1、 Rec. ITU-R BO.1506 1 RECOMMENDATION ITU-R BO.1506 A methodology to evaluate the impact of solar interference on geostationary (GSO) broadcasting-satellite service (BSS) link performance (Question ITU-R 220/11) (2000) The ITU Radiocommunication Assembly, considering a) that the GSO link can be optim
2、ized for arid regions, with very small link margin thus it is sensitive to interference; b) that such interference-sensitive GSO links will have performance levels driven by other sources of fading and degradation than rain; c) that one of these degradations is caused by solar transits in the main b
3、eam of the receiving antennas and that these degradations could be severe for links with small margins or large antenna diameters; d) that the performances of GSO links are used in some methodologies to determine acceptable interference levels between systems, recommends 1 that, in designing GSO BSS
4、 links, the methodology attached in Annex 1 of this Recom-mendation may be used to assess the level of performance degradation on GSO BSS links resulting from solar transit. NOTE 1 It should be noted that some GSO systems can implement operational measures (such as site or satellite diversity) in or
5、der to diminish the impact of the solar transit on the system performances. ANNEX 1 A methodology to evaluate the impact of solar interference on GSO BSS link performance 1 General approach The solar transit in the GSO receiver is a phenomenon that can be easily assessed as the geometry is well know
6、n. The following method is proposed to fully describe the solar transit effect on GSO link budgets thus allowing for proper assessment of the performances of some links that do not need margins to compensate for rain fades. The impact of solar transit is not a fade but an increase of the system nois
7、e temperature that can be significant for some low margin, low noise GSO links. 2 Rec. ITU-R BO.1506 The proposed method is based on the well-defined geometry of Sun position relative to a specified location on the Earth: the Sun is approximately a 0.53 diameter disk as seen from an Earth point. The
8、 solar transit effect is significant when the disk intersects the main beam of the BSS receiving antenna. The impact may be determined by using either a detailed approach or a simplified approach. The detailed approach varies the antenna gain over the optical disc of the Sun in accordance to the ass
9、umed antenna gain pattern. The simplified approach assumes a constant antenna gain over the optical disc of the Sun corresponding to the gain towards the centre of the Suns disk. The geometry is described in Fig. 1. The Sun can be considered as a noise source moving in a well-defined trajectory acro
10、ss the sky. During the period of the spring and autumn equinoxes, the Sun is located at or near the intersection of the equatorial and ecliptic planes. During this time, the Sun is in line with the receiving BSS antenna and the GSO BSS satellite. This leads to an increase of the antenna noise temper
11、ature that affects the BSS receiver figure-of-merit (G/T) of the link and thereby degrading the carrier-to-noise ratio (C/N). Depending on the link clear-sky margin this degradation can result in link outage for a short period of time. 1506-01FIGURE 1Solar orbitSatellite orbit2 Methodology The metho
12、dology is based on the calculation of the relative position of the Sun to the boresight of the receiving antenna, and thus on the estimation of the antenna noise temperature increase due to solar interference. Step 1: Select the duration over which the calculations are performed on the basis of the
13、beginning and end dates. Select the time step as a function of the antenna size for which the calculations are done. Step 2: For each time step considered, determine the orbital parameters of the Sun: w = 282.9404 e = 0.016709 M = 356.0470 + 0.98560 T Rec. ITU-R BO.1506 3 where: w : argument of peri
14、helion (degrees) e : eccentricity M : mean anomaly (degrees) T : time (days). Step 3: Convert these orbital elements into the equatorial, rectangular, geocentric coordinates (XSun, YSun, ZSun). The eccentric anomaly E is defined by: E = M + e sin(M) (1.0 + e cos(M) then: X1 = DSun(cos(E) e) )sin(1Y1
15、2EeDSunGf7Gf8Gf6Ge7Ge8Ge6= and: V = A tan 2 (Y1, X1) 1Y1X22+=R with: DSun: distance between the Earth and Sun centres A tan 2() : function that an x, y coordinate pair to the correct angle. The longitude of the Sun, lonSun, is then determined by: lonSun = V + w and finally: XSun= R cos(lonSun) YSun=
16、 R sin(lonSun) cos(ecl) ZSun= R sin(lonSun) sin(ecl) where ecl is the obliquity of the ecliptic that can be estimated with: ecl = 23.4393. Step 4: Determine the equatorial, rectangular, geocentric coordinates (XES, YES, ZES) for the earth station (ES) and the GSO satellite (SAT): XES= REarthcos(latE
