CC BY
18
UDC 654.165
doi:10.15217/issnl684-8853.2018.1.85
PREDICTION OF OPERATIONAL PARAMETERS OF RADIO SIGNALS PASSING A LAND-SATELLITE LINK THROUGH STORMTIME IONOSPHERE
N. S. Blaunsteina, Dr. Sc., Phys.-Math., Professor, [email protected] Y. Ben-Shimola, PhD, Electrical Engineering, SeniorLecturer, [email protected] aBen-Gurion Universityofthe Negev, P.O.B. 653,1, Ben-Gurion St., Beer-Sheva, 74105, Israel
Introduction: The subject of this research became important during the recent decades due to the increasing demand in globalization of wireless networks by using long-path ionospheric radiowave propagation in land-satellite communication links. Purpose: Analyzing the key parameters of VHF/UHF radio waves in a satellite-land link which determine the fading effects occurring in storm-time mid-latitude ionosphere. Results: For perturbed plasma parameters experimentally caused by a magnetic storm, the absorption and phase fluctuations of the radio signals were examined. The scintillation index corresponding to signal scintillations in multi-path ionospheric communication links with fading was studied on the base of experimental data. It has been shown that -10% density irregularities in the storm-time F region yield strong fast fading of radio signals in VHF/UHF frequency band with considerable signal intensity fluctuations (up to 1%) and phase deviations (up to hundreds of radians). Practical relevance: The obtained results allow you to predict multiplicative fading in ionospheric sub-channels and its impact on the total link budget in a full land-satellite communication link.
Keywords — Ionospheric Sub-Channel, Ionospheric Plasma Density Irregularities, Signal Intensity Scintillation Index, AmplitudeAttenuation, Phase Fluctuation, Root Mean Square, VHF/UHF-Band Radio Waves.
Citation: Blaunstein N., Ben-Shimol Y. Prediction of Operational Parameters of Radio Signals Passing a Land-Satellite Link through Stormtime Ionosphere. Informatsionno-upravliaiushchie sistemy [Information and Control Systems], 2018, no. 1, pp. 85-95. doi:10.15217/issn1684-8853.2018.1.85
Introduction
The ionosphere is one of the sub-channel (additional to the terrestrial and atmospheric sub-channels) of the global (e. g., mega-cell) land-satellite and transionospheric communication systems. It is well known that ionospheric irregularities cause strong fluctuations of the amplitude and phase of radio signals, i.e. fast fading. Enhanced irregularities are usually observed during magnetically disturbed periods, especially in the auroral ionosphere (see [1-9] and bibliography there). As the auroral boundary shifts equatorward during magnetic disturbances, it is usually assumed that mid-latitude scintillation events occur inside the expanded auroral zone. As was expected by many researchers dealing with satellite observations of the perturbed stormtime ionosphere, this phenomenon occurs not only in the proximity of the polar and auroral zones, but also in the sub-auroral and middle-latitude ionosphere [10-14]. This event occurs in about 15-17 % of time per year and usually is observed during several-day to several-week periods. It was found that during magnetic storms strong phase and amplitude scintillations (the scintillation index <S4 > 0.5) of radio signals of the satellite operating at 240 MHz [2] and 1.5 GHz [3] are observed (namely, near Boston, MA, USA and Ithaca, NY, USA) well equatorward of the auroral zone, up to the sub-au-
roral and mid-latitude ionosphere. Satellite and radar observations in the stormtime sub-auroral ionosphere [4-7] revealed irregular structures embedded within fast westward streams, called sub-auroral polarization streams (SAPS) [8]. In particular, strongly irregular plasma density structures are embedded within SAPS wave-like structures (called SAPSWS) [6, 7]. Thus, the Defense Meteorological Satellite Program (DMSP) for satellite observations, which coincided with the [2, 3] scintillation events, revealed SAPSWS-related irregular density troughs. Within the troughs, power spectral densities (PSD) (Snk =10-20 % with respect to non-disturbed ionospheric plasma n0 [9]) of plasma irregularities (electron and ions, since plasma is quasi-neutral) in the range of apparent wave numbers from k = 2n/X = 0.85 km-1 to k = 12 km-1, X is the wavelength, were well-approximated by a plasma density power-law (8nk/n0)2k-p with the spectral index of 5/3 < p < 2 [9].
