1、 Rep. ITU-R RA.2131 1 REPORT ITU-R RA.2131 Supplementary information on the detrimental threshold levels of interference to radio astronomy observations in Recommendation ITU-R RA.769 (2008) 1 Introduction Detrimental threshold levels of interference for radio astronomy were first adopted in 1963 an
2、d were given in ex-CCIR Report 224. Ex-CCIR Report 224 was revised repeatedly in response to advances in radio astronomy techniques and its contents were incorporated into Recommendation ITU-R RA.769 in 1992, when ITU-R replaced the International Radio Consultative Committee (CCIR) within the ITU st
3、ructure. Recommendation ITU-R RA.769 was first revised in 1995 (Recommendation ITU-R RA.769-1) after some experience had been gained with its use. The current version of the Recommendation ITU-R RA.769-2, was adopted in 20031in response to the realignment of allocations above 71 GHz and continuing a
4、dvances in millimetre receiver technology. This Report provides supplementary information on the threshold levels of interference detrimental to radio astronomy observations. The threshold levels contained in the tables in Recommendation ITU-R RA.769 are expressed as power flux density (pfd) and/or
5、spectral power flux density (spfd) in units of dB(W/m2) and dB(W/(m2 Hz), respectively. However, for some applications it is more useful to express these thresholds in terms of the electric field strength: for instance, propagation studies express signal level in those terms. Section 2 provides the
6、conversion formulae necessary to convert pfd and spfd values to electric field strength. Section 3 summarizes the transition from Recommendations ITU-R RA.769-1 to ITU-R RA.769-2. Section 4 gives tables of pfd, spfd and electric field values. To assist in interpolating threshold values between the r
7、ather widely-spaced entries in the tables of Recommendation ITU-R RA.769, these thresholds are also shown graphically in 5. 2 Conversion formulae The basic relation between electric field strength E (V/m) and the pfd (W/m2) is: pfd = E2/120 (1) where the factor 120 represents the impedance of free s
8、pace. The electric field strength expressed in microvolts/m is E(V/m) = 106(120 pfd), which on being expressed in dB, according to generally understood engineering usage2, is E(dB(V/m) = 20 log 106(120 pfd)or: E(dB(V/m) = 145.7633 + 10 log pfd (2) 1Egypt and Syria in conformity with the Arab positio
9、n at Radiocommunication Assembly 2003 regarding Recommendation ITU-R RA.769-2 will object to the content of this Report being based on the language used in this Recommendation, especially the use of the word “detrimental”. 2Engineering convention in the electro-magnetic context relates voltage ratio
10、s of E, when expressed in dB, to the change in E that corresponds to the same dB change in radiated power: hence equation (2). 2 Rep. ITU-R RA.2131 3 Threshold level comparison of Recommendations ITU-R RA.769-1 and ITU-R RA.769-2 Recommendation ITU-R RA.769-1 and the more recent version Recommendati
11、on ITU-R RA.769-2 each contain tables entitled “Threshold levels of interference detrimental to radioastronomy observations” (Tables 1 and 2, in both versions), and “Threshold interference levels for VLBI observations” (Table 3 in Recommendation ITU-R RA.769-2, that corresponds to Table 4 in Recomme
12、ndation ITU-R RA.769-1). The differences between the two versions of the Recommendation (and the values in the tables) are slight and may be summarized as follows: The bands, bandwidths and thresholds in Tables 1 and 2 of both versions of Recommendation ITU-R RA.769 are identical below 89 GHz, excep
