1、 Report ITU-R RA.2189(10/2010)Sharing between the radio astronomy service and active services in the frequency range 275-3 000 GHzRA SeriesRadio astronomyii Rep. ITU-R RA.2189 Foreword The role of the Radiocommunication Sector is to ensure the rational, equitable, efficient and economical use of the
2、 radio-frequency 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 Region
3、al Radiocommunication 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 submis
4、sion of patent statements 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 Rep
5、orts (Also available 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 re
6、lated satellite services 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 app
7、roved in English by the Study Group under the procedure detailed in Resolution ITU-R 1. Electronic Publication Geneva, 2011 ITU 2011 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without written permission of ITU. Rep. ITU-R RA.2189 1 REPORT ITU-R RA.21
8、89 Sharing between the radio astronomy service and active services in the frequency range 275-3 000 GHz (2010) TABLE OF CONTENTS Page 1 Introduction 1 2 Atmospheric absorption 2 3 Antenna beamwidth 4 4 RF power generation 7 5 Sharing between active services and radio astronomy . 7 5.1 Terrestrial tr
9、ansmitter into terrestrial radio telescope . 8 5.2 Airborne transmitter into radio telescope . 10 5.3 Satellite transmitter into radio telescope . 11 6 Conclusions 12 1 Introduction Certain characteristics of the frequency range 275-3 000 GHz combine to reduce the likelihood of interference between
10、the radio astronomy service and active services in this range. The purpose of this Report is to present a basic introduction to those characteristics and how they affect potential sharing scenarios. Based on the analysis in this Report, there is little chance for interference to radio telescopes fro
11、m co-frequency terrestrial, airborne, or satellite transmitters, particularly at frequencies above 1 000 GHz. The results of this study are applicable to current and future discussions relating to extending No. 5.565 of the Radio Regulations to frequencies in the 275-3 000 GHz range, and studies rel
12、ated to the advancement of technology at frequencies above 275 GHz1. 1In this Report, “THz frequencies” refers to the range 275-3 000 GHz. 2 Rep. ITU-R RA.2189 2 Atmospheric absorption In the range 275-3 000 GHz, propagation through the Earths atmosphere is strongly affected by absorption due to atm
13、ospheric molecules. The molecular species most responsible for the absorption are oxygen (O2) and water vapour (H2O). Non-resonant absorption creates a general continuum of absorption that steadily increases with frequency, while exceedingly large values of attenuation are found at specific frequenc
14、ies corresponding to natural resonances of the molecules. At sea level, the general continuum of absorption is approximately 5 dB/km at 275 GHz, 300 dB/km at 1 000 GHz, and 4 000 dB/km at 3 000 GHz. At specific molecular resonances in this range, the attenuation can be as large as 550 000 dB/km. Att
15、enuation will decrease with altitude due to lower concentrations of oxygen and water vapour. Figure 1 shows attenuation in dB/km at 4 different altitudes: sea level, 300 m, 1 000 m, and 3 000 m. The curve assumes the 1976 Standard Atmosphere model2,3, with the addition of a column of 2 cm total prec
16、ipitable water vapour with a scale height of 2 km, at a sea-level relative humidity of 50%. The atmospheric parameters were used in the am atmospheric transmission model to compute the absorption curves4,5. Based on the assumed atmospheric characteristics, the following inputs were used in the am mo
17、del: TABLE 1 Assumed atmospheric properties for calculating absorption over a horizontal path of 1 km in length Altitude (m) Temperature (K) Pressure (mbar) Column density of dry air (cm2) Column density of water vapour (cm2) 0 288.15 1013.25 2.55 1 024 3.34 1 022 300 286.20 977.73 2.47 1 024 2.87 1
18、 022 1 000 281.65 898.75 2.31 1 024 2.03 1 022 3 000 268.65 701.09 1.89 1 024 7.45 1 021 2U.S. Standard Atmosphere 1976 U.S. Government Printing Office, Washington DC, http:/ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770009539_1977009539.pdf. 3Standard atmosphere calculator available at http:/
19、4Paine, Scott, “The am Atmospheric Model”, Submillimeter Array Technical Memo #152 (Revision 3); available at http:/www.cfa.harvard.edu/spaine/am/. 5Recommendation ITU-R P.676 accurately calculates atmospheric attenuation up to a maximum frequency of 1 000 GHz. The am model is more rigorous for freq
20、uencies above 1 000 GHz. For a consistency check, the data in Fig. 1 and those in Fig. 5 of Recommendation ITU-R P.676 agree well in the region of overlapping frequency coverage. Figure 1 of this Report is based on am for the entire range. Rep. ITU-R RA.2189 3 FIGURE 1 Atmospheric attenuation comput
21、ed over horizontal paths of 1 km at four different altitudes, assuming the atmospheric properties of Table 1. For reference, free-space loss over 1 km is also plotted 0 500 1 000 1 500 2 000 2 500 3 000Attenuation (dB/km)1101001 00010 000100 0001 000 0000 m altitude300 m altitude1 000 m altitude3 00
