Environmental Engineering Reference
In-Depth Information
satellites, and it is estimated that initially, its primary operations consisted
of 200 people, approximately three people per spacecraft - a 25% improve-
ment over the WMAP mission. The GlobalStar satellite system consists of 48
satellites and requires approximately 100 people to operate it, approximately
two people per satellite. The Iridium and GlobalStar satellites are mostly ho-
mogeneous, which makes for easier operations than heterogeneous satellites.
Understandably, many of the future NASA missions are proposing multiple
homogeneous spacecraft.
The last two examples in Table
1.3
show future NASA multisatellite mis-
sions along with the operation requirement using current technology, and the
goal. Keeping to the current technology of approximately four people per
spacecraft, the cost of operations will become prohibitive as the number of
spacecraft increases. Missions are currently being planned and proposed that
will include tens and hundreds of spacecraft. The most effective way of avoid-
ing excessive cost of operations is by reducing the operators-to-spacecraft
ratio. To do this, operators need to work at a higher level of abstraction and
be able to monitor and control multiple spacecraft simultaneously.
In addition to saving costs on operations, autonomy can play a vital role
in reducing the size of the communications components. This, in turn, reduces
weight and the cost of the mission not only in components, but also in the
amount of lift needed to put the spacecraft into orbit. Historically, the mis-
sion principal investigator (PI) would want all of the data to be transmitted
back to earth for archival purposes and for rechecking calculations. Earlier,
instruments did not generate as much data as they do now and it was not an
issue in transmitting it because the onboard resources were available. Newer
instruments now produce more data and many mission are now flying more
remote orbits. Both of these require higher-gain antennas and more power,
which increase costs, including greater cost for launch. An alternative is to
do onboard processing of science data or transmit less data by stripping out
nonscience data, both of which result in less data to download. This is an
example of one tradeoff, i.e., in basic terms, design the mission to either (a)
download all of the data, or (b) download only part of it (thereby reducing
the cost of one part of the mission) and doing more science by adding more
instruments.
1.3.2 Communications Delays
Autonomous onboard software is needed when communications can take more
than a few minutes between the spacecraft and the ground. When communica-
tions is lengthened, mission risks increase because monitoring the spacecraft
in real-time (or near real-time) by human operators on earth becomes less
feasible. The mission is then flown less on a real-time, current basis, and the
operator needs to stay ahead of the mission by visualizing what is happening
and confirming it with returned data.
