Listening to the universe: the secrets of radio astronomy

Gilles Brégant, the Managing Director, and Jérôme André, Head of the ANFR Spectrum Engineering Department, gave a presentation to CRAF members on the Agency's missions and its actions in support of the development and protection of radio astronomy: an ideal opportunity to take stock of the importance of radio astronomy!

Astronomy is the science that studies celestial objects such as stars, planets, galaxies and everything else in the Universe. For thousands of years, people have observed the sky to try to understand our place in the cosmos. Traditionally, those observations were made with the naked eye or using optical telescopes that capture visible light.

The universe is "full" of radio waves

So when we think of astronomy, we often imagine optical telescopes pointed at a starry sky. But there is another way to observe the universe: radio astronomy. That discipline captures the radio waves emitted by certain celestial objects to explore the mysteries of the cosmos.

 Why observe radio waves?

Light is an electromagnetic wave, like radio waves, but radio waves make it possible to see things that visible light does not. Some regions of the universe are obscured by dust clouds, or are too far away to be observed by conventional telescopes. Thanks to radio astronomy, scientists can detect invisible phenomena: pulsars, quasars, distant galaxies or the hydrogen clouds that form stars.

Frequencies, the key to listening to the cosmos

The radio waves used in radio astronomy cover a range of frequencies from a few megahertz (MHz) to several hundred gigahertz (GHz).

Radio astronomy uses two main methods to observe the Universe:

Continuum observations

These observations of radio emissions from astronomical objects in a wide frequency band mainly focus on:

• objects that emit a lot of radio waves, such as the remnants of stellar explosions (supernovas), active galaxies or very dense neutron stars (pulsars).
•  the study of the hot gases or dust that also emit radio waves.

Spectral line observations

Spectral lines are specific "imprints" left in radio waves by certain atoms or molecules. This type of observation makes it possible to:

•  know which molecules or elements are present in space such as hydrogen or carbon monoxide for example. The line frequencies are specific to the molecule or the observed element. For example, the 1400-1427 MHz frequency band corresponds to the interstellar neutral hydrogen (HI) line.
•  to measure the temperature and density of gas clouds.

Regulations to protect radio astronomy

The main difficulty with radio astronomy is the very weak level of the signals captured on Earth or in near space once they have crossed truly astronomical distances: to be able to analyse them, they have to remain detectable by radio telescopes - which means avoiding all disruptive emissions in the vicinity of those instruments (both geographical and spectral).

The ITU Radio Regulations (RR) therefore allocate frequency bands to the radio astronomy service which are thus protected from active services.

In Study Group 7, which is in charge of the ITU's scientific services, Working Party 7D (WP 7D) is in charge of radio astronomy service studies.  Those studies cover radio astronomy sensors, including those onboard satellites, and very long baseline interferometry (VLBI).

Three items on the WRC-27 agenda directly concern radio astronomy:

• 1.15 on lunar communications, considering the protection the RR provides for radio astronomy in all bands on the dark side of the Moon, which seems to be a very favourable location for the future installation of radio astronomy systems;
• 1.16, on the protection of radio astronomy in several bands and the recognition of two radio silence zones in Chile and South Africa;
• and 1.18, whose objective is to include the bands above 76 GHz into Resolution 739, which aims to protect radio astronomy from active space systems in adjacent bands. 

For the subjects covering radio astronomy, WP 7D also prepares draft ITU-R Recommendations for adoption by Study Group 7. Those are a set of international technical standards developed by the ITU Radiocommunication Sector, and draft ITU-R Reports, which are technical, operational or procedural documents.

The ITU-R Recommendations and ITU-R Reports covering radio astronomy are part of the RA series.

A radio astronomy manual published in 2013 was drawn up by WP 7D and covers the radio astronomy aspects relevant to frequency coordination, such as management of the use of the radio frequency spectrum to minimise interference between radio services.

Some examples of French radio astronomy stations

• The Nançay decimetric radio telescope

The radio telescope is composed of a directional flat mirror (200 m x 40 m), which reflects the waves onto a spherical mirror (300 m x 35 m), which in turn reflects them onto the focal carriage at the centre where they are collected by cooled receivers observing in the 1 to 3.5 GHz frequency range.

Below is a LOFAR image of a radio galaxy superimposed on a conventional optical image of the night sky:

SKA-Low is sensitive to low-frequency radio signals between 50 and 350 MHz. Those are the weakest signals which have travelled for billions of years and have been 'stretched' to longer wavelengths by the expansion of the Universe. In radio astronomy, the sensitivity of a telescope depends on the collecting surface available to pick up signals from space. SKA-Low has a collection surface area of 400,000 m2, which means that even the weakest signals can be detected, combined and enhanced in a way that was never previously possible, thanks to a technique known as aperture synthesis. High resolution means sharper images that reveal finer detail.