Satellite formation flying concept becoming a reality
Posted: June 4, 2001

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 Observing-1 satellite program.

Illustration of Landsat 7 leading EO-1 in orbit. Photo: NASA-GSFC
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.

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.