1、 Report ITU-R RA.2163(09/2009)Astronomical use of frequencyband 50-350 THz and coexistencewith other applicationsRA SeriesRadio astronomyii Rep. ITU-R RA.2163 Foreword The role of the Radiocommunication Sector is to ensure the rational, equitable, efficient and economical use of the radio-frequency
2、spectrum by all radiocommunication services, including satellite services, and carry out studies without limit of frequency range on the basis of which Recommendations are adopted. The regulatory and policy functions of the Radiocommunication Sector are performed by World and Regional Radiocommunica
3、tion Conferences and Radiocommunication Assemblies supported by Study Groups. Policy on Intellectual Property Right (IPR) ITU-R policy on IPR is described in the Common Patent Policy for ITU-T/ITU-R/ISO/IEC referenced in Annex 1 of Resolution ITU-R 1. Forms to be used for the submission of patent st
4、atements and licensing declarations by patent holders are available from http:/www.itu.int/ITU-R/go/patents/en where the Guidelines for Implementation of the Common Patent Policy for ITU-T/ITU-R/ISO/IEC and the ITU-R patent information database can also be found. Series of ITU-R Reports (Also availa
5、ble online at http:/www.itu.int/publ/R-REP/en) Series Title BO Satellite delivery BR Recording for production, archival and play-out; film for television BS Broadcasting service (sound) BT Broadcasting service (television) F Fixed service M Mobile, radiodetermination, amateur and related satellite s
6、ervices P Radiowave propagation RA Radio astronomy RS Remote sensing systems S Fixed-satellite service SA Space applications and meteorology SF Frequency sharing and coordination between fixed-satellite and fixed service systems SM Spectrum management Note: This ITU-R Report was approved in English
7、by the Study Group under the procedure detailed in Resolution ITU-R 1. Electronic Publication Geneva, 2010 ITU 2010 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without written permission of ITU. Rep. ITU-R RA.2163 1 REPORT ITU-R RA.2163 Astronomical u
8、se of frequency band 50-350 THz and coexistence with other applications (2009) 1 Introduction The infrared and near-infrared Parts of the electromagnetic Spectrum are of large and growing interest for astronomical research. Thermal emission from dust at temperatures of a few Kelvins to hundreds of K
9、elvins, which is the case for most dust clouds and larger rocky bodies, is strong in these bands, which makes them ideal for studies of star and planet formation and interstellar dust clouds. Molecules important in the chemical processes taking place in these clouds produce many spectral lines in th
10、e infrared and near-infrared wavelength range. Consequently this Part of the Spectrum has become of high astronomical interest and has led to the implementation of large ground-based facilities for observing in those frequency bands to which the troposphere is adequately transparent, and airborne an
11、d spaceborne facilities for observations at other frequencies. This Report is intended to provide background information relevant to the task of making bands in this part of the electromagnetic spectrum accessible to future active services while ensuring that astronomical observations are adequately
12、 protected. 2 Atmospheric transmissivity Although only Part of the Spectral Range of interest is accessible from ground-based observatories, the larger size and much greater flexibility for accommodating new projects and equipment changes makes them still a very attractive option, even in bands wher
13、e there is significant atmospheric absorption. Even then, ground-based facilities are highly expensive, so most major observatories are constructed and operated by national or international consortia, at the best available sites. To maximize the amount of usefully accessible spectrum, the ground-bas
14、ed facilities are located at high-altitude locations, such as Mauna Kea in Hawaii (altitude about 4 200 m). Figure 1 shows a plot of atmospheric transmissivity at the site of the Gemini North Telescope on Mauna Kea. The transmissivity () of the atmosphere is the ratio of the power received at the te
15、lescope to that incident upon the top of the atmosphere. The corresponding attenuation is given by: L = 10 log () dB The graph shows the atmospheric transmissivity at the summit of Mauna Kea Hawaii as measured at the site of the United Kingdom Infra-Red Telescope (UKIRT). The thick-lined, cross-hatc
16、hed blocks close to the frequency axis represent the frequency ranges that are covered by the principal detectors on the Gemini North Telescope, which is located close to UKIRT, the thin-lined, open blocks span the bands used as (continuum observation) examples in this study. The instrumentation on
17、the Gemini North telescope was used because it represents the front-line instruments now in use for astronomical observations. The Gemini telescopes and most other infrared facilities are imagers. An image of the patch of sky seen by the telescope is focused on an array of detectors. Each of these d
18、etectors is equivalent to the feed on a conventional, single-antenna radio telescope. The detector array can be used for broad-band imaging or spectral imaging, with the pass-band being chosen by an appropriate filter being 2 Rep. ITU-R BT.2163 placed in front of the detector array. At a typical obs
19、erving wavelength of 1.65 microns, the half-power beamwidth of an 8-m antenna is about 0.05 arc-s, although phase scintillation due to the atmosphere degrades this value. However, with respect to the reception of interference, the angle of note is the solid angle in the sky seen by the entire detect
20、or array, which may be a substantial fraction of a degree across. FIGURE 1 Zenithal atmospheric transmissivity as measured at the United Kingdom Infra-red Telescope on Mauna Kea, Hawaii. Unity indicates full transmission with no (minimal) loss, zero indicates complete absorption Report 2163-016.0 3.
