Transparent management of stored command sequences in SOHO/UVCS ground software.
The UltraViolet Coronagraph Spectrometer (UVCS) on board the Solar and Heliospheric Observatory (SOHO) is a complicated scientific instrument almost continuously conducting spectral observations of the extended solar corona in EUV since 1996. Each such observation requires a precise setup of multiple UVCS mechanisms, photon-counting Cross-Delay Line detectors and numerous software parameters. It takes hundreds of flight software commands to setup the instrument for an observation. The commands are uploaded on-board during daily (or less frequent) sessions of near-real-time commanding, and stored in two software command queues.
The size of those queues are limited, and so complicated or long-running observations may require more instrument commands than the queues can accommodate. To help the situation, a stored command sequences (SCSs) facility is implemented in the UVCS flight software. It is supported by a very minimalistic set of memory control commands, which only allow a SCS to be uploaded into a specified part of the dedicated on-board computer memory; no means of memory management is provided. Direct use of this facility would be very inefficient and error-prone, so a special ground software module was designed and implemented. The unique feature of the module is that it is almost completely transparent. To prepare an observation command sequence, the mission operator just has to indicate that a specific subsequence of commands should be stored and executed as a SCS. The ground SCS management module compiles the ASCII command sequence into a binary representation, checks whether this sequence is already uplinked to the spacecraft, automatically uplinks and installs it if it is not, and substitutes the command block for the SCS call. Such an approach virtually eliminates the possibility of any SCS "mismanagement" and makes this mission critical facility very efficient and robust, as demonstrated by its faultless performance during the nearly 15-year long SOHO mission.
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