1、 Rec. ITU-R PI.531-3 219 SECTION 6F: IONOSPHERIC PROPAGATION PREDICTION AND APPLICATIONS AT FREQUENCIES ABOVE ABOUT 30 MHz RECOMMENDATION ITU-R PI.53 1-3 IONOSPHERIC EFFECTS INFLUENCING RADIO SYSTEMS INVOLVING SPACECRAFT (Question ITU-R 218/3) (1 978- 1990- 1992- 1994) The IT Radiocommunication Asse
2、mbly, considering that ionospheric effects may influence the design and performance of ISDN (Integrated Services Digital a Network) and other radio systems involving spacecraft, recommends 1. systems. that the information contained in Annex 1 should be used as required in the planning and design of
3、such ANNEX 1 Ionospheric effects upon Earth-space propagation 1. Ionospheric effects A signal carrier which penetrates the ionosphere is modified by the medium due to the presence of electrons and the Earths magnetic field. Both large-scale changes due to the variation of electron density, as well a
4、s smaller scale irregularities, affect the carrier. The effects include scintillation, absorption, variation in the direction of arrival, propagation delay, dispersion, frequency change and polarization rotation. These effects on transmission, at frequencies mainly above about 20 MHz, are treated in
5、 this Annex. 2. Scintillation 2.1 Introduction Scintillations, as discussed in this Annex, are variations of amplitude, phase, polarization and angle-of-arrival produced when radio waves pass through electron density irregularities in the ionosphere. Ionospheric scintillations present themselves as
6、fast fluctuations of signal level with peak-to-peak amplitude fluctuations from 1 dB to over i0 dB and lasting for several minutes to several hours. The phenomena are caused by one of two types of ionospheric irregularities: sufficiently high electron density fluctuations at scale sizes comparable t
7、o the Fresnel zone dimension of the propagation path, or sharp gradients of ambient electron density, especially in the direction transverse to the direction of propagation. - - Either type of irregularity is known to occur in the ionosphere under certain solar, geomagnetic and upper atmospheric con
8、ditions, and the scintillations can become so severe that they represent a practical limitation for communication systems. Scintillations have been observed at frequencies from about 10 MHz to about 12 GHz. IT!-? RECMN PI.531-3 9h 4855232 0522956 TOO = COPYRIGHT International Telecommunications Unio
9、n/ITU RadiocommunicationsLicensed by Information Handling Services 220 Rec. ITU-R PI.531-3 For systems applications, scintillations can be characterized by the fading depth and period. A useful index to quantify the severity of scintillation is the scintillation index, S4, which is defined as the st
10、andard deviation of received power divided by the mean value of the received power, i.e., where Z is the carrier intensity, and c denotes ensemble average. The fading period of scintillation varies over quite a large range from less than one tenth of 1 s to several minutes, as the fading period depe
11、nds both upon the apparent motion of the irregularities relative to the ray path, and in the case of strong scintillation, on its severity. The fading period of gigahertz scintillation ranges from approximately 1 to 10 s. Long period (of the order of tens of seconds) components of saturated scintill
12、ation (S4 approaches 1) at VHF and UHF bands have also been observed. 2.2 Modelling/scaling rules for system applications Ionospheric scintillations exhibit a wide range of variations in frequency dependence, morphology patterns, and diurnal, seasonal and solar cycle dependence. Different signai sta
13、tistics have been found in different observations. An enormous amount of literature is available and new findings based on refined measurement techniques and modelling methodology appear each year. System engineers are advised to use reliablehelevant published data for applications. If direct data a
14、ndlor findings are not available or applicable, the modelling/scaling rules in the following sections should be used. 2.3 Frequency dependence of scintillation If results from direct measurement are not available, an f-1.5 frequency dependence of S4 is recommended for engineering applications. 2.4 I
15、nstantaneous statistics and spectrum behaviour 2.4.1 Instantaneous statistics During an ionospheric scintillation event, the Nakagami density function is believed to be adequately close for describing the statistics of the instantaneous variation of amplitude. The density function for the intensity
16、of the signal is given by: where the Nakagami %-coefficient“ is related to the scintillation index, S4 by: (3) 2 m = lIS, In formulating equation (2) the average intensity level of I is normalized to be 1.0. The calculation of the fraction of time that the signal is above or below a given threshold
