AAS Resolution on the Protection of Radio Frequencies Used for Radio Astronomy

Adopted 11 June 1995, Revised 7 June 2025

Radio wavelength observations are crucial to the field of astronomy due to the unique perspectives they offer. Radio telescopes are able to produce high-resolution images via aperture synthesis, revealing the detailed structures of astronomical objects. Studies of quasars, galaxy dynamics, the cosmic microwave background, pulsar timing, and cosmic magnetic fields among others almost uniquely benefit from study by radio waves. However, the radio spectrum is a finite natural resource with many different services vying for access. Distant astronomical signals are many orders of magnitude fainter than human-produced radio transmissions for communications. It is for these reasons that specific astronomically important radio bands have historically been internationally protected.

The continuing protection of radio astronomical observations is of great interest to the American Astronomical Society (AAS). The AAS strongly endorses the maintenance of international allocations for the radio astronomy service and domestic regulations supporting the protection of these allocations and broader frequency bands at the small number of special geographic sites where radio astronomy telescopes are located.  As technology develops, new concerns arise. Of particular concern is the growing congestion of the radio spectrum caused by an ever-increasing demand for wireless communications services, especially in space-based operational paradigms. The expansion of satellite constellations means that traditional methods of mitigating radio frequency interference at telescopes –-- including siting facilities in remote locations and using natural terrain to attenuate transmissions – are becoming less effective. Radio Quiet Zones (RQZs) are regions in which the usual spectrum allocations are modified for the protection of astronomy and other passive services within a certain region. One such RQZ is the National Radio Quiet Zone (NRQZ) around the Green Bank Observatory in West Virginia. However, there are many radio astronomy observatories which have no established RQZ. The AAS endorses the establishment of new RQZs for the Very Large Array, the Very Long Baseline Array, DSA-2000 and other remote sites. Additionally, with the growing population of transmitting satellites in low-Earth orbits, RQZs within the United States and internationally (such as within Chile), which were originally established when the primary requirement was to mitigate impacts from ground-based sources, may provide insufficient protection for astronomical telescopes located within their borders. The AAS endorses the incorporation of coordination requirements within RQZs that will mitigate the impact of satellites on astronomical telescopes. Additionally, protection of radio astronomical bands must account for unwanted emissions from transmissions in nearby bands and harmonics, as such unwanted emissions have the potential to severely disrupt radio astronomical observations.

In response to the novel challenges, especially with expanding satellite constellations, the AAS supports ongoing efforts to develop innovative means of coexistence between radio astronomy systems and wireless communications services.  In particular, dynamic spectrum sharing techniques and active coordination between radio astronomy facilities and wireless operators (both satellite and terrestrial) offer opportunities to preserve the operations of both commercial services and radio astronomy while fostering long-term positive collaborations between academia and industry.  Current efforts working towards these goals should continue.

The AAS also notes with concern that several satellite constellation operators are advancing towards an operational model known as Supplemental Coverage from Space (SCS) in which satellite transmissions are received directly by cell phones rather than from base stations in fixed locations. This model makes use of very high-power satellite transmissions and large spot beams due to the frequency bands and satellite antenna sizes used, making geographic avoidance of radio astronomy sites difficult. Traditional means of coordination within RQZs may become ineffective if SCS is adopted by satellite operators worldwide. Innovative dynamic spectrum sharing projects are therefore critical for ensuring that radio astronomers can continue conducting research from Earth, but dynamic spectrum techniques may not be feasible if too many SCS bands are adopted. The AAS supports the adoption of as few SCS bands as possible and close coordination between SCS operators and radio astronomy observatories to minimize impact.  

The AAS supports comprehensive studies within the International Telecommunication Union (ITU) process leading to recommendations especially for the protection of the RQZ in Chile where ALMA, CMB experiments, and the Event Horizon Telescope operate  from satellite constellation transmissions, and broader recommendations for other radio astronomy facilities worldwide at the 2027 World Radiocommunication Conference (WRC). The AAS endorses the recommendations provided by the National Academies Committee on Radio Frequencies for WRC-27. 

Additionally, the AAS endorses the radio astronomy recommendations in the International Astronomical Union (IAU) Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference (CPS) Call to Protect the Dark and Quiet Sky from Harmful Interference by Satellite Constellations. In particular, the AAS recognizes the importance of limiting unintended electromagnetic radiation (UEMR) from satellites, since UEMR in the 50 – 200 MHz range can be especially problematic to astronomical experiments seeking to confirm the Epoch of Reionization. The AAS supports ongoing efforts to better understand and characterize UEMR to find effective solutions to mitigate impacts.