Environmental Engineering Reference
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improved upon in the 1980s with the introduction of TMON, a telemetry
monitor that also supported an autonomous onboard response capability.
3.2.2 1980s Spacecraft
Relative to the 1970s, the period of 1980s saw the launch of larger, more
expensive, and more sophisticated spacecraft. Many of these spacecraft (e.g.,
HST and the Compton Gamma Ray Observatory (CGRO)) were supposed to
launch in the mid- to late-1980s, but in actuality launched in 1990 because of
delays due to the loss of the shuttle Challenger.
In the case of HST, a more powerful OBC (the DF224) enabled the devel-
opment of more elaborate pointing-related mathematical algorithms as well
as a wider variety of safemode options (supported by a larger number of FDC
checks) than had been present on previous spacecraft. In particular, use of
HST's fine guidance sensors (FGSs) required the development of rather com-
plex (by onboard standards) mathematical algorithms to command the FGSs
and process their data. In fact, the processing demands of the FGS function-
ality was so high that the HST FSW's 10 Hz processing rate was created for
and exclusively dedicated to this purpose. The FGS guide-star acquisition
algorithms were themselves extremely powerful, exploiting the full command-
construct repertoire to achieve the intricate branching/looping logic needed to
optimize the probabilities for acquiring the guide stars essential for performing
HST's science.
The very existence of the 10 Hz processing rate points to an additional
noteworthy aspect of HST's autonomy capabilities that often is taken for
granted, namely its executive function. For the sake of simplicity, most FSW
development efforts try to limit the number of tasks to as few as possible, usu-
ally one or two. However, because of HST's unique computational, precision,
and timing demands, the HST pointing-control subsystem (PCS) software re-
quired five distinct processing rates, namely, 1,000 Hz for the executive, 40 Hz
for primary PCS control laws and gyro processing, 10 Hz for FGS processing,
1 Hz for star tracker processing, ephemeris modeling, and FDC, and 1/300 Hz
for the minimum energy momentum management control law. Just the man-
agement of these very different, and often competing, tasks demonstrated a
significant degree of executive autonomy.
HST's FSW also displayed a high level of autonomy in acquiring science
targets through “conversations” between the NSSC-I computer supporting SI
commanding and processing, and the DF224 computer responsible for space-
craft platform functionality. For example, for science observations where the
target direction was not known to a sucient level of accuracy to guaran-
tee acquisition in an SI's narrow FOV, the DF224 could initiate (through
stored commanding) a small scan of the region of the sky surrounding the
estimated target coordinates. The passing of the target through the FOV of
the SI would then trigger a sudden increase in SI intensity measurements,
which would then be noted by the NSSC-I. On completion of the scan, the
