When Iridium technicians flip the switch to turn on their global satellite-phone service a week from now, the radio-astronomy community will experience the most dramatic limits to date on its ability to view the universe.
The limits are part of three time-share agreements hammered out over the last four years between Iridium and 15 observatories in the continental United States and its territories. Under the agreement, Iridium will reduce the power emitted by its satellite transmissions for a few hours each day, during nonpeak times. This means radio astronomers, once able to make observations at just about any time during a 24-hour period, must cut their observation time by as much as 84 percent, and the spectrum to be shared represents a fraction of what is now set aside for radio astronomers.
The compromise reached between Iridium and the observatories sends the troubling sign that commercial interests may gain the upper hand over science, said Donald Backer, professor of astronomy at the University of California at Berkeley.
Electromagnetic spectrum has become an endangered resource in the '90s since telecommunication deregulation and the new technologies -- such as satellite phones -- that came with it. The new services from these technologies have created an unprecedented demand for spectrum, and signals from different services can interfere with each other.
Despite efforts by regulators like the Federal Communications Commission, to prevent legitimate users from interfering with each other the onslaught of communications services demanding operating space along the airwaves is starting to crowd the spectrum. Now, neighboring services -- each with increasingly sophisticated technologies -- are starting to encroach upon each other's turf.
Satellite-communications networks like Iridium are to relay signals through their orbiting constellations and directly down to a user's phone. Because the antennae on Iridium phones are small, Iridium must transmit high-powered signals from its satellites to Earth to ensure a reliable connection. The power in these signals is so great, it can generate out-of-band emissions, signals that creep into neighboring bands of allocated spectrum.
At the same time, radio telescopes are designed to pick up the weak signals emitted by galactic matter. These telescopes must be sensitive to emissions in space -- the more sensitive the telescope, the more it can "see" in the universe. But its sensitivity makes it vulnerable to interference such as that generated by out-of-band emissions. So, radio telescopes are the victims of their own evolution, noted Mark McKinnon, scientist at the National Radio Astronomy Observatory at Green Bank, West Virginia.
"Anytime a signal is sent toward the ground, like satellite downlinks, it runs the risk of interfering with us," said McKinnon, who helped run a series of interference tests on Iridium satellites.
The out-of-band interference risk is greater the closer two services are within a part of spectrum. Radio astronomers use the part from 1610.6 MHz. to 1613.8 MHz. to pick up emissions of hydroxide molecules. But Iridium requested the neighboring band, from 1613.8 MHz. to 1626.5 MHz. for its satellite-to-Earth transmissions. When Iridium tested its downlinks with radio astronomers, the scientists found the noise generated by out-of-band emissions to be deafening, McKinnon said.
"There was definitely interference -- we had to throw data away," McKinnon explained.
As a solution, radio astronomers and Iridium engineers came up with a threshold for power levels emitted by the downlinks. This set a limit for the strength of Iridium's signals, lest they make observations a complete waste of time. But Iridium still needed to send high-powered signals, especially during peak-use hours. Hence, the time-share agreements.
Through time-share, Iridium can send the high-powered signals at the times it needs to, and during its off-hours, it agrees to reduce the power of the signals. Even with the threshold, the power of Iridium's signal dwarfs the signals radio astronomers seek by a factor of 100 to one. McKinnon concedes that scientists will still be sifting through data to find anything useful even with this threshold. But that's the nature of compromise, noted Mark Davis, a project scientist at the Arecibo Radio Astronomy Observatory in Puerto Rico.
"You have to decide the quality of the quiet," said Davis, who is part of a small band of scientists around the world who spend part of their time helping to protect the designated radio-astronomy bands.
But astronomers like UC Berkeley's Backer take little comfort in the time-share. Nor do they appreciate the part of the agreement in which Iridium provides, at no charge, an interface that will allow scientists to see through its transmissions during peak hours. This interface is added to the existing hardware at the observatories to indicate when the transmitters on board Iridium satellites are active. It has the effect of peeking through a picket fence to observe matter. Unfortunately, this interface reduces the sensitivity of the telescopes, therefore reducing the types of observations that can be made.
Even though the current agreements affect the frequencies of hydroxide molecules, the ability to see other matter could be affected by later satellite systems. Green Bank's McKinnon said despite Iridium's providing the hardware that could aid astronomers with its system, observatories will have to develop new hardware to accommodate satellite systems that will come later. This makes the life of astronomers, many of whom are academics, more complicated, he added.
"Most grants cover pay for graduate students, so adding $100,000 to [a grant application] for developing equipment is going to raise a lot of eyebrows," said McKinnon. "Academics are hired and fired based on their publications, not on the equipment they develop."
