Oct 16, 2013

Commercial Earth Observation | CNES Study Details Pros, Cons of Alternative Imagery Architectures

Estimated Article Reading Time: 3 min.

(SpaceNews, By Peter B. de Selding | Oct. 14, 2013) BEIJING — The French space agency, CNES, is exploring three different types of next-generation high-resolution optical imaging satellite systems, including one that would operate in an unusual elliptical orbit to provide up to 45 minutes of “staring” time over a given target area, with a 1-meter ground resolution.

A second design is of a 3-meter-resolution satellite positioned in geostationary orbit over the equator, which would provide always-on viewing during the satellite’s 15-year life but pose issues of satellite orbit control and image processing.

Astrium Satellites of Europe and Ball Aerospace & Technologies Corp. of the United States, both of which build high-resolution imaging satellites, are looking at the geostationary orbit option but say the cost and size of such a satellite today make it unattractive for most prospective customers.

The third design being looked at by CNES is a low-Earth-orbit system with a ground resolution of 20-30 centimeters that would revisit a given area every day or two. More-frequent overflights of areas of interest are what many customers are asking for, meaning this system likely would need at least two satellites, if not more.

In presentations here Sept. 23-27 during the 64th International Astronautical Congress, officials from CNES and Astrium Satellites said the design work is aimed at fielding the next system around 2023.

One of the design goals for a low-orbiting system is that any satellite be small enough to fit inside the 2.6-meter-diameter fairing of Europe’s Vega rocket. Laure Brooker Lizon-Tati of Astrium said the Vega fairing likely would limit the satellite’s ground sampling distance to 30 centimeters, meaning it could discern objects of that size and larger.

Any sharper resolution, she said, probably would need active optics, possibly including a zoom capability, which would demand a 2-meter telescope — too large for Vega.

Payload data handling is also an issue. CNES and Astrium estimate that a low-orbiting satellite with a ground resolution of between 20 and 35 centimeters and a swath width of 20 kilometers — these were the design objectives — would generate between 100,000 and 57,140 pixels per line.

Instead of gigabytes, the data volume would be measured in the tens of terabytes, requiring higher transmission capacity and possibly an extended ground network, the CNES-Astrium study has determined.

By way of comparison, the two CNES-owned Pleiades high-resolution optical satellites now in orbit have a 20-kilometer swath width but only a 70-centimeter resolution that generates 30,000 pixels per image line.

The WorldView-2 satellite operated by geospatial imagery provider DigitalGlobe of Longmont, Colo., by comparison, produces imagery with a 13.1-kilometer swath width at 31 centimeters’ resolution, generating 42,260 pixels per line.

The elliptical orbit solves some of the problems of the low-orbiting system, but the question of launcher compatibility remains.

CNES and Astrium are looking at an orbit with an apogee of 6,353 kilometers and a perigee of 1,261 kilometers, angled at 116 degrees relative to the equator. It would offer 1-meter-resolution static images and a 3-meter-resolution video capability for the 30- to 45-minute period it spends staring at a given region.

The CNES study centers the satellite’s orbit over about 20 degrees north latitude. The satellite would be able to spend nearly half its orbit with a view over the region of interest.

An elliptical orbit such as this would subject the satellite to regular doses of radiation that could erode its functionality.

Alain Gleyzes, head of the Pleiades high-resolution satellite program at CNES, said the agency has filed for a patent on radiation hardening that could alleviate this problem. Gleyzes declined to discuss the patent in advance of its being issued.

CNES and Astrium have concluded that the telescope needed for the elliptical orbit would be large, but could be a monolithic, silicon carbide design including active optics.

The launcher-related drawback is that a 116-degree orbital inclination might put the satellite beyond Vega’s capability — at least the current Vega design. The vehicle has conducted just two flights, both of them successful, from Europe’s Guiana Space Center in French Guiana.

Astrium and Thales Alenia Space, which build the Pleiades viewing instruments, are teaming up to build the French Defense Ministry’s next-generation optical and infrared surveillance system under a $1.1 billion contract signed in December 2010. The two-satellite system, intended to be in orbit in 2017, may be extended to three if France finds co-investors in Europe.