13、t for the introduction of the 23.8 GHz band, that was added to the tables in Recommendation ITU-R RA.769-2. At or above 89 GHz, the threshold values in Tables 1 and 2 of Recommendation ITU-R RA.769-2 are 2-6 dB more stringent than in Recommendation ITU-R RA.769-1 owing to lower system temperatures a
14、nd increased bandwidths reflecting advances in technology. The set of bands above 89 GHz in Recommendation ITU-R RA.769-2 is more directly reflective of the primary bands allocated to the RAS after the reallocation of spectrum above 71 GHz at WRC-2000. For the VLBI case detrimental threshold levels
15、are given only up to 86 GHz in the Tables. The threshold levels in Recommendation ITU-R RA.769-2 are a few dB more stringent than in the earlier version, in some cases this is due only to the correction of roundoff errors. The detrimental threshold levels below 89 GHz remained unchanged between Reco
16、mmendations ITU-R RA.769-1 and ITU-R RA.769-2. Two compensating changes were made in the formulae and system noise values employed in the two versions: a) The rms power fluctuation in the RAS receiver in a given time and bandwidth (specified by equation (1) in either version of the Recommendation) i
17、s smaller in Recommendation ITU-R RA.769-1 by a factor 1/2, corresponding to the assumption that an unpolarized signal is observed simultaneously in two independent polarizations which are co-added. This assumption was removed in Recommendation ITU-R RA.769-2, recognizing that astronomical signals m
18、ay be polarized, and that observations are commonly performed independently in cross polarized channels in order to measure the polarization, which is an important astronomical parameter. Thus the polarization channels of a radio astronomy receiver must be treated independently even when two polariz
19、ations are being observed. b) The assumed receiver noise values were lowered, corresponding to advances in the state of the art of receiver construction. A factor 2 = 1.5 dB reduction in the system noise was deemed to be appropriate and this generalized reduction was applied. 4 Tables of threshold v
20、alues as pfd, spfd and dB(V/m) Tables 1 and 2 in this Report correspond to Tables 1 and 2 in Recommendation ITU-R RA.769. They repeat the centre frequencies, bandwidths, pfd and spfd values from Recommendation ITU-R RA.769, making clear the differences which exist at/above 89 GHz, and add a column o
21、f electric field values derived using equations (2) and (3). The tables are split into two parts. Tables 1a and 1b list the bands where the detrimental level remains unchanged in going from Recommendation ITU-R RA.769-1 to Recommendation ITU-R RA.769-2, while Tables 1b and 2b list the bands where th
22、e detrimental level did change. Table 3 of this Report provides a similar conversion of spfd values, for the more interference-tolerant VLBI continuum case. Rep. ITU-R RA.2131 3 The pfd values (dB(W/m2) in Tables 1 and 2 of Recommendation ITU-R RA.769, represented by TV(769), are converted to electr
23、ic field values by: E(dB(V/m) = 145.7633 + TV(769) (3) For the VLBI threshold levels in Table 4 of Recommendation ITU-R RA.769-1 and Table 3 of Recommendation ITU-R RA.769-2, it is necessary to convert the tabulated spfd values to pfd values, taking into account that pfd = spfd bandwidth. Below 86 G
24、Hz the relevant bandwidths are those given in Table 1 of Recommendation ITU-R RA.769. At 86 GHz, the bandwidth used was 1 000 MHz because wider bandwidths cannot presently be employed in this type of observation. TABLE 1a Threshold levels of interference detrimental to radio astronomy continuum obse
25、rvations Entries common to Recommendations ITU-R RA.769-1 and ITU-R RA.769-2 Centre frequency (MHz) Bandwidth (MHz) pfd (dB(W/m2) Spectral pfd (dB(W/(m2 Hz) Electric field (dB(V/m) 13.385 25.610 73.8 151.525 325.3 408.05 611 1 413.5 1 665 2 695 4 995 10 650 15 375 23 800 31 550 43 000 0.05 0.12 1.6
