Environmental Engineering Reference
In-Depth Information
evolved over time has been the accuracy and sophistication with which those
measurements have been made. In early spacecraft, onboard storage and CPU
capabilities limited onboard attitude calculations to coarse attitude determi-
nation using sensors such as Sun sensors and magnetometers, or limited fine-
attitude determination to processing star-tracker output using very small on-
board star catalogs, or using reference stars carefully preselected by the ground
system. In the 1990s, more powerful onboard processing capabilities and cheap
onboard storage facilitated the enhancement of this capability. For example,
for the Rossi X-ray Timing Explorer (RXTE) mission, the FSW included a
star catalog and star identification capabilities permitting the FSW to cal-
culate the spacecraft's own fine attitude without ground support. Later still,
the Wilkinson Microwave Anisotropy Probe (WMAP) mission's star tracker
contained its own star catalog and star identification algorithms, and so could
output the spacecraft attitude directly (although it did not compensate for
the star tracker's misalignment relative to the spacecraft), providing a “Lost
in Space” capability.
Knowing the spacecraft position and orientation is only half of the guid-
ance, navigation, and control (GN&C) problem. The other half is controlling
the spacecraft position and attitude such that the desired science can be per-
formed. As with orbit determination, planning orbit maneuvers historically
has been solely a ground function because of the CPU intensive and look-
ahead nature of the problem. According to the traditional paradigm (again
in the NASA/Goddard context), after determining the current spacecraft or-
bit, Flight Dynamics would evaluate whether that orbit satisfied the mission
requirements, and if it did, would calculate when orbit perturbations were
likely to drive the orbit outside those requirements. In collaboration with the
flight operations team (FOT), Flight Dynamics would then plan and sched-
ule (as necessary) a small stationkeeping orbit maneuver designed to restore
the orbit to its operational geometry. The stationkeeping plan created by this
process would include highly detailed instructions (i.e., which thrusters to
fire, how long they should fire, the thruster configuration when firing, etc.),
which would be uplinked to the spacecraft as “Delta-V” (change-of-velocity)
commands, and would be executed open-loop by the FSW, which would have
no way to evaluate the success/failure of the orbit maneuver. Lastly, Flight
Dynamics would evaluate the postburn orbit to determine whether further
corrections were necessary. This same basic procedure also applied to the ma-
jor orbit maneuvers required to acquire mission orbit, except the planning
and scheduling would be more elaborate, typically requiring several burns to
complete. Recently, however, serious consideration has been given to plan-
ning and scheduling routine stationkeeping orbit maneuvers autonomously
onboard. Migrating this function to today's more capable flight computers
not only would reduce the cost associated with using ground staff to perform
a routine operation, but also would reduce operational risk by eliminating
unnecessary command uplinks.
