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Ask Dr. SETI ®

Chapter 5: Sociology

Iridium Interference

Dear Dr. SETI:
Do you know how much the Iridium satellites will influence the amateur radio astronomy and SETI? I've learned that some radio observatories (in the north hemisphere) had signed agreements with Motorola. But, what will happen with those who work in the south, especially with the amateurs?

Rolando, Peru

The Doctor Responds:
I hate to cast a pall upon your amateur SETI activities, Rolando, but the situation is pretty bleak. Over the past year Motorola launched an array of 66 Personal Communications System (PCS) satellites into low-earth orbit, to provide global telephone and internet access. These satellites, which become operational in September 1998, have downlinks in the 1621.25 to 1626.5 MHz band. Many radio astronomers observe the hydroxyl radical radiation line at 1610.6-1613.8 MHz, a segment internationally allocated to radio astronomy on a primary basis. These frequencies are 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.

Even though Iridium does not actually violate the International Telecommunications Union allocations, It is entirely possible that radio astronomy activities around that frequency will be impacted by adjacent-channel interference. The agreement to which you refer was reached between the European Science Foundation (ESF) and Iridium LLC, operators of the Motorola satellites. A press release issued by ESF on 13 August 1998 states:

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. Under its terms, Iridium guarantees Europe's radio astronomers 24 hours a day of 'unpolluted' observation time from 1 January 2006. Both parties are also committed 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 as requested by the radio astronomers. However, in practice, even these levels imply a concession by radio astronomers to satellite-enabled services as the sensitivity of current state-of-the art radio astronomy equipment would imply that they should be set considerably lower.

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 radioastronomers 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 astronomy. "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 is taken into account early in the design of new satellite systems our science could face a difficult future."

Although this agreement appears to provide some relief to European radio astronomers, I see no such protection being proposed for the rest of the world. Satellite interference is not new to the SETI community. Radiation at 1575 MHz from the constellation of 24 Global Positioning System (GPS) satellites is a well known pollutant in the water-hole spectrum. Fortunately, observations around the 1420 MHz hydrogen line, and the hydroxyl component around 1660 MHz, should not be affected by Iridium. As those are the two most popular frequencies for SETI, the impact on our own Project Argus search should be tolerable. However, Iridium is only the beginning, and other, competing PCS satellite constellations are planned. Things can only get worse.


As of the third quarter of 2000, the Iridium commercial venture has gone bankrupt, and the satellites are scheduled to be de-orbited (i.e., burned up in the Earth's atmosphere) over a two-year period. I suppose this is good news for radio astronomy and SETI, and bad news for Iridium's investors. Yet, despite Iridium posting a $5 Billion loss, other constellations of low Earth orbit personal communications satellites are planned, and many of them propose to operate in or near the prime radio astronomy bands. So Iridium is simply Round One of an ongoing battle between science and commerce.

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