R. J. Cohen
For centuries humankind studied the Universe using visible light, but in the twentieth century the technology became available to study essentially the whole electromagnetic spectrum. Radio astronomy was the first of the new astronomies. It opened up a dramatic new view of the Universe, revealing many phenomena that are invisible to optical telescopes, such as the birth of stars, the existence of massive black-holes, and the big-bang origin of the Universe. The technical progress has been dramatic and relentless. Radio astronomy now achieves the sharpest images of any branch of science. Sensitivity has improved more than a million times in the last forty years. And unlike the other new astronomies, all this has been done cheaply and cost-effectively from the ground.
At the start of the twenty-first century radio astronomy is now poised to take the next major steps forward in sensitivity and angular resolution. The Atacama Large Millimetre Array (ALMA), under construction in Northern Chile, will be ten times more powerful than any previous millimetre-wave facility, with unique imaging capabilities. ALMA is a truly global project involving North and South America, Europe, Japan and perhaps others. Atmospheric absorption by water, oxygen and other molecules is a major issue at millimetre-waves, so high and dry sites must be found for millimetre-wave observatories. The site for ALMA is a plateau in the Atacama Desert, at an altitude of 5,000 metres. It has the best millimetre-wave transparency of any site we know about on the planet. Special measures are needed to ensure the long-term protection of this site from radio interference.
At longer wavelengths the Square Kilometre Array (SKA), still in its design phase, will provide a hundred-fold increase in collecting area and sensitivity. A number of possible sites around the world are being tested and evaluated, and the international SKA consortium hopes to make a final selection in 2006. ALMA and SKA are global projects, beyond the reach of any one nation, bringing together resources and expertise from radio astronomers around the world. However there are strong concerns that the issue of radio interference should not limit the performance of these new instruments.
Radio astronomy is vulnerable to interference because the signals it studies are incredibly faint, and dwarfed by the signals used for telecommunications. A mobile phone placed on the Moon would be one of the brightest objects in the radio sky seen from Earth. Radio astronomy needs quiet frequency bands, just as optical astronomy needs dark skies free from light pollution. Use of the radio spectrum is regulated by the International Telecommunication Union (ITU). The ITU allocates frequency bands to particular applications or services at World Radio Conferences (WRCs), where the international Radio Regulations are agreed. The radio astronomy service entered the Radio Regulations in 1959, when the first frequency bands were reserved for exclusively passive use. Among the first bands to be protected in this way was a frequency band covering the 1420-MHz spectral line of neutral atomic hydrogen. Today the radio astronomy service is allocated 2% of the spectrum below 50 GHz, but much more in the millimetre-wave bands where commercial pressures are less. Protection criteria for radio astronomy have been established, but these are contained only in Recommendations, not in the Radio Regulations. They are advisory, not mandatory.
The Radio Regulations provide worldwide access to narrow slices of spectrum, thus encouraging the growth of many radio observatories in many countries, and facilitating the global cooperation of radio observatories in very-long-baseline interferometry (VLBI) and other experiments. However the new mega-facilities such as ALMA and SKA will require access to essentially all of the radio spectrum for some of the time, in order to achieve their full potential. The expansion of the Universe means that signals from distant objects are red-shifted to successively lower frequencies as we look further out in space. Spectral lines such as the 1420-MHz hydrogen line are simply red-shifted out of their protected bands into frequency bands used for radar, mobile phones, television, radio, etc. In order to study the most distant reaches of the Universe, other approaches are needed.
The Recommendations on radio astronomy give guidance on how radio astronomy can be protected through the establishment of radio-quiet zones. Radio astronomers are encouraged to choose remote sites as free as possible from interference. Some frequency bands which the radio astronomy service shares with terrestrial services can be protected on a site-by-site basis through coordination, which means that transmitters are kept outside a calculated distance from the observatory. This is how my own observatory, Jodrell Bank in the UK, is able to observe in a frequency band shared with television broadcasts. There are no UK broadcasts in this band (TV channel 38), and with the assistance of our European neighbours the signals from European television are kept to a low enough level for our work to proceed. At millimetre waves it is actually feasible to protect all frequencies in this way, since the sites are few, the propagation is essentially by line-of-sight, and atmospheric attenuation is significant.