17、S) cos(lonES+ (EarthT) YES= REarthcos(latES) sin(lonES+ (EarthT) ZES= REarthsin(latES) 4 Rec. ITU-R BO.1506 where: REarth: Earth radius (6 378 km) latES, lonES: latitude and longitude of the earth station Earth: angular speed of the Earth, (rad/days) (2 = 6.2831) T : time step considered (days). and
18、 for the GSO satellite: XSAT= (REarth+ H) cos(latSAT) cos(lonSAT+ (EarthT) = (REarth+ H) cos(lonSAT+ (EarthT) YSAT= (REarth+ H) cos(latSAT) sin(lonSAT+ (EarthT) = (REarth+ H) sin(lonSAT+ (EarthT) ZSAT= (REarth+ H) sin(latSAT) = 0 with: latSAT= 0 H : altitude of the GSO satellite (35 786 km). Step 5:
19、 The angle between the Sun, S, the earth station, E, and the GSO satellite, G, can be obtained by: )cos(| = EGESEGES so: ( )( )()( )()( )()()()()()()GefGefGefGfeGefGefGefGfdGfcGefGefGefGeeGefGefGefGedGec+=222222ZZYXXZZ+YYXXZZZZYYYYXXXXcosESSATESSATESSATESSunESSunESSunESSATESSunESSATESSunESSATESSunYA
20、This angle is the off-axis angle of the Sun viewed from the antenna. Step 6: Determine the value of the antenna gain over the Suns disk: Gf2Gf2Sun,G d)( where is the off-axis angle and is the azimuth angle. Rec. ITU-R BO.1506 5 a) Detailed approach The Sun is modelled by a disk positioned on a spher
21、e centred on the receive earth station. The sphere represents the space seen by the antenna using the spherical angles and . 1506-02zyxP2P1FIGURE 2Sun diskThe sphere onwhich the Sun ispositioned: azimuth angle: off-boresight angle (boresight is defined by z-axis): elevation angle between the center
22、of the Sun to the z-axisz-axis: boresight direction of a receiving stationP1: centre point of the SunP2: point on the Sun diskBoresight direction of a receiving antennaThe z-axis is in the direction of the pointing direction of the receive antenna. The computation can use the axis symmetry of the ge
23、ometry: the points with the same gain form arcs. These result from the intersection of a plan perpendicular to the axis antenna z with the portion of sphere containing the Sun. The value of the integral is so determined by the addition of the different lengths of the iso-gain arcs, multiplied by the
24、 value of the gain for the arc. 6 Rec. ITU-R BO.1506 If is the half angle of view of the Sun (0.266), there are two cases: Case 1: If : 1506-03yxR: projection of solar diskcentred on the pointingdirectionS: projection ofsolar diskFIGURE 3Projection of the Sun on the xy plan* For simplificatioin, the
25、 picture shows a projection of the solar disk which is circular.In reality it is not circular. The disk R is the projection of the Sun when it is centred on the z axis. When the Sun is not on the z-axis, the calculations of the receive antenna gain in the direction of the Sun are done through the in
26、tegration over iso-gain arcs, which have an half-aperture that can vary from 0 to . To determine the overall gain in the direction of the Suns disk Gf2Gf2Sun,G d)( , the following formula applies when is smaller than : =Ge5Gf2Gf2+=)()sin(2d)( G,GSunwhere: GfeGfdGfcGeeGedGec=)sin()sin()cos()cos()cos(
27、cosA Rec. ITU-R BO.1506 7 Case 2: If : 1506-04yxRSTFIGURE 4The calculation above is valid for all the arcs which correspond to iso- lines of less than ( ) (represented by the dotted circle T above). For lower values of , the computation of the gain over the portion of Sun disk, is simplified by the
28、z axial symmetry of the geometry: =Ge5Gf2Gf2=)()sin(2d),(0GG where: G() : linear isotropic antenna gain (function of the off-axis angle ) : angular increment. b) Simplified approach The Sun only subtends approximately 0.53 (Sun) as viewed from the Earth and if we assume that over Sunthe normalized a
29、ntenna gain (Gn) will average out to be Gntowards the centre of the Sun (GnSun), then :d)Gf2Gf2Sun,G( can be approximated by: GfaGfbGf9GeaGebGe9Gf7Gf8Gf6Ge7Ge8Ge6 =Gf2Gf22cos12d),(SunSunnSunGG Step 7: Determine the value of the gain over the entire space: Gf2Gf2SpaceG d),( 8 Rec. ITU-R BO.1506 Due t
30、o the z-axis of the antenna patterns from the ITU-R Recommendations, the calculation is straightforward: =Ge5Gf2Gf2=)()sin(2d),(0GGSpacewhere: G() : the linear isotropic antenna gain, only depending on the off-axis angle : the increment of angle. Step 8: Determine the temperature of the Sun: 75.0000
31、120= fTSunwhere f is the frequency and is the polarization factor, set here to 0.5, due to the fixed polarization of the antenna and the random polarization of the Sun. Step 9: Determine the temperature increase at the receive antenna: Gf2Gf2Gf2Gf2Gf2Gf2Gf2Gf2=SpaceSunSunSpaceSunSunGGTGGTTd),(d),(d)
32、d),(Step 10: Determine the degradation of the receivers C/N ratio as follows: Gf7Gf7Gf8Gf6Ge7Ge7Ge8Ge6 +=00log10)/(TTTNC where T0is the initial system noise temperature. 3 Algorithm to take into account the solar transits in the link budgets In the analysis that uses dynamic link budgets for the i
