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Agenda Item 1.8: 71 – 238 GHz
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 1.8‐1 and 1.8‐2, this band is extremely important for a wide range of scientific
problems, both for RAS and EESS.
RAS
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.10
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 1.8‐1.
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
10
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 – 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.
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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 are invaluable in investigating galactic chemical evolution and nucleosynthesis in
stars. Maser action often occurs in certain molecules in star‐forming regions and in envelopes of
evolved stars, such as in SiO or CH3OH. Observations of maser lines provide information on small‐scale
structure and time variability of emitting sources.
1.4 37 Indentified Features
C H 3C H O
35 Unidentified Features
~6 lines per 100 km/s
C H 3C H O + C 2H 5C N
HCO O CH 3
HCO O CH 3
(C H 3)2O
TRMS = 0.003 K (theoretical)
1.2
C 2H 3C N
HCO O CH 3
C 2H 3C N
H C O O C H 3 + C 2H 3C N
C 2H 5C N
C 2 H 5 O H + (C H 2 O H ) 2
C H 3C H O + C 2H 3C N
C 2H 3C N + C 2H 5C N
1.0
C H 3C H O
C 2H 3C N
HCO O CH 3
C H 3N H 2
C 2H 5C N
C 2H 3C N
C 2H 5C N
HCO O CH 3
T R * ( K)
0.8
C 2H 5O H
C 2H 3C N
C 2H 3C N
HCO O H
C 2H 5C N
C H 3C H O
0.6
N H 2C H O
C 2H 5O H
(C H 3 )2 O
C 2H 5O H
U
HNCO
C H 3C H O
U
0.4
U
U
U
U
U
U
U
0.2
CS
O CS
13
0.0
231000 231200 231400 231600 231800 232000
Frequency (MHz)
Figure 1.8‐1: Typical spectrum of a dense molecular cloud, Sgr B2(N), obtained in a
portion of the 71‐ 238 GHz band, using the Sub‐Millimeter Telescope (SMT) of the Arizona
Radio Observatory using a single‐sideband (SSB) receiver covering a 1 GHz band.
Integration time is 2 hours. A large fraction, but not all, of the molecular lines have been
labeled on the spectrum. “U” indicates unidentified lines. Because radio telescopes use
heterodyne receivers with multiplexing spectrometers, spectral resolution as high as 1
part in 108 can be achieved, particularly in cold, quiescent astronomical sources. This
resolution is invaluable for chemical identification of molecules, and for investigations of
velocity structure in astronomical objects. SOURCE: Lucy Ziurys, University of Arizona.
The 71‐238 GHz region also covers the premier spectral windows for astrochemistry. Many new
interstellar molecules are discovered by observations at these frequencies, as illustrated by the recent
identification of CCP, PO, HSCN, AlO, and AlOH. It is naturally difficult to predict a priori the transition
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frequencies of any possible new molecule. Therefore, protecting given lines does not fully cover the
science that comes from observations in these bands.
Finally, this spectral region is important for astrobiology, and the study of life in the universe. It is
commonly thought that Earth lost its original carbon in the form of a methane atmosphere. The carbon
on this planet today had to come from exogenous delivery via comets, meteorites, and interplanetary
dust particles, which appear to carry debris from the original molecular cloud in which the solar system
was created. Observations of organic molecules in molecular clouds are enabling the link to be made
between the carbon inventory on planets and that of the presolar nebula.
Summarized below in Table 1‐8.1 are some of the important astronomical uses of this spectral region.
