Environmental Engineering Reference
In-Depth Information
sensor measurements, with autonomous mode transition capabilities to famil-
iar safemodes, such as Sunpoint.
In the area of pointing control, for the case of HEAO-2, pointing control
and ground attitude determination accuracy (using star tracker data) were re-
quired to be good to 1 and 0.1 arcmin, respectively - a very demanding require-
ment for the time. And the SMM fine Sun sensor was so well calibrated that its
attitude could be controlled autonomously to 5 arcsec with respect to the Sun-
line. Attitude control was achieved using onboard closed-loop proportional-
integral-derivative (PID) control laws (including feed-forward specification of
environmental torques) and Kalman filters (for optimization of attitude-error
determination and gyro-drift bias calibration). So fundamentally, the control
approaches used on these spacecraft from the 1970s were quite similar to
those currently used on modern spacecraft in support of spacecraft slews, tar-
get acquisition, angular momentum management, and maintenance of attitude
during science observations.
Originally SMM also possessed the rudiments of an autonomous target
identification and acquisition capability that presaged the more elaborate ca-
pability implemented in HST (as discussed later), which, in turn, was the next
step on the road to a fully autonomous TOO response capability. Specifically,
when SMM's SI detected a Solar flare, the data were processed onboard, the
flare's location was determined, and the spacecraft was autonomously reori-
ented to observe the phenomenon. This feature enabled a far quicker response
than would have been the case if the data processing and commanding respon-
sibility resided in the ground system, allowing time-critical measurements to
be made during the early stages of the flare's duration. The Orbiting Solar
Observatory-8 (OSO-8), launched in 1975, also could steer its payload plat-
form independently to expedite acquisition of its short-lived targets.
The evolving nature of flight autonomy can be seen within the decade of the
1970s itself just by noting the significant increase in pointing independence be-
tween HEAO-1 and HEAO-2. Specifically, HEAO-1 (launched in 1977) relied
on the ground to provide it periodic attitude reference updates (every 12 h)
based on ground attitude determination. Just 2 years later, HEAO-2 already
possessed the capability to compute its own attitude reference update, based
on ground-supplied guide-star reference information, a capability also imple-
mented in SMM's ACS for autonomous control of roll about the Sunline.
Furthermore, HEAO-2 could autonomously sequence through a weekly target
list, adjusting the order of the targets so as to economize on the use of thruster
fuel used in momentum dumping, a remarkable degree of independence even
relative to the 1990s missions discussed later.
Examination of FDC also shows a dynamic quality. Although many space-
craft flew hard-coded limit checking and response code, SMM's statistical
monitor performance function provided additional flexibility. It allowed op-
erations personnel to specify additional FSW parameters to be monitored
autonomously onboard beyond those specified in the at-launch flight code,
without making a modification to the code itself. This function was itself
