Radio astronomy: Rethinking spectrum for passive services

By Busang Sethole, Spectrum and Telecom Engineer, South African Radio Astronomy Observatory, and Harvey Liszt, Chair, Scientific Committee on Frequency Allocations for Radio Astronomy and Space Science (IUCAF), United States

The radio astronomy service (RAS) is a distinct element of the international radiocommunication framework. It is, by definition, passive: RAS receives naturally occurring radio waves from cosmic sources and does not transmit.

Since its formal recognition in the ITU Radio Regulations, administrations and the scientific community have relied on allocations, regulatory provisions, technical recommendations and, very importantly, on radio quite zones (RQZ) established under national arrangements to protect these extremely weak signals. The recent rapid growth of large non‑geostationary satellite constellations exposes fundamental tensions between the regulatory framework and the practical reality of passive observation.

Sensitivity without flexibility

Radio astronomy is constrained by physics in ways that active services are not. Natural spectral lines and other cosmic radiation occur at specific frequencies fixed by the laws of nature and receivers are optimized to detect signals many orders of magnitude below operational noise levels.

Observational sensitivity follows strict physical rules and is heavily dependent on both the total instantaneous bandwidth and the spectral resolution. Confining RAS to a narrow set of bands or restricting available bandwidth reduces scientific return.

These are key concerns for the International Telecommunication Union (ITU) and its Radiocommunication Sector (ITU-R) in the run-up to the next World Radiocommunication Conference, WRC-27.

ITU‑R Recommendation RA.769 establishes interference thresholds for radio astronomy that are far lower than levels considered acceptable for active services. Signals that are negligible for conventional communications can corrupt or mask astronomical data.

Regulatory symmetry vs. physical asymmetry

Provisions of the Radio Regulations illustrate a key regulatory tension:

• Although the RAS is not a radiocommunication service, provision No. 4.6 says that it shall be treated as one for the purpose of interference resolution procedures.
• No. 4.4 allows administrations to notify (register) frequency assignments outside allocated bands, on condition that harmful interference is not caused to stations operating in accordance with the Radio Regulations (easy for a passive service) and protection is not requested from them (suffer in silence).
• 11.12 allows notification of any frequency for the purpose of reception at a radio telescope.

Together these provisions create procedural symmetry, each service has comparable formal rights and obligations in interference discussions. In practice, however, symmetry in procedure does not yield equivalence in outcome.

Passive services cannot change observing frequencies, reduce their “transmission” power, or negotiate operational adjustments in the way transmitters can. Many valuable RAS observations occur in bands without primary allocations or under footnotes (e.g., No. 5.149 or 5.465), where RAS protection can be weak or contingent. When administrations authorize out‑of‑allocation transmitter uses under No. 4.4, RAS may lack practical recourse despite being recognized under No. 4.6. This mismatch between regulatory procedure and observation becomes more acute as spectrum use intensifies.

The satellite perspective: Constrained spectrum

Satellite operators face strong technical and commercial drivers: delivering ubiquitous broadband, mobility, and direct‑to‑device services require multi‑band operation, high spectral efficiency, continuous coverage, and large constellations. These imperatives limit opportunities to avoid spectral overlap with passive services, particularly when radio astronomy seeks wide contiguous bandwidths or access outside traditional protected bands.

Space‑to‑Earth emissions are also difficult to geographically confine, signals from satellites impinge on ground stations and radio telescopes across national borders. This complicates bilateral or regional coordination and highlights the need for global, physics‑aware solutions.

Emerging interference pathways

Modern systems introduce new interference mechanisms, on scales that earlier frameworks did not fully anticipate:

• Direct illumination of main beams: airborne and space borne transmitters can enter the main lobe of highly directive radio astronomy antennas, producing strong, localized contamination that may saturate or even damage a receiver.
• Aggregate noise rise: large constellations and dense terrestrial deployments can raise the baseline noise floor through many low‑level contributions.
• Spurious, harmonic and out‑of‑band emissions: nonlinearities and imperfect filtering of transmitters can create interference in bands adjacent, nearby and well-removed from the intended transmission.
• Electrical noise from onboard electronics (Unintended Electromagnetic Radiation or UEMR) is not regulated at ITU-R but appears as harmful interference in allocated bands
• Increased probability of persistent or repeated events as system density grows.