Table 1.8‐1: Important Astronomical Uses of the 71 – 238 GHz Bands11, 12
Scientific Topic Common Molecular Probes Frequencies (GHz)
CO, HCN, HCO+, N2H+, CS, H2CO, 115, 230, 88,13 89,14 93, 96, 144,
Star formation, mass loss from
young proto‐stars, protostellar HC3N, CH3OH, CH3CN, SO2 234, 72, 142, 218, 72, 91,100,
disks 109, 136, 145, 154 , multiple
lines for CH3OH, CH3CN, SO2
CO, H2CO, HCO+, HCN 115, 230, 72, 141, 89, 8815
Galactic structure, molecular
clouds
CO, HCN, HCO+, CH3OH 115, 230, 8816, 89,17 multiple
External galaxies, molecules at
high red shift lines for CH3OH
+
115, 230, 88,18 89, 73, 91, 109,
Evolved stars, planetary nebulae CO, HCN, HCO , SiS, SiO, SiC2,
CCH, C4H, HC5N, SO2, 142, 87, 174, multiple for C4H
HC3N through HC3N
+
Diffuse clouds CO, HCN, HCO , SO, CN, CCH, 115, 88, 89, 100, 113, 87, 88
C3H2
Comets, planetary atmospheres HCN, HCO+, CS, H2CO 88,19 89,20 96, 144, 234, 144, 218
Astrochemistry Numerous molecules
Interstellar masers SiO, H2O, CH3OH, HCN 86, 130, 217, 107, etc
Isotope ratios CO, HCN, CN, H2CO, MgNC, NaCl 113/115, 88/86, 110/113,
multiple for CH3OH, MgNC, NaCl
Astrobiology PO, CP, PN, H2CO, all organics 109, 152, 174, 95, 97, 143, 238,
97, 140, 234, 71, 142, 218,
various
11
Important transitions also exist in the frequency range 238 – 275 GHz, for example, the J = 3 → 2 transitions of
HCN and HCO+.
12
This region is used for both filled aperture spectroscopy and imaging, and covers several of the prime bands
for ALMA: Band 3 (84‐115 GHz) and Band 6 (210‐275 GHz).
13
The J = 3 → 2 transition of HCN is at 265 GHz.
14
The J = 3 → 2 transition of HCO+ is at 267 GHz.
15
HCN has an important transition frequency at 265 GHz.
16
Ibid.
17
HCO+ has an important transition frequency at 267 GHz.
18
HCN has an important transition frequency at 265 GHz.
19
Ibid.
20
HCO+ has an important transition frequency at 267 GHz.
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EESS
The Earth exploration satellite service (passive) spectrum in the range between 71 and 238 GHz is used
operationally for several vital measurements of the Earth and its atmosphere for weather and climate
applications. Global water vapor profiles are essential to the numerical weather prediction of rainfall
and drought and help constrain such predictions in general.21 Water vapor is the primary mechanism for
energy storage and its movement within the atmosphere as it drives extreme weather events.
Combined microwave and infrared spectral data can yield what is nearly all‐weather global
observations, even in most cloudy conditions, of water vapor and air temperature profiles.22 The critical
importance of the oxygen and water vapor lines at 115‐122 and 176‐190 GHz cannot be over
emphasized: due to the unique molecular properties of oxygen and water vapor, atmospheric
temperature and humidity in particular cannot be measured in bands other than those currently
allocated. A list of additional scientific applications is given in Table 1.8‐1 and Table 1.8‐2 gives a list of
corresponding applicable frequencies. Given the importance of weather forecasting and climate
monitoring to the public, it is important that we protect this region of the spectrum for continued
successful EESS use.
Table 1.8‐2 EESS uses of spectrum from 71 to 238 GHz23
Frequencies (GHz)
Use/Measurement/Target Significance
Atmospheric humidity and Used operationally in numerical 115‐122, 176‐190
temperature profiles weather prediction, forecasting of
severe storms, and monitoring
climate
Cloud ice content Forecasting of severe storms, and 8585‐92, 150‐160
monitoring climate
Precipitation Planning operations and monitoring 8585‐92
climate
Land surface type Monitoring climate 8585‐92
Trace gases Mapping of key atmospheric 177.26, 181.59, 200.98, 204.35,
constituents (HCN, NHO3, N2O, ClO, 206.13, 230.54, 233.95, 235.71
CO, O3) tied to carbon cycle, global
climate, ozone depletion, pollution,
atmospheric transport
Sea ice extent and Monitoring climate 85 – 92
concentration
21
National Research Council, Spectrum Management for Science in the 21st Century, The National Academies
Press, Washington, D.C., 2010, pp. 29‐30.
22
Ibid.
23
Adapted from Tables 2.1 and 2.2 found in National Research Council, Spectrum Management for Science in the
21st Century, The National Academies Press, Washington, D.C., 2010, pp.137‐138.
20
