Communications Essays - Channel Simulator


Use of a channel simulator to evaluate data transmission via Satellite under Ionospheric Scintillation conditions

There were several a studies conducted by different Universities and organizations to determine the effects of Ionospheric scintillation and Total Electron Content. This paper would show the different procedures and calculations that each study created. This paper would also show any advancements or solutions that were developed to address this issue.

The University of Nottingham for example setup 4 networks of four state-of-the art GPS Ionospheric Scintillation and TEC Monitor receivers (the NovAtel/AJ Systems GSV4004). They placed these receivers in strategic locations in Norway and in the UK. Their task was to collect samples or data and compare them with the data that has been collected by other GPS networks collected by the British Isles GPS archive facility.

Problem Statement

Two areas of the globe are particularly troubled by fading, i.e., sub-auroral to polar latitudes and a belt surrounding the geomagnetic equator. Amplitude fluctuations of signals above 100 MHz are occasionally noted in middle latitudes but neither their depth of fade nor the frequency of their occurrence is disruptive in proposed or experimental systems. A map of the nighttime world viewed from the parochial view of F-layer irregularities might reveal areas of disturbance as seen in Fig. 1 ; hatch density is roughly proportional to the Occurrence of deep fades. It can be seen that the equatorial zone, during the time depicted (1968, a year of high sunspot number) encompassed plus and minus 10“ to 15” from the geomagnetic equator. The equator forward boundary of the high-latitude region moves to a low of 57” invariant latitude at midnight. The polar cap appears to show diminished. Scintillation compared to the irregularity region of the auroral zone but the data do not conclusively show this.

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Ionospheric scintillation enhances the satellite signal level by up to 6-8 dB and can bring the amplitude down to sky noise, background. The fluctuation is produced by the radio wave traversal of small scalier regularities of electron density about 1 km in their shortest dimension. The irregularities may be enhancements or depletions of electron density relative to ambient. These lay, for the most part, in the Flayer of the ionosphere at heights predominantly ranging from 225-400 km. Irregularities produced by sporadic Eand by small traveling disturbances in the Flayer also produce scintillations but from the viewpoint of the systems designer these are relatively unimportant. The signal increase for short periods is a characteristic of scintillation which could be utilized where systems are designed specifically for latitudes suffering considerable scintillation. The decrease or fade becomes significant only during the short period of time when the signal falls below the allowed system limit. Even during this time a properly modulated signal, taking into account the fading period of the scintillation, could overcome the deficiency. Knowledge, therefore, of the fading amplitude and rate as a function of latitude and magnetic conditions is an important factor in some system applications. Several indices have been designed to characterize the amplitude excursions. One measure is to use a scale from CL5 and by visual inspection, without actual measurement, assign a value to the record. Other ways involve scaling the deviation of the signal amplitude from the mean amplitude. Four measures are thus possible depending on whether mean deviation or root-mean-square deviation is used and whether the record is proportional to voltage or to power. If the probability of the amplitude deviation is known, it is possible to relate the four measures of scintillation index. While it is difficult to relate these measures theoretically because the probability distribution of the amplitude deviation is not, in general, known, experimental comparisons show that they are proportional to each other. For a statistical analysis that involves a long period of time, an index has been adopted by the Air Force Cambridge Research Laboratories (AFCRL) and the Joint Satellite Studies

Scintillation index

where P, is the power amplitude of the third peak down from the maximum excursion and Pmin is the power amplitude of the third level up from the minimum excursion. A sample record analyzed in this fashion is shown in Fig. 2.

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Using the experimental comparison of Bischoff and Chytil [5], it is possible to convert scintillation index used in this paper to the more precise indices of power level excursions. The theory of scintillation that has been most extensively developed is restricted to the “weak scattering” approximation, where the random phase deviations in the medium are required to be smaller than one radian. In this approximation, Briggs and Parkin [6] have shown that the magnitude of the scintillations may be expressed by

Scintillations cause both enhancements and fadings about the median level of the signal as the radio signals sweep across the irregular ionosphere. When these fadings exceed the specified fade margin of a link, its performance is degraded. The degree of degradation will depend on the magnitude of fadings relative to the margin, the duration of the fade, the rate of fading, the type of modulation, and the criteria of acceptability. On a global scale, the degradations are most serious for propagation paths which transit the low latitude irregularity belt around the magnetic equator and the high latitude environment encompassing the auroral oval and polar cap regions

The morphology of scintillations has been studied for several years and documented for the equatorial, mid-latitude, auroral, and polar cap regions. Intensity scintillation measurements with orbiting and geostationary satellites provided the major database for such long-term studies.

There’s much attention has recently been given to the fluctuations of radio waves from stars and satellites imposed by their passage through irregularities in the ionosphere. These effects depend on the position of the source relative to the observer. When the source is at a large zenith angle, the fluctuations are increased as compared with observations made at the zenith under similar conditions. Only amplitude fluctuations will be considered because these are much easier to observe than phase fluctuations, and many experimental observations are available for comparison with the theory.

In considering the fluctuations imposed on a wave in its passage through an irregular medium there are two possible approaches. In the diffraction method

The medium is considered to be equivalent to a certain thin diffracting screen.

Because the absorption in the ionosphere is negligible for the frequencies normally used for the observation of scintillations, this screen will produce across the emerging wave front variations of phase only, with no variations of amplitude.

As the wave propagates beyond the screen, fluctuations of amplitude begin to develop, and this part of the problem is essentially a matter of diffraction theory.

In the alternative approach, this may be called the scattering method, the wave at the observing point is considered to be the sum of the unscattered wave and waves scattered by the irregularities in the medium. This type of theory has been used by WHEELON (1959); it is reasonably simple so long as the scattering is weak, but becomes complicated when the scattering is strong. Both methods are equally valid and give identical results, as has been shown for a simple ease by BOOKER(195s).

