The Committee on Radio Astronomy Frequencies (CRAF) is a committee of
the European Science Foundation (ESF).
The Radio Astronomy Service is a passive service. This means that it is a receiver-only radiocommunication application since the transmitter is a physical process. It cannot have any influence on the transmitting source nor on the frequency it likes to transmit. All other radiocommunication services control both a transmitter and receiver which can both be manipulated by man. They are called active services. The difference between the nature of an active service and a passive service is the major cause of the vulnerability of radio astronomy to interference from active services. For radio astronomy, this vulnerability is enhanced by the fact that the received signals from celestial radio sources are of the order of 109 times weaker than common practice in active service applications.
Man-made interference has a certain "climatology" in frequency, spatial coordinates, time and propagation conditions. This implies that this problem has a dynamic character, which requires a dynamic solution to be achieved as a joint effort of the Administrations, active spectrum users and radio astronomers. These solutions have technological, commercial and political aspects.
The radio interference experienced by radio astronomy in a specific frequency band can be a result of sharing between two services operating within the same frequency band, of out-of-band emission, or of spurious emission. In geographical terms, this interference can have a global, regional or local origin. The challenge for the active and passive spectrum users, for administrations and industry is to find ways to reduce, remove and prevent this interference considering the specific characteristics of the service involved. To achieve this, each spectrum user has to improve his system to a state-of-the-art situation with respect to protection of other frequency users. When the available possibilities are not sufficient a coordinated effort is needed for further technological development.
The future of radio astronomy and other sciences making passive use of the radio spectrum requires strong pressure on adequate technological developments to cope with the "multi-dimensional problem space" of multi-service frequency use. The intrinsic vulnerability of passive spectrum use should be considered in any solution to the challenges facing scientific use of radio.
Radio astronomy is in several aspects an international scientific activity.
In the context of Very Long Baseline Interferometry, VLBI groups of radio telescopes form an interferometer network in which the different interferometer elements can have mutual distances up to several thousand kilometer. The final instrument which is formed by such a network has an angular resolution as a single instrument of this dimension. The different radio telescopes participating in such a network are usually located in a large variety of countries - even in different continents and since 12 February 1997 also in space. For each VLBI observation it is mandatory that each radio telescope involved can produce measurement data with the same quality. Therefore, the status of protection of the frequency of the observation should also be the same in the participating countries.
Another international aspect of radio astronomy is that it is general practice that radio astronomers do their observations with the instrument needed to answer the scientific questions. This implies that each observatory receives astronomers from other countries to use its facilities.
This international aspect of the science of radio astronomy implies that a policy of protection of radio frequencies allocated to the Radio Astronomy Service in one country always has impact on radio astronomical progress in other countries.
The radio astronomical requirements for spectrum and observing time are determined by the scientific question in a way beyond the control of a radio observatory.
The current status of radio astronomy and the perspective of its future development make it clear that much more radio spectrum is needed to answer the fundamental questions of physics and the Universe. Radio astronomers understand that it is unrealistic to request that more spectrum is allocated to the Radio Astronomy Service. However, the development of research requires that more and more radio astronomy observations are done and will be done outside the frequency bands allocated to the Radio Astronomy Service. Radio astronomers are, therefore, actively working on the development of interference suppression technologies to enable access to frequency bands where no allocation to radio astronomy exists.
This development does not mean that the protection of radio astronomy observations become less important - on the contrary: The dynamic occupancy of the radio spectrum implies that radio astronomy observations outside the frequency bands allocated to the Radio Astronomy Service can be done at some price. This price consists of at least three aspects:  much longer observations because a large fraction of the measurement data is affected by interference and has to be rejected;  very intensive and interactive data processing has to be done to extract the astronomical information from the data and separate this from unwanted man-made transmissions;  the fact that since proper calibration of the observations and of the instruments is not possible when the data are affected by interference, this calibration has to be done outside the frequency band used for the observation in the nearest frequency band allocated to the Radio Astronomy Service. Besides the fact that frequency bands allocated to active radiocommunication services are not always used, the reason to obtain reliable observations outside the allocated bands is the possibility of calibrating these observations and the instruments with observations in frequency bands which are free of interference, e.g. exclusive passive frequency bands.