22、0 m altitudeFree space loss over 1 kmFrequency (GHz)Because atmospheric absorption is a strong factor for terrestrial systems at THz frequencies, calculation of path loss between a transmitter and receiver must include this factor. The signal level at the receiver is: APLGGPPRTTR+= (1) where: PR: th
23、e power at the output port of the receive antenna PT: the power at the input port of the transmit antenna GT: the gain of the transmit antenna in the direction of the receive antenna GR: the gain of the receive antenna in the direction of the transmit antenna PL: the “traditional” path loss between
24、transmit and receive antennas due to geometric spreading and terrain blockage A: the additional loss factor due to atmospheric absorption. All terms are expressed in logarithmic units. Due to extreme atmospheric absorption, typically the only possible interference scenarios involve a transmitter and
25、 victim receiver that are line-of-sight to one another, and therefore the PL factor is free-space loss: 44.92log20log20)dB(GHzkm+= fDPL (2) 4 Rep. ITU-R RA.2189 where: Dkm: the distance between the transmitter and the receiver (km) fGHz: the frequency (GHz). At sea level, the minimum baseline absorp
26、tion rate is approximately 5 dB/km at 275 GHz (i.e. A 5 Dkm). Solving for Dkmat which PL = A shows that atmospheric absorption A will be greater than free-space loss PL for any distance greater than approximately 34 km (free-space loss and atmospheric absorption are both 172 dB at this distance). At
27、 1 000 GHz, the baseline absorption rate is approximately 300 dB/km, and the distance at which free-space loss and atmospheric absorption are the same (150 dB) is approximately 0.5 km. At 3 THz, the baseline absorption rate is approximately 4 000 dB/km, and the corresponding distance at which absorp
28、tion is greater than the calculated free-space loss is about 33 m (loss/absorption are both 132 dB), although this is less than the near field distance of a small 10 cm diameter antenna and the free-space loss formula breaks down. At specific absorption resonance peaks, these distances shrink dramat
29、ically. Consider for example a resonance near 1 411 GHz, where sea level attenuation exceeds 65 000 dB/km. Attenuation exceeds the calculated free-space loss at a distance of only 1.6 m, which is again less than the near field distance of a very small antenna. At higher elevations the conclusions ar
30、e similar. At 3 000 m altitude and 275 GHz frequency, the baseline absorption rate is approximately 1 dB/km, and atmospheric attenuation exceeds free-space loss for distances over about 186 km. At 1 000 GHz frequency, the absorption rate is approximately 100 dB/km, and atmospheric attenuation exceed
31、s free-space loss for distances over about 1.6 km. At 3 000 GHz, the baseline absorption rate is approximately 1 000 dB/km, and the distance is about 150 m. The conclusion is that for frequencies above about 1 000 GHz, atmospheric absorption is typically a more significant factor than geometric spre
32、ading (free space loss). This is especially true for sites that are not on high and dry mountaintops. 3 Antenna beamwidth In addition to atmospheric absorption, small antenna beam sizes also reduce the chances of accidental interference. The beamwidth of a dish antenna, measured in degrees, is given
33、 by the approximate formula: cmGHzdeg1720df (3) where: deg: the approximate beamwidth (degrees) fGHz: the frequency (GHz) dcm: the antennas physical diameter (cm) : a parameter ( 1) that is effectively the fraction of the diameter of the dish illuminated by the feed. A given size antenna will produc
34、e a smaller beamwidth with increasing frequency; alternatively, at a given frequency, a larger dish will create a smaller beamwidth (assuming remains constant). At frequencies near or above 275 GHz, antenna beamwidths are very small, even for small dishes. As an example, a 30 cm diameter dish (about
35、 the size of a large dinner plate) will create a beam of only 0.28 at a frequency of 275 GHz, assuming = 0.75. For illustration, 0.28 corresponds to the approximate angular extent of a medium-sized computer monitor when viewed from the opposite end of a 105 m football playing field. At 1 000 GHz, th
36、e beamwidth of the same antenna is 0.08 Rep. ITU-R RA.2189 5 which corresponds to the angular extent of about 2/3 the diameter of a football when viewed from the opposite end of the playing field. At 3 000 GHz, the beamwidth of the same antenna is about 0.025. To gauge this angle, it is equivalent t
37、o less than the diameter of a tennis ball when viewed from the far end of a football field. Assuming that the antenna beam and a potential interfering emission source are constrained to be in the same plane (for example, an antenna pointed along a horizontal path and an emission source on the ground
38、), the likelihood that a random point source of emission falls within the main beam of a horizontally-pointed antenna is approximately deg/360. The computed probability is approximately 4.6 103for a 5 cm antenna at 275 GHz, to 6.5 104for a 10 cm antenna at 1 000 GHz, to 7 105for a 30 cm antenna at 3
39、 000 GHz (see Table 2). Because of strong absorption in this frequency range, rapid free-space fall-off, and low RF power generation ( 4), it is likely that if any interference occurs between two active terrestrial systems, it would require their antenna beams to be pointing directly at each other.
