The 32nd International Astronomical Union General Assembly (IAU GA) brought astronomers from around the world together in Cape Town, South Africa this past August. The scope and number of talks and sessions at the IAU GA was astounding, with 2648 participants from 107 countries attending the 211 science sessions. This year’s IAU GA focused especially on the issue of “dark and quiet skies”, a term used to characterize the night sky as a natural environment. Like other environments, say a lake or forest, the night sky can subject to human made pollution, in this case light pollution and radio frequency interference from technologies like cell phones or airplanes.
Due to the rapid increase in the number of satellites deployed in low Earth orbit, starting most significantly in 2019 with the first launch of SpaceX’s Starlink satellite constellation, there has been concern within the astronomical community about the impact of these satellites on observatories and the ability of astronomers and the public alike to enjoy the night sky without a constant stream of satellites coming into view. Recognizing the importance of this issue, the IAU established the Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference (CPS) in 2022.
At the IAU GA, the CPS led a day-long session that featured astronomers, advocacy groups, and industry representatives, all offering their perspective on the current state of affairs and what needs to change. There was also a special session on the “future of radio astronomy in an increasingly crowded spectrum”, whose focus is the battle of sorts happening over allocation of frequencies between different groups worldwide, as frequency bands often offer value to more than one group, and sometimes with competing, even antithetical, interests.
Figure 1: The frequency allocations for the US radio spectrum. Each color represents a different activity that specific band is allocated for. The bright yellow is reserved for radio astronomy. (Source credit, US National Telecommunications and Information Administration)
One topic that came up over and over again is how direct-to-device (D2D), also referred to as supplemental coverage from space (SCS), will impact radio astronomy. (You can tell how fresh the debate around this is just based on the lack of agreement on acronyms). D2D promises to connect satellites directly to smartphones, allowing for cell coverage when users are out of reach from cell towers. This could, for example, be used by a stranded hiker lost in the woods to call for help. However, to connect to the smartphone, satellites will need to beam down signals to users, creating a possible source of radio frequency interference (RFI) for ground based radio observatories.
What might be the effects of these satellites on radio data? Harry Qiu, a radio spectrum scientist at the Square Kilometer Array (SKA), a next generation radio telescope based in Australia and South Africa, presented the results of a study on the impact of the OneWeb satellite constellation. They find that these satellites can cause interference even outside of the telescope field of view, in the sidelobe of the telescope. The way radio telescopes receive a signal is through a main lobe, which is where the highest power is received. Side lobes are minor lobes where signal strength is weaker, but unwanted signals can still be detected. Looking through 60 hours of data, Qiu finds that these satellites can have a flux density of 10^4 Jy! For comparison, pulsars, a prominent astronomical radio source, are typically detected on the order of mJy. Qiu also notes there are also difficulties with knowing the technical specifications, like the gain, of these satellites, since they are often kept private as proprietary technologies. Work like this will hopefully improve understanding of satellite flux measurements for simulations and help with ways for observatories to avoid satellites when they are overhead.
Some observatories have opted for a cooperative approach with the satellite industry in an attempt to better understand the impacts and mitigate effects. For example, the National Radio Astronomy Observatory (NRAO), which operates the Very Large Array (VLA) in New Mexico and the Green Bank Observatory (GBO) in West Virginia, has spent the last few years developing a data sharing system with SpaceX. The VLA and GBO share data from their current observations with SpaceX, and SpaceX in turn updates in real time its satellites to avoid creating strong interference. For example, Starlink satellites will not beam down to areas around the VLA or GBO, or they will use less channels when beaming down. When in the boresight of the telescope (the line of sight with the maximum power), the Starlink satellites will also turn off temporarily, which due to the speed of travel, is normally only for 10s of seconds. NRAO also finds Starlink emission to be >10^4 Jy when transmitting, and also notes they have seen emission from outside the assigned bands for Starlink due to unintended electromagnetic emission of the satellites onboard electronics. This unintended emission has also been seen by the International Low Frequency Array (LOFAR) in Europe and the SKA.
Regulations at the local, national, and international level were also discussed. Radio quiet zones, such as the one around the Green Bank Telescope in West Virginia, are especially important regulated areas that can help maintain low amounts of radio interference. Boris Sorokin, presenting about upcoming challenges for radio astronomy spectrum management, highlighted how the SKA of Australia and the Atacama Large Millimeter Array (ALMA) of Chile have both had their radio quiet zones internationally recognized. On the more challenging side, supplemental satellite coverage has been added for terrestrial mobile networks, meaning more frequencies available to cell phone companies. Also notable is that harmonics, which are integer multiples of a fundamental frequency (ex: 1600 Hz is a harmonic of 800 Hz) are still barely considered in international radio frequency allocations! This means that though a signal may be intended to emit within a specific radio band, its harmonics may still be interfering in other bands.
Bevin Vanderley from the National Science Foundation suggests that establishing some best practices to complement regulations can help push industry and governments to improve their methods. For example, satellite manufacturers could measure and characterize unintended emission from their satellites prior to launch by testing them in an anechoic chamber (a shielded box that prevents radio waves from entering or leaving). To highlight the urgency of this problem, Vanderley notes that from 2014-2023, there was an increase in the total fractional loss across all bands (1-25 GHz) of the VLA, from 10% to 15%. This is a 50% increase over 9 years in discarded data due to RFI at the VLA.
Andrew Williams from the European Southern Observatory summarized some of the efforts made by governments and nongovernmental organizations to tackle the issue of dark and quiet skies. The US Federal Communications Commission now requires operators to make an astronomy coordination agreement with NSF to get a license and ESA has developed a Space Debris Mitigation standard and launched a Clean Space Programme. There is also the EU Space Label, (a space law of sorts for the entire EU), the Earth Space Sustainability Initiative, and the Space Sustainability Rating. The IAU has also developed a free online informational course to learn about satellite constellations, Sat Cons 101.
These are just a few highlights from one sub section of the giant that was the 2024 IAU GA. One amazing aspect of this meeting is how publicly available all the content is. All the abstracts and talks can be viewed online, through the website or on Youtube! I would highly recommend taking a look through the content, there is something for everyone and anyone interested in astronomy and astrophysics!
Figure 2: A map showing how Starlink avoids the VLA observatory (shown here in the green and blue dots) by allocating specific zones where it cannot beam down. The red is where no downlink is permitted, while the light blue is where a smaller number of channels is allowed. This is part of the NRAO’s effort to coordinate with SpaceX and reduce satellite interference on observations (Source Credit, NRAO).
Astrobite edited by Diana Solano-Oropeza
Featured image credit: Jay Caboz
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