Environmental Engineering Reference
In-Depth Information
NSSC-I could then request (through a limited interface between it and the
DF224) that the spacecraft attitude be returned to that pointing. The DF2244
would then create and initiate a realtime slew command satisfying the NSSC-
I's attitude change. As with the SMM autonomous target acquisition feature,
HST's capability provided a far quicker response than would have been the
case had data processing and commanding responsibility resided in the ground
system, a very important consideration given the high cost of HST operations
and the extraordinary demand for HST observing time.
By contrast, because of its lower pointing accuracy requirements, the
Extreme Ultraviolet Explorer (EUVE), also originally scheduled for the 1980s
but actually launched in 1991, did not require a level of sophistication in its
ACS subsystem as high as that required on HST. However, it did provide a
higher level of flexibility with respect to onboard data monitoring and limit
checking. EUVE's telemetry monitoring capability (referred to as TMON, and
also flown on CGRO and the Upper Atmosphere Research Satellite (UARS))
permitted the user to select, after launch, specific data points to monitor.
In addition, TMON provided a limited logic capability to respond onboard
to detected spacecraft conditions (observed via limit checks on data or flag
checking) through autonomous generation of commands. EUVE's FSW also
included a separate statistics monitor program (called SMP) that was later
combined with TMON and flown on MIDEX spacecraft as the TSM program
driven limit-checking/response system that has served as the model for later
missions in the explorer series.
As an early predecessor of true event-driven operations, EUVE utilized
an Orbit Time Processor (a table-driven task) that allowed its FSW to de-
fine orbit-based events, a variation on the relative-time-based commanding
discussed earlier. Occurrence of the event could then trigger a relative time
sequence (RTS), a task, or set an event flag that in turn could be monitored by
a running task. The EUVE FOT employed this enhancement to the standard
stored commanding infrastructure to re-phase the timing of EUVE's survey
mode, which operated within a third of an orbit duty cycle. EUVE also used its
Orbit Time Processor to send dusk/dawn commands to the survey instrument.
Although HST's FDC capabilities are not as flexible as EUVE's, HST
checks for a much wider spectrum of anomalous conditions, with a larger
range of autonomous responses. For example, HST provides four distinct
software safemodes: inertial hold, a multistaged Sunpoint, zero-gyro (derived
from Sunpoint), and spin-stabilized. Also, in response to guide-star reacqui-
sition problems associated with radiation hits on its fine guidance electronics
(FGE) following SAA entrances, HST's FSW developers have implemented an
FGE memory-refresh function that restores key FGS commanding parameters
to their latest values prior to the SAA entrance. Note that many of HST's
FDC capabilities were added postlaunch in response to problems experienced
inflight, which not only illustrates the power of FSW to solve unanticipated
operational problems that can be dealt with no other way, but also provides
