Environmental Engineering Reference
In-Depth Information
through the FSW, with the exception of the cases of some classes of emer-
gency conditions and some special applications. (For example, if the Main
C&DH computer goes down, the ground can communicate directly to the
spacecraft via the uplink-downlink card and issue commands such as Turn
On Back-up Main C&DH Computer and Switch to Back-up). The FSW pro-
vides both access to the spacecraft for commanding purposes and insight into
ongoing spacecraft operations via telemetry. Furthermore, because the FSW
“sees” in realtime what is happening onboard and can take immediate action
in response to what it sees, the FSW often is capable of performing tasks that,
if assigned to the ground system and FOT, would be much more expensive
to do. These cost savings may arise from lower software costs due to simpler
modeling requirements onboard or from effective replacement of human staff
hours with FSW functionality. Additionally, where those reductions pertain
to replacement of repetitive FOT manual activities, one obtains a cost savings
multiplier over the entire mission duration. In the following subsections, each
of these three services to the operations team will be discussed in more detail.
Autonomy Enablers of Access to Spacecraft Systems
FSW is the mechanism enabling nearly all access to the spacecraft by the FOT.
The FSW provides a commanding infrastructure that translates very precisely
what the FOT wants done into appropriate hardware and/or software com-
mands, as discussed in previous sections. By this means, the FSW to a certain
degree provides the FOT “hands-on” capability with respect to the various
spacecraft hardware elements and subsystems. However, while it creates FOT
access to spacecraft systems, the commanding infrastructure (through its val-
idation capabilities) protects the spacecraft from the occasional operational
errors that might otherwise lead to irreparable damage to delicate hardware.
The FSW even allows the FOT to modify the operation of FSW functions
by changing the values of the key parameters that drive the functions' models.
For example, most spacecraft model their own orbital position and the posi-
tions of other spacecraft, such as tracking and data relay satellites (TDRS),
through the use of onboard orbit propagators. The starting position and veloc-
ity from which future positions are calculated can be specified by the ground
in a table, updates to which are uplinked as frequently as required in order
to maintain ephemeris accuracy requirements. For other missions, ephemeris
updates are performed via command structures rather than tables, but the
basic process is effectively the same. Also, as a protection against missing a
routine ephemeris update (either due to inflight problems or simply a ground
operations error), FSW for the medium-class explorers (MIDEX) missions
notify the ground if the onboard parameters have grown “stale,” eventually
causing the FSW to terminate that ephemeris model's processing. And if an
