1、Rec. ITU-R S.733-2 1 RECOMMENDATION ITU-R S.733-2 DETERMlNATION OF THE G/T RATIO FOR EARTH STATIONS OPERATlNG lN THE FIXED-SATELLITE SERVICE (Question ITU-R 42/4) (1992-1993-2000) The ITU Radiocommunication Assembly, considering a) antenna power gain-to-system noise temperature (Gm; that the primary
2、 figure of merit for earth stations operating in the fiied-satellite service is the ratio of the b) different situations and one method for its prediction, that there are two commonly used methods for measuring earth station G/T, each of which has advantages for recommends 1 measurement of noise pow
3、er emanating from a radio star, using the method explained in Annex 1; 2 geostationary satellite, using the method explained in Annex 2; that one method of measuring the ratio of antenna power gain-to-system noise temperature (Gm is by the that an alternative method for measuring this ratio is the m
4、easurement of a reference signal from a 3 the antenna gain and an estimation of the system noise temperature; 4 NOTE 1 - The G/T of an earth station can be degraded by various naturally occurring processes. Increases in receiving noise temperature due to the atmosphere and precipitation, ground radi
5、ation and cosmic sources are treated in Appendix 1 to this Recommendation. NOTE 2 - Information on determining the G/T of earth stations operating at frequencies greater than 10 GHz and the effects of various noise sources on the performance of earth stations operating in this frequency range is giv
6、en in Annex 3 of this Recommendation. that when neither of the methods explained are applicable, the ratio must be determined by a measurement of that the following Notes should be regarded as part of this Recommendation. NOTE 3 - The accuracy of the alternative method in 5 2 depends on the measurin
7、g accuracy of the power flux-density of satellite emissions at the reference earth station, which is of the order of I1 dB. Further information regarding G/Tmeasurements of receiving systems is given in ex-CCIR Report 276 in Volume 1 (Monitoring of radio emissions from spacecraft at fiied monitoring
8、 stations) and International Electrotechnical Commission (IEC) Publication 835 Part 3. ANNEX 1 Measurement of the G/T ratio with the aid of radio stars 1 Introduction It is desirable to establish a practical method of measuring the G/T ratio with high accuracy, which will permit comparison of values
9、 measured at various stations. This Annex describes a method for the direct measurement of the G/T ratio using radio stars. It should be noted however, that the radio star method is not practical in certain cases (see 5 5). 2 Rec. ITU-R S.733-2 2 Method of measurement By measuring the ratio, r, of t
10、he noise powers at the receiver output, the G/T ratio can be determined using the formula: Radio source where: k: 1: Wf) Y= Flux-density atf GHz (W/(m2 . Hz) G 87ck(r-1) Taurus A Cygnus A Boltzmanns constant (1.38 x JE-) wavelength (m) radiation flux-density of the radio star as a function off, freq
11、uency (W/(m2 . Hz) (P, + Pst) 1 p, P, : noise power corresponding to the system noise temperature T PSt: additional noise power when the antenna is in exact alignment with the radio star 10-26 103.794-0.27810go(l OOOf) D(f)TauA = 10-26 107.256 - 1.279 loglo(l OOOf) (.f )CygA = G (antenna gain) and T
12、 (system noise temperature) are referred to the receiver input. In equation (1), account is taken of the fact that the radiation of the star is generally randomly polarized and only a portion corresponding to the received polarization is received. The radiation flux-density Qu) is obtained by radio
13、astronomical measurements. Omega This method has a basic advantage when compared with the calculation of G/T from G and T measured separately as only one relative measurement is necessary to determine the ratio, instead of two absolute measurements. 10-26 104.056 - 0.378 log10 (1 OOOf) Nf )Omega = 3
14、 Suitable radio stars The discrete radio sources Cassiopeia A, Cygnus A and Taurus A appear to be the most appropriate for measurements of G/T by earth stations in the Northern Hemisphere, while Orion, Virgo and Omega are similarly appropriate for earth stations in the Southern Hemisphere. The flux-
15、densities of Cygnus A and Virgo, however, may not be sufficient in every case. Table 1 gives values of the flux-density of the radio stars indicated, where the frequency is between 1 and 20 GHz. TABLE 1 Flux-densities from radio sources Value of January 1980 (see 0 4.2). Rec. ITU-R S.733-2 3 For the
16、 measurements at frequencies above 10 GHz, the use of the radio waves from planets, Venus for example, as well as above-mentioned radio stars could be advantageous. Flux-densities of the radio waves from planets increase with frequency and their solid angle is very small giving rise to negligible co
17、rrection errors due to angular extension. The flux-density Q(f) is expressed by: where: Tb(f) : brightness temperature of a planet (K) w : semi-diameter. The value of Q(f) derived from equation (2), is substituted in equation (1) to obtain the value of G/T of an earth station. The value of w can be
