French military planners are studying the use of higher orbits for optical reconnaissance satellites as a way of assuring more-frequent coverage of a given area and saving on the costs of launching satellite constellations.

 

No decisions have been made, but initial research is being done on whether satellites at 3,000 kilometers in altitude, and ultimately even geostationary orbit, could be used for delivering sufficiently sharp ground resolutions to be of military interest.

 

"We are looking at a range of potential orbits, and the 3,000-kilometer orbit strikes us as particularly interesting," said Sylvain Equilbec, a satellite-reconnaissance program manager at the French arms-procurement agency, DGA. "It is part of the work we are doing as we study what comes after Helios 2 and Pleiades."

The first of two optical and infrared Helios 2 reconnaissance satellites is scheduled for launch in December and will operate in an 800-kilometer polar orbit. The smaller Pleiades optical satellites, to be operated alongside radar spacecraft being made for the Italian government, are scheduled to be launched starting in 2008 and to be placed into 695-kilometer orbits.

 

In a series of presentations here Oct. 31 about using space imagery for defense and security, government and industry officials said it is no longer far-fetched to start considering much higher orbits for high-resolution optical imagers. The event was organized by the French Foundation for Strategic Research (FRS).

Most Earth observation satellites orbit in polar or near-polar low Earth orbits of less than 1,000 kilometers in altitude to place the imager as close as possible to the Earth's surface. That proximity permits the satellites to produce images with a ground resolution of several centimeters up to a meter or more, which means objects that large can be identified in the images. However, low orbits mean a satellite does not revisit its coverage area as often as military planners -- and some civilian agencies -- would like.

 

It is for this reason that constellations of smaller satellites working together are finding increased popularity. They can guarantee a quick revisit of a target area and, with advances in telescope construction, maintain a small-satellite's lightness while preserving the sharp resolution.

At the other end of the imagery system are meteorological satellites in geostationary orbit 36,000 kilometers above the equator, their imagers permanently trained on a given section of the globe. But the resolution is several kilometers.

 

Philippe Guyot, director of optical and radar instruments at Alcatel Space of Paris, said today's technology in Europe could produce a satellite that, from geostationary orbit, could produce an image with a ground resolution of 15-20 meters. By 2010 or 2015, he said, the feasibility of a geostationary satellite with a resolution of 5 meters will be demonstrated.

 

For military use, images with 1-meter resolution or better are required for some purposes. "For that level of resolution, you would need to recombine several images after they are taken, and we can look forward to that around 2020," Guyot said. "A satellite carrying six separate telescopes could produce a 2-meter image given the advances in materials and detector electronics."

 

In addition to the distance from the Earth, geostationary-orbiting high-resolution satellites would need to compensate for their poor angle of view. Located over the equator, they would not be able to peer straight down at many areas of interest.

 

Michel Bouffard, director of Earth observation and science at EADS Astrium's French division in Toulouse, France, said European governments' Global Monitoring for Environment and Security (GMES) program includes funds to investigate geostationary observation satellites. But Bouffard cautioned that the tradeoffs required for such a satellite -- telescope size, launch cost and image sharpness versus a constellation of smaller satellites in lower orbit -- had not yet been made.

 

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