Committee on Radio Astronomy Frequencies

Astrophysical importance of the band 15.35 - 15.4 GHz

The frequency band 10 to 15 GHz provides some of the best angular resolutions (~2 arc minutes) using many large and accurate radio telescopes. Many of the nonthermal synchrotron sources are just detectable at higher frequencies, and this frequency range gives us observational information at the highest frequency where these can be detected reliably. This high-frequency range is also important for monitoring the intensity variability of the enigmatic quasars. These objects, which could be the farthest celestial objects that we can detect and which produce surprisingly large amounts of energy, have been found to vary in intensity with periods of weeks and months. Such observations lead to estimates of the sizes of these sources, which turn out to be very small for the amount of energy they produce. The variability of quasars (and some peculiar galaxies) is more pronounced at high frequencies and observations at such frequencies facilitate the discovery and the monitoring of such events. The energy emitted during any one burst from a quasar is equivalent to completely destroying a few hundred million stars in a period of a few weeks or months. We do not yet understand the fundamental physics that can produce such events - observations of the size and variability of these sources are the only ways that can assist us in solving such problems. Such observations are now best performed in the frequency range 10 to 15 GHz.

The small sizes of the quasars are revealed from the VLBI observations mentioned earlier. Such observations are also being made in the frequency band 15.35 - 15.40 GHz. The higher frequencies provide us with better angular resolution and enable us to determine more accurately the sizes and structure of quasars.

The band 15.35 - 15.40 GHz is an important radio astronomy band in the continuum series and needs to be widened to 15.30-15.55 GHz by sharing with compatible services. The possibility of moving it down in frequency to contain the above formaldehyde line near 14.5 GHz has been discussed. A band at least 200 MHz wide would then be sought, so located to avoid the risk of interference from radio navigation satellites below 14.4 GHz. This location would have the advantage of being within a band currently allocated to the Fixed and Mobile Services, rather than the present location between bands available for Space Research (space-to-Earth) and Aeronautical Radionavigation Service, which makes an extension of the present band difficult. The Fixed-Satellite Service in the lower band is designated as Earth-to-space and should not be a serious source of interference.

Threshold levels of interference detrimental to radio astronomy continuum observations:

	System Sensitivity (noise fluctuations)		Threshold Interference Levels
	Temperature	Power Spectral Density	Input Power	Power Flux Density	Spectral Power Flux Density
Single Dish	0.095 mK	-269 dB(WHz-1)	-202 dBW	-156 dB(Wm-2)	-233 dB(Wm-2Hz-1)
VLBI					-187 dB(Wm-2Hz-1)

NOTE:

For further details the reader is referred to ITU-R Recommendation RA769. It should, however, be noted that the observed spectral power flux densities are much lower than the harmful interference levels because they are made in the main beam of the radio astronomy antennas whereas the harmful levels correspond to reception in the sidelobes, i.e. the differences are related to the gain of the antennas. The differences can be of the order of 50 to 80 dB W m^-2 Hz^-1 (see ITU-R "Handbook on Radio Astronomy", 1995/2003, chapter 4).

In Europe, the band 15.35 - 15.4 GHz is used by the Radio Astronomy Service in:

• Germany
• Italy
• Sweden
• United Kingdom

Threats to the Radio Astronomy Service:

Out-of-band emission from the following services:

• Aeronautical Radionavigation Service
• Fixed Service
• Mobile Service