Environmental Engineering Reference
In-Depth Information
not a function that must be resident in the FSW backbone. For spacecraft in
near-earth orbits, global positioning system (GPS) can be used for orbit deter-
mination. GPS also provides a time fix, simplifying onboard clock correlation.
The GPS solution is purely a hardware one, constituting a fully independent,
modular agent.
On the other hand, if future orbit information must be predicted, or if GPS
cannot be used (as, for example with a mission at the L2 Lagrange Point), a
software solution, often requiring input from the ground, must be used instead.
The simplest implementation of this capability would involve creating an orbit
determination agent that continuously generated orbit solutions, independent
of the need of any other agent. These solutions could then be stored in a data
“archive” pending a request by other agents. Similar to attitude data, old
data could be maintained onboard until downlinked, or if not needed on the
ground, simply overwritten periodically.
In addition to the spacecraft ephemeris already discussed, there often is
an onboard need for Solar, Lunar, ground station, and/or tracking and data
relay satellites (TDRS) position information as well. These data are usually
computed onboard via analytical models and would supplement the spacecraft
orbit data already supplied by the orbit agent. In a similar fashion, other
reference information such as geomagnetic field strength and South Atlantic
Anomaly (SAA) entrance/exit times (for a set of SAA contours) could be
supplied by the agent, as required by the mission.
6.2.2 Attitude Sensor/Actuator and Science Instrument
Calibration
Currently, very few calibrations are carried out autonomously onboard. For
nearly all current GSFC missions, gyro drift biases are calibrated at high
cycling rates via a Kalman filter, and for small explorer (SMEX) missions,
onboard magnetometer calibration is standard. Neither of these is required to
be performed (at least immediately) when in safemode or inertial hold, so they
need not be part of the FSW backbone. In the future, however, the dynamic
quality of other spacecraft hardware may require more elaborate onboard
calibrations, both engineering-related and SI-related. In such circumstances,
consolidating all calibration functionality within a single Remote Agent would
facilitate interaction with (for example) a data monitoring-and-trending agent
or a planning-and-scheduling agent, whether those agents are located onboard
or on the ground.
6.2.3 Attitude Control
Other than for responding to ground realtime commands, spacecraft slews are
performed in support of scheduled science observations or calibration activi-
ties. So, the attitude slew function can safely be assigned to a Remote Agent
as long as a basic slewing capability is included in the FSW backbone as well.
