While the Green Bank Observatory is located in a radio quiet zone to shield it from terrestrial interference, it and other radio telescopes facing growing interference from satellites. (credit: Green Bank Observatory/Jee Seymour)

A solution to the growing problem of satellite interference with radio astronomy

by Christopher Gordon De Pree, Christopher R. Anderson, and Mariya Zheleva
Monday, March 20, 2023

Visible light is just one part of the electromagnetic spectrum that astronomers use to study the universe. The James Webb Space Telescope was built to see infrared light, other space telescopes capture X-ray images, and observatories like the Green Bank Telescope, the Very Large Array, the Atacama Large Millimeter Array and dozens of other observatories around the world work at radio wavelengths.

Radio telescopes are facing a problem. All satellites, whatever their function, use radio waves to transmit information to the surface of the Earth. Just as light pollution can hide a starry night sky, radio transmissions can swamp out the radio waves astronomers use to learn about black holes, newly forming stars, and the evolution of galaxies.

We are three scientists who work in astronomy and wireless technology. With tens of thousands of satellites expected to go into orbit in the coming years and increasing use on the ground, the radio spectrum is getting crowded. Radio quiet zones—regions, usually located in remote areas, where ground-based radio transmissions are limited or prohibited—have protected radio astronomy in the past.

As the problem of radio pollution continues to grow, scientists, engineers, and policymakers will need to figure out how everyone can effectively share the limited range of radio frequencies. One solution that we have been working on for the past few years is to create a facility where astronomers and engineers can test new technologies to prevent radio interference from blocking out the night sky.

Astronomy with radio waves

Radio waves are the longest wavelength emissions on the electromagnetic spectrum, meaning that the distance between two peaks of the wave is relatively far apart. Radio telescopes collect radio waves in wavelengths from millimeters to meters.

Even if you are unfamiliar with radio telescopes, you have probably heard about some of the research they do. The fantastic first images of accretion disks around black holes were both produced by the Event Horizon Telescope. This telescope is a global network of eight radio telescopes, and each of the individual telescopes that make up the Event Horizon Telescope is in a place with very little radio frequency interference: a radio quiet zone.

A radio quiet zone is a region where ground-based transmitters, like cellphone towers, are required to lower their power levels so as not to affect sensitive radio equipment. The US has two such zones. The largest is the National Radio Quiet Zone, which covers 34,000 square kilometers, mostly in West Virginia and Virginia. It contains the Green Bank Observatory. The other, the Table Mountain Field Site and Radio Quiet Zone, in Colorado, supports research by a number of federal agencies. Similar radio quiet zones are home to telescopes in Australia, South Africa, and China.

A satellite boom

On October 4, 1957, the Soviet Union launched Sputnik into orbit. As the small satellite circled the globe, amateur radio enthusiasts all over the world were able to pick up the radio signals it was beaming back to Earth. Since that historic flight, wireless signals have become part of almost every aspect of modern life, from aircraft navigation to wifi, and the number of satellites has grown exponentially.

The more radio transmissions there are, the more challenging it becomes to deal with interference in radio quiet zones. Existing laws do not protect these zones from satellite transmitters, which can have devastating effects. In one example, transmissions from an Iridium satellite completely obscured the observations of a faint star made in a protected band allocated to radio astronomy.

Satellite internet networks like Starlink, OneWeb, and others will eventually be flying over every location on Earth and transmitting radio waves down to the surface. Soon, no location will be truly quiet for radio astronomy.

Interference in the sky and on the ground

The problem of radio interference is not new.

In the 1980s, the Russian Global Navigation Satellite System—essentially the Soviet Union’s version of GPS—began transmitting at a frequency that was officially protected for radio astronomy. Researchers recommended a number of fixes for this interference. By the time operators of the Russian navigation system agreed to change the transmitting frequency of the satellites, a lot of harm had already been done due to the lack of testing and communication.

Many satellites look down at Earth using parts of the radio spectrum to monitor characteristics like surface soil moisture that are important for weather prediction and climate research. The frequencies they rely on are protected under international agreements but are also under threat from radio interference.

A recent study showed that a large fraction of NASA’s soil moisture measurements experience interference from ground-based radar systems and consumer electronics. There are systems in place to monitor and account for the interference, but avoiding the problem altogether through international communication and prelaunch testing would be a better option for astronomy.

Solutions to a crowded radio spectrum

As the radio spectrum continues to get more crowded, users will have to share. This could involve sharing in time, in space or in frequency. Regardless of the specifics, solutions will need to be tested in a controlled environment. There are early signs of cooperation. The National Science Foundation and SpaceX recently announced an astronomy coordination agreement to benefit radio astronomy.

Working with astronomers, engineers, software and wireless specialists, and with the support of the National Science Foundation, we have been leading a series of workshops to develop what a national radio dynamic zone could provide. This zone would be similar to existing radio quiet zones, covering a large area with restrictions on radio transmissions nearby. Unlike a quiet zone, the facility would be outfitted with sensitive spectrum monitors that would allow astronomers, satellite companies, and technology developers to test receivers and transmitters together at large scales. The goal would be to support creative and cooperative uses of the radio spectrum. For example, a zone established near a radio telescope could test schemes to provide broader bandwidth access for both active uses, like cell towers, and passive uses, like radio telescopes.

For a new paper our team just published, we spoke with users and regulators of the radio spectrum, ranging from radio astronomers to satellite operators. We found that most agreed that a radio dynamic zone could help solve, and potentially avoid, many critical interference issues in the coming decades.

Such a zone doesn’t exist yet, but our team and many people across the US are working to refine the concept so that radio astronomy, Earth sensing satellites, and government and commercial wireless systems can find ways to share the precious natural resource that is the radio spectrum.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Christopher Gordon De Pree is Deputy Electromagnetic Spectrum Manager at the National Radio Astronomy Observatory. He previously served as Professor of Physics and Astronomy and Director of Bradley Observatory at Agnes Scott College from 1996-2021. His research interests are high mass star formation as studied with radio telescopes including ALMA and the Very Large Array.

Christopher R. Anderson is Associate Professor of Electrical Engineering at the United States Naval Academy. He has over two decades of experience in radio wave propagation measurements and modeling, software-defined radios, and dynamic spectrum sharing. He is the Founder and Director of the Wireless Measurements Group (WMG), a focused research group that specializes in spectrum, propagation, and field strength measurements in diverse environments and at frequencies ranging from 300 MHz to 28 GHz.

Mariya Zheleva is Assistant Professor of Computer Science at the University at Albany, State University of New York. Her research focus is on wireless networks for infrastructure-challenged regions, characterized with low-bandwidth Internet gateways, lack of reliable electricity and sparse populations. Dr. Zheleva has previously worked on other projects related to wireless networking including monitoring, medium access control for 60 GHz networks and smart phones.

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