33、nterference analysis like non-GSO/GSO scenarios, the solar interference can be inserted in the link budgets according to the following algorithm. This algorithm can thus provide the solar impact on the performance (percentage of time during which a C/N level is met) of the links analysed. It also ca
34、n be used to compute the duration of degradation as well as occurrences. Rec. ITU-R BO.1506 9 1506-05Inputs:Frequency of the interestDiameter of the antennaLatitude of the earth StationLongitude of the earth StationLongitude of the satelliteBeginning dateEnd dateTime incrementT = T + incrementDeterm
35、ination of thedegradationDetermination of thetemperature increaseComputation of the gain overthe optical diskTSun= 120 000 f 0.75Gf2Gf2 G(, ) dT : beginning dateIf T end dateComputation of the orbital elements of the Sunw, e, MComputation of XSun, YSun, ZSunComputation of XSAT, YSAT, ZSATComputation
36、 of the angleFIGURE 5EndNoYesComputation of XES, YES, ZESSpace4 Application of the methodology to different antenna sizes The detailed approach described in the previous sections has been applied to different antenna sizes. In all the cases the initial noise temperature used is 155 K, at 12.5 GHz an
37、d the antenna patterns used are according to Fig. 8, Annex 5 of RR Appendix S30. 10 Rec. ITU-R BO.1506 For 3 m: 1506-05a01002003004005006007001 3 5 7 9 111315171921232527290123456781 3 5 7 9 11131517192123252729(Days)Noise temperature increaseindegrees(K)Degradation of(C/N) down(dB)(Days)Rec. ITU-R
38、BO.1506 11 For 1.8 m: 1506-05b012345670501001502002503003504004505001 3 5 7 9 11131517192123252 4 6 8 10 12 14 16 18 20 22 241 3 5 7 9 11131517192123252 4 6 8 10 12 14 16 18 20 22 24(Days)Noise temperature increaseindegrees(K)Degradation of(C/N) down(dB)(Days)12 Rec. ITU-R BO.1506 For 0.6 m: 1506-05
39、c01231 3 5 7 9 111315171921232527290204060801001201401601 3 5 7 9 11131517192123252729(Days)Degradation of(C/N) down(dB)Noise temperature increaseindegrees(K)(Days)0.51.52.53.5As was expected the results show that the depth of the degradation of the C/N is a function of the antenna size and the dura
40、tion of the solar transit increases as the antenna diameter decreases. 5 Variation during a day Computations have been done based on the detailed approach to show a time profile of the degradation of C/N as a function of the time of the day close to the equinox period. The time step is set to 1 s. R
41、ec. ITU-R BO.1506 13 For a 3 m antenna: 1506-05d01002003004005006007001 501 1 001 1 501 2 001 2 501 3 001 3 501(s)Noise temperature increaseindegrees(K)For a 0.6 m antenna: 1506-05e2002040608010012014012104196288371 0461 2551 4641 6731 8822 0912 3002 5092 7182 9271603 1363 3453 554Noise temperature
42、increaseindegrees(K)(s)6 Comparison of two approaches for different antenna sizes The examples below show the results obtained using the two approaches to assess the Sun interference to GSO BSS links. Table 1 shows the estimated antenna noise temperature increases obtained using two approaches. In t
43、he simplified approach, the antenna gain over the Suns optical disc is assumed to be constant and corresponds to the gain towards the centre of the Suns disk. This generally results in the estimated maximum temperature increase to be higher than that obtained using the detailed approach. As shown in
44、 Fig. 6, for smaller antenna apertures typically used in BSS systems, e.g. 1.8 m, there is a maximum difference of about 5%. However, this difference rapidly rises to 30% for a 3 m antenna. 14 Rec. ITU-R BO.1506 TABLE 1 Maximum temperature increase via different antenna diameters 1506-06051015202530
45、3545 60 75 90 120 180 300Antenna diameter (cm)Differencesbetweentwoapproaches(%)FIGURE 6Percentage differences in the temperature between two approaches7 Conclusion The solar transit can cause a significant degradation of a GSO BSS link during the period of the spring and autumn equinoxes during whi
46、ch the Sun is seen from time to time in close alignment to the pointing direction of GSO receive earth station antennas. The impact on a link performance depends on the size of the antenna and the initial noise temperature of the link. For large antennas with high gain, the degradation of the C/N ca
47、n be up to about 7 dB (for a link with initial noise temperature of 155 K) but occurs fewer times than for small antennas with wider beams. The detailed approach gives greater detail and accuracy but increases the analysis complexity. Whereas the simplified approach is less complex to implement. For
48、 small antenna apertures typically used in BSS systems (i.e. 1.8 m) both approaches may be used to assess the Sun interference into a GSO BSS link with an error of less than 0.1 dB. Antenna diameter Maximum temperature increase (detailed approach) (K) Maximum temperature increase (simplified approach) (K) 45 cm 91.8 93.0 60 cm 146.6 148.8 75 cm 200.1 202.9 90 cm 254.2 257.6 1.8 m 529.0 557.1 3.0 m 594.6 780.5
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