A mega-constellation is a large group of satellites, with hundreds or thousands of individual satellites. Starlink and OneWeb are examples on such systems. Their aim is to provide a global telecommunication coverage, also in rural and isolated locations. However, such mega-constellation are causing disturbances to astronomy: both optical and radio. In this work, we are only focusing on interference and compatibility aspects between radio astronomy and satellite mega-constellations.

The International Telecommunications Union (ITU) regulates the use of the radio frequency spectrum, also in satellite orbits. Some bands are allocated for the radio astronomy service (RAS) use with a primary and secondary status. The primary status forbids all radio transmission inside of the band. However, only about 2 percent of radio frequency bands are allocated to RAS. Current astronomical scientific goals require wide-band observations, also outside of RAS allocated bands, which have no protection from the ITU level. This is causing challenges for RAS operation. A possible way to improve the situation is establishing radio quiet zones (RQZ) around observatories. However, RQZ decisions will be only made at the national level, which is not an optimal solution. It is also difficult to create RQZ for air and space-borne transmitters. In addition, various technical and mitigation solutions could improve interference situation, e.g., notch filters. One option is to use the dynamic scheduling. The dynamic scheduling means, for instance, that satellite’s orbits are considered in a scheduling stage. This is an effective method, but in the other hand, it is very challenging and complicated. Sometimes it is not even possible solution, because scientific goals cannot be achieved.

Large satellite mega-constellations are increasing the satellite density per sky area. This increasing number of satellites could lead to a high probability of having a satellite in the main beam of an antenna. Even if satellite down-link frequencies (intended emission) are different than radio astronomy service (RAS) frequencies, there are some major potential risks, which should be considered. Satellites could produce so called Out-of-Band (OoB) emission when they are interfering other frequency services. This problem comes more likely when devices are aging and faulting. Satellites may also produce harmonics outside their allocated frequency bands. Other potential risk for RAS is unintentional electromagnetic radiation (EMR) from satellites. Electronic circuit and components, for instance the central processing unit (CPU), could produce a wide-band interfering transmission. The sensitivity of radio astronomy system is high enough to detect such EMR based transmissions. If the transmission from satellites goes to RAS bands, we can classify that as a radio frequency interference (RFI).

In Finland, the situation is still rather good, since for instance Starlink is not yet operational. In Figure 1, OneWeb satellite orbits on November 17, 2022 are shown, seen from Aalto University Metsähovi Radio Observatory (MRO). This figure shows that there are already tens of OneWeb satellites in the sky. But globally, the situation is not so positive anymore. OneWeb’s down-link frequency is 10.7–12.7 GHz and MRO observes solar total flux (10.7–11.7 GHz) partly at the same frequency range. Some compatibility issues have been already noticed between OneWeb satellite operations and solar radio astronomy observations in MRO.

More compatibility studies are required between RAS and satellite services (SS). The recently established International Astronomical Union (IAU) global outreach project “Dark and Quiet Skies” aims to raise awareness of these issues. The Committee of Radio Astronomy Frequencies (CRAF) have made compatibility studies over several decades, including recent work particularly related to satellite mega-constellations. Nevertheless, a fluent dialogue between the RAS operator and the national frequency administration is also required to achieve best possible protection to RAS.