Answer: how to use a radio telescope for compatibility tests or standard development?

The protection criteria used for radio astronomical measurements and the methodology to determine the power flux density levels harmful to radio astronomy are given in Recommendation ITU-R RA.769. In Administrative compatibility studies involving radio astronomy stations, usually the protection criteria of this Recommendation are used. The telescope characteristics assumed in this Recommendation, such as integration time and antenna pattern are considered as “reasonably representative” of what is actually used in radio astronomy. It should be pointed out, however, that the parameter values used in practice cover a wide range of values. It is considered that Recommendation ITU-R RA.769 gives an adequate representation of the requirements for the protection of radio astronomy.

Each radio telescope has its own technical and operational characteristics, and none of them identical: “each radio telescope is its own prototype”. Furthermore, each radio telescope was built for a specific kind of research. Its technical characteristics are set by the specific scientific demands of the research projects to which the instrument is dedicated, coupled with the implementation of the latest technology with adequate flexibility to serve a specified range of research projects.

The flexibility required for most radio astronomy telescopes implies that it is virtually impossible to define their characteristics for compatibility studies as a contribution to the Administrative co-ordination process. This applies also to the development of technical standards for radio emitting devices.

Recommendation ITU-R RA.769, considered to be a reasonable representation of the protection criteria for radio astronomy, was developed to avoid the burden on Administrations of determining the detailed telescope parameters for individual telescopes required for protecting the operations of the Radio Astronomy Service and to develop a generic opinion from very different and usually incomparable individual scenarios.

In cases where it is deemed necessary that tests should be made (such as for very very specific questions by a national regulatory authority), the range of questions needs to be posed

    • What question needs to be answered by the anticipated tests?
    • What technical and operational conditions are required to lead to interpretable results for the anticipated tests?
    • Can these conditions be realised at the radio astronomy station under practicable circumstances?
    • What kind of information will tests add to the Administrative co-ordination process and compatibility studies using ITU-R Recommendations?
    • Can the interpretation of the measurement data be accurate, representative for the issue at stake and reliable, vis-a-vis the model assumptions in Recommendation ITU-R RA769?

Very likely, such tests will show that the sensitivity of a specific radio telescope differs from the levels given in Recommendation ITU-R RA.769, as the characteristics of this instrument differ from the “model assumptions” used for this Recommendation. The sensitivity of some radio telescopes may be such that, for these specific antennas the “769-levels” could be relaxed somewhat, while for other instruments these levels should be tightened significantly. Usually, CRAF or a qualified expert at a radio astronomy station is able to evaluate the sensitivity of an instrument at station with respect to the levels listed in Recommendation ITU-R RA.769.

Any test will give only information about the impact of the specific transmitter used on a specific radio telescope under specific conditions, and cannot be expected to yield a generically applicable result for use in the co-ordination process or standard development.

Committee on Radio Astronomy Frequencies