The spectral properties of plasma perturbations allow for predicting spectral characteristics of the radio signal with data (called the bandpass signal [15]), and finally predict the type of signal fading, flat or frequency selective, in such a perturbed ionospheric channel. These aspects are very actual for advanced wireless networks design based on modern techniques of signal processing, namely, orthogonal frequency division multiplexing (OFDM),
where each subscriber obtained very narrow bandwidth. The main goal of such a technique is to eliminate effects of the noise caused by multipath phenomena occurring in the channel with fading and by inter-subscriber interference [16]. Therefore, it is important to predict a-priory the type of the channel, frequency or time dispersive (or both), and to estimate the coherent bandwidth and time of coherency of such a channel with fading for the future performance of OFDM access technique [16]. Above example shows the actuality of this paper and research carried out in it.
This work describes fading phenomena and their effects on key parameters of radio signals in the land-satellite communication link passing through the structured storm-induced plasma region, filled by the "typical" small- and moderate-scale irregularities. The basic effects of irregular plasma structures, embedded within SAPS, on radio signals propagation via the perturbed stormtime ionosphere are briefly discussed in the first Section. In the Section following it presents the results of computation of signal integral absorption, amplitude and phase "point-to-point" deviations, and variations in the intensity and phase of radio signals propagating through this SAPSWS-related "typical" link. These results help to understand the fading phenomena effects for signals passing mul-tipath ionospheric links during magnetic storms. Then, we conclude main effects of radio signals of various frequency bands occurring in the perturbed ionosphere during magnetic storms.
Satellite Observations of Stormtime SAPS-related Density Irregularities
We start our discussions with analysis of several satellite campaigns carried out during period from the end of 20 century to beginning of 21 century. Thus, Fig. 1 (rearranged from [9]) shows a snapshot of plasma observations from the DMSP F14 satellite near the scintillation event near Boston, MA, USA during the 23 September 1999 magnetic storm [2]. Shown in the bottom panel is the plasma density variation along the satellite track measured by a spherical Langmuir probe sampled at the rate of 24 Hz. A detailed description of the probe and the whole DMSP/SSIES (Special Sensor for Ions, Electrons, and Scintillations) suite can be found in [10-13]. The top panel shows the corresponding waveform of plasma density deviations, Sn/n0, obtained applying 0.1-9.5 Hz bandpass elliptic filter (see details in [2]). Here Sn is the perturbation of the background ionospheric non-disturbed plasma density (denoted by n0). One can see also that enhanced density irregularities were present well equatorward of the boundary of auroral zone indicated by the vertical dashed
0.2
0.1
0
-0.1 7 6
5 4
3 2
00:5712
:5736 :5800
:5824 :5848 UT
49.4
50.7
52.0
53.3
54.7MLat
■ Fig. 1. DMSP F14 observations during the 23 September 1999 magnetic storm near Boston, MA, USA: (top) waveform of relative density variations 8n/n and (bottom) the density variation along the satellite track
a
л
0.2 0.1 0
23
Min after 1700 UT
■ Fig. 2. (top) Horizontal (positive westward) component VH of the convection velocity and (bottom) the plasma density from the DMSP F15 satellite on March 31, 2001 (extracted from [3])
line (i.e., beyond this zone the sub-auroral inono-sphere is seen to be also perturbed).
We present now another example of stormtime sub-aurora density irregularities observed by the DMSP F13 and F15 satellites during the major magnetic storm of March 31, 2001 [3]. Fig. 2 shows a 5-min snapshot of the horizontal (positive westward) component of the convection velocity VH (the top panel) and plasma density ni (bottom) from F15/SSIES obtained by the ion drift meter and Langmuir probe sampled at the rates of 6 and 24 Hz, respectively.
