1、 Rec. ITU-R S.1525-1 1 RECOMMENDATION ITU-R S.1525-1 Impact of interference from the Sun into a geostationary-satellite orbit fixed-satellite service link (Question ITU-R 236/4) (2001-2002) The ITU Radiocommunication Assembly, considering a) that Sun transits are a natural phenomena in geostationary
2、-satellite orbit fixed-satellite service (GSO FSS) networks, which occur over a period of 3-9 days twice a year, depending on the antenna diameter; b) that GSO FSS earth station operators and customers plan for the Sun transits and implement appropriate means to minimize their impact; c) that GSO FS
3、S earth station operators should have a methodology available to analyse the magnitude of the interference from the Sun and the timing of the interference events, recommends 1 that, in designing GSO FSS links, the methodology given in Annex 1 may be used to assess the level of carrier-to-noise ratio
4、 (C/N) degradation of a GSO link resulting from Sun transit; 2 that GSO FSS earth station operators may use the method in Annex 2 to predict the date and time of a Sun transit at an earth station. ANNEX 1 Calculation of the level of interference from the Sun into a GSO FSS link Sun transits occur tw
5、ice a year near the spring and autumn equinoxes when the Sun passes close to the main beam of the receiving GSO earth station. During these Sun transits, the microwave radiation from the Sun acts as a source of interference, increasing the effective noise temperature of 2 Rec. ITU-R S.1525-1 the sat
6、ellite link and therefore degrading the link performance. If the amount of degradation exceeds the clear-sky margin of the link, then the link will suffer an outage usually termed a Sun-outage. For frequencies below about 30 GHz, emission from the Sun can be considered as having three components: th
7、e thermal emission from the “quiet” Sun, a slowly-varying component related to the number and size of sunspots, and occasional intense bursts of emission due to Sun flares. All three components are time-varying, and so it is extremely difficult to use the Sun as a reference source for accurate evalu
8、ation of the performance of earth station antennas. In the satellite communication bands, the thermal emission from the quiet Sun decreases with increasing frequency. The emission is generally unpolarized. Sunspots are magnetic regions on the Sun, appearing as dark spots on its surface. They typical
9、ly last for several days, although very large ones may last for several weeks. Although the number of sunspots varies greatly from day to day, there is an underlying pattern with a period of approximately 11 years. Every 11 years, the Sun undergoes a period of activity called the “Sun maximum”, foll
10、owed by a period of quiet called the “Sun minimum”. During the Sun maximum there are many sunspots, Sun flares, and coronal mass ejections, all of which can affect communications and the weather on Earth. There is a rough correlation between the total solar flux and the number of sunspots. As this i
11、ncreased flux is associated with several small areas of the Suns surface, it is unsafe to assume a uniform brightness distribution across the face of the Sun. In fact, if the GSO earth station has an antenna beamwidth smaller than the apparent diameter of the Sun, then it could experience varying le
12、vels of interference during a single Sun transit event. Emission due to sunspots is somewhat circularly polarized, although this effect is diluted by the random polarization of the thermal emissions from the quiet Sun. Sun flares or bursts may double or triple the Sun flux, occasionally reaching fac
13、tors of 100 times the usual level. In the satellite communication bands, most events are fairly short 5 min to about an hour. Occurrence is unpredictable, but as already noted the events are more frequent around the Sun maximum. Typically there could be a couple of events per day. The apparent diame
14、ter of the Sun at microwave frequencies is slightly larger than the optical diameter. Also, the Earth-Sun distance varies slightly during the year, resulting in a variation in the apparent diameter of the Sun and thus in apparent brightness temperature. However these effects are small compared with
15、other uncertainties (such as the sunspot number), and may thus be neglected. A reasonable estimate of the apparent diameter of the Sun at the equinox is 0.53. Rec. ITU-R S.1525-1 3 Although the foregoing material indicates that significant and unpredictable variations exist in the effective level of
16、 the Sun flux in the satellite communication bands, a number of simple models have been proposed for the average level. These models are adequate for assessment of the typical levels of interference which can be expected during Sun transits. One expression for the brightness temperature of the quiet
