PARIS — European Earth scientists organizing a funding competition among three finalist missions on March 5 were told that the biggest issue facing one of them is not its advanced technology, but the U.S. Defense Department’s priority allocation of radio frequencies.

Meeting in Graz, Austria, to select the the 7th Earth Explorer mission to be flown by the 20-nation European Space Agency (ESA), backers of the Biomass mission were pelted with questions about how badly the U.S. network of missile warning and space-tracking radars in North America, Greenland and Europe would undermine Biomass’ global carbon-monitoring objectives.

Europe’s Earth observation satellite system may be the world’s most dynamic, but as it pushes its operating envelope into new areas, it is learning a lesson long ago taught to satellite telecommunications operators: Radio frequency is scarce, and once users have a piece of it they hold fast.

In the case of Biomass, backers appear to have structured a polar-orbiting mission that preserves its essential qualities even if it sacrifices coverage of certain large forested areas.

The issue was not unexpected. In fact, until global frequency regulators in 2003 gave Earth observation science satellite missions the right to use a six-megahertz sliver of spectrum, a mission like Biomass could never have been proposed.

Biomass seeks to use a P-band synthetic aperture radar and a 12-meter-diameter deployable reflector — to be purchased in the United States from either Northrop Grumman or Harris Corp. — to study many of the world’s forests to determine carbon levels. While carbon and methane are both widely viewed as key metrics in global warming, satellites designed to monitor them are not yet numerous.

The problem is that these frequencies, between 420 and 450 megahertz, are already in use. Radars installed in the United States, Greenland, Britain and Turkey by the United States and its allies as part of a ballistic missile warning and space surveillance network have been operating in this spectrum for decades.

It was only after long debate at the International Telecommunication Union (ITU ) that frequency regulators agreed to let these promising Earth observation radars use a piece of the already-reserved spectrum. But the allocation was on a secondary basis, meaning the environmental satellites could not interfere with the military radars, sometimes referred to as the Space Object Tracking Radar network, and would need to stay out of the way.

As a practical matter, that meant ESA and the Biomass backers need to work around possible interference with the military radars by not covering North America and Europe.

An initial analysis has concluded that the military radars would be compromised if the Biomass radar flew over them.

The Biomass team, led by Shaun Quegan of Britain’s University of Sheffield, and ESA officials said the current configuration of the military radars is unlikely to change, or to occupy more territory and thus further reduce Biomass’s operational territory. If any new sources of interference in the frequency band were to materialize, ESA would have at least the moral high ground in negotiating with the responsible governments. ITU has no real power to enforce its rules.

Biomass backers further said that while the military radars’ locations meant the satellite would essentially be deprived of measurements over North America and Europe, this will not scuttle the mission’s essential value.

“The most critical forest regions, such as the tropical forest belt, the boreal forest of Siberia and the temperate forests of China, would still be covered,” the Biomass team wrote in a document in support of their mission. “The worst-case operational restriction that could arise following a request of the U.S. [Department of Defense] would not significantly affect the primary objectives of the mission.”

It is not the first time an ESA mission confronted problems with frequency interference. The Soil Moisture and Ocean Salinity (SMOS) satellite operating in L-band and launched in November 2009 encountered multiple radio frequency interference sources when it first started operations.

SMOS managers had little idea before launch that so many broadcasts around the world were apparently operating outside their allocated frequencies. By raising the issue with the ITU and with individual governments, ESA was able to reduce much of the interference. But the battle, according to one SMOS scientist, is never definitively won.

“It looks like Japan’s 4G mobile network is now a problem,” this official said. “We could tell them to please add filters to their broadcast towers, and they could remind us how many billions that would cost. It’s a continuing effort.”

Peter B. de Selding was the Paris Bureau Chief for SpaceNews.