Environmental Engineering Reference
In-Depth Information
scheduling assignment within the high priority time demands specified by the
ground without impact to their own science responsibilities. Rather than at-
tempting to segment the planning and scheduling work, parceling out pieces
to the individual GEOs, and assigning one of the three GEOs to coordinate
the effort, they decide instead to utilize the idle processing power of the fourth
(spare) GEO and assign the job to GEO-4, leaving to GEO-1 responsibility
for interfacing with GEO-4 when its task is completed.
While GEO-4 is performing the new intermediate-term planning and
scheduling task, the other GEOs concentrate on their immediate routine jobs,
namely receiving science data from LEOs for relaying to the ground and per-
forming their own science assignments (and communicating the results to the
ground) according to their current operating instructions.
For the first duty, the GEOs function in a manner quite similar to the tra-
ditional ground station, which has knowledge of the time and angle at which
to view any given LEO as it comes into view over the horizon, thereby starting
a “view period” during which communications can take place. For the GEOs
to perform their similar communications function with the LEOs, switching
curves (in latitude and longitude) are maintained onboard the GEOs for use
by the planning and scheduling agents in conjunction with the look-ahead
modeling and data-monitoring and trending agents. The switching curves di-
vide the “sky” into three 120
◦
slices (where, it may be noted, none of the
GEOs will be able to “see” the LEOs near the north and south poles). The
curves defining the segment boundaries are padded so that when a LEO enters
a padded region, preparations to initiate communications with the new GEO
are begun and completed before the LEO leaves the padded region. To effect
change in control, the GEO currently directing the LEO's actions instructs
(via its communications agent) the LEO to terminate its current telemetry
link and reorient its main antenna toward the new GEO. The new GEO then
initiates a link with the LEO and requests that flow of completed science
products be renewed. Data received from LEOs are then formatted by the
individual GEOs in observation files by the SI data-storage and communi-
cations agents and are relayed to a designated single GEO (say, GEO-3) for
integration into an overall global picture/assessment. As the GEOs themselves
conduct their own science observations (as discussed in a previous section),
the data from GEO-1 and GEO-2 are relayed to GEO-3 for merger with the
LEO data.
When GEO-3 has completed the integration process, it also performs the
data reduction processing required to convert the raw measurements into sci-
ence end products. These results are then transmitted to the lights-out ground
station for archiving and dissemination. Optionally, the raw measurements
themselves may be transmitted to the ground for archiving. Note that GEO-3
does not integrate and process all the data all the time. Once GEO-3 be-
gins its integration job, another of the three GEOs (say GEO-2) will be the
collection point for new data as it is generated by the constellation. So at any
given time, one GEO will be collecting science data from the constellation,
