Signal Sleuths: Uncovering Radio Frequency Interference
The Role of Radio Astronomy
Let’s talk about radio astronomy. You may have heard of optical astronomy, which studies stars using light we can see with our eyes. Radio astronomy, on the other hand, studies the universe with radio waves. Many objects in space, like black holes, pulsars, and weak signals from the Big Bang, send out radio waves. Because these waves are weak, astronomers use large dishes to collect them and strong electronics to boost the signals. Then, computers help study the signals to learn more about the universe.
Radio astronomers have made many discoveries that changed how we understand space. One famous find was made by Jocelyn Bell Burnell, a graduate student at Cambridge University. She discovered pulsars. Pulsars are what is left of stars that exploded, which are called supernovae. They spin very fast and send out beams of energy. This discovery proved that these strange objects exist and helped scientists measure distances in space very accurately.1
Radio astronomy has helped us learn about some of the universe’s biggest mysteries. One big win was taking the first picture of a black hole and its dark shadow (Figure 1). Scientists used eight large radio telescopes from around the world to collect these data. This image first showed an object that many people once thought was only a theory. The picture proved that black holes exist and showed that radio astronomy is a unique way to explore the universe. 2
Figure 1: The first image of a black hole and its shadow (image courtesy of The Event Horizon Telescope Collaboration).
Beyond finding new things in space, radio astronomy also helps us learn more about objects we already know. For example, the Crab Nebula is the remains of a star that exploded in what is called a supernova. The Crab sends out energy across the electromagnetic spectrum. Many kinds of astronomy—from X-ray to radio—have studied this area in detail, and each one shows something different. By putting all this information together, astronomers can understand the Crab Nebula better. We only see how complex it is when we look at data from different fields of astronomy (Figure 2).3
Figure 2: A composite image of the crab nebula. This image was created using the data from five different observatories across the spectrum. (Image courtesy of NASA, ESA, NRAO, and the University of Buenos Aires).
The National Radio Astronomy Observatory (NRAO) is funded by the US National Science Foundation (NSF). It uses radio waves to study the universe. NRAO builds and operates some of the best radio telescopes in the world. One of them is the Robert C. Byrd Green Bank Telescope in West Virginia. This is the largest radio telescope that can move to point in any direction (Figure 3).
Figure 3: The Greenbank Telescope (GBT). The telescope dish is 100m or 328 feet across.4 This image makes it appear that the GBT is higher than the surrounding mountains. While it is very tall at 485 feet it is still much shorter than the surrounding Allegheny mountains which range from 3100 to 4800 feet tall.5 (Image courtesy of NRAO/AUI/NSF).
The Green Bank Observatory is in the center of the National Radio Quiet Zone (Figure 4). The Federal Communications Commission (FCC) made this zone in 1958 to protect the observatory and a nearby military base from radio noise. The FCC must approve almost every new radio transmitters before it is used. Because of these rules, it is important to watch all radio signals generated inside the zone to keep it quiet and secure.
Figure 4: A map of the National Radio Quiet Zone around the GBT.
Moving the telescope lets astronomers study any part of the universe and make maps of objects like the distant galaxies, and even the center of our own Galaxy, the Milky Way While exploring, the GBT found a radio source at the center of our galaxy.6 The GBT is a great place to study radio pulsars because of its location and sensitivity. Pulsars are fast-spinning neutron stars that send out strong radio waves.1 Using the GBT, scientists discovered the largest pulsar ever recorded. This pulsar has almost twice the mass of the sun. You can learn more about this finding in a short video from the researchers. Besides pulsars, the GBT helps scientists explore dark energy, galaxy formation, the growth of galaxy clusters, star formation, and many other types of objects.7
Another key facility at NRAO is the Very Large Array (VLA), which is near Socorro, New Mexico (Figure 5). The VLA has 27 radio telescopes, and each dish is 25 meters (82 feet) wide.8 They work together to make clear images of objects that give off radio waves. Scientists from around the world are chosen through a competitive process to use the VLA for new discoveries.
Figure 5: The VLA (Image courtesy of NSF/AUI/NRAO). There are four configurations of the VLA: A, B, C and D. A is the largest at 22 miles across while D is the smallest at less than 1 mile across.9
The VLA is an interferometer. This means it is a telescope made of many smaller telescopes that work together as one. When they work together, astronomers can see objects in the sky in far more detail. The VLA has been used since 1980. In that time, it has helped scientists make many discoveries, like finding a black hole that is one million times heavier than the sun. It has also helped discover an Einstein Ring. An Einstein Ring happens when light from a faraway galaxy or star bends around a massive object, such as another galaxy or black hole. This bending of light makes the object look like a ring. This effect is called gravitational lensing, and Einstein predicted it in his theory of general relativity. The VLA is also used to map gas and clouds in space, track spacecraft in our solar system, study micro-quasars, and explore the center of our galaxy.10
NRAO supports ALMA in Chile’s Atacama Desert (see Figure 6). ALMA is made up of 66 antennas. Fifty-four antennas are 12 meters (39 feet) wide, and 12 antennas are 7 meters (23 feet) wide. ALMA lets scientists study the universe using millimeter and submillimeter radio waves. This radio interferometer gathers information that is hard to get with other telescopes.11
Figure 6: Atacama Large Millimeter/submillimeter Array ((Image courtesy of NSF/AUI/NRAO). These telescopes are all a little smaller than the VLA telescopes and much smaller than the GBT telescope.