Recent technical assessments suggest aggregate, harmonic and airborne contributions require greater attention in compatibility studies and regulatory discussions.

Limits of existing protections

The strongest international protection for RAS is the set of 20-some frequency bands exclusively allocated to passive services under No. 5.340, where “all emissions are prohibited.” But these only cover 1-2 % of the spectrum below 86. Other protections take the form of primary allocations (of which below 86 there again are few), more specific footnotes like No. 5.372 in certain bands, and the use of RA.769 thresholds in compatibility assessments. However:

• Many protections are only effective if implemented and enforced nationally.
• Compatibility analyses often permit a degree of threshold exceedance that is hard to verify and may require mitigation that is difficult to realize operationally.
• It is hard to assess the aggregate interference from even one sufficiently complex other system or set of simultaneously interfering systems, or to enforce existing limits on aggregation of interference from more than system (i.e. Recommendation ITU-R RA.1513).
• Enforcement is usually retrospective: RAS identifies interference after observations are degraded and then must prove causality.
• Automatic, real‑time mechanisms to prevent interference from transiting space borne systems are in their infancy.

These features place passive services at a practical disadvantage despite formal recognition.

The role and limits of radio quiet zones

Administrations realized as early as 1957 that radio astronomy would especially thrive in environments allowing the widest possible access to pristine spectrum. For this reason, they began to create radio quiet zones where domestic spectrum regulation was modified to limit transmitters working outside radio astronomy’s allocated frequency bands.

There are now more than fifteen of these, summarized in Report ITU-R RA.2259.

Radio quiet zones remain one of the most effective terrestrial mitigation tools. Yet they face limits and are imperfect:

• Satellite and airborne sources are generally excluded from RQZ rules by national regulators even when they could be.
• The global licensing and coordination of satellite systems means RQZ requirements need international recognition to be fully effective.
• Protecting wide bandwidths and transient observations imposes additional demands on any local exclusion regime.

Towards a more balanced, physics‑aware framework

The regulatory toolkit has historically balanced competing needs well, but evolving technologies show that regulatory parity alone is insufficient where services are physically asymmetric. Future international discussions should consider measures that reflect practical realities without unduly privileging any stakeholder. Practical options include:

• Explicit acknowledgment in regulatory texts of the asymmetry between passive and active services and the particular nature of passive band protection.
• Incorporation of aggregate‑interference and airborne‑source models into ITU‑R compatibility methodologies.
• Stronger emphasis on spurious, harmonic and out‑of‑band emissions in passive band protection, especially when dealing with air-borne emissions.
• Recognition of all the mechanisms of interference on air borne and space borne stations including the electrical noise that produces radiation at harmful levels in protected radio astronomy bands
• Reassessment of how RA.769 thresholds are applied in coordination and national implementation, potentially enhancing their status in compatibility studies and mitigation planning.
• Mechanisms to globally recognize and support Radio Quiet Zones, including coordination processes for satellite operators.
• Development of best‑practice operational measures (e.g., satellite quiet windows over key facilities, improved on‑board filtering, and boresight avoidance routines) combined with monitoring and faster dispute resolution.

Safeguarding future knowledge and exploration

Radio astronomy and satellite communications both serve vital, global public interests. One expands humanity’s scientific knowledge of the Universe, while the other connects the unconnected on Earth.

Ensuring their continued coexistence requires regulatory approaches that go beyond procedural symmetry to account for physical asymmetry — recognizing that passive services cannot “move” in frequency or increase signal power and therefore need protection strategies calibrated to their sensitivity and observing practices.

A combination of improved technical methods, stronger international recognition of radio quiet zones, tighter attention to aggregate and airborne interference, and operational mitigations can enable both communities to thrive without sacrificing core public benefits.

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