17、 Sun at microwave frequencies is: 75.0000120= fTSunwhere: TSun: equivalent brightness temperature (K) f : frequency (GHz) : factor to account for the polarization of the emissions from the Sun, which could be taken to be 0.5 for the reasons given above. This model gives a value of around 21 000 K fo
18、r the quiet Sun at a frequency of 4 GHz. By comparison, a typical value at sunspot maximum would be 90 000 K. 1 General approach The Sun transit of a GSO receiver is a phenomenon that can be easily assessed as the geometry is well known. The method described in detail in Recommendation ITU-R BO.1506
19、 which evaluates the impact of solar inteference into broadcasting-satellite service (BSS) links may be used also in the case of FSS links. That Recommendation fully describes the Sun transit effect on GSO FSS link budgets. The impact of Sun transit is not a fade but an increase of the system noise
20、temperature that can be significant for some low margin, low noise GSO links. 2 Application of the methodology to different antenna sizes The detailed approach described in Recommendation ITU-R BO.1506 has been applied to different antenna sizes. Two possible approaches could be used to generate the
21、 solar noise temperatures for the off-axis angle and the azimuth angle as seen at the focal point of an FSS earth station antenna: Full simulation of the motion of the Sun, using for example the algorithm developed in Annex 2. A simplified approach based on the fact that the declination angle of the
22、 Sun changes at approximately 0.4 per day at the equinoxes and its hour angle changes at approximately 0.25 per min. The results in Figs. 1 to 6 are typical of those produced using these methods. In all the cases the initial noise temperature used is 150 K and the antenna patterns used are according
23、 to Recom-mendation ITU-R S.465 at 11 GHz. 4 Rec. ITU-R S.1525-1 1525-011 3 5 7 9 1 13151719212325272931333501 0002 0003 0004 0005 0006 000DaysSky noisetemperature (K)FIGURE 1Daily maximum sky noise temperature increase for a 10 m antenna1525-021 3 5 7 9 1 13151719212325272931333537390246810121416Da
24、ysDegradation ofC/N(dB)FIGURE 2Daily maximum degradation of the received C/N of a 10 m antennaRec. ITU-R S.1525-1 5 1525-031 3 5 7 9 1 1315171921232527293133353705001 0001 5002 0002 5003 000DaysSky noisetemperature (K)FIGURE 3Daily maximum sky noise temperature increase for a 3 m antenna1525-041 3 5
25、 7 9 1 131517192123252729313335373902468101214DaysDegradationofC/N(dB)FIGURE 4Daily maximum degradation of the received C/N of a 3 m antenna6 Rec. ITU-R S.1525-1 1525-051 3 5 7 9 1 1315171921232527293133353739020120406080100DaysSky noisetemperature (K)FIGURE 5Daily maximum sky noise temperature incr
26、ease for a 0.6 m antenna1525-061 3 5 7 9 1 1315171921232527293133353739DaysDegradationofC/N(dB)FIGURE 6Daily maximum degradation of the received C/N of a 0.6 m antenna02.521.510.5The impact on a link performance depends on the size of the antenna and the initial noise temperature of the link. For la
27、rge antennas with high gain, the degradation of the C/N can be up to 15 dB (as shown in Fig. 2) but occurs fewer times than for small antennas with wider beams (e.g. Fig. 6). As was expected the results show that the depth of the degradation of the C/N is a function of the antenna size and the durat
28、ion of the Sun transit increases as the antenna diameter decreases. Rec. ITU-R S.1525-1 7 3 Variation during a day Computations have been done 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. 1525-07(h)082
29、46101214160 6 12 2418Degradation ofC/N(dB)FIGURE 7Degradation of the received C/N for a 10 m antenna during a day1525-08(h)0210 6 12 18 24Degradation ofC/N(dB)FIGURE 8Degradation of the received C/N for a 0.6 m antenna during a day2.51.50.5ANNEX 2 Simplified method to calculate the Sun transit perio
30、d for a GSO earth station 1 Introduction GSO FSS earth station operators have accepted that Sun interference is a natural phenomenon that occurs for a short period 0 to 21 days before or after the equinoxes, depending on whether the station is located in the Northern or Southern Hemisphere. 8 Rec. I
31、TU-R S.1525-1 Most operators use simplified algorithms such as the one given below, which does not require a specific carrier link budget, to estimate the day and times when a Sun transit will occur. With this information, they can take proactive action to mitigate the effects of Sun interference. 2
32、 Satellite ephemeris data Satellite operators use a number of different mathematical models to represent the movement of a satellite. A simplified approach was developed by one GSO satellite operator whereby, instead of computing all physical effects acting on a satellite, these effects are defined
33、in terms of three equations. This approximation contains 11 parameters obtained with a least-squares curve fit. It has been demonstrated that this simplified model approximates the full prediction results to better than 0.01 for a period of up to seven days. In this approach, the three equations tha