18、found elsewhere in American Ephemeris and Nautical Almanac (US Government Printing Office, Washington DC 20402). In the case of the planet Venus, the values Tb(f) are thought to be about 580 K and 506 K at 15.5 and 31.6 GHz, respectively. Since the values of Tb(f) are based on a limited amount of me
19、asured data at the frequencies mentioned, and have not yet been determined for other frequencies, further study is required to confii and extend the results given here. 4 Correction factors The corrected value of G/T is given by: (G/T)c = G/T + Ci + C2 + C3 where: Cl: correction for atmospheric abso
20、rption C2 : correction for angular extension of radio stars C3 : correction for change of flux with time. All factors to be given in decibels. The value of atmospheric absorption C1 can be estimated using 5 2.2 of Recommendation ITU-R P.676. (3) 4.1 Angular extension of radio stars If the angular ex
21、tension of the radio star in the sky is significant compared with the antenna beamwidth, a correction must be applied. The following equations are close approximations for the angular extension correction factor, C2, also plotted in Fig. 1. where: : 3 dB beamwidth (degrees) h : wavelength (m) D : an
22、tenna diameter (m). 4 Rec. ITU-R S.733-2 0.30 g 0.20 v LY 9 Q !3 o u 0.10 L“ o .+ Y o e, 0.00 FIGURE 1 Correction factor for the angular extension of radio stars 0.00 0.10 0.20 0.30 0.40 0.50 Half-power beamwidth of antenna (degrees) 2.00 1.50 UN 3 1.00 !3 o L“ 9 .+ Y o e, u 0.50 0.00 Cassiopeia A,
23、Taurus A, Orion, Omega, Virgo Cygnus A - - - - - - - - 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Half-power beamwidth of antenna (degrees) 0733-01 The measured brightness distribution for Cygnus A can be adequately described by a dual columnar shape with 0.02 min of arc in each columns diameter and 2.06 mi
24、n of arc in angular distance. If the annular model for Cassiopeia A and the dual columnar model for Cygnus A are adopted, a convenient approximation is available for the correction factor. These models may also be useful to measure the half-power beamwidth of antennas by observing the half intensity
25、 width of the drift curve. This also means that the correction factor for the angular extension of radio stars can be determined from the observed drift curve itself without the knowledge of the half-power beamwidth of the antenna. 4.2 Change of flux with time Cassiopeia A is subject to a frequency
26、dependent reduction of flux with time. The correction may be obtained from: 0.97 - 0.3 log10 f 1 O0 (4) where: n: number of years elapsed, with n = O in January 1980 f: frequency (GHz). Rec. ITU-R S.733-2 5 4.3 Polarization effects Taurus A, Cygnus A, Orion, Virgo and Omega are elliptically polarize
27、d and it is necessary to use the mean of two readings taken in two orthogonal directions. These precautions are not necessary when using Cassiopeia A. 5 Limitations of the radio star method The method described in this Annex has several disadvantages. These are: - accuracy is not very good for small
28、er earth stations, however, given modern equipment, and careful measurement setup, consistently accurate antenna gain measurements are achievable with y-factors 0.2 dB (see Table 2 for approximate minimum antenna sizes); this technique may not be possible for stations with limited steerability. - TA
29、BLE 2 Minimum allowable antenna diameter for using a radio star to measure antenna gain, assuming 25“ elevation angle andy-factors 0.2 dB Radio star Cassiopeia A Taurus A Cygnus A Minimum antenna diameter at C-band Cassegrain I Prime focus 4.6 I 5.4 5.1 I 5.9 6.0 I 6.0 Minimum antenna diameter at Ku
30、-band Cassegrain I Prime focus 9.3 I 11.0 8.0 I 9.5 16.0 I 18.5 APPENDIX 1 TO ANNEX 1 Contributions to the noise temperature of an earth-station receiving antenna 1 Introduction The noise temperature of an earth-station antenna is one of the factors contributing to the system noise temperature of a
31、receiving system, and it may include contributions associated with atmospheric constituents such as water vapour, clouds and precipitation, in addition to noise originating from extra-terrestrial sources such as solar and cosmic noise. The ground and other features of the antenna environment, man-ma
32、de noise and unwanted signals, and thermal noise generated by the receiving system, which may be referred back to the antenna terminals, could also make a contribution to the noise temperature of the earth-station antenna. Numerous factors contributing to antenna noise, particularly those governed b
33、y meteorological conditions, are not stable and the resulting noise will therefore exhibit some form of statistical distribution with time. A knowledge of these factors and their predicted variation would be a valuable aid to earth-station designers, and there is therefore the need to gather informa
