginning, and encounter phase at the end. It is a time during which ground system upgrades and tests may be conducted, and spacecraft flight software modifications are implemented and tested. Cruise operations are typically carried out from the Space Flight Operations Facility at JPL.

Spacecraft Checkout and Characterization

After launch, the spacecraft is commanded to configure for cruise. Appendages which might have been stowed in order to fit within the launch vehicle are deployed either fully or to intermediate cruise positions. Telemetry is analyzed to determine the health of the spacecraft, indicating how well it survived its launch. Any components which appear questionable might be put through tests specially designed and commanded in real time, and their actual state determined as closely as possible by subsequent telemetry analysis.

During the cruise period, additional command sequences are uplinked and loaded aboard for execution, taking the spacecraft through its routine operations, such as tracking Earth with the HGA and monitoring celestial references. The flight team members begin to get the feel of their spacecraft in flight. Inevitably, problems arise which had been unforeseen, and the onboard fault protection algorithms receive their inadvertent tests; the spacecraft will, more likely than not, go into safing or contingency modes , and it must be painstakingly recovered.

TCMs are executed to fine tune the trajectory. Eventually, as the spacecraft nears its target, the science instruments are powered on and calibrated, if they have not already been powered on earlier during cruise.

Real-time Commanding

Frequently, commands stored on board during cruise or other phases must be augmented by real-time commands, as new activities become desirable, or, rarely, as mistakes are discovered in the on-board command sequence. There is an inherent risk in real-time commanding; it is always possible that the wrong commands may be sent. The longer, planned sequences of commands (generally just called "sequences") typically benefit from a long process of extensive debate and selection, testing and checking and simulation prior to uplink. These factors may limit the desirability of undertaking many activities by real-time commands that do not have the benefit of the full sequence development process, but the necessity, as well as the convenience, of real-time commanding frequently prevails.

Typical Daily Operations

Usually, at least one person is on duty 24 hours a day, seven days a week at JPL to watch the spacecraft while in flight, and respond to any anomalous indications. The person so designated is typically the mission controller or ACE. The ACE is a person on the Mission Control Team who is the single point of contact between the entire flight team, consisting of the Spacecraft Team, the Navigation Team, Science and other teams, and the teams and facilities external to the project such as DSN, SFOF Facilities, Mission Control and Computing Center ("MC-cubed"), Operations Planning and Control Team (OPCT), Ground Communications Facility (GCF), AMMOS, and the Information Processing Center (IPC).

"ACE" is not an acronym, despite all attempts to make it one. It simply refers to one single point of contact for a project's real-time flight operations, not too inappropriately a pun for an expert combat pilot. The ACEs are multimission personnel, and one ACE may be serving more than one flight project at a time. For example, in 1993, one ACE was in charge of Magellan, Voyager 1, and Voyager 2 simultaneously. The ACE executes commanding, manages the ground systems, insures the capture of telemetry and tracking data, watches for alarms, evaluates data quality, performs real-time analyses to determine maneuver effectiveness and spacecraft health, and coordinates the activities of the DSN and other teams in support of the projects. Typically, a large portion of the ACE's interactions are with the Spacecraft Team, the DSN, and the OPCT.

Monitoring Spacecraft and Ground Events

An accurate list of expected events is needed to compare with spacecraft events as they occur in real time, in order to make sure the spacecraft is operating as planned. It is also required for the purpose of directing DSN station activity, and to be able to plan command uplinks and other real-time operations. That list is called the integrated sequence of events (ISOE). Integrated means that it contains both spacecraft events and DSN ground events. Compiling an ISOE begins with a list of the commands which will be placed in the spacecraft's memory, and which will be executing over a period of typically a week or two into the future. Times of the events are included with the commands. These are supplied in a spacecraft event file (SEF) supplied by the spacecraft team. They are adjusted for light time, and are combined with DSN station information and events. A subset of the list is provided to the DSN as a keyword file (KWF), which the DSN then combines with similar listings from other projects to create a sequence of events (SOE) for each particular station. The illustration below shows an excerpt of activities typical on a flight project which contribute to the generation of SOE products.

Tracking the Spacecraft in Flight

DSN tracking schedules have been negotiated months or years in advance. Now the spacecraft is in flight. Near the time when the spacecraft will be rising in the sky due to Earth's rotation, its assigned DSN tracking activity begins. During the period allotted for "precal" activities, the Link Monitor and Control (LMC) operator sits down at his or her console in the Signal Processing Center (SPC) of one the DSN's three Deep Space Communications Complexes (DSCC). The operator will be controlling and monitoring the assigned antenna, called a Deep Space Station (DSS), an assigned set of computers which control its pointing, tracking, commanding, receiving, telemetry processing, ground communications, and other functions.

This string of equipment from the antenna to the project people at JPL is called a link, referring to the two-way communications link between the spacecraft and the project. Prior to the LMC operator's arrival, the Complex Monitor and Control (CMC) operator will have assigned, via directives sent out to the station components over a local area network (LAN), applicable equipment to become part of the link. Now the LMC operator begins sending more directives over the LAN to configure each of the link components specifically for the upcoming support. Predict sets containing uplink and downlink frequencies and Doppler bias ramp rates, pointing angles and bit rates, command modulation levels, and hundreds of other parameters are all sent to the link components. Problems are identified and corrected.

At the end of the precal period, the LMC operator checks the DSS area via closed circuit TV, makes a warning announcement over its outdoor loudspeakers, and the DSS antenna swings to point precisely to the spacecraft's location in the eastern sky. The transmitter comes on, and red beacons on the antenna illuminate as a warning. Upon locking the receivers, telemetry, and tracking equipment to the spacecraft's signal, the link is established. This marks the Beginning of Track (BOT) and Acquisition of Signal (AOS). Depending on the nature of the spacecraft's activities, there may be Loss of Signal (LOS) when the spacecraft turns away to maneuver, or if it goes into occultation behind a planet. This LOS would presumably be followed by another AOS when the maneuver or occultation is complete. During the day, the DSS antenna moves slowly to follow, or track, the spacecraft as the Earth rotates.

Near the end of the LMC operator's shift, the DSS is pointing lower on the western horizon. At the same time, another LMC operator inside the SPC of another DSCC a third of the way around the world, is doing his or her precal as the same spacecraft is rising in the east. To accomplish an uplink transfer, the setting DSS's transmitter is turned off precisely two seconds after the rising DSS's transmitter comes on. Scheduled End of Track (EOT) arrives, and the LMC operator at the setting DSS begins postcal activities, idling the link components and returning control of them to the CMC operator.

Preparation for Encounter

Command loads uplinked to the spacecraft are valid for varying lengths of time. So-called quiescent periods such as the lengthy cruises between planets require relatively few activities, and a command load may be valid for several weeks. By comparison, during the closest-approach part of a flyby encounter, a very long and complex load may execute in a matter of hours. Prior to encounter, the spacecraft is generally sent a command sequence which takes it through activities simulating the activities of encounter. Changes in data rate and format, and spacecraft maneuvers, are designed to put the flight team and ground systems through their paces during a realistic simulation, in order to provide some practice for readiness, to shake down the systems and procedures, and to try to uncover flaws or areas for improvement.

Instrument calibrations are undertaken prior to encounter to be sure that experiments are being carried out in a controlled fashion. Optical instruments, for example, may be commanded to take observations of empty space, in order to gain knowledge of flaws or idiosyncrasies in the instrument, which can then be removed from later encounter observations.