«Bonn, 16/02/2011 Report on Wind Farms and Radio Astronomy Axel Jessner Max-Planck-Institute for Radio Astronomy, Auf dem Hügel 69, D-53121 Bonn, ...»
Committee on Radio Astronomy Frequencies
Expert Committee for Radio Astronomical Frequency Management
Report on Wind Farms and Radio Astronomy
Max-Planck-Institute for Radio Astronomy,
Auf dem Hügel 69, D-53121 Bonn, Germany
Harvesting wind power for electricity generation is one of the few sustainable ways of power generation with
minimal CO2 emissions. The grave environmental and energy problems facing humanity on a global scale mean that all efforts to utilize sustainable energy sources ought to be supported. However, the special requirements of radio astronomical observatories could impose a restriction on the deployment of tall radio-reflecting structures, such as wind turbines, near radio telescopes. Nevertheless, this is likely to affect only a small fraction of the areas suitable for the location of wind turbines, and should therefore have a negligible impact on national or global wind power capacities.
Introduction Radio astronomy and radio frequency interference The radio signals from cosmic objects (stars, galaxies, etc.) that radio astronomers observe are extremely weak because the objects are very distant, often millions or billions of light years away from us. The large radio telescopes used are highly sensitive and routinely detect minute signals that have flux densities of the order of 1 mJy = 10−29 Wm−2Hz-1, which corresponds to the signal received from a UMTS mobile phone radiating 1 W at a distance of 40 million km (i.e. at approximately a hundred times the Earth-Moon distance).
Radio astronomical telescopes detect radio emission mainly in the forward direction, but it is impossible to avoid signal reception from most other directions. In technical terms, a radio telescope is highly directional with a very high „gain‟ (typically 105 – 106) in the forward or „main beam‟ direction. Astronomical radio antennas are however not pointed at low level terrestrial objects. Their sensitivity is so high, that they can even detect the radio part of the thermal spectrum of an object at ambient temperatures. Designers of radio telescopes take great pains to avoid receiving the thermal and other radiation from the local environment. However, a small fraction of it, about a million times smaller than what is received through the main beam can still find its way into the receiver. It is this ambient reception which is also the pathway for radio interference coming from all directions.
Local interference sources can easily be a million times stronger than the signal from remote cosmic sources, but at the receiver they will appear to similar strength. Being the result of an ambient reception, their direction cannot easily be determined or avoided. One can only implement effective regulatory and preventive measures to keep local rfi below the detection threshold. Local means terrestrial in this context and can mean distances of hundreds of kilometres.
In order to detect distant cosmic radio sources, radio observatories require sufficient frequency bandwidth which is free of man-made radiation for a sufficiently long time (and that includes even weak and distant man-made sources). These very stringent requirements mean that radio telescopes are usually placed in carefully selected remote areas. Some countries, like the USA, Chile, Australia, and South Africa, have created large radio quiet zones around their current or future radio observatories, where human radio emissions are very strictly controlled. This is not an option in densely populated European countries, where the regulatory administrations attempt to coordinate the placement of radio transmitters so that they do not cause radio frequency interference (RFI) at the radio observatory.
Radio astronomers expend considerable money and time to avoid the creation of RFI on their sites by building shielded rooms for their electronic equipment, which is also carefully checked. The use of mobile phones and all other wireless equipment is forbidden, and regular interference surveys are undertaken in order to find any interfering equipment. However, all the efforts of regulators and radio astronomers are not always successful, as the example in Fig. 1, from Onsala Space Observatory (Sweden), shows.
Figure 1: A spectrum observed with the Onsala 20 m radio telescope in Sweden. Spectra are used to detect and determine properties of, e.g., molecular gas surrounding old stars. In this case, the molecule being studied is C4H. The radio signals from the molecular gas (the ‘peaks’ in the spectrum) are weak compared with the RFI. It is often impossible to separate the cosmic signal from RFI, which makes the observational data useless. The source of the RFI is unknown.
Another example, from the 100 m radio telescope in Effelsberg (Germany), is RFI that appeared in 2009, in a very important and protected frequency band for radio observations at ~1420 MHz (Fig. 2). The RFI is time variable, which makes it even more harmful, and its origin is still under investigation.
Figure 2: A spectrogram showing a pattern of time variable interference of unknown origin at ~1420 MHz. A wavy pattern is seen, which indicates reflections of a signal from an unknown transmitter by an unknown object. The amplitude of the signal is colour coded with frequency on the horizontal scale and time increasing on the vertical scale (data from Effelsberg, Germany, July 2009). The origin of this interference is still unknown.
International regulations When combined with state-of-the-art signal processing and detection, a large radio telescope equipped with a cryogenically cooled radio astronomical receiver is much more sensitive than any industrial radio surveillance equipment. It will detect interference where other equipment shows only noise. Anything that can be detected with industrial radio equipment will almost certainly cause significant RFI in a radio telescope (note that a radio telescope is not very directional for local sources and therefore cannot easily be used to pin-point the origin of the interference). RFI can make sensitive radio observations impossible and the efforts to locate and neutralise the sources of RFI are great and can take a lot of resources that would otherwise be dedicated to scientific work.
It is because of this that preventive regulatory and technical measures are undertaken in consultation with radio astronomers before a potentially disastrous situation arises.