34、t predict the satellites position at any relative time, t, from the start of the epoch are: Satellite east longitude: L = L0+ L1t + L2t2+ (Lc+ Lc1t) cos(Wt) + (Ls+ Ls1t) sin(Wt) + (K/2) (lc2 ls2) sin(2Wt) K lclscos(2Wt) (1) Satellite geocentric latitude: l = (lc+ lc1t) cos(Wt) + (ls + ls1t) sin(Wt)
35、(2) Satellite radius: rsat= rg(1 2L1/3(W L1) (1 + KLcsin(Wt) KLscos(Wt) (3) where: W = L1+ 360/trdegrees/day tr: rotation period of the Earth, in days rg: nominal radius of GSO satellite orbit (km) K = /360 t : time in days and the eleven parameters are: L0: mean longitude (East of Greenwich) (degre
36、es) L1: drift rate (degrees/day) L2: drift acceleration (degrees/day/day) Lc: longitude oscillation-amplitude for the cosine term (degrees) Lc1: rate of change of longitude, for the cosine term (degrees/day) Ls: longitude oscillation-amplitude for the sine term (degrees) Ls1: rate of change of longi
37、tude, for the sine term (degrees/day) lc: latitude oscillation-amplitude for the cosine term (degrees) lc1: rate of change of latitude, for the cosine term (degrees/day) ls: latitude oscillation-amplitude for the sine term (degrees) ls1: rate of change of latitude, for the sine term (degrees/day). R
38、ec. ITU-R S.1525-1 9 With the satellite position defined as a function of time in terms of a geocentric system aligned with the Greenwich Meridian, the satellite position with respect to the earth station and the appropriate pointing angles are calculated as follows: r = rsat rsta(4) rx= rsat cos(sa
39、t) cos(sat sta) Ra(5) ry= rsat cos(sat) sin(sat sta) (6) rz= rsatsin(sat) Rz(7)where: sat: satellite latitude (geocentric) sat: satellite longitude sta: station longitude East of Greenwich Ra: station radial distance from Earth rotational axis Rz: station axial distance above Earth equatorial plane.
40、 The calculation of the earth station coordinates are generally known in terms of its geodetic latitude, longitude and height above a reference ellipsoid (altitude). This reference ellipsoid is based on an equatorial radius and a flattening constant. The earth station position is calculated in terms
41、 of the radial distance from the rotational axis of the Earth Raand the axial distance north of the equatorial plane of the Earth, Rz, which are calculated as follows: Ra= (R + h) cos(sta) (8) Rz= R(1 f )2+ h sin(sta) (9) 2/12)(sin()2(1staequffRR= (10) where: h : geodetic height of the earth station
42、 above the ellipsoid (km) sta: geodetic latitude of the Earth station f : flattening of the Earth ellipsoid Requ: equatorial radius of Earth ellipsoid. From the above, the azimuth and elevation pointing is determined as follows: Earth station azimuth angle: AZ = arctan (ry /rNorth) (11) Earth statio
43、n elevation angle: ELgeometric = arctan (rzenith /(rNorth2+ ry2)1/2) (12) where: rNorth= rx sin(sta) + rzcos(sta) (13) rzenith= rxcos(sta) + rz sin(sta) (14) 10 Rec. ITU-R S.1525-1 3 Predicted Sun transit periods In order to calculate the timing of the interference from the Sun, earth station pointi
44、ng angles in terms of the equatorial coordinate system (ECS) are required, which will be described in more detail in the following paragraph. This is the same coordinate system used for polar antenna mounts. The hour angle and declination in the ECS are calculated from AZ and EL above as: =)(cos)(si
45、n)(cos)(cos)sin()(sin)(cosarctanangleHourAZELELAZELstastaDeclination = arcsin(sin(EL) sin(sta) + cos(EL) cos(sta) cos(AZ) 3.1 Coordinate system for calculating Sun transit The calculations for Sun interference are based on the equatorial coordinate system, where the Earths equator is the reference p
46、lane and the vernal equinox is the reference direction. The vernal equinox or The First Point of Aries, is the intersection of the ecliptic (mean plane of the Earths orbit) with the celestial equator, at which the Sun crosses the equator from south to north. The Earths centre is the origin of this s
47、ystem, which is illustrated in Fig. 9. 1525-09RADEquatorFirst Point of AriesSunHour angleEarth stationSuns MeridianGreenwich MeridianHorizon planeZenithEclipticFIGURE 9Celestial sphereD: Declination with respect to the EquatorRA: Right ascension of the SunVernalequinoxDeclination and right ascension
48、 give the coordinates of the Sun. Declination is the angle between the equatorial plane and the Sun. Right ascension is the angle, measured counterclockwise relative to celestial north in the equatorial plane from the vernal equinox to the current position of the Sun. The hour angle is the angular d
49、ifference between the observers longitude and the longitude of the Sun. Rec. ITU-R S.1525-1 11 3.2 Calculating the coordinates of the Sun Time is measured with respect to the period of the Earths rotation in terms of the Sun day, which is the period between successive Sun passages through the observers meridian. Since the Earth also circles the Sun in a one-year period, the Sun day is not the true period of the Earths rotation. The Earth travels 1/365 of the way around its or