26、2.95 6.6 3.9 6.0 27 10 10 10 100 50 400 500 1 000 201 199 196 194 189 189 185 180 181 177 171 160 156 147 141 137 248 249 258 259 258 255 253 255 251 247 241 240 233 233 228 227 55.2 53.2 50.2 48.2 43.2 43.2 39.2 34.2 35.2 31.2 25.2 14.2 10.2 1.2 +4.8 +8.8 4 Rep. ITU-R RA.2131 TABLE 1b Threshold lev
27、els of interference detrimental to radio astronomy continuum observations Entries differing between Recommendations ITU-R RA.769-1 and ITU-R RA.769-2* Centre frequency (MHz) Bandwidth (MHz) pfd (dB(W/m2) Spectral pfd (dB(W/(m2 Hz) Electric field (dB(V/m) 22 355 (769-2 only) 290 146 231 0.2 89 000 6
28、000 8 000 125 129 222 228 +20.8 +16.8 110 500 (769-1 only) 11 000 121 222 +24.8 150 000 (769-2 only) 8 000 124 223 +21.8 166 000 (769-1 only) 4 000 120 216 +25.8 224 000 14 000 8 000 114 119 215 218 +31.8 +26.8 270 000 10 000 8 000 113 117 213 216 +33.8 +28.8 * Arrows have been used to indicate chan
29、ges in the sense Recommendations ITU-RRA.769-1 ITU-R RA.769-2. TABLE 2a Threshold levels of interference detrimental to radio astronomy spectral-line observations Entries common to Recommendations ITU-R RA.769-1 and ITU-R RA.769-2* Centre frequency (MHz) Bandwidth (kHz) pfd (dB(W/m2) Spectral pfd (d
30、B(W/m2 Hz) Electric field (dB(V/m) 327 1 420 1 612 1 665 4 830 14 488 22 200 23 700 43 000 48 000 10 20 20 20 50 150 250 250 500 500 204 196 194 194 183 169 162 161 153 152 244 239 238 237 230 221 216 215 210 209 58.2 50.2 48.2 48.2 37.2 23.2 16.2 15.2 07.2 06.2 * Arrows have been used to indicate c
31、hanges in the sense Recommendations ITU-RRA.769-1 ITU-R RA.769-2 Rep. ITU-R RA.2131 5 TABLE 2b Threshold levels of interference detrimental to radio astronomy spectral-line observations Entries differing between Recommendations ITU-R RA.769-1 and ITU-R RA.769-2* Centre frequency (MHz) Bandwidth (kHz
32、) pfd (dB(W/m2) Spectral pfd (dB(W/(m2 Hz) Electric field (dB(V/m) 88 600 1 000 144 148 204 208 58.2 62,2 98 000 (769-1 only) 1 000 143 203 57.2 115 000 (769-1 only) 1 000 141 201 55.2 140 000 (769-1 only) 1 500 139 200 54.2 150 000 (769-2 only) 1 000 144 204 58.2 178 000 (769-1 only) 1 500 136 198
33、52.2 220 000 2 500 1 000 133 139 197 199 51.2 53.2 265 000 2 500 1 000 131 137 195 197 49.2 51.2 * Arrows have been used to indicate changes in the sense Recommendations ITU-RRA.769-1 ITU-R RA.769-2. TABLE 3 Threshold levels of interference detrimental to VLBI (continuum) observations Entries differ
34、ing between Recommendations ITU-R RA.769-1 and ITU-R RA.769-2* Centre frequency (MHz) Bandwidth (MHz) pfd (dB(W/(m2 Hz) Electric field (dB(V/m) 325.3 6.6 215 217 1.0 3.0 611 6 211 212 +2.5 +1.5 1 413.5 27 209 211 +11.1 +9.1 2 695 10 204 205 +11.8 +10.8 4 995 10 198 200 +17.8 +15.8 10 650 100 192 193
35、 +33.8 +32.8 15 375 50 187 189 +35.8 +33.8 23 800 400 182 183 +49.8 +48.8 43 000 1 000 173 175 +62.8 +60.8 86 000 1 000(1)166 172 +69.8 +63.8 * Arrows have been used to indicate changes in the sense Recommendations ITU-RRA.769-1 ITU-R RA.769-2. (1) VLBI observations are presently limited to bandwidt
36、hs of 1 GHz or less for technical reasons. 6 Rep. ITU-R RA.2131 5 Graphs of threshold values Figures 1 and 2 in this Report present the threshold values in Tables 1 and 2 of Recommendations ITU-R RA.769-1 and ITU-R RA.769-2 in graphical form. In each figure, threshold values common to both versions
37、of Recommendation ITU-R RA.769 at lower frequencies are labelled “769” and the values unique to either Recommendation ITU-R RA.769-1 or Recommendation ITU-R RA.769-2 at higher frequencies are shown by the dotted and dashed blue or red lines, respectively. Symbols are shown at tabulated frequency bands and are connected in order to assist in interpolation to non-tabulated bands.