Note that quite similar SAPS/trough patterns were observed throughout the storm also by the DMSP F13 satellite. As in [2, 3, 9, 10], we obtained the density waveforms in the SAPS regions applying 0.1-9.5 Hz bandpass elliptic filter. Their PSD are shown in Fig. 3 as a function of apparent frequencies. Likewise [9, 10], PSD is well represented by the law 8nk/n0 ~ k-p, p = 2 (or ?>n/n0 ~ f-2), where k = 2fc, and p is parameter of PSD [2-9].
ИНФОРМАЦИОННЫЕ КАНАЛЫ И СРЕДЫ
10-1
10-2
И 10-3 A
10-4 10-5
Frequency, Hz
■ Fig. 3. Power spectral densities of plasma irregularities vs. the apparent frequency from the DMSP satellites and near the times designated in label: e. g., 130521 stand for the satellite F13 at 0521 UT (extracted from [9])
We emphasize that spatial and temporal variations cannot be separated in data from a single spacecraft and thus frequencies (wavelengths) mean apparent frequencies (scale-lengths). Given the satellite speed vs = 7.5 km/s, apparent frequencies 1-10 Hz correspond to wavelengths Xs = 2tc/ ks = v/f= 7.5-0.75 km along the satellite track. Note also that after about 1900 UT short-scale oscillations faded away, whereas "smooth" SAPS/ trough plasma structures remained throughout and even after the storm recovery phase. Such behavior is typical of SAPSWS that develop within 10-20 min and decay is ~1 hour after substorm expansion onsets [6, 7, 9].
Prediction of Main Parameters of Signals Passing Stormtime Perturbed Ionosphere
Main Parameters of the Radio Channel
In predicting the key parameters of radio signals passing through irregular ionosphere characterized by a spectral index p = 1 + <n> (<n> is the mean value of the plasma refractive index) or by the PSD index p'=p - 2 [15-21] it is important to show that the signal intensity fluctuations at a given frequency can be described by the Ricean K-parameter as a ratio of coherent and incoherent components of the total signal intensity [22]:
K = (Ico)/(Iinc)-
(1)
Here (Ico) is the line-of-sight (LOS) component of the signal (called also the coherent part [14, 15, 22]) and (Iini) is the incoherent component of the total signal intensity:
where
{I) = {To) + {Lnc),
Y-inc) \J-co
(2a)
(2b)
The scintillation index
=
/ n -( I)2
-1
(2c)
denoted in [1-9] as S4, defines the normalized intensity of the signal fast fading.
For Gaussian zero-mean random signal amplitudes, the relationship between K and ct| (denoted in experimental works [1-5] as S4) is simply
K2 = 1 / CT? = 1 / S4.
(3)
We note, according to [15] that for Gaussian nonzero-mean random signal amplitude distribution, as clear seen from relations (1)-(2c), formula (3) becomes more complicated. However, as was shown by many researchers investigating stochastic processes occurring in the perturbed ionosphere [15, 1621, 23-36], the spatial distribution of ionospheric irregularities of natural and artificial nature can be correctly described by zero-mean Gaussian law, which according to ergodic theorem leads to the same Gaussian distribution of scattered radio signals in the time domain.
The basic characteristics of signal fading are derived by means of the approach developed in [1620, 23-32] and summarized in [15, 21]. They are: 1) mean square phase fluctuation ((A®)2); 2) outer scale L0 of the disturbed ionospheric plasma structure; 3) inner scale l0 of the plasma density irregularities.
In addition, to describe radio wave propagation in irregular plasmas, the outer scale L0 should be compared with the Fresnel scale dF =,JXZ0 [15]. Here X is the signal wavelength and Z0 is the distance from the ionospheric layer filled by plasma inhomogeneities to the ground reception plane [15, 25-27]).
