PROTECTING THE FUTURE OF RADIO ASTRONOMY

Transcription

1 PROTECTING THE FUTURE OF RADIO ASTRONOMY An overview of radio spectrum management issues Summary of Issue Prepared by the Megascience Forum s Working Group on Radio Astronomy November 1998 Telecommunications companies and radio astronomers have to share the finite resource of the radio spectrum. Until recently astronomers have been able to carry out their measurements by locating observatories at remote sites, and by observing within narrow, specially assigned portions of the spectrum. Now, with the advent of powerful, omni-present fleets of low-orbiting communications satellites, the future of the entire field of radio astronomy is in jeopardy. During the coming years, a coordinated effort by scientists, industry and governments will be required to preserve and advance the ability to probe the farthest reaches of the Universe through radio astronomy, while continuing to develop the use of the radio spectrum for commercial and other purposes. Background To study the Universe beyond the immediate vicinity of Earth, astronomers capture and analyze the electromagnetic signals emitted by distant objects, such as stars and galaxies. Giant telescopes gather and focus this radiation, revealing information about the evolution and the physics of the Universe. Each different type of signal is characterized by a specific range of wavelengths, the most familiar being visible light. At wavelengths too long for the human eye to detect (so-called radio waves the same ones used for TV transmissions, ship-to-shore communications, airplane radar, mobile telephones, etc.) a vast range of phenomena, including the early history of the Universe, can be observed. The special problems that relate to the future of radio astronomy are the subject of this brief overview document. Astronomy has been revolutionized in recent years by the realization that some of the electromagnetic signals reaching the Earth today have been traveling since shortly after the Big Bang, at the origin of the Universe. These signals contain information about the formation of the first galaxies and stars more than ten billion years ago. By observing or listening to these faint radio signals, scientists can essentially look back in time to just after the Big Bang. Radio astronomers believe that gains in reception sensitivity of their instruments, achievable in the next generation of observational facilities, will permit observation of nearly the whole history of the Universe, all the way back to just after the Big Bang. This new window on the history of the Cosmos will reveal the forces that have driven its evolution. Improved sensitivities on the order of a hundred times those of existing telescopes will be required, but technology has advanced to the point where these levels of sensitivity are within reach. Although electromagnetic signals from the origins of the Universe are reaching Earth, the ability to listen to these signals by improved instrumentation is jeopardized by the growing use of the radio spectrum for telecommunications. Because signals from the early Universe are often hundreds of millions of times weaker than typical communications signals, use of the radio spectrum for telecommunications is increasingly causing difficulties in observing the faintest, most distant possible objects, which are among the most fascinating and important, with the most information about Annex 1 Page 1 of 5

2 origins and history. International Coordination of Spectrum Use Radio astronomers have been active for at least 30 years in the international regulatory process of the International Telecommunications Union (ITU). Today, approximately 2% of the radio spectrum has been allocated for use by radio astronomy. This total allocation is divided up into a series of windows frequency bands where radio astronomers should be able to carry out their observations without interference. However, radio emissions from communication systems operating in other bands often spill over into bands allocated to radio astronomy. Though these emissions can be controlled, it is always at some cost to the commercial operator. Consequently, protection of radio astronomy has, for the most part, been limited to encouraging government regulatory administrations to take all practicable steps to ensure the ability to make sensitive radio astronomical observations. In addition, specific standards for permitted levels of one form of unwanted radio emissions known as spurious emissions have also been established internationally. While radio astronomers participate actively in the relevant international regulatory discussions, regulatory structures alone will almost certainly not provide adequate solutions to the interference phenomenon in the future. Radio astronomers believe that reserving frequency windows and developing emission standards will not adequately protect the future of the field, for the following reasons: 1. Soon no place on Earth will be free from strong, man-made signals from the sky. The planned constellations of global personal telecommunications satellites will ensure that every location on Earth will be exposed to strong, rapidly moving signals which emanate from the same general direction as the objects astronomers study. These signals will be some ten million billion (10 16 ) times stronger than the faint signals from the early Universe that astronomers need to record. An appreciation of the magnitude of the interference problem can be obtained by considering a simple hand-held mobile telephone unit. If this unit were located as far away as the Moon, its signal would appear on Earth as one of the brightest radio sources in the sky at its particular transmission frequency, compared with naturally emitting astronomical objects. The signals from a commercial satellite can be a hundred million times stronger, making observations at those and adjacent frequencies effectively impossible. 2. It will be necessary to observe at frequencies across the entire radio spectrum. As explained above, radio astronomers currently observe in several specially allocated, narrow bands of the spectrum, chosen decades ago when only the nearby Universe (in the vicinity of our own Milky Way Galaxy) could be studied, due to the limited sensitivity of radio telescopes at that time. However, radio waves from the more distant and exotic objects arrive at Earth at significantly different frequencies, due to a complex phenomenon ( the cosmological redshift ) linked to the overall expansion of the Universe. To take full advantage of the technological capabilities of the next generation of radio telescopes, and to study some of the most fascinating questions of modern science, astronomers must find ways of accessing the entire radio spectrum, even though only a small fraction of it is officially reserved for them. In this regard, it is important to note that radio astronomers generally only receive that is, their operation is passive and they will disturb no one while listening, regardless of where in the radio spectrum that occurs. Annex 1 Page 2 of 5