There are also two Recommendations dealing with radio-quiet zones in space: the shielded zone of the Moon, and the L2 Earth-Sun Lagrangian point. The L2 point is naturally radio-quiet because it lies 1.5 million kilometres from Earth, in the opposite direction to the Sun. It is also a stable point in terms of the gravitational potential of the Earth-Sun system, which means that quasi-stable orbits of up to 250,000 km are possible with relatively little station-keeping. From the L2 point the Earth, Sun, Moon and most artificial radio transmitters lie in a small cone on the sky, so sensitive observations can be made by looking away from these sources. The Microwave Anisotropy Probe MAP is currently mapping the cosmic microwave background radiation from the L2 point and has just released some of its first results, showing that the Universe is 13.7 thousand million years old and will continue to expand forever.
However, to return to Earth, there are two remaining problems for ground-base radio observatories. The first is electrical equiment not necessarily classified as a radio transmitter, and therefore outside the scope of the ITU. To achieve protection from radiations of this kind it is necessary for the observatory to have some other form of legislative protection, so that new factories or mines are not built at the perimeter. This is a local problem to be solved with local measures. The ALMA site has been protected in this way since 1998 under Chilean national law, which established the Cerro-Chacón Science Preserve.
Transmitters on satellites are a global problem for radio astronomy. They can block out large frequency bands and they can be seen from anywhere on Earth. There is a long history of interference to radio astronomy from satellites. Unwanted emissions from satellites often spread far beyond the frequency range actually used for the satellite service, causing radio pollution of the surrounding frequency bands, including passive bands. Until recently there were no limits whatsoever on these unwanted emissions. The first limits were agreed at WRC-2000 in Istanbul, and came into force this year. They are lowest common denominator limits acceptable to all satellite operators, and they do not protect radio astronomy. The satellite operators were able to accept tighter limits (ten thousand times tighter!) in order to secure allocations for new satellite systems, at WRC-2000, but they would not accept tighter general limits to apply to all satellites. The question of setting tighter limits for particular satellites in particular frequency bands is still under discussion.
A new approach is needed to protect the mega-facilities such as ALMA and SKA from satellites. Recognizing this, the Organization for Economic Cooperation and Development (OECD) set up a high-level Task Force on Radio Astronomy which brought together senior representatives of the telecommunications industry, the scientific community and international regulators, to try to map out a strategy that would ensure that radio astronomy and the telecommunications industry can each continue to thrive and develop. The report of the Task Group, which is shortly to be published, is expected to recommend among other things the creation of one or more international radio-quiet zones. These would be remote areas of the Earth, above which transmissions from satellites would be coordinated, that is, restricted in time, frequency and geographical coverage, in such a way that access to the radio spectrum is provided for radio astronomy without compromising the satellite services. ITU already has precedents for protecting special sites from satellite transmissions, but some technical and regulatory aspects of this proposal require careful study. It is also imperative that the existing levels of protection via passive bands be retained for the protection of existing radio observatories, which will continue to have an important place in the development of radio astronomy.
The Union Radio-Scientifique International (URSI) has called for the issue of international radio-quiet zones to be discussed at the forthcoming WRC-2003, to be held in Geneva in June-July. URSI calls for the draft agenda of the following WRC (WRC-2007) to include an item: "to consider the possibility of creating one or more internationally recognized radio quiet reserves, and take appropriate action" and for the ITU to undertake the necessary preparatory studies. Steps are already being taken within Europe to draft a suitable proposal for WRC-03 to initiate this process.
R. J. Cohen,
University of Manchester,
Jodrell Bank Observatory,
Macclesfield,
Cheshire SK11 9DL,
U.K.