The spectrum of the signal intensity fluctuations
can be derived via
'Й5)
[16-19, 21, 23, 30, 32,
34-36] by assuming that the root mean square (RMS) of phase fluctuations is larger than one radian. Moreover, in [15, 18, 19, 21, 30] have shown theoretically that at k >> 1/L0 the power spectrum of phase fluctuations S(k) in the perturbed inhomo-geneous ionosphere is proportional to k-p (e. g., f-p), where p = 2 (see Fig. 3). This condition was proved by numerous observations of perturbed stormtime ionosphere (see [9, 21] and bibliography there).
Main Characteristics of the Radio Signal Fading
We analyze now propagation characteristics of the radio signal passing disturbed ionospheric region perturbed by geomagnetic storm. As it follows
from experimental data presented above, the background ionospheric plasma can be perturbed essentially in period of magnetic storm preparation. During this period the observed deviations of plasma density around its non-disturbed background can achieve 10-20 % (see Figs. 1, 2). There are several features should be estimated for predicting fading phenomena in the satellite communication links passing through the perturbed ionosphere, such as absorption, point-to point attenuation and phase deviation of radio signal amplitude along the radio path, as well as scattering phenomena caused signal intensity and random phase variations.
Absorption of Radio Signal in the Disturbed Ionosphere
To estimate losses due to absorption, i.e., the part of energy of radio signal, which is absorbed in the perturbed region of the ionosphere, from D to F, we propose here, following [15, 21, 35], to use a special value of absorption in decibels:
A*= lOlog ^ -
4.3
2 (
®pe (vem
+ vei + vee )
j 2 2 ds[dB],
c [ffl + (vem +Vei +vee) ]
(4)
0.174593
b)
0.174533
5 6 7 f, Hz
ffl
250 200 150 100
50 0
d)
f)
70
60
50
ffl 40
s 30
20
10
0
50
45
40
35
ffl 30
Td 25
rt
20
15
10
5
0
5 6 7 f, Hz
0.628319
5 6 7 f, Hz
1.0472
5 6 7 f, Hz
■ Fig. 4. Absorption of signal vs. frequency for weak magnetic storm (a, c, e) and strong magnetic storm (corresponding to Alaska region) (b, d, f): a, b — grazing angle y« 0.175 rad (y « 10°); c, d — y « 0.628 rad (y « 36°); e, f — y « 1.05 rad (y « 60°)
where vem, vei and vee are the frequencies of electron-neutral, electron-ion and electron-electron collisions, respectively.
The parameter of absorption fully characterizes real ionospheric radio traces and can usually be estimated by measuring radiometric absorption at the fixed frequencies and by knowledge of frequency dependence of coefficient of absorption k. We should note that in integrand of (4) we account not only for the effects of electron-neutral and electron-ion collisions, but also ion-neutral and electron-electron collisions, based on the elements of kinetic theory [37-40], but not a magneto-ioning theory usually used in such computations based on hydrodynamic classical approach.
Then, accounting in (4) for the fact that the length of radio path, s, and the height of the ionosphere, h, are related as h = ssiny, where y is the grazing angle of the satellite antenna with respect to the ground-based antenna, we can estimate the total absorption of radio signal along the radiopath. In Figs. 4, a-f this parameter is presented in [dB] vs. the carrier frequency of probing radio signal sent via stormtime ionosphere under different grazing angles, respectively. The corresponding profile of disturbed plasma density N(h) is taken from Fig. 2 and normalized on the non-disturbed plasma density N0(h) taken from [15, 21] for the mid-latitude regular ionosphere, and accounting in computations of integral (4) for the limits hmin = 50 km and hmax = 500 km. We consider below two scenarios, weak and the strong magnetic storm, which are shown in Figs. 4, a, c, e and 4, b, d, f respectively. The corresponding data were taken from observations of magnetic storm, described above for weak storms observed at the middle latitude ionosphere above USA and at the northern regions of USA and Alaska.
