Agenda Item 1.8 is the consideration of the progress of ITU R studies concerning the technical and regulatory issues relative to the fixed service in the bands between 71 GHz and 238 GHz, taking into account Resolutions 731 (WRC 2000) and 732 (WRC 2000). The millimeter wave spectrum above 70 GHz has become the subject of increasing interest for fixed wireless services due to its propagation characteristics and the wide bandwidth available for carrying communications traffic. New technology is now emerging that offers the possibility of using these higher bands for fixed wireless applications. Therefore, it is important the use of this frequency range for passive scientific observation be recognized.
Recommendation: Administrations are urged to protect the passive services from harmful interference in 71-238 GHz. Per Tables 2.4 and 2.5, this band is extremely important for a wide range of scientific problems, both for Radio Astronomy Service and Earth Exploration-Satellite Service.
Radio Astronomy Service
This spectral region, which contains the 3 mm, 2 mm, and a large section of the 1.2 mm atmospheric windows, is extremely important for studies of virtually every aspect of the dense interstellar medium.1 In fact, almost any given interstellar molecule has favorable transitions in this frequency band. Thus, this region is rich in spectral lines and high spectral resolution can be achieved, as shown in Figure 2.1.
Millimeter molecular lines in the 71-238 GHz bands serve as important probes of dense gas in a wide variety of astronomical settings. High resolution spectroscopy and the Doppler effect allow the velocity structure of an astronomical source to be readily discerned through spectral line measurements. Millimeter transitions of molecules such as CO and H2CO have been used to trace galactic structure and the distribution of dense gas in our Galaxy and in external galaxies. Because stars form in dense molecular clouds, molecular lines in these bands are very useful probes of star formation, and have been used to locate young protostars and protostellar disks. Molecules are also common constituents of dying (or evolved) stars, and are present in large quantities in stellar ejecta of red giant and asymptotic giant branch stars. Molecular spectra have been successfully used to study the mass loss mechanisms from such stars and how they develop into white dwarfs and planetary nebulae. The low energy transitions of many molecules have also been used to examine the structure and chemical composition of diffuse clouds and cold, dense globules. Because multiple transitions of a given molecule can be observed in many of these objects, radiative transfer modeling can be done to accurately determine gas temperatures and densities, important physical quantities. Gaseous vapors emitted by comets are also investigated by observations of spectral lines at these wavelengths, and the climatology of planetary atmospheres in our solar system as well. Isotope ratios are also successfully probed in a wide variety of environments using mm molecular lines, such as 13CO and 12CO, HC14N and HC15N, and even Na35Cl and Na37Cl. Such ratios
1 This frequency range takes on this prominence because of fundamental quantum mechanics, and the nature of the dense interstellar medium. Dense interstellar gas (n ~ 103 to 107 cm-3) is typically cold, with temperatures in the range T ~ 10-100 K. Under such conditions, atomic energy levels are not populated, and only the very lowest energy levels of molecules can be accessed, namely, the rotational levels, as opposed to vibrational or electronic. Rotational energies of any given molecule are proportional to 1/ I, where I is the moment of inertia. Most simple molecules containing the cosmically-abundant elements H, C, N, O, and S have moments of inertia that place their rotational spectra in the 1-3 mm region (about 71-300 GHz). For example, the fundamental rotational transitions (i.e., J = 1 → 0) of the most abundant interstellar molecules, including CO (115 GHz), HCN (88 GHz), HCO+ (89 GHz), N2H+ (93 GHz), CN (113 GHz), NO (150 GHz), H2CO (72 GHz) and H2S (169 GHz) occur in these bands. The 71-238 GHz band also contains the next higher transitions (2 → 1, 3→ 2) of many of these molecules, as well.