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Close call: next March these dishes at Westerbork will begin competing with mobile phones. Credit: NFRA

Radioastronomers have gained a small victory in their battle with the telecommunications satellite operator Iridium over use of an important radiofrequency. But victory comes at a cost: they have had to accept a significant loss in observing time.

Iridium launches its global mobile telephone service next month. It is based on a system of 66 satellites operating both uplinks and downlinks in a single frequency band, adjacent to the 1,612 MHz band reserved by international law for radioastronomers.

Technical inadequacies in Iridium's system mean that overspill emissions from the downlink will drown out the weak 1,612- MHz signals from deep space (Nature 380, 569; 1996).

While US radioastronomers have reached a time-sharing agreement with Iridium, their European counterparts have fought hard to protect their frequencies. They have won the support of their governments, which have withheld operating licences from Iridium until agreement is reached over acceptable pollution levels.

After six months of acrimonious negotiations, Iridium this week signed an agreement with the European Science Foundation (ESF) on behalf of its associated Committee on Radio Astronomy Frequencies (CRAF). This should keep the 1,612 MHz band clear of interference until next March. The situation should improve after January 2006, the deadline by which Iridium must replace its 66 satellites with an improved non-polluting system.

According to the terms of the agreement, Iridium and radioastronomers will have to work together to develop non-polluting next-generation satellites, and to develop technical fixes to reduce the susceptibility of radioastronomy equipment to overspill emissions.

During the next few months Iridium and radioastronomers will have to reach a further agreement on time-sharing during the period between March 1999 and 2006. With both sides determined to keep the lion's share of the time, the battle will be hard fought.

Willem Baan, chairman of the Inter-Union Commission on Frequency Allocations for Radioastronomy and Space Science, and director of the Westerbork Observatory in the Netherlands, is worried that the requirements of radioastronomers are more than Iridium will want to concede. He is also unhappy about the spirit of the negotiations so far. “Iridium knew of the radioastronomy problem as early as 1991,” he says. “Yet it did little to solve it in the design phase of their system and so now we have a serious problem.”

He believes that radioastronomers are paying for Iridium's failure to solve its own problems. “It is still not clear that radioastronomers will be able to perform very sensitive observations in the immediate vicinity of the strong main transmission of Iridium,” he says. “Other first-generation mobile satellite service systems avoid these problems by using other downlink frequencies.”

Not only will radioastronomers be forced to accept time-sharing, but, even with a guarantee of ‘no pollution’ after 2006, they will lose about eight per cent of observation time as Iridium's armada of satellites pass directly across the piece of sky they are looking at. This is a loss that any telecommunications company would find unacceptable, says Baan.

However, ESF secretary general Enric Banda puts a positive spin on the agreement, which “underlines the willingness of Europe's radioastronomers to work constructively with the growing number of satellite companies, to find technical solutions that will allow science and industry to continue to profitably coexist in space”.

But interference from satellites remains an increasing threat. “This is not an isolated problem,” says Jim Cohen, CRAF's chairman. “Unless the protection of radioastronomy is taken into account early in the design of new satellite systems, our science could face a difficult future.” Even with full cooperation from the telecommunications industry, the sheer number of satellites in systems like Iridium will eventually add up to a serious physical obstruction in the radio sky.