Responding to concerns by radio astronomers that its transmission signals will overpower those from the distant celestial objects they track, Iridium L.L.C. has moved forward with a series of compromise agreements that it hopes will resolve the issue.
A small portion of spectrum Iridium’s satellite constellation will use to transmit its voice and data signals borders the frequency that emits signals from hydroxyl ions, which astronomers use to track giant suns and newly formed stars.
Astronomers study these faint signals on the 1610.6 MHz through 1613.8 MHz frequencies. Iridium’s constellation will transmit its much more powerful signals on the nearby 1621.35 MHz through 1626.5 MHz frequencies. It is very likely that the more powerful Iridium signals will “bleed” into the frequency range so critical to the astronomers.
According to officials at Motorola Inc.’s Satellite Communications Group-the entity building the system that Iridium will operate-scientists on both sides of the issue worked together to reach a solution.
First, Iridium designed transponders that would reduce the chance of frequency bleed.
Second, Motorola’s wholly owned subsidiary, Satellite Systems License Inc., signed time-sharing agreements with 14 of 15 radio astronomy observatories in the country. The remaining observatory relies on a different frequency band than the others and is not in danger of interference from Iridium’s system.
The company made three coordination agreements. One agreement encompassed 12 U.S. observatories while the other two agreements were made with two separate observatories.
The time-sharing agreements establish times when Iridium will be at peak use and times when it will not. During off-peak hours, radio astronomers will have priority. For instance, Arecibo Observatory in Puerto Rico will have priority from 10 p.m. to 6 a.m. EST, during which time Iridium will take necessary steps to quell its signals, which may disrupt radio observations.
Astronomers, in turn, will try not to schedule observations during hours of peak phone use.
The third aspect to the solution involves an air-interface agreement. Astronomers will be able to read signals through the bursts of data sent by Iridium satellites during peak time, sort of like looking at an object through a picket fence. The astronomers can then go back and get more detailed and clear readings during the off-peak hours.
Iridium has yet to reach agreements with astronomers in Europe and Asia, who are considered less willing to compromise than their U.S. counterparts, who they have accused of “dancing with the devil.”