21、0 2.0 1.5 1.2 1.0Wavelength (microns)1.00.80.60.40.20.050 100 150 200 250 300 350Frequency (THz)Transmissivity3 Noise In radio telescopes operating at lower frequencies, the sensitivity is ultimately limited by the combination of sky noise, ground noise, interference where present and noise produced
22、 in the receiver system. The combination of these values is the basis for calculation of the interference thresholds listed in Recommendation ITU-R RA.769. That listing gives threshold values for both continuum (integrating the power over the whole band allocation) and spectral observations, where t
23、he bandwidth used is that of a spectrometer channel. For telescopes operating at frequencies above 50 THz (1 Terahertz, or THz is equal to 1012 Hz) the situation is different: 1 most optical/infra-red telescopes are imagers, using charge-coupled device (CCD) arrays at the focus of the dish/mirror, w
24、hereas most single-antenna radio telescopes are single-pixel devices, although this is now changing; 2 the noise levels in radio telescopes using bands currently allocated to the radio astronomy service are dictated by a combination of ground, sky and receiver noise. In infra-red telescopes the dete
25、ctor noise is generally negligible compared with the noise from the sky (the troposphere); Rep. ITU-R RA.2163 3 3 because the sky noise comprises multiple spectral lines from atmospheric gases, continuum noise thresholds are larger than those in spectral observations because the continuum bandwidth
26、covers multiple spectral line noise peaks whereas the narrow channel bandwidths used in spectral observations can often be fitted into the low-noise valleys between the noise peaks, so the difference is not merely a factor of (bandwidth ratio)1/2; 4 the main lobe of the “antenna pattern” often has a
27、 half-power bandwidth less than an arc-second, so the main lobe and the inner side lobes are all closely grouped in the sky. Telescopes are usually in buildings, so interference is not usually picked up by back lobes (unless locally generated); it gets in through the aperture in the dome where the t
28、elescope looks out; 5 some of the formulae used in conventional radio astronomy are invalid for THz frequencies. For example, the frequently-used Rayleigh-Jeans approximation to the black body may or may not be applicable, depending upon what is being observed. These factors suggest an alternative a
29、pproach would be more appropriate in considering protection criteria and interference thresholds. 4 Interference thresholds Since the noise level is largely dictated by sky noise rather than detector noise, the sensitivity of the telescope is essentially dictated by the size of the mirror/antenna. T
30、he approach to defining thresholds is therefore based on observations made using a telescope (Gemini North, located on Mauna Kea, Hawaii). We then use these to produce quantities that are independent of the telescope and can therefore be applied generically in the same manner as the thresholds in Re
31、commendation ITU-R RA.769 are applicable at longer wavelengths. In general a measurement of an astronomical quantity consists of the value plus superimposed noise. In typical observations made at THz systems using the Gemini North Telescope, a measurement is deemed valid when it stands out above the
32、 background noise by about 5 times the r.m.s. noise level. Due to the low noise levels produced by the detectors, the noise degrading the desired measurement is almost entirely due to sky noise entering the main beam of the antenna pattern. The interference threshold spfd corresponding to the method
33、ology described in Recommendation ITU-R RA.769 is given by: 00dB17 GSS += where S corresponds to the detection threshold spfd for the detector, which is defined as 5- above the noise and G0 is the boresight gain of the telescope dish/mirror in dB. There is a 7 dB difference between the defined detec
34、tion criterion and the r.m.s. background noise fluctuation spfd, which corresponds to 1-. The acceptable level of interference as defined according to the methodology in Recommendation ITU-R RA.769 is 10 dB below that value. This approach is used in estimating the spfd thresholds tabulated below. Th
35、e mirrors on the Gemini telescopes have a diameter of 8 m and we assume here an aperture efficiency of 50%. The spectral power flux-densities are in units dB(W/m Hz). Whereas at the longer wavelengths it is general practice to express the position of a wave in the electromagnetic spectrum in terms o
