JWST Attitude Control Subsystem
Pointing control and slewing of JWST is performed by the attitude control subsystem (ACS). Fine guiding additionally involves the Fine Guidance Sensor (FGS).
Pointing and slewing of JWST is done by the spacecraft flight software, which processes data from attitude sensors, instructions from the ISIM and the JWST ground system, and issues commands to actuators. The attitude control subsystem (ACS) is responsible for maintaining attitude and pointing, slew maneuvers, momentum unloading, Delta-V (orbit correction) maneuver control, high gain antenna pointing, observatory safe modes, and ensuring that the observatory remains within Sun avoidance constraints.
This page provides a functional summary how JWST controls pointing and slewing to conduct science operations. Related pages describe the predicted pointing stability and slew accuracy, as well as the predicted slew times and overheads.
The ACS uses sun sensors, star trackers, and gyroscopes to sense the observatory orientation and movement, as well as reaction wheels and/or thrusters to apply force or torque to the observatory for pointing control or maneuvers. The reaction wheels provide the control torques needed to maintain attitude and pointing as well as to slew. The spacecraft's star trackers provide stellar inertial attitude reference for three-axis coarse pointing control. The ACS points the telescope boresight to within 8″ (1-σ, per axis) of the commanded position prior to guide star acquisition, without any position reference or input from the Fine Guidance Sensor (FGS).
Control of the roll orientation about the telescope's optical axis is provided by input from the spacecraft's two star trackers. The star trackers each have a ~16° diameter FOV, projected on to a 512 × 512 pixel CCD detector. They are oriented over 45° from the telescope boresight and each other. The star trackers compare the observed positions of bright stars (V < 6) to an internal star catalog. This allows the use of a single star for fine guidance within the FGS field of view (FOV) while still maintaining roll control.
The duration of slews is a function of the length of the motion. The rate of motion is determined in part by the need to keep settling times within certain limits as well as the desire to reach the new pointing as soon as possible. For slews between 25" and 3°, the slew rate is slower than for shorter or longer slews, to avoid exciting slosh modes of the propellant in the tanks. Once excited, propellant slosh can take a long time to damp (more than 20 minutes in some cases).
Fine guidance is a closed loop system, in which a guide star in the FGS FOV is used to stabilize the observatory during science exposures. The FGS makes measurements of the guide star position in the plane of the sky and sends these to the ACS every 64 ms. Using the FGS data, the ACS determines the telescope pointing error to be removed, using a combination of the fine steering mirror (FSM) and the spacecraft's reaction wheels.
Each science visit uses a single guide star. Pointing changes within the FGS FOV (dithers, target acquisition motions, etc.) are specified to the spacecraft in terms of the change in the guide star location (Delta X, Delta Y) in the FGS FOV, and the change in the position angle (Delta PA) about the guide star's position.
For stationary targets, the ACS controls the FSM and reaction wheels so that the guide star remains at a fixed location in the FGS detector.
In order to change the telescope pointing orientation by more than one FGS pixel (about 0.06"), the ACS must exit the "Fine Guide" mode, execute the pointing change, and then reestablish fine guidance. Very small offsets <0.06" can be executed by the FSM, while the ACS remains in closed-loop fine guidance control.
Guiding for moving targets
For moving targets (in our Solar System), the process is similar, except that the FGS measures the guide star position in "Track" mode, which is less accurate compared to "Fine Guide" mode for a given guide star brightness. For moving targets, the ground system computes a trajectory for the guide star that keeps the solar system target stationary in the science instrument. The ACS then updates the control position of the guide star every 64 ms, and the FGS in "Track" mode adjusts the position of the guide star track box to follow the guide star.
The planning and scheduling system provides predictive management of the expected momentum, but actual timelines may differ. Hence, the ACS participates in real-time management of the momentum on the observatory by monitoring the momentum as a function of time and taking autonomous action as needed to keep the observatory safe.