The methodology to determine the levels of interference detrimental to radio astronomy is given in Recommendation ITU-R RA.769. This Recommendation also contains a table of power flux-density, pfd, and spectral power flux-density, spfd, values for the protection of radio astronomy which must not be exceeded.

The pfd and spfd levels presented in Recommendation ITU-R RA.769 have traditionally served to indicate the levels of detrimental interference for radio astronomy stations. Although these numbers may appear extremely stringent for certain active operators in the spectrum, they are essential for the protection of radio astronomy operations. While the technical characteristics of the radio astronomy stations have changed dramatically in recent years, the numbers of Recommendation ITU-R RA.769 have not changed since 1979.

Routine operations of radio astronomy stations support scientific research on signals that are much weaker than the spfd levels presented in Recommendation ITU-R RA.769 – for example, in astronomical terms, the level for emissions detrimental to radio astronomical HI-line observations in the 21-cm band (i.e. 1400-1427 MHz) is 125 Janskys (1 Jansky = 10^-26 Wm^-2Hz^-1), while the sensitivity of routine observations of this line are expressed in milliJansky.

In practice, man-made in-band signals that exceed the levels of detrimental interference given in this Recommendation cannot be easily removed from the data by mitigation techniques, if at all. These thresholds of interference to the Radio Astronomy Service as calculated from the methodology given in this Recommendation are a realistic assessment of the values of unwanted emissions above which radio astronomical data are degraded or completely obliterated.

Therefore the levels presented in this Recommendation should not be considered as a “starting position” for negotiations and studies between passive service operators and active service operators.

Although these levels are inadequate to protect all radio astronomy experiments, they do serve as adequate “upper boundaries” for protecting general operations of the current radio astronomy stations and of the next generation telescopes. The figure below explains the degradation in sensitivity of radio astronomy observations due to interference exceeding the level of detrimental interference of Recommendation ITU-R RA.769. When discrepancies exist between “what is desired” by passive services and “what can be achieved” by active services, an increase of the levels in Recommendation ITU-R RA.769 cannot be a viable mitigation option if radio astronomy is to remain a viable branch of science.

Reduction in radio astronomy channel capacity due to interference

Further sensitivity improvements on current telescopes and the introduction of the next generation telescopes will only further increase the difference between the operational sensitivity levels and the detrimental levels given in Recommendation ITU-R RA.769. It is crucial for preserving the future of radio astronomy as a science not to increase the levels given in this Recommendation.

See also the document “Scaling of Interference Limits given in ITU-R RA.769