James Webb Telescope Issue - 14

Feature
Commanding the James Webb Space Telescope: OSS and Event-Driven Operations
(AD) scripts, and the Dictionary Interface (DI) scripts. The
OPE scripts process visit files in the order specified in
the OP segment file. They launch visit file-specified AD
scripts with specified parameters to carry out the actual
observations. Once invoked by the OPE, these toplevel
scripts carry out their functions with the assistance
of various lower-level AD scripts. The AD scripts are
categorized according to the subsystems that drive science
the most: each science instrument (SI), which includes
the Near-Infrared Camera (NIRCam), the Near-Infrared
Spectrograph (NIRSpec), the Near-Infrared Imager and
Slitless Spectrograph (NIRISS), and the Mid-Infrared
Imager (MIRI); the Fine Guidance Sensors (FGS); the
Optical Telescope Element (OTE) and spacecraft bus; and
common utility scripts used by all scripts ranging from file
operations to coordinate transformations. Abstracting away
the specifics of CCSDS command packet construction from
the AD scripts, the DI scripts serve as wrapper scripts for
sending commands to various application flight software
with the aid of onboard dictionary files containing the
translations from mnemonics to parameters required by the
Script Processor in the construction of a command packet [3].
Thus, commands or telemetry queries are communicated
in the following manner:
1. OSS sends a command or telemetry query to the SP Task
2. The SP Task sends the corresponding packet to the
Command Manager Task
3. The Command Manager Task notes the application ID
in the packet and passes it along to the appropriate
Application flight software Task (e.g., that of one of the
science instruments, the mirrors, or a subsystem that is
part of the spacecraft bus such as attitude control)
4. The application flight software Task generates commands
to the hardware to complete the requested operation
- these commands are sent back to the Command
Manager Task, which then passes them to the Bus
Interface Task (either 1553 or SpaceWire) to connect to
the hardware being commanded physically
Commands originating from the ground, including those
that interface with the OPE for adjustments to the onboard
observation plan, are received by the spacecraft RF
communications subsystem and transferred to the ISIM
via the Command and Telemetry Processor (CTP) using
the ISIM flight software CTP Interface Task and Command
Manager Task [4].
IV. Visit Files - The Entry Point to OSS
Normal operations are dictated by the interplay of the
Proposal Planning Subsystem (PPS), the Flight Operations
Subsystem (FOS), and OSS. PPS consists of multiple
subsystems, including the Astronomer's Proposal Tool
THE BRIDGE
(APT), which enables astronomers to request the
observations they would like to execute, the Visit Planning
System (VPS) for extended range visit plans, the Visit
Scheduling System (VSS) for short term visit plans, and
the Observation Plan Generation System (OPGS) which
Figure 3: Science Operations Center Subsystems
generates the observation plans and visit files to be
interpreted by OSS [3].
Visits have a hierarchical structure. A serial list of one or
more activities constitutes a " sequence. " Up to three
sequences can be executed in parallel within a " group. "
One or more groups are executed serially within a visit. This
structure facilitates proper control of activity ordering while
providing the ability to perform certain activities in parallel,
thus improving observing efficiency (see example visit file
in Figure 2).
There are multiple types of visits: science, engineering,
calibrations, and real-time handoffs.
* Science: Science visits typically include a slew to the
science target, guide star acquisition, science target
acquisition, mechanism moves (e.g., filter/grating wheel
moves), detector and data acquisition configurations,
the SI exposures, and small spacecraft pointing offsets to
support dithered observations. Dithering helps improve
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