By Gregory Hellbourg, Scientist, Cahill Center for Astronomy and Astrophysics, California Institute of Technology (Caltech), US, and Kaushal Buch, Senior Engineer and Group Coordinator, GMRT Backend Systems, Giant Metrewave Radio Telescope, National Centre for Radio Astrophysics (NCRA-TIFR), India

Radio astronomy detects extremely weak electromagnetic signals, often far below receiver noise. These signals carry essential information about phenomena ranging from distant galaxies to transient cosmic events.

In practice, however, radio telescopes must operate in an increasingly complex radio-frequency spectrum landscape shaped by numerous active and interconnected communication systems.

Recovering cosmic signals from a noisy world

Radio frequency interference (RFI) has become a central challenge for scientists dealing with outer space – and not least for radio astronomers. Signals from telecommunication systems, broadcasting, and satellites can be many orders of magnitude stronger than cosmic signals.

The key question is: How much astronomical information can still be recovered?

Radio telescopes are passive receivers reliant on a limited range of protected frequency allocations. While some bands are protected under the Radio Regulations, scientific observations often require broader access, creating a coexistence challenge with active services.

A spectrum of interference

RFI affects radio astronomy depending on its strength and characteristics. Astronomical signals are extremely weak, many tens of decibels below typical communication signals. Weak interference may remain within the receiver’s linear response and simply add unwanted power.

Even then, contaminated channels are often excluded, reducing sensitivity. Stronger signals can generate harmonics and intermodulation products, contaminating frequencies beyond the original emission, within the emitter and receiver.

At higher power levels, interference can cause compression or saturation. In extreme cases, it can damage sensitive front-end components, leading to irreversible data loss.

RFI is broadly classified as broadband and narrowband. Broadband RFI includes lightning, electrical discharges, or unintended emissions from on-board electronics, while narrowband RFI is typically associated with communication and broadcasting systems.

The toolbox for mitigation

At the hardware level, filtering, front-end limiters, and high dynamic range receiver design suppress or tolerate strong interference before it reaches the backend. Site selection and geographic avoidance remain effective for terrestrial transmitters but are significantly less effective against satellites.

Operational avoidance also helps: Telescopes can be scheduled or steered to reduce exposure to transmitters and satellites, supported by coordination, illumination constraints, and radio quiet zones.

In the digital domain, time- and frequency-domain processing, often implemented in real-time to handle data rates and pre-integration mitigation, suppresses impulsive and narrowband interference using techniques such as Median Absolute Deviation (MAD), spectral kurtosis, adaptive filtering, and subspace methods such as the Karhunen-Loève transform or singular value decomposition, though with reduced sensitivity to low-level interference.

A widely used approach is “flagging,” which consists of identifying and excluding corrupted data samples based on statistical properties.

In interferometric arrays, mitigation can also be applied in the “visibility” domain, the correlations between antenna pairs, which encode spatial information enabling adaptive nulling and spatial filtering. Reference antenna subtraction can also be applied to both single- and multi-dish systems.

What do these techniques achieve?

The figure below illustrates a first-of-its-kind real-time filtering system deployed at the upgraded Giant Metrewave Radio Telescope (uGMRT) in India, applied to broadband impulsive interference from power lines. The filtered data show reduced noise and improved signal-to-noise ratio, enabling the recovery of an astronomical signal otherwise difficult to detect. However, residual artefacts remain visible, likely due to low-level interference or from narrowband interference which this filter is not meant to filter, showing that mitigation improves usability but does not fully restore the original signal.

The cost of mitigation

When data are flagged or excluded, effective observing time is reduced. When filters remove parts of the spectrum, information is lost. For largely publicly funded observatories, this directly reduces scientific return.

Mitigation also requires significant investment. Engineers design and deploy hardware solutions, often requiring commissioning and recalibration, while software specialists maintain processing pipelines, sometimes in real time.

Astronomers are also affected. A growing fraction of their effort is devoted to pre-processing to remove residual interference and avoid biases, reducing the rate of scientific production.

Mitigation therefore involves a balancing act – minimizing impact while preserving as much information as possible.

Fundamental limits

No mitigation technique can perfectly reconstruct an astronomical signal once contaminated. All approaches leave residual errors that are rarely negligible compared to the extremely weak signals of interest.

The most effective mitigation approach remains prevention through spectrum management, which is coordinated globally through the International Telecommunication Union (ITU).

The role of regulation

The ITU Radio Regulations define radio astronomy as a passive service and protect frequency bands allocated on a primary basis, while Radio Regulations No. 11.12 and No. 5.149 recognize its operation beyond these bands. Regulation No. 4.5 addresses protection from interference in adjacent bands.

Recommendations from the ITU Radiocommunication Sector (ITU-R) support compatibility studies, notably on detrimental interference thresholds (RA.769) and on data loss impacts (RA.1513).

As spectrum use intensifies, particularly with non-geostationary satellite systems, coordination becomes increasingly critical. Growing spectrum management challenges include boresight avoidance, pre-launch compatibility testing, and establishing radio quiet zones as a regulatory instrument distinct from geographic siting.

These aspects are reflected in the agenda of ITU’s upcoming World Radiocommunication Conference (WRC-27), where several agenda items involve radio astronomy. Agenda Item 1.16 addresses protection from aggregate interference from large satellite constellations.

Additional scientific-service items, including 1.15, 1.17, 1.18 and 1.19, involve contributions from ITU’s dedicated radio astronomy group (ITU-R Working Party 7D) alongside broader compatibility studies.

These efforts highlight ongoing ITU-R work to ensure the viability of passive services in an increasingly congested spectrum environment.

Looking ahead

RFI mitigation techniques continue to evolve, including machine learning-based detection, spatial filtering, and operator coordination, many of which are already deployed.

However, mitigation alone cannot solve the interference problem. Radio astronomy operates at the limits of detectability, and preserving its capabilities requires both technological innovation and regulatory protection.

Ensuring access to usable radio spectrum remains essential for continued exploration of the Universe.

About the ITU World Radiocommunication Conference 2027

Header image credit: Keith Miller (Caltech/IPAC – SELab), Annie C. Mejia Roa (Caltech)