34、tion on the antenna noise characteristics of existing earth stations in a form which can best be interpreted for future use. This Appendix presents results of antenna noise measurements made at 11.45 GHz, 11.75 GHz, 17.6 GHz, 18.4 GHz, 18.75 GHz and 31.65 GHz. From the results measured at 17.6 GHz a
35、nd 11.75 GHz, cumulative distributions of temperatures have been derived together with the dependency of the clear-sky noise temperature on the elevation angle. 6 Rec. ITU-R S.733-2 2 Measuring equipment The antenna noise temperature measurements have been performed in the Netherlands using a series
36、 of radiometers equipped with a 10 m Cassegrain antenna fed by a corrugated horn. These measurements have also been performed in Japan using noise adding type and Dicke type radiometers equipped with 13 m and 10 m Cassegrain antennas, and an 1 1.5 m offset Cassegrain antenna. Noise measurements made
37、 in Germany were carried out on a 18.3 m diameter antenna using the y-factor method, under clear-sky conditions. 3 Results of measurements Figure 2 shows the cumulative time distribution of the measured antenna noise temperature at 11.75 GHz and 17.6 GHz. The noise temperature shown in Fig. 2 is the
38、 value measured at the output flange of the feedhorn. FIGURE 2 Measured antenna temperature as a function of the percentage of time each level was exceeded 25 1 0-2 2: 10-1 5 5 2 1 Time percentage Curves A: 17.6 GHz, 7200 h B: 11.75 GHz, 8100 h Antenna diameter: 10 m Angle of elevation: 30“ 10 2 5 0
39、733-Q2 The main contribution to the antenna noise temperature is caused by atmospheric attenuation. Other contributions are caused by cosmic effects and radiation from the ground. The measurements presented in Fig.2 have been performed at an angle of elevation of the antenna of 30“. The measurement
40、period was between August 1975 and June 1977. The conditions during the measuring period can be considered as being typical for the local rain conditions. Figure 3 shows the elevation dependence of the antenna noise temperature under clear sky conditions. The value of antenna noise temperature of Fi
41、g. 3 corresponds to those of Fig. 2 at the 50% time percentage. An analysis of the measurement results given in Fig. 3 showed that the antenna noise temperature consists of an elevation dependent part and a component which is roughly constant. Rec. ITU-R S.733-2 7 130 120 110 100 90 6 v k 80 .$ Y i3
42、 70 8 60 .+ 40 30 20 10 O FIGURE 3 Antenna noise temperature, TA, as a function of the angle of elevation, a of the antenna under clear-sky conditions 1 O“ 5“ 10“ 15“ 20“ 25“ 30“ 35“ 40“ 45“ Elevation angle, a (degrees) Note 1 - Curves 1 to 6 are identified by reference to Table 3. Note 2 -Measureme
43、nt conditions were as follows: Characteristics Curves 1 and 3 Curve 2 A: calculated B : measured Curve 4 A : measured Curve 5 o : measured Curve 6 0: measured Temperature o( 279 294 296 290 281.5 Relative humidity (%) 82 51 50 49 66 Absolute humidity (g/m3) 6 10 10 I 6 Barometric pressure (mbar) 101
44、6 1 O18 1006 1013 1017 8 Rec. ITU-R S.733-2 1 This constant part is formed by: - - cosmic background microwave radiation having a value of the order of 2.8 Ky noise resulting fiom earth radiation. This contribution changes slightly with the angle of elevation of the antenna due to the side-lobe perf
45、ormance of the radiation diagram. A value of the order of 4 to 6 K is expected fiom this source; a noise contribution due to ohmic losses of the antenna system which is of the order of 0.04 dB. This component is expected to be 3 to 4 K. - 11.75 The elevation dependent part of the antenna noise tempe
46、rature is caused by losses due to water and oxygen in the atmosphere and in order to estimate this elevation dependent part the curves of measured points in Fig. 3 may be approximated by the following function which is accurate to 1% for elevation angles greater than 15“: 0.9858 TA =T, +Tm(l-o cosec
47、cil ) Radiometer K 0.988 where: TA : antenna noise temperature T, : constant part of the noise temperature T, : mean radiating temperature of the absorbing medium o : transmission coefficient of the atmosphere in the zenith direction a: angle of elevation of the antenna. In the range of angles of el
48、evation between 5“ and 90“, the constants of the functi y-factor TA 0.9738 (5) Radiometer re as given in Table 3. 0.940 Based on the constants given in Table 3 and for a = 90“ in equation (5), the second term in this expression leads to the value of the zenith sky temperature caused by atmospheric a
49、ttenuation. The zenith brightness temperature can be found by the addition of the zenith sky temperature and the cosmic microwave background radiation temperature. In this particular case, where atmospheric losses are very low, simple addition is allowed. TABLE 3 Radiometer (see Fig. 3) 5 31.65 11.45 21 0.934 I 17.6 3 Radiometer I 18.4 0.970 Radiometer 18.75 61 I I 8.3 10 18.3 I 7.3 I I 8.3 10 l3 I 9.3 I I 10 I 4.5 Measuring technique I Reference station 10 m OTS Netherlands 18.3 m OTShS-V i in the receiving band of the frequencies F for at least (1 O0 - Pi)% of the time. Li, e