ALMA is also an interferometer. It has made many important discoveries. One major discovery was finding oxygen more than 13.28 billion light years from Earth. This is the farthest place where oxygen has been seen. ALMA has also been used to map the nearby universe. It helps scientists learn about different kinds of galaxies where stars are born (Figure 7). ALMA also helps scientists study how galaxies and stars form and change. It studies planet formation, the solar system, black holes, and other objects in space.12
Figure 7: The shapes of star forming galaxies (image courtesy of ALMA (ESO/NAOJ/NRAO)/PHANGS. S. Dagnello (NRAO).
NRAO also works to stop radio interference, called RFI. Radio interference can ruin the data from their sensitive instruments. The team uses many ways to fight RFI. In the NRQZ, way is to check for new FCC licenses near the observatories. If these signals cause problems, the team contacts the license holder to ask them to lower the signal or change its direction. If that does not work or the signal source is unknown, other methods are used. These include avoiding the times when interference happens, adding filters to the telescope, or using special software to remove unwanted signals (S. Wasik, personal communication, February 26, 2025). To find out more about these efforts, click here.
CHIME, the Canadian Hydrogen Intensity Mapping Experiment, is a radio telescope built to study neutral hydrogen. Neutral hydrogen is one of the most common elements in space, and learning about it helps scientists understand how the universe has changed over time. Later, researchers discovered that CHIME’s design also works well for studying other space events. For example, the telescope can detect pulsars and fast radio bursts (FRBs), which are strong bursts of radio waves coming from faraway galaxies. CHIME’s main telescope is unique. It has four large, fixed cylindrical antennas, and each one points to a different part of the sky (Figure 8). This design lets CHIME watch large areas of the sky for long periods, making it great for spotting rare cosmic events.13
Figure 8: The CHIME telescope located at the Dominion Radio Astrophysical Observatory in Kaleden British Columbia. (Image courtesy of CHIME)
CHIME also added three extra telescopes called outriggers. One is in Canada near Princeton, British Columbia. The other two are in the United States. One U.S. telescope is at the Hat Creek Radio Observatory in California, and the other is at the Greenbank Observatory in West Virginia. Each outrigger has one cylindrical antenna that gathers extra data. When scientists add this data to that from the main CHIME telescope, they can locate sources in the sky more easily (Figure 9).14
Figure 9: The CHIME Outrigger in Greenbank, WV (Image courtesy of Jill Malusky)
Researchers at Green Bank Observatory found that this extra antenna is very good at watching for radio frequency interference (RFI). Because the antenna operates 24 hours a day and stays in one spot, it gathers a lot of data. These data are very useful for tracking and studying RFI.
References
1. The Royal Society. (n.d). Dame Jocelyn Bell Burnell DBE FRS. https://royalsociety.org/people/jocelyn-bell-burnell-11066/
2. Choi, C. Q. (2019, December 19). Historic 1st Photo of a Black Hole Named Science Breakthrough of 2019. https://www.space.com/first-black-hole-photo-science-breakthrough-2019.html
3. The National Radio Astronomy Observatory. (n.d.). Cosmic Coloring Compositor. https://public.nrao.edu/color/
4. The National Radio Astronomy Observatory (n.d). Bigger Than a Barn. https://public.nrao.edu/gallery/bigger-than-a-barn/#:~:text=The%20Green%20Bank%20Telescope%20is,unmistakable%20landmark%20in%20West%20Virginia
5. Lists of John. (n.d.) Pocahontas County WV Peaks List. Lists of John. https://listsofjohn.com/searchres?c=1555
6. American Physical Society. (n.d.) Historic Site: Green Bank Observatory. American Physical Society. https://www.aps.org/funding-recognition/historic-sites/green-bank-observatory
7. The National Radio Astronomy Observatory (n.d.). Green Bank Telescope Discoveries. https://science.nrao.edu/about/news/green-bank-telescope-discoveries
8. The National Radio Astronomy Observatory (n.d.) Welcome to the Very Large Array! https://www.vla.nrao.edu/#:~:text=Welcome%20to%20the%20Very%20Large,(422%20feet)%20in%20diameter.
9. The National Radio Astronomy Observatory (1/21/21). The Very Large Array: Astronomical Shapeshifter. AUI. https://aui.edu/the-very-large-array-astronomical-shapeshifter/#:~:text=There%20are%20four%20primary%20configurations,hybrid%20configuration%20known%20as%20BnA
10. The National Radio Astronomy Observatory. (n.d.). VLA Science. https://public.nrao.edu/telescopes/vla/science/
11. The National Radio Astronomy Observatory. (n.d.). Atacama Large Millimeter/submillimeter Array. https://public.nrao.edu/telescopes/alma/
12. ALMA Observatory. (2021). Discoveries. https://www.almaobservatory.org/en/about-alma/how-alma-works/early-discoveries/
13. CHIME Collaboration. (2020). The Canadian Hydrogen Intensity Mapping Experiment is a revolutionary new Canadian radio telescope designed to answer major questions in astrophysics and cosmology. https://chime-experiment.ca/en
14. Greenbank Observatory. (3/31/2022). Green Bank Hosts New Telescope for CHIME. https://greenbankobservatory.org/news/green-bank-hosts-new-telescope-for-chime/
This work is funded through NSF Award # 2232159