Radio regulators recognised the outstanding protection requirements of radio astronomy as early as 1959, and devised a framework of international agreements and recommendations at the ITU* to that effect. Most important * ITU: the International Telecommunication Union, in this context is the recommendation ITU-R RA 769-2 which specifies limits of ambient radio power at a radio observatory above which harmful interference occurs. These limits are several orders of magnitude lower than the interference limits for other radio services such as broadcasting or mobile communications. Particular measures such as power limitation or minimum separation distances for transmitters from radio observatories have to be employed in order to keep the RFI power below the internationally agreed limits. The ESF* expert committee on radio astronomy frequencies (CRAF) is consulted on radio astronomical protection questions by regulatory administrations as a recognized member of the radio communication sector of the ITU. CRAF has observer status in CEPT†and participates in a consultative capacity in ECC‡ meetings.
Wind turbines In the above context, the operation of wind turbines close to radio observatories is a new problem that has to be addressed. The recent and laudable national and European initiatives to increase the utilisation of wind power for electricity generation have given an incentive for the development of new possible sites for wind power generators. These now include areas closer to radio observatories. The impact of wind turbines on radio astronomical operations caused by their radio emissions and reflections is described in the next section.
As pointed out by the great Swedish scientist Svante Arrhenius as long ago as 1900, a rise in carbon dioxide in the atmosphere, which has already taken place and is continuing, will have a consequent rise in average global temperature with possible catastrophic consequences for mankind. It is also well known that without sufficient energy our civilization would collapse and certainly radio astronomy isn‟t conceivable without electricity.
Consequently radio astronomers welcome the use of renewable sources for electricity generation. On the other hand, they still need to make sure that they can continue to observe without interference, thereby providing unique data for the understanding of the origin, the structure, and the future evolution of the universe. A local planning process with rules that take account of the vulnerability of radio astronomical observatories, the expected radio emissions and reflections from a wind farm and its shielding by topography and distance is the rational solution to the question of how science can continue to provide answers for us without the destruction of the environment on which depend for survival.
Sources of Radio Frequency Interference
Radio astronomy aims to perfect the detection of the very weakest signals from very distant radio sources, often billions of light years away. Any detectable radio emission of man-made origin can obscure these signals and constitutes RFI for a radio astronomer. The radio frequency range stretches (in broad terms) from ~10 MHz to above 100 GHz, and unique astronomical information is present throughout this frequency range.
In the past, radio astronomy suffered a great deal from interference from distant TV transmissions, direct broadcasting TV satellites, mobile satellite communication systems (IRIDIUM, still continuing) and navigational satellites (GLONASS). In addition to these „remote‟ sources there are many diverse cases of local interference from radar transmitters, fixed radio links, electric trains, malfunctioning communications, TV equipment, industrial equipment and even electric cattle fences, that cause problems. The radio emission limits for industrial and consumer equipment have been created to ensure interference free operation for broadcasting and communication services. However the BS EN 61800-3:2004 document on 'Adjustable speed electrical power drive systems, Part 3: EMC requirements and specific test methods' draws attention to the fact that these limits are insufficient to protect domestic equipment from interference: (Section 6.4.3: 'Warning: In a domestic environment, this product may cause radio interference, in which case supplementary mitigation measures may be required.'). For radio astronomy the interference potential is much more severe and additional shielding or large separation distances are required for frequencies up to several GHz. Hence the radio emission limits for industrial equipment do not take the higher requirements of radio astronomy into consideration. Industrial equipment certified to CISPR-11 and CISPR-22 standards§ can therefore cause harmful RFI to radio astronomy (see figures 3 and 4). It is one of the reasons why radio astronomers have to make great efforts to shield their own electronic equipment.
* ESF: European Science Foundation,
min. separation (km) frequency (MHz)
Figure 3: Free space separation distances between CISPR-11 electronic equipment and a radio observatory. Black:
Minimum line of sight distance needed to shield a 50m high radio astronomical antenna from equipment at ground level as a function of frequency. Diamonds indicate radioastronomical bands. For ground-level ( 2-5 m) the local sub-urban clutter (buildings, trees etc.) can provide additional interference attenuation of about 20 dB. However separation distances of many km may be needed even in this case. Blue: optical horizon from a height of 50 m. Red: similar separation distances for an industrial emitter at a height of 100 m. Any equipment within the optical visibility range (mauve) of 61 km can become the source of significant interference.
The figure above illustrates why additional shielding of industrial emissions close to the radio telescope is needed. This can be achieved either by additional rfi suppression measures within the industrial plant, extra shielding or attenuation by the local topography or by a combination of various measures. Depending on the kind of terrain, the topographical attenuation can be very high (30 dB) and it had traditionally been the resort of radio astronomers to select remote, preferably mountainous sites for their observatories. Regional planning restrictions then helped to prevent large scale industrial and other developments so that a low interference site could be maintained. But it becomes quite clear, that large structures, potentially emitting radio waves above the top of the local buildings and tress etc. significantly increases the range of possible interference and that can overcome the benefits of a remote location. Note that topographical attenuation (shadowing) will only reduce the interfering signals, but never fully suppress them. If they are strong enough, they can still be received, even when there is no optical line of sight to the transmitter.
a) Tall structures, such as wind power generators within the line of sight of a radio telescope, can function as primary (i.e. radio emission from the generator electronics, Fig. 2) and/or
c) a large rotating structure close to a telescope can even modulate the near-field background signal of the telescope because of its periodically varying electrical characteristics. It can disturb the antenna pattern and present a variable source of thermal radio emission for low elevations of the radio antenna.
Figure 4: Radio emission detected from wind power plants.
Left: Emission at 1420 MHz measured at a distance of 200 m from a wind turbine near Euskirchen (Germany). The horizontal scale shows time in minutes and seconds, and the receiver is pointing to the source for the first minute, then away for the second minute. The vertical scale shows the received radio power.