Environmental Engineering Reference
In-Depth Information
2.2.5 Safemode
The last item, safemode, may include several independent subfunctions, de-
pending on the cost and complexity of the spacecraft in question. Typical
kinds of safemode algorithms include Sun acquisition modes (to maintain
power positive, maintain healthy thermal geometry, and protect sensitive op-
tics), spin-stabilized modes (to maintain attitude stability), and inertial hold
mode (to provide minimal perturbation to current spacecraft state). Usually,
the processing for one or more of these modes is located in the main space-
craft bus processor, but often in the past, there has been a fall back mode in a
special safemode processor, the attitude control electronics (ACE) processor,
in case the main processor itself has gone down. In addition to its safemode
responsibilities, the ACE was the interface with the coarse attitude sensors
and actuator hardware, obtaining their output data, and providing command
access. The individual SIs themselves also have separate safemode capabili-
ties, executed out of their own processor(s). Anomalies causing the main bus
processor to become unavailable are dealt with via a special uplink-downlink
card, which, in the absence of the main processor, enables continued (though
limited) ground communication with the spacecraft.
2.3 Flight vs. Ground Implementation
Increasing levels of onboard autonomy are being enabled by increases in flight
data system capacities (CPU, I/O, storage, etc.), as well as by the new ap-
proaches/structures for FSW design and development (object-oriented design,
expert systems, remote agents, etc.). In particular, operational activity cat-
egories that previously were virtually the private domain of the ground sys-
tems (such as P&S, engineering data analysis and calibration, and science
data processing and calibration) now provide exciting opportunities for shift-
ing responsibility from the ground to the flight component in order to take
advantage of the strengths inherent in a realtime software system in direct
contact with the flight hardware.
The key advantages possessed by the flight component over the ground
component are immediacy, currency, and completeness. Only the flight
component can instantly access flight hardware measurements, process the
information, and respond in realtime. For example, for performance of basic
spacecraft functions such as attitude control and thermal/power management,
only the FSW has direct access in realtime to critical information needed to
define the spacecraft's operational state, as well as the direct access to the
spacecraft actuator hardware required to create and maintain the desired
state. The FSW is also the only component of the integrated flight/ground
operational system with full-time access to all relevant information for appli-
cations such as fault detection and SI target acquisition.
By contrast, in the past, the advantage of the ground over the flight seg-
ment has been the larger, more powerful ground computers that (for example)