As follows from above illustrations, with increase of the magnetic storm strength, from weak to strong, absorption of radio signals increases roughly three times at lower frequencies from 100 to 500 MHz, while at 500-600 MHz, absorption does not depend on frequency; only on the grazing angle. At the same time, with increase of the grazing angle, the tendency of decrease of signal absorption for week storms compared with strong storms is the same, per twice or three times. Results of computations presented above allow us to conclude that magnetic storms occurring in the northern mid-latitude ionosphere can significantly perturb ionospheric plasma and thus cause a dramatic attenuation of radio signals up to 500 MHz. This effect depends on the grazing angle and can be insignificant for grazing angles bigger than y > 20°. Hence with increase of the radiated frequency of radio signal for the same grazing angle or for increase of grazing angle for the same frequency, the tendency of
significant decrease of signal energy absorption is evident.
Signal Point-to-Point Amplitude Attenuation and Phase Deviations
At the same time, we should point out that for radar and radio communication applications, other characteristics of signal fading can give information about radiophysical effects, dealing not with the cumulative intensity integral absorption along the radiopath, but with the point-to-point amplitude and phase deviations of the signal along the radio-path. In this case, it is important to understand of how perturbed parameters of the ionospheric plasma attenuate radio wave amplitude and change its phase. To understand these radiophysical effects, we will consider an arbitrary monochromatic radio wave [15]:
U(z, t) =A(z, t)exp[/(rot - kz)].
(5)
In this case, the wave number k can be described through the coefficient of wave amplitude attenuation, a, and the coefficient determining changes of wave phase in real time, p, i.e.,
k = a +'p.
(6)
Unlike usual description of parameters a and p, we will present them through collision frequencies of plasma particles using elements of kinetic theory (see [15, 21]). For numerical computations, we introduce the additional notations and parameters to describe a and p, that is, X = ra2 / ro2, Z = v/ra, rap is the background plasma density in the perturbed ionosphere [15, 37-39]. In these notations, after straightforward derivations, we obtained that
(7a)
or
ю2 \l - x 1 ю2 "1 X(1 + iZ)
с2 1 - iZ _ c2 1 + Z2
Ю
1 -
X + iZX) 1 + Z2
k2- — * " c2
1 --
X
--1-
1 + z '
1 + z2
(7b)
By introducing now normalized parameters (to the wave number in free space k0), a = — and
~ P ko
P = —, we finally get:
k0
1 +
2
1 +
2,2 2
Ю +V -Юp
? ?
(8a)
Р =
ZX 1 + Z1
VI®
Р
иуг
1 +
V I2 ю(ю2 +V2 )
(8b)
Here v = vem + vei + vee = vem(1 + V + V'l where V = vei/ vem and = vee/vem.
Deviations of radio signal parameters, a and p, in the perturbed ionospheric region, were found by using parameters of the disturbed ionosphere, i. e., variations in the plasma frequency rop/rop0, plasma concentration N/N0 (N = N0 + 8N, SN < N0), and plasma conductivity ct/ct0, where rap0, N0, and ct0 are the background non-disturbed ionospheric parameters [15, 21]
rap0 = (4*N0e2/me)1/2; (9)
e0 = 1 -
/ (®2 + (vem +v e; )2)
ct0 =
2
Ю0
(Ует + vei)/ 4л(ю2 + (Vem +veí)2 )
(10)
. (11)
Here e and me are the charge and mass of plasma electron; other parameters are defined above.
Thus, in Fig. 5, a, b we present deviations of radio signal parameters, a and p, compared with those, a0 and p0, obtained for non-disturbed background ionospheric plasma. The computation results are shown in Fig 5, a (for normalized attenuation) and Fig. 5, b (for normalized phase velocity) versus altitude of the ionosphere for different frequencies of probing waves varied from 50 MHz to 2 GHz (i.e., covering also HF-bandwidth which is important for transionospheric propagation). Thus, we use
a) Frequencey = 50, 100 MHz 1
0.9 0.8 0.7 0.6 0.5 0.4
Frequencey = 200, 500 MHz 1
0.975 0.97 0.965 0.96
0.955
100
200 300
h, km
100
200 300
h, km
Frequencey = 1000, 2000 MHz 1
0.9998
0.9996
0.9994
0.9992
0.999
0.9988
0.9986
0.9984
0.9982 100
200 300
h, km
b) Frequencey = 50, 100 MHz
xl(T7
9
Frequencey = 200, 500 MHz Frequencey = 1000, 2000 MHz
h, km
x10-
1.4 1.2 1
-s? 0.8 0.6 0.4 0.2
200 MHz 500 MHz
200 300
h, km
x10
10
0 100
h, km
■ Fig. 5. Normalized attenuation coefficient (a) and phase velocity (b) of probing wave vs. ionospheric altitudes for frequencies varied from 50 MHz to 2 GHz
8
those frequencies which are actual both in land-ionosphere-land navigation (f < 500-600 MHz) and those which usually used in GPS radio monitoring of the ionosphere (f > 900 MHz).