17、is greatly facilitated by the fact that the distribution function corresponding to the Nakagami density has a closed form expression which is given by: I where r(ni, nil) and T(m) are the incomplete gamma function and gamma function, respectively. Using equation (4), it is possible to compute the fr
18、action of time that the signal is above or below a given threshold during an ionospheric event. For example, the fraction of time that the signal is more than X dB below the mean is given by P( 10“ lo and the fraction of time that the signal is more than X dB above the mean is given by 1 - P(lO-x/ *
19、O). COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesRec. ITU-R PI.531-3 221 2.4.2 Spectrum behaviour Since ionospheric scintillations are believed to be caused by relatively stationary refractive-index irregularities moving horizontall
20、y past the radio wave path, the spatial and temporal power spectra are related by the drift velocity. The actual relationship depends on the irregularity composition (power spectra) and a number of other physical factors. As a result, the power spectra exhibit a wide range of slopes, from f- to f-6
21、as have been reported from different observations. A typical spectrum behaviour is shown in Fig. 1. The f-3 slope as shown is recommended for system applications if direct measurement results are not available. FiGURE 1 Power spectral density estiniates for a geostationary satellite (hiekat-IV) at 4
22、 GHz II j 1 Leai fluctuation Fluctuation frequency (Hz) The scintillation event was observed during the evenings of 28-29 April 1977 at Taipei earth station A: 30 min before event onset B: at the beginning C: 1 h after D: 2 hafter E: 3 h after F: 4 h after 1 COPYRIGHT International Telecommunication
23、s Union/ITU RadiocommunicationsLicensed by Information Handling Services222 Rec. ITU-R PI.531-3 2.5 Geomeric consideration 2.5.1 Zenith angle dependence In most models, ,$ is shown to be proportinal to the secant of the zenith angle, i, of the propagation path. This relationship is believed to be va
24、lid up to i = 70“. At greater zenith angles, a dependence ranging between 1/2 and first power of sec i should be used. 2.5.2 Seasonal-longitudinal dependence The occurrence of scintillations and magnitude of S, have a longitudinal as weil as seasonal dependence that can be parameterized by the angle
25、, b, shown in Fig. 2. It is the angle between the sunset terminator and local magnetic meridian at the apex of the field line passing through the line-of-sight at the height of the irregularity slab. The weighting function for seasonal-longitudinal dependence is given by: where W is a weighting cons
26、tant depending on location as well as calendar day of the year. As an example, using the data available from Tangua, Hong Kong and Kwajalein, the numerical value of the weighting constant can be modelled as shown in Fig. 3. 2.6 Morphology Scintillation can be categorized according to geomagnetic lat
27、itude, longitude, and local time. The latitudinal categorization consists of - - - equatorial scintillations, which are caused by plasma bubbles in the equatorial anomaly regions; mid-latitude scintillations, which are correlated with the occurrence of ionospheric spread-F; high-latitude scintillati
28、ons with effects that differ between the auroral and polar regions, though both are closely related to auroral activity and geomagnetic activities. A general sketch is provided in Fig. 4 and a summary of scintillation characteristics is given in Table 1. Observations with the GPS satellites at frequ
29、encies 1.2 and 1.6 GHz showed that in and near the auroral oval, the most severe scintillations occur from noon to 14 MLT (magnetic local time), and from 22 to midnight h4LT. Even near solar maximum, the morning sector (03 to 09 MLT) is relatively free of scintillation activity. These can be seen on
30、 Fig. 5. This figure shows the average scintillation activity observed during six weeks in February and March 1991 (soon after the solar maximum) as a function of geomagnetic latitude and local geomagnetic time. The total electron content (TEC) was measured every 0.8 s, and the standard deviation co
31、mputed from samples of 100 s is coded in grey- scale. 2.7 Cumulative statistics In design of ISDN and other radio systems, communications engineers have concerns not only with the system degradation during an event such as those described in Q 2.3, 2.4 and 2.5, but also with the long term cumulative
32、 occurrence statistics. For communications systems involving a geostationary satellite, which is the simplest radio system configuration, Figs. 6 and 7 are recommended for the assessment and scaling of occurrence statistics. The sunspot numbers (SSN) cited are the 6-month averaged sunspot numbers av
33、ailable from the NTIA/ITS of the United States of America. 2.8 Simultaneous occurrence of ionospheric scintilhtion and rain fading Ionospheric scintillation and rain fading are two impairments of completely different physical origin. However, in equatorial regions at years of high sunspot number, th
