Environmental Engineering Reference
In-Depth Information
a strong argument in favor of creating and maintaining a strong FSW main-
tenance team capable of responding to those operational problems when they
do occur.
3.2.3 1990s Spacecraft
Relative to the 1970s and 1980s, the 1990s witnessed major hardware and
infrastructure advances that enabled greater onboard capabilities. The flight
computers were more powerful, with larger memories, and were faster, en-
abling more sophisticated algorithms and models. Floating point arithmetic
and higher level languages (such as C, C++, and Ada) allowed FSW code to
be written more like comparable ground system code. For example, object-
oriented design concepts can now be used to make flight code more re-usable,
and in the long run, potentially cheaper. Thanks to high capacity, lightweight,
and cheap solid state storage devices, larger amounts of science data may be
stored onboard and packaged more conveniently (with respect to end-user
needs) without undue concern for added overhead space costs (although in
practice this gain has been largely offset by corresponding increases in SI out-
put data volume). More sophisticated operating systems are now available to
handle the masses of data and manage the more elaborate computations. The
cumulative result of this technological progress has been to enable a series of
new individual flight autonomy capabilities targeted to the needs of specific
missions, as well as to support the development of entirely new FSW concepts.
To meet demanding time requirements for TOO response, the RXTE FSW
included three new flight autonomy capabilities: onboard target quaternion
computation, target quaternion validation, and the antenna manager. The
first two enable a science user to specify simply the target's right ascension and
declination, and whether there are any special roll coordinate needs. The FSW
then takes this targeting information expressed in the natural “language” of
the user, transforms it appropriately (i.e., into quaternion format) for use
in slewing to and acquiring the target, quality-assures the attitude vs. Sun-
angle avoidance, and then slews the spacecraft to point to the target at the
ground-specified time.
A new RXTE autonomy capability was proposed as a post-launch update,
but could not be funded, which would have greatly enhanced RXTE's already
superb TOO response time. If RXTE's all sky monitor (ASM) detected the
signature of a possible gamma ray burster (GRB), the ASM FSW could have
determined the celestial coordinates of the potential TOO. After verifying
that those coordinates had not previously been observed, the ASM could then
have communicated the GRB celestial coordinates to the OBC, which could
then have utilized RXTE's existing capabilities to compute and validate the
new target quaternion. Next, the FSW could have autonomously determined
the right time to break away from currently scheduled observations, slew to
that target, and then generate the appropriate SI configuration commands
so that observations by RXTE's proportional counter array (PCA) could