3 Scientists Proposed Solutions Radio astronomers believe that practical solutions to these new problems can be developed and, given adequate funding and a measure of cooperation between commercial and scientific users of the radio spectrum, can be realized in practice. In addition, international cooperation will be needed, since the challenges are world-wide. Specifically, solution to the spectrum management problems will involve some combination of the following measures: 1. Development of innovative technologies: to separate out and filter man-made signals from those of astronomical interest. Improved filtering technologies, adaptive nulling using large phased array antennas, and on-line interference subtraction/circumcision that make use of signal modulation schemes are among the possible solutions. These technologies could also be of interest to telecom companies, who already experience interference from one another, as well as from government, aviation, defense and other spectrum users. 2. Forward-looking regulatory approaches: for sharing the spectrum among all users. This could be possible because radio astronomy does not need to own the spectrum, only to listen in it during agreed periods of time. This approach could involve sharing in frequency, in time and/or in geographical location. As radio astronomy does not involve transmitting signals, it will not interfere with other receiving systems, at any frequency. 3. International radio quiet zones: designation of one or more places on Earth (with dimensions of several thousand kilometers) where radio astronomy observations could be made without interference from man-made signals. In such a zone, no transmissions in certain broad frequency ranges would be permitted (including from space) during agreed upon periods of time. These geographical regions could be set up for a finite period and might be selected because they are de facto radio quiet already. Instruments and Facilities In addition to addressing the interference problem discussed above, radio astronomers are striving to expand their observational capability through technological advances in instrumentation. Current trends in instrumental development include: 1. Increasing the range of observable phenomenon: making sensitive observations at ever shorter wavelengths, as the technology is developed to make this possible (as described in more detail below). 2. Increasing sensitivity over today s radio telescopes by a factor of nearly 100: by enlarging signal band-widths and by increasing the total collecting area of telescopes, that is, by increasing the physical size of telescopes. Annex 1 Page 3 of 5

4 3. Increasing angular resolution: (the sharpness of images) by networking telescopes across continents with a technique called Very Long Baseline Interferometry (VLBI). This involves splitting the total collecting area up into smaller units that are spread over large distances, and then combining the signals coherently from the separate detector units. The larger the total distances, the higher the angular resolution and the sharper the images. To achieve sharpness comparable to the Hubble Space Telescope, distances from ten kilometers up to a thousand kilometers are required depending on the wavelength of the radio waves. Even higher resolutions are attainable by temporarily incorporating new telescopes in a VLBI network and by basing some of the additional radio telescopes on orbiting satellites. Each part of the electromagnetic spectrum gives clues to different aspects of the origins, history and structure of the Universe, and each type of electromagnetic signal is best detected using a specific technology optimized for the relevant range of wavelengths. Accordingly, astronomers are engaged in internationally coordinated efforts to develop new ground-based instruments for sensing in several broad regions of the electromagnetic spectrum: 1. Optical and near-infrared range of wavelengths: a new generation of very large, ground-based telescopes with 6-m to 10-m primary mirrors is already under construction, which will yield at least a dozen instruments in the world by the turn of the century. These instruments will significantly extend penetrating power at optical wavelengths, and will complement the Hubble Space Telescope by providing spectroscopic capabilities not possible with that instrument. Most of these new optical telescopes are being developed within the framework of international bi- or tri-lateral agreements. The next major development at these wavelengths will likely be the Next Generation Space Telescope (NGST), for which discussions have begun between NASA and ESA with a view to a launch in 2007 or later. 2. Sub-mm and mm range of wavelengths: new radio astronomy instruments are planned for satellite and ground-based observatories. Satellite instruments avoid the distortions and limited transparency of the Earth s atmosphere. However, ground-based facilities are still needed to achieve the high angular resolutions required to probe the physically important dimensions of the objects being observed. Three new, very large, ground-based mm-wave telescope projects (being developed in Europe, Japan and the USA) are currently in various stages of planning, financing and implementation. It is hoped that the full realization of these projects will follow completion of the large optical facilities, in the first decade of the coming century. The preferred site for all three of these radio facilities is the high Andean desert in Chile. Substantive discussions are now taking place on the potential benefits of international cooperation in these efforts. 3. Cm-to-meter range of wavelengths: early-stage discussions have begun in many countries concerning the required specifications for a new instrument optimized for these wavelengths. Although recent decades have seen incremental increases in the sensitivity of instruments, the scientific imperative is now for sensitivity up to a hundred times the current capability. A major project under discussion is the Square Kilometer Radio Telescope. Here, fundamental considerations relating to the signals expected from objects in the early Universe have led to specifying a desired total collecting area of about a million square meters, i.e. one square kilometer. This total area should be sub-divided into several tens of individual sub-units, spread over an area hundreds of kilometers in total extent, with the individual signals combined interferometrically in real time. Progress toward solutions for the man-made interference problem is especially necessary for this instrument. Eight institutes in six countries have recently agreed to cooperate in the development of technologies for a radio mega-telescope for this wavelength region. It is proposed to be the premiere instrument in the field, becoming a vast source of discoveries about the Universe soon after its birth and as it exists today. The goal of the participating institutes is to reach consensus on a concrete development plan in the year 2000, with the expectation that substantial sums for construction purposes could become available in the second half of the coming decade. While Annex 1 Page 4 of 5

5 planned as a major discovery instrument in and of itself, significant added value will accrue if the Square Kilometer Radio Telescope can come into operation in the same time frame as the Next Generation Space Telescope (the planned follow-on to the spectacularly productive Hubble Space Telescope), i.e., approximately ten years from now. This is then also the time frame in which progress in ensuring access to the radio spectrum for scientific use should be achieved. Conclusion In planning a balanced, rational, and coordinated advance of radio astronomy, it is necessary and appropriate for government agencies to be involved in the discussions that are taking place in the scientific community, both as concerns planning of future facilities and as concerns protecting access to the radio spectrum. Governments should promote structured joint activities between science funding agencies, the scientific community, the telecommunications industries, and governmental regulatory bodies. These activities should be directed towards finding effective technological and regulatory solutions that will preserve access by scientists to the radio spectrum, while allowing continued expansion of world-wide telecommunications. Annex 1 Page 5 of 5