Environmental Engineering Reference
In-Depth Information
1.
Fine attitude determination (as discussed)
2.
Orbit determination (as discussed)
3.
Attitude sensor/actuator and SI calibration (as discussed)
4.
Attitude control (execute slew requests; fine pointing)
5.
Orbit maneuvering (plan and execute orbit stationkeeping)
6.
Data monitoring and trending (as discussed)
7.
“Smart” fault detection, diagnosis, isolation, and correction (as discussed)
8.
Look-ahead modeling (probably not required)
9.
Target planning and scheduling (not required)
10.
SI commanding and configuration (execute science calibration requests)
11.
SI data storage and communications (in cooperation with ground agent)
12.
SI data processing (just for target acquisition)
LEO Earth Pointers
The scheduling problem for LEO earth pointers is much simpler than for LEO
celestial pointers. Long duration look-ahead is no longer an issue since the
spacecraft's orbit cannot be predicted to a high level of accuracy very far in
advance. The planning aspect of the problem, however, is very target depen-
dent, and might vary with time depending on science prerogatives. One can,
therefore, imagine a set of templates (with ground tunable parameters) asso-
ciated with individual targets (or target types) that an onboard scheduling
agent could invoke whenever the onboard orbit-determination agent (a GPS
receiver coupled with a short-duration orbit propagator) in conjunction with a
data monitoring-and-trending agent deemed the target was coming into view.
Because fuel available for stationkeeping maneuvers is directly equivalent to
mission lifetime, it is unlikely that most NASA LEO earth pointers would ex-
pend fuel for large orbit-change maneuvers for the purpose of observing TOOs
(short-duration events like volcanoes, for example), though the requirements
on TOO response for some non-NASA spacecraft (for example, military imag-
ing spacecraft) may be more demanding. Given that GPS receivers now give
the spacecraft itself more immediate access to accurate, current spacecraft
orbit data than does the control center, migrating some portion of the short-
term LEO earth-pointer scheduling responsibility to the spacecraft to improve
observational eciency could be justified on a cost-benefit basis for some mis-
sions of this type, and migrating routine orbit-stationkeeping maneuvers to
the spacecraft could reduce operations costs as well.
As in the case of the LEO celestial pointer in the previous section, one can
easily imagine additional calibration functions being migrated to the flight
system for LEO earth pointers. There also may be significant advantages in
providing an SI data processing capability/agent onboard for the purpose of
distinguishing between useful data-taking opportunities (for example, for a
Landsat spacecraft, in the no-cloud-cover situation) and unusable conditions
(in this case, a full-cloud-cover situation).
