LEO Boom Increasingly Concerns Radio Astronomy

"Our lives are going to get a lot more complicated" with the LEO boom, said Harvey Liszt, National Radio Astronomy Observatory spectrum manager and chairman of the international Scientific Committee on Frequency Allocations for Radio Astronomy and Space Science. Radio astronomy has dealt with satellite interference since the 1970s, when the spectrum allocation table wasn't as clear and practices were sloppier, such as little filtering, he said. The first GPS satellite "emitted right over" much of the L band, he said. The expected proliferation of LEO satellites creates a problem an order of magnitude greater, he said.

Even small out-of-band emissions "can completely drown out the incredibly faint hiss" of other galaxies, said Harvard astrophysicist Jonathan McDowell. He said LEO constellations' effects on radio astronomy operations won't be known until those constellations start full operations. University of California, Berkeley research astronomer Dave DeBoer said even if radio telescopes aren't aimed in the direction of a LEO satellite, downlinks will be far stronger than any celestial signal and corrupt the data the telescope takes in. Broadband satellites and their global coverage are "the great equalizers," making pretty much everywhere on Earth problematic for radio astronomy and could render it blind to large swaths of frequency, he said.

The University of Arizona Radio Observatory is increasingly challenged by the bands used for mobile communications increasing in frequency toward the sub-millimeter bands used by its telescopes on Kitt Peak and Mount Graham, emailed astronomer Remo Tilanus. He said satellite constellations such as SpaceX's Starlink with proposed frequencies of up to 75 GHz and global coverage pose similar concerns. "Terrestrial 5G, point-to-point communications, and car collision radars are additional sources of concern, but for those in principle there are some options to use spot-blanking or other shielding to somewhat alleviate the problem," he said.

While dynamic temporal frequency coordination might be possible, "no amount of preparation by radio astronomy will suffice to allow it to operate in the presence of many thousands or tens of thousands of satellites in LEO," the International Astronomical Union reported in April to the U.N.'s Committee on the Peaceful Uses of Outer Space's Science and Technology Sub-Committee. It said spectrum management protections fall short because of the small amount of scientific spectrum receiving protection, as well as "the poor historical record of the national and international spectrum management regime in remediating problems of satellite interference to radio astronomy."

Some Fixes

There are some possible fixes. Experts said there's no silver bullet, however.

The International Astronomical Union made two recommendations. They are that non-geostationary orbit satellites must be able to avoid direct illumination of radio telescopes and radio-quiet zones, and they need sidelobe levels low enough that their indirect illuminations of radio telescopes and radio-quiet zones don't interfere.

FCC rules require satellite operators to protect radio astronomy when operating in relevant spectrum bands. SpaceX and the National Science Foundation in 2019 announced a spectrum coordination agreement protecting NSF radio astronomy telescopes operating in the 10.6-10.7 GHz band from SpaceX satellite operations in adjacent spectrum. Asked about the adequacy of FCC radio astronomy protections, the FCC didn't comment this week.

OneWeb emailed that it has always been important for the operator to work with the astronomy community to safeguard observations and measurements. "In both cases of optical and radio astronomy we are in active conversations and taking steps to ensure we can both realize the potential of our network while protecting their work as well," it said. "For radio astronomers specifically, OneWeb has worked closely with the community and determined that it will protect the radioastronomy sites in 10.6-10.7 GHz primary allocation by not transmitting in the adjacent frequency channel when a OneWeb satellite is in the visibility of the radioastronomy receiver." It said for the 14.47-14.5 GHz radioastronomy secondary allocation, it protects radioastronomy sites "by limiting its satellite terminal emissions by agreed regulatory conditions with the concerned administration where the radio astronomy site resides.” SpaceX didn't comment.

The 10.6-10.7 GHz band represents "a tiny slice" of the spectrum radio astronomy uses, Liszt said. For the rest of the bands that radio astronomers use but aren't allocated, "the sky will be very, very bright," he said. While radio astronomy doesn't try to claim protection in bands where it doesn't have an allocation, it still needs to find usable bands and will "have to try to sneak under the radar, as it were," he said.

Berkeley's DeBoer said it's unknown if filters and coordination efforts with satellite operators to angle their beams away from particular areas where radio telescopes are located will work. He said the satellite industry has been "willing to talk, but solutions are somewhat [technically] challenging."

The radio astronomy discussions come as optical astronomers continue to talk with SpaceX about routes to mitigating reflectivity issues with its Starlink constellation, said Liszt. Harvard's McDowell said experiments with a nonreflective coating painted on Starlink satellites caused overheating problems with the satellites, so SpaceX for the past 12 months has been launching satellites with visors that shade shinier parts like antennas. The company also is modifying orientations, pointing satellites in different directions during parts of the mission such as orbit raising to make them fainter. McDowell said those steps have generally made the satellites invisible to the naked eye, and problems with astronomy observation images are lessened, "not solved."

McDowell said optical astronomy is "less totally hosed in the near term than we thought we might be" but big longer-term worries remain, such as China's planned GW constellation of thousands of LEO satellites. “Ultimately, this issue needs to be regulated in some way” to account for company and astronomy needs, McDowell said. “If you are going to change the night sky for everyone in the world, that is something that needs to be discussed at the international governance level.”