R. J. Cohen
Most people have some idea what an astronomer is: a person who studies the stars with an (optical) telescope. But in fact there are two windows for ground-based astronomy, the optical window and the radio window. I am a radio astronomer. I use large radio telescopes to study the Universe in a different way. Radio telescopes show you many things you can't see with an optical telescope, from the birth of stars out to the Big Bang, when our Universe began. And they also reveal unfamiliar aspects of objects we already knew. Quasars, for example, are the most powerful energy sources known to man, so far away that they appear as point sources to an optical telescope. Nowadays, using radio telescopes linked up on different continents, we can image the ejection of matter from the quasar core at almost the speed of light. The imaging technique, called Very Long Baseline Interferometry (VLBI), achieves the sharpest images of any branch of science, measured nowadays in microarcseconds (the angle that would be subtended by a small coin placed on the Moon).
Radio astronomy is a modern science that is growing very fast. In my scientific lifetime the sensitivity of our measurements has increased almost one million times. So too has the sharpness of our images. And you can do all this from the ground, in most weather.
Telecommunications are also growing fast. The radio spectrum is a finite resource and it is filling up. The cosmic signals are billions of times weaker than the telecommunications signals. The frequencies and power levels are fixed by nature. The cosmos cannot compete with the local din. New radio telescopes are built in remote sites, to reduce the risk of interference. Even so, we are now worried that our new generation of instruments will not be able to achieve the next advance in sensitivity because of man-made interference. Today it is radio astronomers who see the effects of radio pollution, but tomorrow others will suffer.
Radio astronomy is particularly vulnerable to radio interference. It needs quiet frequency bands just as optical astronomy needs dark skies. The global body charged with regulating the use of radio frequencies is the International Telecommunciation Union (ITU). ITU draws up its rules, the Radio Regulations, at World Radio Conferences (WRCs), held every three years. Radio astronomy first entered the Radio Regulations in 1959, when the first frequency bands were set aside for passive use. Today some 2% of the radio spectrum below 50 GHz is allocated for passive use. The ITU has nine recommendations on the protection of radio astronomy, including interference thresholds. Unfortunately this has failed to protect radio astronomy from satellites.
Transmissions from satellites are a particularly difficult challenge. Satellites usually transmit down to Earth, while we are trying to look up at sources which are many billions of times weaker. The satellite operators usually want global coverage, so all terrestrial sites are affected, however remote they are. Radio transmissions from satellites can spread from their allocated frequency bands into nearby passive bands: a form of radio pollution. This can happen due to the type of modulation used in the transmitter, due to imperfections in the hardware, or due to malfunctions. Once interference is identified from an operating satellite the timescale for change is very long. Things cannot be easily fixed out in space. With over one hundred satellites launched each year the problems are growing. And to make matters worse, there is no pre-launch verification that new satellites will be environmentally safe.
The Russian global navigation system GLONASS illustrates several of the problems. The first GLONASS satellite was launched in 1982, for military navigation. As the constellation of satellites filled up, radio astronomers around the world noticed severe interference from the GLONASS transmissions near 1.6 GHz. Emissions from GLONASS at this frequency are modulated in a way that causes them to spread over many times the bandwidth needed by the navigation receiver. In addition, spikes appear periodically throughout the emission spectrum, due to imperfections in the hardware. These are what the ITU terms ``unwanted emissions''. The Russians were initially unaware of the global disruption caused to radio astronomy by their satellites. Once the relevant authorities had been contacted, discussions began, and after a joint GLONASS-RadioAstonomy experiment the Russians were convinced and sympathetic. A plan to ``clean up'' the GLONASS transmissions was agreed in 1993 and is still in the process of being implemented. The Russians have stuck by the plan, which is projected to be complete in 2006.
There are many other examples. The US TEX satellite was retired from operations ten years ago, but due to malfunction it started to transmit worldwide into a passive band at 328 MHz (close to the frequency of a deuterium spectral line). There is no off switch for the transmitter. The cure in this case involves ongoing maintenance from ground control.
The ASTRA television broadcasts at 10.7 GHz spread unwanted emissions across an adjacent passive frequency band, making it unusable for high-sensitivity radio astronomy. The German radio astronomers have been allowed by their national administration to retune their sensitive receivers to lower frequencies, moving into someone else’s frequency band.
The Iridium system brought new dimensions to the problem. The protection of radio astronomy was clearly stated as a goal when the satellite allocation was being discussed in 1992. By 1995 however, calculations showed that at full capacity the satellite downlink would exceed the ITU recommended interference levels substantially, so corporate lawyers made their entrance and attempted to talk the problem away through their interpretation of the Radio Regulations. In the event there were too few users to generate the high levels of interference we feared.
There has been growing awareness within the ITU of the problems caused by unwanted emissions, but action has been slow in forthcoming. For the whole twentieth century there were no limits at all on the unwanted emissions from spacecraft. Not only radio astronomers, but other users of passive bands such as Earth-observation scientists and meteorologists, and even safety-of-life services such as the COSPAS-SARSAT emergency beacon at 406 MHz are affected by unwanted emissions. The ITU first called for action on the issues in 1979, and in 1995 set up the first of three special Task Groups to study the problem. As a result, WRC-2000 placed the first limit to spurious emissions from satellites. The limits are not sufficient to guarantee protection of passive services from all satellite systems, but they are far better than no limits at all. The limits will apply to new satellites from the year 2003, and to all satellites from 2012. They are relaxed limits acceptable to all operators, and do not reflect what can be achieved with modern technology. Task Group 1/7 will study how the limits might be improved to protect specific passive bands from specific satellite systems. However there is no guarantee that the ITU limits will be met in practice, since pre-launch tests of unwanted emissions by an independent regulator simply do not happen. The process would cost time and money, and there are no incentives for any operator or administration to introduce them.
The ITU in fact has limited scope in its capacity to solve problems like this. There is no mechanism by which the ITU can enforce its regulations and recommendations, especially in space. Environmentally safe engineering may add to costs and delay projects. Individual operators have no incentive to use the cleanest technology. The power of multinational corporations is growing at a time when governments around the world are seeking to reduce their role and responsibilities. And with the demise and rebirth of the company operating Iridium, the question of responsibility looms large. Who is responsible for the problems caused by commercial satellites once the company has gone bankrupt?
The problems can only be solved at the highest global level. Access to the radio spectrum is vital resource to be preserved for future generations, rich and poor alike. The ITU can recommend technical standards, but has no powers or sanctions to enforce them. Those can only come from the governments of the world, acting in concert to ensure that there is equal access to the radio spectrum for all, a level playing field for big and small players. The governments of the world can prevent the workings of the ITU being dominated by the strong commercial interests of multinational companies. Governments can insist that the best technological standards are set. For technology really does have most of the answers, if we are all prepared to use it. No one knows this better than radio astronomers, who continue to benefit from advances in the technology of receivers, antennas, computers, etc. We must continue to develop techniques to reduce the effects of interference one our measurements, and we in turn look to satellite designers and operators to develop cleaner transmitters and systems to reduce the problem at source. If the international rules are made wisely, applied evenly and policed effectively, everyone will benefit.
R. J. Cohen,
University of Manchester,
Jodrell Bank Observatory,
Macclesfield,
Cheshire SK11 9DL,
U.K.