Geodetic radio astronomy: Key for monitoring Earth and space

By Vincenza Tornatore, Senior Scientist, Politecnico di Milano, Italy, and Hayo Hase, German Head of the Argentinean-German Geodetic Observatory, La Plata, Federal Agency for Cartography and Geodesy, Germany

“Radio astronomy” is defined in the ITU Radio Regulations under provision 1.13 as astronomy based on the reception of radio waves of cosmic origin, with 1.58 stipulating a dedicated “radio astronomy service” for such uses.

The Radio Regulations treaty, administered by the International Telecommunication Union (ITU), governs the use of radio-frequency spectrum and satellite orbit resources globally. Its provisions are reviewed and updated approximately every four years at ITU’s World Radiocommunication Conference (WRC), with agenda items addressing specific spectrum-management issues, such as frequency allocations for space, science and radio astronomy services.

The radio astronomical measurement technique known as very long baseline interferometry (VLBI), is not only used to map radio sources in the universe but also plays an indispensable role in the day-to-day monitoring of the Earth.

“Geodesy” is the science of accurately measuring and monitoring the Earth’s geometric shape, orientation in space, and gravity field of the Earth. “Geodetic VLBI” refers to the use of the VLBI to reach the first two goals related to geometry and orientation of the Earth, mandatory for reference frames.

Radio astronomers study objects in space that emit radiation at the radio wavelengths, while geodesists use cosmic radio sources named quasars (most distant and brightest galactic active nuclei) as fixed points in the sky with respect to measuring and monitoring the moving Earth.

What is geodetic VLBI?

Geodetic VLBI determines the Earth orientation parameters (EOP) that link the inertial celestial reference frame (ICRF) materialized by quasars with the non-inertial terrestrial reference frame (ITRF) materialized by a network of points on the Earth. Some nodes of this network are the VLBI radio telescopes.

The Earth’s rotation is constantly changing due to mass shifts within the Earth, ocean and atmospheric tides and currents, the melting of ice masses, as well as floods and droughts. These affect not only the length of the day, but also the position of the Earth’s rotational axis. Universal Time 1 (UT1) ꟷ the absolute rotational phase of Earth ꟷ is measured most accurately by VLBI.

Daily variations in the Earth’s orientation are subtle, but if not properly tracked they can accumulate and affect the accuracy of satellite and space-based systems that depend on precise timing. Earth orientation parameters derived from geodetic VLBI ensure consistency, quality, and interoperability across global navigation satellite systems (GNSS) and other satellite-based observations.

Beyond navigation and timing, Earth observation parameters play a crucial role in observing and understanding global phenomena such as climate change and natural hazards and are fundamental for reliable navigation in deep-space missions to the Moon, Mars, and further.

Lack of protection in the Radio Regulations

Geodetic VLBI detects only faint cosmic radiation using highly sensitive receivers on radio telescopes. Whilst active radiocommunication services initially require an allocation, the passive radio astronomy service can operate outside allocated bands (RR 11.12).

For geodetic VLBI, this has been possible for over four decades. However, the exploration of space by private investors and the simultaneous expansion of wireless telecommunications have altered the electromagnetic environment to such an extent that geodetic radio telescope stations are increasingly affected by unwanted radiation.

Who coordinates the global VGOS radio telescope network?

The International VLBI Service (IVS) coordinates the VLBI Global Observing System (VGOS), a network of currently around 25 worldwide distributed radio telescopes, which observe as one global sensor dozens of quasars during a 24-hour VLBI session. It uses receiving systems in the range of 2–14 gigahertz (GHz).

This frequency range allows unattenuated access to the faint cosmic rays due to a transparent atmosphere. Geodetic VLBI is a time measurement whose resolution is proportional to the observed bandwidth.

Unfortunately, the lower range of 2–3 GHz is so heavily utilized by active radio services that, in practice, observations can only be made between 3 and 14 GHz. In this range, observations are carried out using frequency bandwidth synthesis across just 32 channels, each with a bandwidth of 32 megahertz (MHz), meaning that only about 10 per cent of the covered bandwidth of 11 GHz is effectively observed.

Technical details can be found in Report ITU-R RA.2507.

No single national spectrum administration can achieve global protection for geodetic VLBI on its own. Nevertheless, new allocations for mobile communications and satellite constellations pose a major challenge to the geodetic data supply chain. In its policy brief, the UN-Global Geodetic Center of Excellence (GGCE) has called for efforts to safeguard VLBI, as have the International Astronomical Union and the International Union of Geodesy and Geophysics in their resolutions.

Consequently, several countries have proposed that the radio astronomy service should receive further allocations to protect geodetic VLBI and, in consequence, space missions. Adopting such safeguards will require an agenda item for ITU’s next radiocommunication conference cycle, culminating in 2031.

The upcoming WRC-27 next year in Shanghai, China, could put the topic on the agenda for WRC-31.

What is very long baseline interferometry?

• Radio telescopes on different continents observe simultaneously the same cosmic radio source (quasar) in the distance of 10 billion light years.
• A plane wavefront of cosmic radiation arrives at telescopes at different times causing delays.
• While observing the Earth rotates, causing delay rates.
• The network of radio telescopes is one global sensor for monitoring of Earth rotation length of day, rotational axis – and baseline lengths.

Who coordinates the VLBI Global Observing System (VGOS) radio telescope network?

• VGOS stations are coordinated by the International VLBI Service for Geodesy and Astrometry (IVS).
• National infrastructure is committed to the IVS.
• IVS is a non-profit organization.
• The VGOS network is growing and global coverage is important.
• VGOS observations require the same frequency sequence globally.
• Protection is needed only for observatory sites and observation channels.

Header image credit: Bundesamt für Kartographie und Geodäsie (BKG) ꟷ The Twin-Telescope Wettzell, Germany, a network station for the VLBI Global Observing System (VGOS)