Motivation for SPHERES
Developing autonomous formation flight and docking control algorithms is an important step in making many future space missions possible. The ability to autonomously coordinate and synchronize multiple spacecraft in tightly controlled spatial configurations enables a variety of new and innovative mission operations concepts.
Separated Spacecraft Interferometers
One application of formation flight technology is separated spacecraft interferometry.
By combining light from two or more telescopes, interferometry yields an
image that has a resolution equilvalent to a telescope far bigger (and far
more expensive) than the separate telescopes that made the image. Several
future missions, such as NASA's Starlight and Terrestrial Planet Finder, will take
advantage of the high resolution afforded by interferometric techniques.
To achieve the desired resolutions for these missions, long baselines
are required. Unfortunately, the long baselines required make a single spacecraft
impractical from both cost and technological standpoints. This has led mission
designers to consider separated spacecraft operations; however, since interferometry
relies upon controlling the pathlength of light, controlling the relative
position and orientation of the separated spacecraft is critical to mission
success. Given the high cost of operating spacecraft from the ground, there
is strong incentive to perform the tight control of relative position and
orientation autonomously.
Satellite Clusters
Another new way to perform missions from space is the concept of clusters
of microsatellites that operate cooperatively to perform the function of
a larger, single satellite. Each smaller satellite communicates with the
others and shares the processing, communications, and payload or mission
functions. Thus, the cluster of satellites forms "virtual satellite."
The cluster concept provides greater utility and flexibility by permitting the cluster to reconfigure and optimize its geometry for a given mission, enhance survivability, and increase reliability. Clusters will reduce life cycle cost by using mass-produced satellites and minimizing the launch cost by optimizing the launch vehicle's cargo capacity. The cluster concept also eases performance upgrades by allowing upgraded satellites to join a cluster, increasing the overall performance of the virtual satellite rather than replacing a single, large satellite or the entire cluster.
Satellite cluster operations maybe applied to a variety of applications,
including surveillance, passive radiometry, terrain mapping, navigation,
and communications. The Air Force's TechSat 21
mission will apply the cluster concept to space-based radar observation of
ground targets. The mission requires microsatellites to orbit in close
formation and often perform coordinated maneuvers autonomously; this level
of distributed control has never been attempted with a real space system.
Autonomous Docking
Previously, the vast majority of satellites have not been resupplied, serviced, upgraded, or reconfigured while on orbit. This default operational concept could be changed by developing robust autonomous docking control algorithms. The algorithms would eliminate complicated maneuvers executed by large and expensive ground operations teams. Reliable algorithms could then be used by a variety of satellite systems, both government and commercial.
DARPA's Orbital
Express Space Operations Architecture hopes to develop and demonstrate
techniques for on-orbit autonmous rendezvous and docking. Eventually, refueling
and upgrade modules could be added to exisiting satellites to extend lifetimes
and increase utility. However, failure to develop reliable docking algorithms
could result in accidental damage to or loss of existing satellites. Additionally,
resources used to build and launch docking modules will likely be lost.
Tethered Spacecraft Interferometer
Another formation flight concept that has been considered for a SSI system
is the use of tether. To image a target, measurements must be made in all
directions orthogonal to the line-of-sight of the array. The balance between
using a structurally connected interferometer, which allows for very limited
baseline changes, and a SSI system where the usage of propellant can be prohibitively
expensive, seems to be using a tethered system. Such a system is currently
being considered for NASA’s Submillimeter Probe of the Evolution of Cosmic
Structure (SPECS) mission. The SPECS mission attempts to address fundmental
questions about our universe (eg. How did galaxies form and evolve)14. One
mission concept is to use a Tethered Spacecraft Interferometer (TSI) system
to maneuver the sub-apertures out to separations of a kilometer, thereby
achieving very high resolution. Since power, maneuvering loads and data can
be supported by the tether, these typical spacecraft functions are not required
on the maneuvering vehicles. This reduces replication of sub-systems across
the various sub-apertures and eliminates the need for propellant. Furthermore,
the mass per unit length of the tether is much smaller than that of a deployed
truss making it much more mass-efficient, particularly for long baselines.
However, all of these benefits are lost if the control needed to achieve
the precision required by the array proves to be too complex. It is expected
that vibratory motion, consisting of pendulum and violin modes, will be observed
during operations of a TSI. Highly maneuverable spacecraft are particularly
problematic since beam control in the optics system will need to be maintained
to the requisite precision while thrusters fire, tethers vibrate, and reaction
wheels, or CMGs, change momentum. These introduce harsh disturbances that
necessitate the coupling of attitude and optical control.
Buying Down Risk
As with any emerging technology, developing and applying formation flight and docking control algorithms to real systems carries a high degree of risk. When this degree of risk is applied to multi-million or even multi-billion dollar flight systems, the risk can reach unacceptable levels.
The complexity of the algorithms being developed for formation flight and
docking missions is unprecented, as is the algorithm development process.
To buy down the risk associated with developing and applying these algorithms,
the SPHERES testbed provides a useful intermediate step where algorithms
can be verified and the algorithm development process can be validated.