Environmental Engineering Reference
In-Depth Information
costs are likely to remain significantly higher than those of ground software of
corresponding size and complexity, this mission type is probably not a good
candidate for an onboard Remote Agent implementation on a cost-benefit
basis. Again, Remote Agents may find many useful applications within the
ground system's lights-out control center.
6.6.4 Survey Missions
The nominal operation of a survey mission is by its very nature very straight-
forward and predictable. Planning and scheduling are well-defined for ex-
tremely long durations. For a properly designed spacecraft, calibrations should
be stable and easily monitored/trended by the ground. SI data need only be
collected and dumped to the ground for processing and archiving; no on-
board processing would be required, unless contact time and downlink band-
width/onboard data storage availability is an issue. Again, in a cost-benefit
trade, the relatively higher cost of FSW software over equivalent ground soft-
ware will always be an argument in favor of a ground implementation decision
(at least initially, with migration as an option). The one exception might be
in the area of fault detection, diagnosis, isolation, and correction (supported
by data monitoring and trending), where given the likelihood of nonfulltime
contact between flight and ground, an autonomous capability to deal with
a wide range of anomalies and still keep the mission going could prove very
useful. Otherwise, it would probably be more cost ecient to maintain most
potential Remote Agent functionality in the ground system.
6.6.5 Lagrange Point Celestial Pointers
Lagrange points are points of stable or unstable equilibrium relative to the
influence of the combined gravitational fields of two celestial objects acting on
a third object in orbit about the two objects. For simplicity of presentation, we
will restrict our discussion to the sun-earth Lagrange points, though Lagrange
points can be defined for any two celestial objects close enough in proximity for
both of their gravitational fields to affect a third orbiting object (for example,
the behavior of the Trojan asteroids relative to the sun-Jupiter system). For
a given celestial system, there will be a total of five Lagrange points, three
(L1, L2, and L3) in-line with the two objects and two (L4 and L5) off-axis, as
illustrated in Fig.
3.2
. Objects at the on-axis points (L1, L2, and L3) are in
unstable equilibrium (i.e., objects at these points will drift off in response to
perturbations), while those at the two off-axis points are in stable equilibrium.
The simple orbital geometry and benign environmental conditions (rela-
tive to those of a near-Earth orbit) make Lagrange-point orbits good choices
for spacecraft with sensitive thermal constraints. The absence of occultations
facilitates long duration observations of dim objects. In addition, for spacecraft
in halo orbits about L1 or L2, the distance to the Earth is as low as a million
