The European Science Foundation (ESF) and Iridium LLC, a telecommunication satellite operator and a subsidiary of Motorola Inc, have reached an agreement today (10 August 1998) to protect an important radio astronomy frequency band from harmful interference from Iridium's flotilla of 66 low-earth orbiting satellites.
The agreement addresses harmful interference from unwanted emissions of the Iridium satellite system into the band 1610.6-1613.8 MHz allocated to radio astronomy on a primary basis. This is the frequency used by astronomers to study the distribution of the hydroxyl radical, one of the most common interstellar molecules, enabling them to investigate a wide range of issues including the evaporation of comets and the birth and death of stars.
Iridium will launch its global mobile telephone service this September. However, close to the Iridium operating frequency, the radio astronomers want to detect signals with an intensity more than 109 times weaker than those from an Iridium satellite. Leakage from this neighbouring Iridium band 1621.35 - 1626.5 MHz, which will be used to link the satellites to individual mobile phones, into the adjacent radio astronomy band could therefore be devastating.
The agreement signed by the ESF, on behalf of its associated Committee on Radio Astronomy Frequencies (CRAF), and Iridium LLC is the result of six months of intense negotiations and was reached only after CRAF agreed to make a number of significant concessions (see notes - [1]). However, under the terms of the new agreement, Iridium guarantees Europe's radio astronomers 24 hours a day of "unpolluted" observation time from 1 January 2006. It also commits both parties to reaching a further agreement by 1 March 1999 on transitional arrangements, covering the number of hours each day, during which Iridium unwanted emissions are to be restricted and an agreed maximum interference level at other times, for the period 1 March 1999 to 31 December 2005. For the six months from Iridium's start-up in September 1998 until 1 March 1999, the satellite company has agreed to keep emission levels below harmful interference levels.
In addition, under the terms of the agreement, both parties will continue to work together to find adequate and technically practical solutions for reducing both the out-of-band emissions of the Iridium satellite system and the susceptibility of radio astronomy equipment to these emissions.
After signing the agreement, ESF Secretary General, Professor Enric Banda commented: "This is an important agreement for radio astronomy and provides welcome guarantees. Radio astronomy, as a passive service, is uniquely vulnerable to radio interference and CRAF's success, in representing the interests of Europe's different radio astronomy observatories during these negotiations, has once again demonstrated the value for Europe's scientific community of cooperation and of speaking with one voice."
He added: "The agreement also underlines the willingness of CRAF and Europe's radio astronomers to work constructively with the growing number of satellite-enabled companies to find sustainable technical solutions that will allow science and industry to continue to profitably coexist in space."
However, despite this agreement, interference from satellites remains an increasing threat to radio astronomy observations. "This is not an isolated problem," said Dr. Jim Cohen, CRAF's chairman. "The number of cases of interference to radio astronomy from satellites is growing steadily. Unless the protection of radio astronomy from satellites is taken into account early in the design of new satellite systems our science could face a difficult future."
Notes:
[1] The criteria defining harmful levels of interference for radio astronomy in the
1610.6-1613.8 MHz band have been set by the International Telecommunication
Union (ITU-R RA769-1) at -238 dB(W m-2 Hz-1). Even this
represents a significant concession by radio astronomers to satellite-enabled
industries as current state-of-the-art sensitivities would imply that these
should be several orders of magnitude more stringent.
[2] further reading: The CRAF Handbook for Radio Astronomy - second edition (1997), published by the ESF, ISBN: 2-903148-94-5.