36、f the frequency of the wave, in infra-red and near-infrared astronomy it is the convention to use wavelength. The tables show the band parameters in terms of wavelength, as received from the infra-red astronomy community contributing to this Report, and also in terms of frequency. Conveniently, the
37、frequencies fL and fHin THz are given by 300 divided by the wavelength in microns. 4 Rep. ITU-R BT.2163 Continuum observations Wavelength () Frequency (THz) Threshold spfd Band LCHBW fLfCfHBW Gain 5- spfd J 1.15 1.24 1.33 0.18 225.56 243.22 260.87 35.31 143 326 199 H 1.49 1.64 1.78 0.29 168.54 184.9
38、4 201.34 32.80 141 320 197 K 2.00 2.18 2.36 0.36 127.12 138.56 150.00 22.88 138 324 202 (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) Spectral line observations Wavelength () Frequency (THz) Threshold spfd Band LCHBW fLfCfHBW Gain 5- spfd J 1.15 1.24 1.33 2.5E-4 225.56 243.22 260.87 0.048 143 3
39、24 197 H 1.49 1.64 1.78 3.2E-4 168.54 184.94 201/34 0.036 141 324 200 K 2.00 2.18 2.36 4.3E-4 127.12 138.56 150.00 0.027 138 324 202 L 3.43 3.78 4.13 6.7E-4 72.64 80.05 87.46 0.014 133 324 207 M 4.55 4.67 4.79 9.4E-4 62.63 64.28 65.93 0.013 132 324 209 (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (
40、12) (1)Band designation. Unfortunately the band designations in this wavelength regime duplicate some of those applied at lower frequencies. (2)Lower wavelength limit of detector operating band. (3)Wavelength of centre of detector operating band. (4)Upper wavelength limit of detector operating band.
41、 (5)“Wavelength bandwidth”, difference in wavelength between the lower and upper operating wavelength limits for the detector. (6)Lower frequency limit of detector operating band. (7)Centre frequency of the detector operating band. (8)Upper frequency limit of the detector operating band. (9)Frequenc
42、y Bandwidth covered by the detector. (10)The gain of the 8-m mirror/antenna of a Gemini Telescope at the operating frequency, assuming an aperture efficiency of 50%. (11)The spectral power flux-density of an observed signal exceeding the rms noise level by a factor of 5, for an integrating time of 2
43、 000 s. (12)Using the antenna gain and the 5-sigma detection spectral power flux-density, we obtain this value for the level of additional noise or interference that received through the far side lobes, would degrade the rms noise fluctuation by 10%. (13)In the case of spectral line observations, th
44、e bandwidth in this column is the wavelength interval covered by one spectrometer channel. The bandwidth in the continuum case is simply the difference between the low and high frequencies or wavelengths defining the operating band. In the case of spectral observations the channel bandwidth is the b
45、andwidth of the spectrometer sensor. This is given in microns, so the equivalent bandwidth in THz is: 2/3002/300+=f where is the wavelength of that channel and is its bandwidth in microns (as per table). Rep. ITU-R RA.2163 5 5 Current observing sites There are many telescopes in the world with the c
46、apability to make observations in the THz bands, and the number is increasing. Telescopes with mirrors as large as 30 m are under development. However, the number of sites combining high-altitude, low levels of manmade interference and good accessibility is comparatively small. The list below shows
47、current sites around the world where THz telescopes are in operation, under construction or envisaged. North latitudes and East Longitudes are labelled +, and South latitudes and West longitudes are labelled. Site Country Latitude Longitude Height (m) AAO Australia 31:16:37 +149:03:58 1 164Almeria S
48、pain +37:13:25 02:32:46 2 168 Apache Point USA +32:46:49 105:49:13 2 788Armazones Chile 24:35:52 70:11:46 2 701 Las Campanas Chile 29:00:54 70:44:12 2 722Canary Islands Spain +28:45:24 17:53:30 2 400 Cerro Pachon Chile 30:14:27 70:44:12 2 722Cerro Tololo Chile 30:10:09 70:48:21 2 200 Chajnator Chile
49、 23:01:22 67:45:18 5 062Dome Circe Antarctica 75:06:00 +123:21:00 3 233 Flagstaff USA +35:05:49 111:32:09 2 163Kitt Peak USA +31:57:30 111:38:48 2 096 Mauna Kea USA +19:49:16 155:28:06 4 205Mount Graham USA +32:42:06 109:52:19 1 926 Mount Hopkins USA +31:41:18 110:53:05 2 617 Paranal Chile 24:37:38 70:24:15 2 635 Pic du Midi France +42:56:11 +00:08:34 2 877 San Pedro USA +30:45:00 115:13:00 2 830 La Silla Chile 29:15:15 70:44:22 2 4006 Mitigation The interference situation is quite different at THz frequencies
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