Environmental Engineering Reference
In-Depth Information
star trackers has provided spacecraft like the Willsinson Microwave Anisotropy
Probe (WMAP) (launched in 2001), a true “Lost in Space” capability. Pre-
viously, the limited star catalogs flown on GSFC spacecraft, along with the
simple star identification algorithms utilized in the FSW, required that fairly
accurate
a priori
spacecraft attitude knowledge be available onboard for reli-
able fine attitude updates to be performed. Now, however, star trackers are
available that use more extensive internal catalogs and more powerful star
identification algorithms to provide quaternion information to the FSW with-
out previous attitude knowledge. This new autonomy capability both sup-
ports ongoing science observing and streamlines recovery from safemode entry.
These new star trackers also output the change in attitude, providing a di-
rect back-up and sanity check to the primary body-rate data supplied by the
gyros.
Second, the earth observing spacecraft (EOS) Aqua (formerly EOS-PM,
launched in 2002) has implemented an autonomous communication capabil-
ity referred to as “Call 911.” When a serious anomaly occurs on the space-
craft, a stored command sequence reconfigures the communications downlink
(from the spacecraft to the TDRS system (TDRSS) to the ground station)
and broadcasts an unscheduled multiple access (MA) message via the TDRS
Demand Access capability. The message is forwarded from White Sands to
the EOS Aqua control center, triggering an alarm that unexpected telemetry
has been received. The telemetry provides a status message describing the
anomaly. The ground can then be ready for contingency commanding at the
next scheduled ground contact, or declare an emergency and schedule TDRSS
S-band single access (SSA) contact time.
Third, on the Swift spacecraft (launched 2004), the key to the rapid TOO
response to detected GRBs was a capability considered previously as a post-
instrument (the burst alert telescope (BAT)) detects the signature of a possi-
ble GRB, the FSW determines the celestial coordinates of the potential TOO.
After verifying that those coordinates had not previously been observed, the
FSW communicates the GRB celestial coordinates to the OBC, which then
computes and validates the new target quaternion. The FSW autonomously
determines the right time to break away from currently scheduled observa-
tions, “swiftly” slews to that target, and generates the appropriate SI con-
figuration commands so that high precision observations by Swift's narrow
field instruments (NFI) (the X-ray telescope (XRT) and UV/optical telescope
(UVOT)) can be made. Swift also (via TDRSS) can respond to TOO alerts
identified by other observatories. This new autonomy function is a very signif-
icant first step in the direction of “smart” SIs controlling the science mission
and all resources required to perform the science mission, as opposed to the
traditional operational approach in which the spacecraft/ground system con-
trols the mission and configures the SIs to perform the observations.
Fourth, a capability was considered for the Triana mission (launch post-
poned indefinitely for budgetary reasons) that would have utilized science