40、Given the small beam sizes, the chances that two unrelated horizontally pointed terrestrial antennas have beams that point at each other is very small. Roughly speaking, the chance that one antenna is in the beam of the other is approximately deg/360, so that the probability P2Dthat two antennas are
41、 simultaneously within the beams of each other is then: 2GHz2cm222128.22)360( fdPD= (4) where for simplicity the last term has assumed that the antennas are identical in size and illumination efficiency. The following table summarizes the resulting probability for various antenna sizes and frequenci
42、es, assuming = 0.75. TABLE 2 Probability, within the same plane, that a point source of emission falls within the main beam of a randomly-pointed antenna (deg/360), and the probability that two identical unrelated antennas happen to be pointed within each others beams, as a function of frequency and
43、 antenna diameter Frequency (GHz) Antenna diameter (cm) Probability of isotropic sourcein main beam (deg/360) Probability of main beam coupling (P2D) 275 5 5 103 2 105 275 10 2 103 5 106 275 30 8 104 6 107 1 000 5 1 103 2 106 1 000 10 6 104 4 107 1 000 30 2 104 5 108 2 000 5 6 104 4 107 2 000 10 3 1
44、04 1 107 2 000 30 1 104 1 108 3 000 5 4 104 2 107 3 000 10 2 104 5 108 3 000 30 7 105 5 109 6 Rep. ITU-R RA.2189 Similar considerations can be made for satellite antennas, where the probabilities are even smaller because the antennas are not constrained to operate in one plane. In the satellite case
45、, beam solid angles (in steradians or square degrees) are taken into account. For small beams, the fraction of a sphere occupied by the beam (terrestrial or satellite) is given by: 2GHz2cm22deg52deg3.56109.123.57414fd=(5) where is the beam solid angle in steradians, the factor of 57.3 is for convers
46、ion of the beamwidth from degrees to radians, and 4 is the number of steradians in a sphere. Equation (5) is then the fraction of a sphere covered by the beam, which is the inverse of the antennas isotropic gain: 5.17log20log20log204log10)dBi(GHzcm+= fdG (6) The probability that a random source of e
47、mission falls within the main beam is /4, which, for a 10 cm antenna, equals 1.0 105at 275 GHz, 1.0 106at 1 000 GHz, and 1.0 107at 3 000 GHz (see Table 3). The probability that two unrelated antennas are randomly pointed at each other is (1/4) (2/4): 4GHz4cm433170fdPD(7) where for simplicity as in t
48、he 2-dimensional case, it has been assumed the antennas are identical. The 3-dimensional probability computes to the following values as a function of frequency and antenna size. TABLE 3 Probability that a random source of emission falls within the main beam of an antenna (/4), and the probability t
49、hat two identical antennas happen to be pointed directly within each others beams, P3D= (/4)2, as a function of frequency and antenna diameter. The gain of the antenna, = 10log(4/), is also listed Frequency (GHz) Antenna diameter (cm) G (dBi) /4 Probability of main beam coupling (P3D) 275 5 43 5 1053 109275 10 49 1 1052 1010275 30 5
