Environmental Engineering Reference
In-Depth Information
erroneous update is made, a continuity check in the FSW (implemented on
most missions) can detect the mistake, allowing the FSW to reject the update
and continue using the existing parameters until an accurate set is supplied
by the ground.
Specifically with respect to missions developed by NASA Goddard, the
telemetry monitor (TMON) (and its more recent variant, the telemetry and
statistics monitor (TSM)) capability provides a yet more powerful access into
spacecraft function. TMON not only allows the FSW to monitor ongoing
spacecraft behavior by checking the values of specific telemetry data points,
it also includes a command interpreter program that can execute logical state-
ments and act upon them. As the operation of TMON is driven by uplinked
data table loads, TMON can be used to add new onboard functionality with-
out modifying the FSW itself, thereby providing the FOT with a way to work
around onboard hardware or software problems in a manner less demanding
on the FSW maintenance team.
In addition to supplying a means to influence the spacecraft and FSW
behavior, the ground can indirectly change the FSW itself. For small or local-
ized changes, a FSW maintenance development team can modify the FSW,
uplinking new code that effectively bypasses some existing code elements and
substitutes modified, or even entirely new functionality in its place. If the
number of modifications becomes excessive, or if the scale of the upgrade is
extremely large, a new version of the FSW program may be developed by the
maintenance team and uplinked, which, in turn, may be modified as future
changes are required. For long duration missions having extensive cruise du-
rations (e.g., Jet Propulsion Lab (JPL) missions that may take years to get on
station), the cruise phase often is used to re-write the flight code to compensate
for major hardware anomalies experienced after launch, or even to complete
development and testing of major functionality not finished prior to launch.
By these means (command infrastructure, table modification/command up-
link, TMON/TSM, and actual FSW coding changes), the FSW affords the
ground a remarkably high capacity for accessing, influencing, and modifying
spacecraft behavior in flight.
Flight Autonomy Enablers of Insight into Spacecraft Systems
To support the safe and effective utilization of the access to spacecraft systems
afforded by FSW, the FSW must also allow the ground a comparably high
level of insight into ongoing spacecraft operations. To this end, spacecraft
typically are designed so as to ensure that, catastrophic failures aside, the
ground will always receive, at the very least, some minimal level of health
and safety telemetry that summarizes the current spacecraft state, along with
significant error messages describing what, if anything, has gone wrong.
When operating in a more nominal state, the spacecraft regularly sup-
plies the ground with fairly detailed information concerning the operational
behavior of the spacecraft and its various hardware components. Additional