34、e simultaneous occurrence of the two effects may have an annual percentage time that is significant to system design. The cumulative simultaneous occurrence time was about 0.06% annually as noted at 4 GHz at Djutiluhar earth station in Indonesia. This value is unacceptably high for ISDN types of app
35、lications. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesRec. ITU-R PI.531-3 223 FIGURE 2a The intersection of the propagation path with a magnetic field line at the F-region height FIGURE 2b The angle between the local magnetic meri
36、dian at the apex of the field line shown in Fig. 2a and the sunset terminator Sunset terminator The simultaneous events have signatures that are often vastly different from those when only a single impairment, either scintillation or rain alone, is present. While ionospheric scintillation alone is n
37、ot a depolarization phenomenon, and rain fading alone is not a signal fluctuation phenomenon, the simultaneous events produce a significant amount of signal fluctuations in the cross-polarization channel. Recognition of these simultaneous events is needed for applications to satellite-Earth radio sy
38、stems which require high availability. ITU-R RECMN PI.531-3 94 m 4855232 0522960 431 = COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services224 Rec. ITU-R PI.531-3 FIGURE 3 fongitude sectors Seasonal weightin functions for stations in diffe
39、rent 1 .o 0.8 0.6 0.4 0.2 O 1 61 121 181 24 1 30 1 36 1 Day :I, FIGURE 4 Illustrative picture of scintillation occurrence based on observations at Gband (1.6 GHz) Solar maximum Solar minimum . . _._._._ . -m COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Informat
40、ion Handling ServicesRec. ITU-R PI.531-3 TABLE i Mid-latitude I 225 High-latitude Parame ter Maximum - spring Minimum - winter Tokyo, Japan Maximum - summer Minimum -winter Activity level Diurnal Seasonal Pattern a function Solar cycle Magnetic activity Solar-geographical and temporal dependence of
41、ionospheric scintillation Equatorial Y Exhibits greatest extremes Maximum - night-time Minimum -daytime Longitude dependent Peaks in equinoxes Accra, Ghana Maximum - November and March Minimum -solstices Huancayo, Peru Maximum -October to March Minimum - May to July Maximum -May Minimum - November a
42、nd December Kwajalein Islands Occurrence and intensity increase strongly with sunspot number Longitudinal dependent Accra (Ghana) Occurrence decreases with Kp Huancayo (Peru) March equinox Occurrence decreases with Kp June solstice Occurrence increases with Kp September equinox 0000-0400 (local time
43、) Occurrence increases with Kp Generally very quiet to moderately active Auroral %enerally moderately active to very active Maximum - night-time Sporadic -daytime Maximum - night-time Maximum - daytime Minimum in the morning (O349 MLT) Tokyo, Japan Occurrence in night- time decreases with sunspot nu
44、mber; occurrence in daytime has little dependence Independent of Kp Occurrence and intensity increase strongly with sunspot number Occurrence increases with Kp Polar Great intensity in high sunspot years Maximum - night-time Minimum in rhe morning (03-09 MLT) Pattern a function of longitude sector O
45、ccurrence and intensity increase strongly with sunspot number Occurrence increases only slightly with Kp 1TLD-R RECNN PH.531-3 74 4855212 0522762 204 COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services226 18 Rec. ITU-R PI.531-3 FIGURE 5 O
46、bserved scintillation activity as a function of geomagnetic latitude and local apparent geomagnetic time . . 12 MLT O0 0.20 O. 15-0.20 O. 10-0.15 .,., . : .:“ 0.00-0.10 . 3. Absorption When direct information is not available, ionospheric absorption loss can be estimated from available models accord
47、ing to the (sec i)lf2 relationship for frequencies above 30 MHz, where i is the zenith angle of the propagation path in the ionosphere. For equatorial and mid-latitude regions, radiowaves of frequencies above 70 MHz will assure penetration of the ionosphere without significant absorption. ITU-R RECM
48、N PI.533-3 94 4855232 05229b3 340 = COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services ITU-R RECMN PI.533-3 94 4855232 0522764 087 W Rec. ITU-R PI.531-3 227 I I I I l I Measurements at middle latitudes indicate that, for a one-way traver
49、se of the ionosphere at vertical incidence, the absorption at 30 MHz under normal conditions is typically 0.2 to 0.5 dB. During a solar flare, the absorption will increase but will be less than 5 dB. Enhanced absorption can occur at high latitudes due to polar cap and auroral events; these two phenomena occur at random intervals, last for different periods of time, and their effects are functions of the locations of the terminals and the elevation angle of the path. Therefore for the most effective system design these phenomena should be treated statisticall