As is seen from Fig. 5, a with the increase of frequency of the probing wave, the effect of attenuation of the wave energy becomes weaker, and the probing radio wave propagates at the same manner as in the non-disturbed ionosphere. This effect depends strongly on what altitudes the propagation process is observed. Thus, with increase of ionospheric altitude, attenuation affects stronger for all frequencies under consideration.
The same tendency of decreasing phase velocity of radio wave passing the disturbed ionospheric region with increase of frequency of probing wave is clear seen from Fig. 5, b. In fact, with increase of frequency from 50 MHz to 2 GHz, the normalized phase velocity, P/P0, becomes smaller, i. e., the phase velocity of the probing wave becomes smaller in disturbed plasma compared with that in non-disturbed plasma.
This effect can be easily explained. When the frequency increases (or the wavelength decreases compared to the dimensions of plasma irregularities), the effects of "diffractive scattering" become weaker and, instead of defocusing with strong fading, we observe focusing with weak fading [15, 21].
Signal Intensity Fluctuations
To investigate the signal intensity deviations in the perturbed ionosphere and the corresponding scintillation index evaluations, the plane-screen [16-20, 23, 33, 34] and curved-screen ionospheric model [26, 27, 39] were performed and then summarized in [15]. Based on the theoretical self-consistent framework described there, one can estimate the effects of magnetic storm on the above two parameters of the radio signal propagating in perturbed ionospheric communication channel. Therefore, dealing only with situation occurring during magnetic storm described by the following PSD parameter p = 4 (p' = 2) introduced in first Section (see Fig. 3), we finally get, following [15, 21]
I{k) = 32((ДФ)2^)
(1 + k2L2)2
sin2 l-ik2d% |. (12)
Accounting for self-consistent theoretical framework described in [15, 21] for various scenarios occurring in the strongly perturbed ionosphere, we get for p' = 2 (p = 4) the following expression of the scintillation index:
8>/2
3\/л£о
4<(лфг
(13)
a) vt 0.4
0.35 0.3 0.25 0.2 0.15 0.1 0.05 0
b) 1.2
1 0.8 0.6 0.4 0.2 0
Scintillation index/Outer scale
5 10 15 20 25 30 35 40 45 50 55 60
Ln
Vd
Scintillation index/Outer scale
5 10 15 20 25 30 35 40 45 50
L°/dF
Fig. 6. The RMS of the scintillation index, vs. the
outer scale L0 for 1.5dF < L0 < 50dF, ((АФ) ) = 0.1 rad
(a) and /((АФ) ) = 1 rad (b)
Computations of the scintillation index presented by (13) were compared in [15, 21] with those formulas obtained in [16-20, 23] for weak and moderate plasma perturbations [e. g., for p = 2 (p' = 0) and p = 3 (p' = 1), respectively], for an outer scale L0 = 10dF and inner scale l0 = 10-2dF. Figures 6, a and 6, b present the RMS of the scintillation index,
ctj , computed for weak I J/(AO)2] = 0.1 rad]
and moderate I MAOyJ= 1 rad I signal phase fluctuations versus square mean deviations of signal phase for various PSD parameters p = 2, 3, 4 and different scales of ionospheric irregularities.
