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CONCEPT OF SATELLITE
FORMATION FLYING BECOMING A REALITY The
intensive planning activities and calculations that are currently done in control
rooms on Earth in preparation to maneuver Earth-orbiting research satellites are
about to become a thing of the past. NASA’s first-ever autonomous formation flying
mission is now under way as part of the Earth Observer-1 satellite program. The
EO-1 formation flying effort is demonstrating the capability of satellites to
react to each other and maintain a close proximity without human intervention.
This advancement allows satellites to autonomously react to each other’s orbit
changes quickly and more efficiently. It permits scientists to obtain unique measurements
by combining data from several satellites rather than flying all the instruments
on one costly satellite. It also enables the collection of different types of
scientific data unavailable from a single satellite, such as stereo views or simultaneously
collecting data of the same ground scene at different angles.  EO-1
was launched on November 21, 2000 as a technology mission designed to fly in formation
with another NASA satellite called Landsat-7. Both satellites carry instruments
that enable scientists to study high-resolution images and climatic trends in
the Earth’s environment. The EO-1 satellite flies only 60 seconds (450 kilometers)
behind Landsat-7 and maintains the separation within 2 seconds. This separation
is necessary for EO-1 to observe the same ground location through the same atmosphere
region. It also demonstrates significantly improved return of science data. The
mission allows engineers to compare technological advances made in ground observing
instruments that are smaller, cheaper, and more powerful. EO-1 also demonstrates
technologies for propulsion, onboard processing and data storage.
Click
on all pictures below to enlarge.
| Onboard
EO-1 is an advanced technological controller that is capable of autonomously planning,
executing and calibrating satellite orbit maneuvers. |  |
On EO-1
it is used for the computation of maneuvers to maintain the separation between
the two satellites.The idea and mathematical algorithm for this NASA first was
developed by Dave Folta, John Bristow and Dave Quinn, three aerospace engineers
who work at NASA’s Goddard Space Flight Center (GSFC) in Greenbelt, Md.  "The
technological controller is designed as a universal 3-Dimensional method for controlling
the relative motion of multiple satellites in any orbit," said Folta. "Our
idea was then combined with a new flight software that is the predecessor of a
GSFC sponsored commercial software called FreeFlyer that was produced by a local
Lanham, Md., company called a.i.-solutions inc."
The
new flight software provides for the ingest of real-time navigation data from
the onboard Global Positioning System (GPS) and the transfer of data from the
maneuver algorithm for maneuver commands, onboard predictions of where the satellites
will be in the future. It also provides the necessary attitude pointing, and actual
onboard commanding of the thruster firings Folta added. Formation
flying technologies are primarily concerned with the maintenance of the relative
location between many satellites. Much shorter and more precise baselines can
be established between the satellites. The satellites can then be combined as
part of a "virtual satellite" that should provide previously unobtainable
science data using mass produced, single-string, relatively cheap satellite.  Multiple
scientific instruments often present competing and conflicting requirements on
a satellite design and its operation. So much science at stake for a single satellite
often requires a great deal of onboard redundancy, which imposes its own overhead
on the design process. Separating scientific payloads onto several simpler single-string
satellites can accomplish the same complex missions without the added design and
operational overhead, while risking only one payload at a time.
The
proposed approach for onboard formation control will enable a large number of
satellites to be managed with a minimum of ground support. The result will be
a group of satellites with the ability to detect errors and cooperatively agree
on the appropriate maneuver to maintain the desired positions and orientations. Another
reason to use formations is due to the sensitivity of scientific instruments,
which can often be increased by expanding the effective observation baselines
(separation distances). This can be achieved by distributing the scientific instruments
over many separate satellites. The formation flying technologies flown onboard
EO-1will make these missions routine and cost effective.  Since
this formation flying technology is now fully developed and demonstrated, synchronous
science measurements occurring on multiple space vehicles will become commonplace
and the concept of Earth observing ‘virtual platforms’ will become a reality.
In the process, this technology enables the development of autonomous rendezvous.
Scientific payloads could be launched from any launch vehicle, rendezvous with
and join a formation already in place, and then autonomously maintain this condition
or respond to specific requests for science data collection by altering its own
orbit. Thus, this technology addresses all of the NASA directives to build revolutionary
satellites that are better, faster and cheaper.
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