Environmental Engineering Reference
In-Depth Information
depletable resources, though the first item more strictly describes the phys-
ical behavior or state of the spacecraft. For LEO spacecraft, angular mo-
mentum (for reaction wheel-based spacecraft) management typically is fully
autonomous and is performed via the interaction of magnetic torquer coils
with the geomagnetic field. For orbital geometries where the geomagnetic field
strength has diminished below useful levels, excess angular momentum must
be dumped via a propulsion system of some sort (hot gas thrusters, cold gas
thrusters, or ion jets). Where a propulsion system is utilized to dump angular
momentum, often the ground's planning and scheduling system will play a
role (even dominate) when angular momentum dumping will occur because of
safety concerns regarding autonomous thruster commanding. However, even
for missions following this conservative operational philosophy, there often
will be a contingency mode/state in which autonomous angular-momentum
reduction via thruster firing is enabled to deal with inflight anomalies jeop-
ardizing the control and safety of the spacecraft. For spacecraft not using
reaction wheels (for example, the future Laser Interferometer Space Antenna
(LISA)), angular momentum management is not an issue.
By contrast, management of thruster fuel resources is traditionally almost
exclusively a ground responsibility. This allocation of functionality, histori-
cally, has been due to the mathematical complexity of orbit maneuver planning
and the earlier limited computational power of OBCs. So, if the planning of
orbit maneuvers (the activity expending the bulk of the onboard fuel supply)
is a province of the ground system, then management of the propulsion subsys-
tem's fuel budget quite logically would belong to the ground as well. Recently
however, considerable interest has been generated regarding the feasibility of
autonomous performance of spacecraft orbit stationkeeping activities. In its
more elaborate form, autonomous orbit stationkeeping may even be performed
in support of maintenance of a spacecraft constellation, coordinating the or-
bital motions of several independent spacecraft to achieve a common goal,
also referred to as formation flying. For these applications where planning
and scheduling of the orbit maneuvering function itself are moved onboard,
migrating management of the fuel resources to the flight system will be nec-
essary as well.
Flight Autonomy Enablers of Health and Safety Maintenance
Although each spacecraft has ideally been designed to support completion
of its assigned science program within its nominal mission lifetime, unpre-
dictable, potentially damaging events threatening termination of spacecraft
operations inevitably will occur sporadically throughout the course of the
mission. Many of these events will develop so quickly that by the time the
ground would have recognized the onset of the threat, developed a solution,
and initiates a response, conditions would have worsened to the point that loss
of the spacecraft is unavoidable. To deal with these highly dangerous potential