It is seen that for p = 2 the scintillation index with increase of phase fluctuations limits to the unit. In the last case, for more higher spectral index (p > 2) ct7 exceeds the unit within the range of 0 < L0/dF < 1, which explain us the focusing properties of the ionospheric layer consisting various irregularities and strong variations of signal phase after passing the perturbed ionosphere.
Signal Phase Deviations
We model the ionospheric F-region, according to [15, 21, 27], as a spherical 3-D layer of mean ionization density N and with the outer scale L0 (instead of a plane-layer 1-D model, as was performed in [16-20, 24, 29-34]) with the standard fluctuations of ionization density, (AN/N0)2 = (8N/N0)2. We take the outer scale L0 eqauals H, assuming latter as the thickness of the disturbed ionospheric F-layer. In such notations, the mean square fluctuation of phase of radio signal can be derived with the help of the general model [15, 21, 27], which finally gives:
(ДФ)' = 4г/ Щ
Nn
|X2H2 secX-
(14)
Here re is the radius of electron. For numerical computations we take H = 100 km and N0 = 1011 m-3. In Figs. 7, a and b the RMS of the signal phase fluc-
tuations, _ AO) y, [in radian], are shown versus the zenith angle of a satellite for various frequen-
a)
cd
ft Чч
О Й
cd 3
о 3
И
Й
104
103
102
101
100
Ionization density = 0.01 (1%)
10
-1
Wave frequency = 32 MHz
100 320
10 000 32 000
b)
0 10 20 30 40 50 60 70 80
Zenith angle, deg
Ionization density = 0.1 (10%)
d a
a
x ft Чч
o n
a
3
О
3
И
Й
104
103
102
101
100
10
-1
Wave frequency = 32 MHz
100
320
1000
3200
10 000 32 000
0 10 20 30 40 50 60 70 80
Zenith angle, deg
■ Fig. 7. RMS of signal phase fluctuation vs. zenith angle, for 8N/N0 = 0.01 (a) and for SN/Nq = 0.1 (b) and for frequencies varied from 32 MHz to 60 GHz
cies from 32 MHz (HF-band) to 60 GHz (UHF-band) and for different SN/N0 in percentages to the total plasma content of N0 = 1011 m-3, from 1 % (weak magnetic storm) to 10 % (strong magnetic storm), respectively.
Here, we should mention that the frequency band of signals that we investigate covers whole spectra of useful frequencies operating in existing land-ionosphere-land communication links, GPS satellite links and also in new networks beyond 3-G (third generation), operating at frequencies of 10 GHz to 60 GHz. As is clearly seen from the illustrations presented, the frequency dependence of RMS fluctuations of the signal phase is sufficient only for zenith angles greater 60-65° for mean and large perturbations of the background ionospheric plasma, i. e. 1 to 10 %. With increase of the radiated frequency, the phase fluctuations become strongly depending on 8N/N0. Thus, for the frequency band from 1 to 10 GHz usually used in satellite communications, for zenith angles of 50-60°, the RMS of phase fluctuations increases from 1-5 rad (8N/N0 = 0.1 %) to 20 rad (8N/N0 = 1 %) and 100 rad (8N/N0 = 10 %).
Effects of Fast Fading on the Radio Signal
Based now on the magnetic storm effects experimentally observed during sounding of the disturbed stormtime ionosphere, we will analyze the effects of fading of radio signals determined by the ^-factor of fading, defined above in Section "Main Parameters of the Radio Channel".
For these purposes, we will use relations between the parameter of signal intensity scintillations ct| and the ^-factor described by formula (3), as parameter of fast fading phenomena within the channel [15, 21]. Taking into account variations of scintillation parameter, S4 , observed experimentally in [1, 2], from 0.4 to 0.8 (see also Figs. 1, 2), we get that the corresponding ^-parameter varies in the range of 1.1 to 1.6, indicating the existence of direct visibility between the ground-based and satellite antenna. In the above cases the LOS component is at the same order or higher than the NLOS component.
