For years, the commercial space sector has been abuzz about the prospect of satellite “super constellations” in Earth orbit. These satellites would provide everything from communications and navigation to broadband internet services.

Meanwhile, developments in small satellites (aka. CubeSats) and rideshare programs have made space more accessible to research institutes, universities, and organizations. With so many satellites in orbit, many are concerned about the impact this could have on space debris and astronomy.

Radio astronomy, which observes extremely faint emissions from astronomical objects, could suffer from all the added satellites in orbit. This was in a recent paper published on the arXiv preprint server by an international team of researchers who considered the impact of megaconstellations on radio astronomy.

While many scientists advocate establishing radio observatories on the far side of the moon, a more complete understanding of the impact satellite transmissions have on radio telescopes is needed to ensure future access to “dark” and “quiet skies.”

The study was led by Mike Peel, a postdoctoral researcher with the Blackett Laboratory at Imperial College London and the co-lead of Sathub, part of the IAU Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference (CPS). He was joined by colleagues from the Blackett Lab and researchers from the University of Illinois at Urbana-Champaign, the University of Washington, and the Square Kilometer Array Observatory (SKA). The paper describing their findings was presented at the 9th European Conference on Space Debris and was published on the conference proceedings website.

To break it down, radio astronomy relies on many different types of antennas and receivers to observe the sky in various resolutions and frequencies. These observations allow astronomers to see the “hidden universe,” where the presence of interstellar dust and debris obscures optical light. Traditionally, radio telescopes operate passively to coexist with other actors that rely on the radio spectrum (for transmissions, communications, etc). Observatories are built in remote regions to avoid radio interference (RDI) and improve signal-to-noise ratio (SNR).

This includes the Very Large Array (VLA) in the deserts of New Mexico, and the Square Kilometer Array Observatory (SKA) in the Karoo desert in South Africa and the Murchison region in Australia, or the Atacama Large Millimeter-submillimeter Array (ALMA) located in the Atacama Desert in northern Chile. The International Telecommunication Union (ITU) also reserves some small frequency bands involving spectral lines and physical processes at certain fixed frequencies.

As Peel explained to Universe Today via email: “‘Radio quiet zones’ are sometimes legislated by governments around radio observatories to minimize ground-based transmissions nearby. However, satellite constellations by their very nature are global—there is no escape from them anywhere on Earth.

“While some satellite operators do have agreements in place to avoid certain radio quiet zones, this is still the exception. This is why it’s important to understand the impact their transmissions, intended or unintended, have on radio astronomy instruments.”

Traditionally, satellites restrict their transmissions to frequencies in the X, Ku, and K bands of the radio spectrum (10 to 20 GHz). However, this will change as the number of satellites in LEO increases and satellite operators expand their services across a broader range of frequencies. While the far side of the moon is considered a promising remote location for radio astronomy, this can only be achieved if the location is protected from satellite transmissions.

Ergo, understanding the radio frequency (RF) properties of satellite constellations is crucial for the continued operation of remote radio observatories on Earth and the far side of the moon.

But as Peel explained, this is challenging since very little information is currently available from satellite operators: “A lot of information about satellites and their transmitters is commercially confidential. We know the main transmission bands they are using, and the maximum power they are allowed to transmit in those bands (which can be extremely high—they can appear brighter than the sun at those frequencies.).

“However, we don’t know much about out-of-band transmissions. For example, there can be some leakage on either side of the official band allocated to transmission, or unintentional emissions caused by the onboard electronics, which we can detect at low frequencies, even from 400 km away.”

Another challenge is how the published positions of satellites are frequently off by a few fractions of a degree (arcminutes), where even small differences can make a big difference. While some radio telescopes will avoid observing parts of the sky that coincide with the predicted orbital paths, this will become more challenging as the number of satellites in orbit increases dramatically.

“There are over a million satellites that have been proposed to ITU now, although not all of these will launch,” said Peel. “We still expect there to be 50,000–200,000 satellites in the near future.”

Fortunately, research into satellite radio interference is enabled through the IAU CPS, the Committee on Radio Astronomy Frequencies (CRAF), and similar radio astronomy-related organizations worldwide. There have also been multiple conferences where experts assemble to address the issue, like the Satellite Constellations 1 (SATCON 1) and SATCON 2 workshops and the Dark and Quiet Skies I and II conferences. The subject of out-of-band transmissions was also explored in a study led by researchers from the IAU CPS.

Using the Low Frequency Array (LOFAR) radio telescope, the team observed 68 Starlink satellites for signs of “unintended electromagnetic radiation.” Their observations revealed that the satellites emitted radiation at frequencies between 110 and 188 MHz, well below the 10.7–12.7 GHz radio frequencies used for downlink communication signals.

As Peel said, this latest study determined that satellites can be seen across the electromagnetic spectrum: “At radio frequencies, they are particularly bright at the frequencies they actively transmit at, and these are increasing and going to higher frequencies. Each new satellite constellation wants its frequencies and increasing amounts of bandwidth.

“The more frequencies used, the more radio astronomy observations are impacted. They are also seen at low frequencies, where they are not intentionally transmitting. However, the onboard electronics generate a radio ‘hum’ detected by low frequency radio telescopes.”

Unfortunately, there are no perfect mitigation strategies, and it is entirely impractical to require that satellites not operate above the horizon (as seen from a radio telescope). But the biggest impacts can be minimized through active cooperation between satellite operators through the IAU CPS and international bodies like the International Telecommunication Union (ITU) and the Committee on the Peaceful Uses of Outer Space (COPUOS)—part of the UN Office of Outer Space Affairs (UNOOSA). In the meantime, the available mitigation strategies are taking shape.

“Several operators are looking into operational data sharing, particularly boresight avoidance, where operators know the direction radio telescopes are pointing in, and switch their satellites off when passing through the telescope’s beam,” said Peel.

“Some also avoid transmissions towards radio telescope sites in general—certain countries are developing legislation to require satellite operators to coordinate with national research bodies to minimize impact. To avoid lower frequency emission, better radio frequency shielding and ground-based satellite testing are being investigated.”