ments, which depend on averaging for noise reduction.² The Doppler shift of spectral lines that is due to the expansion of the universe also necessitates going outside of these allocated bands.

• Most bands are shared with active services. Obvious strong signals of terrestrial origin can often be excised from collected data, but weak signals defy editing and may therefore by more pernicious, contaminating long-term wideband averages without being apparent in individual data.

• The radio astronomy bands are not adequately protected from transmissions in adjacent bands. This is particularly a problem with orbiting transmitters, because their modulation techniques are often inefficient and terrain around observatories does not provide shielding.

• Some allocations apply to limited areas of the world, providing no protection at all in other areas.

• There are large intervals between some of the allocated bands. In order to determine the spectral distribution of radio source emission, bands were assigned to radio astronomy at approximately octave intervals.

The frequency range of radio astronomy observations now extends above 3 THz (with measurement of the 3438 GHz transition of ¹³CO and the detection of the 13→12 transition of CO at 1497 GHz), blurring the distinction between radio astronomy and infrared astronomy. With the advent of Earth-to-space telecommunications near 1 µm wavelength (300 THz) and the need to regulate for avoiding interference with optical observatories, the distinction between different kinds of astronomy will eventually vanish from the regulatory perspective. Use of the radio spectrum for communications and other commercial purposes developed first at the lower frequencies (longer wavelengths) because the technol-