In other words, the coherent component of signal intensity is accompanied by additional effects of multipath phenomena (i. e., by the incoherent component) caused by diffractive scattering of radio signals on small- and moderate-scale plasma density irregularities SN/N0, which exist in storm-time ionosphere (see Figs. 1-3). This implies that the coherent (LOS) component exceeds the incoherent (NLOS) component even in stormtime perturbed ionospheric links. As was shown in [15, 21], knowing the ^-factor of fast fading, one can predict deviations of the signal data passing through the disturbed ionospheric radio channel.
Conclusions
In this work, we presented theoretical predictions of the signal intensity and phase fluctuations of radio signals propagating in the stormtime sub-auroral ionosphere. The background ionospheric parameters and density irregularities used in the calculations are taken from the DMSP satellite observations during several magnetic storms that include two events where coincident intense radio-signal scintillations were observed (see first Section and the corresponding bibliography in [1-9]).
Based on experimental observations carried out in [1-9] and summarized in [21], as well as on theoretical framework developed in [15-20, 23-27, 31-34], the following phenomena can be predicted using these results.
1. Deviation of plasma density: weak storm — Sn/n0 = 2 +5% ; strongstorm — 8n/n0 = 10 +20% .
2. Attenuation of signal amplitude: the change is 3 times from weak to strong magnetic storm (see
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Fig. 4); the tendency which does not depend on grazing angle. With decrease of grazing angle, for frequencies up to 600 MHz, the attenuation of radio signal increases.
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УДК 654.165
doi:10.15217/issn1684-8853.2018.1.85
Прогнозирование эксплуатационных параметров прохождения радиосигналов в канале «Земля — спутник» через возмущенную ионосферу
Блаунштейн Н. Ш.а, доктор физ.-мат. наук, профессор, [email protected] Бен-Шимол И.а, PhD, техн., старший преподаватель, [email protected]
аНегевский университет им. Бен-Гуриона, П.О.Б. 653, Бен-Гуриона ул., 1, г. Беэр-Шева, 74105, Израиль
Введение: тема исследования стала актуальной в последнее время ввиду глобализации систем беспроводной связи за счет использования дальнего ионосферного распространения радиоволн в каналах связи «Земля — спутник». Цель: анализ ключевых параметров ультракоротких радиоволн в канале «Земля — спутник», которые определяют эффекты фединга при магнитных бурях, происходящих в среднеширотной ионосфере. Результат: при возмущённых параметрах плазмы, вызванных экспериментально магнитным штормом, проанализированы поглощение и фазовые флуктуации радиосигналов. На основе экспериментальных дан-
р
ных исследован индекс сцинтилляции соответствующим сцинтилляциям сигналов в многолучевых каналах связи с федингом. Показано, что ~10 % нерегулярностеи плотности плазмы во время шторма в области F вызывают сильный фединг высокочастотных радиосигналов диапазона ультракоротких радиоволн со значительными флуктуациями интенсивности сигнала (до 1 %) и изменениями фазы сигнала (до сотен радиан). Практическая значимость: полученные результаты позволяют прогнозировать мультипликативный фединг в ионосферных субканалах и его вклад в определение потерь в полном канале связи «Земля — спутник».
Ключевые слова — ионосферный субканал, ионосферные нерегулярности плотности плазмы, индекс сцинтилляции интенсивности сигнала, затухание амплитуды, флуктуации фазы, среднеквадратическая величина, радиоволны УКВ-диапазона.
Цитирование: Blaunstein N., Ben-Shimol Y. Prediction of Operational Parameters of Radio Signals Passing a Land-Satellite Link through StormTime Ionosphere // Информационно-управляющие системы. 2018. № 1. С. 85-95. doi:10.15217/issn1684-8853.2018.1.85 Citation: Blaunstein N., Ben-Shimol Y. Prediction of Operational Parameters of Radio Signals Passing a Land-Satellite Link through StormTime Ionosphere. Informatsionno-upravliaiushchie sistemy [Information and Control Systems], 2018, no. 1, pp. 85-95. doi:10.15217/issn1684-8853.2018.1.85
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