Scintillation has been observed at frequencies ranging from 10 MHz to 6 GHz. At 137 MHz, scintillation with fades in excess of 6 dB occurs on zenith paths for less than 20 percent of the time near the geomagnetic equator, less than 2 percent of the time in the auroral regions, and less than 0.1 percent of the time at middle latitudes. Scintillation occurrence depends upon frequency, location, propagation path geometry, and the measure used to describe scintillation as well as on the geophysical conditions that cause scintillation. Much of our knowledge of ionospheric scintillation and the irregularities that cause the scintillation has been derived from a large number of observations taken over the last two decades. This paper considers the characteristics of scintillation as it has been experimentally observed and theoretically modeled. Mention is made of the geophysical processes thought to be responsible for scintillation.

Although considerable literature is available on studying theoretically the effects of low-elevation-angle multipath transmissions over land and sea, measurement data for establishing practical models applicable to L-band maritime mobile satellite communications systems at low elevation angles are inadequate. In particular, for the upcoming INMARSAT operation, a technical issue yet to be resolved is a minimal (critical) value of elevation angle from ship to satellite at which the system can still provide an agreed standard of communications quality. While elevation angles affect the extension of coverage of the satellite communications system, an agreed standard elevation angle has not been determined accurately, because of limited experimental data.

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An experiment conducted by COMSAT to determine the extent of degradation of satellite-to-ship and ship-to-satellite signals in the 1535-1660 MHz band at low elevation angles, using a 1.2 m MARISAT antenna with a GfT of -4 dB/K. The paper is not intended to be a pure propagation assessment, which is reported elsewhere, but rather an evaluation of practical maritime communication system effects, including low-angle propagation, antenna pointing, TWT saturation, and interference. The results as provided in this paper are useful for assessment of the system performance at low elevation angles.

The experiment was performed in October 1978 using the Atlantic Ocean MARISAT satellite and the S.S. Mobil Aero, an 18,600 ton oil tanker en route from Norfolk, VA to Texas City, TX, with elevation angles changing from 17’ to 0’. Baseline measurements were made during the initial portion of the trip. On a straight line sailing from Tampa, FL, to Texas City, TX, on a heading of 274 degrees, the elevation angle from the ship to the satellite decreased steadily from 11’ to 0.3’ at an average rate of 0.3’/h. During this 40 h period the experiment was conducted without interruption. The S.S. Mobil Aero was equipped with a MARISAT ship earth station having a G/Tof -4 dB/K. The TDM carrier signal, obtained directly from the filtered IF monitor point at the front panel of the console, has a bandwidth of about 6.7 kHz centered at 455 kHz. The signal was monitored by an rms voltmeter and recorded on a strip chart as well as on magnetic tape. The measured carrier signal level at the monitor point was between -30 and 0 dBm, depending on the strength of the incident signal. The voice carrier was monitored at the 1.5 MHz IF test point at the voice modulation/demodulation module. The IF signal was routed through a 29 kHz bandpass filter and a 20 dB amplifier before being measured (as was the TDM signal)by an rms voltmeter and recorded on strip chart and magnetic tape. Monitoring of the voice carrier was available only when there was telephone traffic between the ship and the shore station. A data test set was used to transmit and receive either 2400 bit/s or 1200 bit/s data for input to a DPSK modem or an FSK modem, respectively. The input-output point was at the data jack on the console.

Strong amplitude scintillation may cause the signal arriving at a GNSS receiver to drop below a threshold that may lead to loss of lock. Strong phase scintillation may cause the frequency Doppler shift in the signal carrier to exceed the receiver’s Phase-Lock-Loop (PLL) bandwidth and loss of phase lock may be observed. A measure of the intensity of amplitude and phase scintillation may be given by scintillation indices which the IESSG continuously recorded during this project. Both phase and amplitude scintillation contributes to the RMS phase tracking error in the output of the PLL. It is when this RMS error exceeds a threshold that loss of lock is bound to occur. Statistics revealed levels of phase scintillation significantly higher than amplitude scintillation in Northern Europe. The jitter introduced by phase scintillations more significantly affects the carrier tracking loop than the code tracking loop due to the much shorter wavelength of the carrier. In addition the narrower bandwidth of the code loop should improve its immunity to amplitude scintillations. However, as carrier aiding of the code loop is present in every GPS receiver one may also assume that loss of carrier lock is followed soon after by loss of code lock. The GNSS users in auroral regions should therefore be more concerned with phase scintillation or a combination of phase and amplitude scintillations, and in particular during geomagnetic storms. Also, both amplitude and phase scintillation affect the L2 frequency more adversely than the L1, due to its lower signal to noise ratio, narrower tracking bandwidth and to the inverse frequency scaling of scintillations, making it clearly more vulnerable.

Scintillations are most severe and prevalent in and north of the auroral zone and near the geomagnetic equator (Aarons, 1982). The equatorial region extends approximately from −20° to 20° and auroral regions from 55° to 90°. These boundaries change with the time of the day, the season of the year, the sunspot number and the magnetic activity.

Equatorial scintillations– Scintillation is predominantly a nighttime phenomenon in the equatorial region occurring for more than 40% of the year during the 20:00-02:00 local time period. It also shows a strong seasonal dependence with a pronounced minimum at the southern solstice and relatively high scintillation activity at the northern solstice. Equatorial scintillations also show a tendency to occur more often during years in which the sunspot number is high. The rms amplitude of electron density irregularities is equal to 20% in the most severe cases. Two regimes may be identified. For values of the scintillation index (S4) below approximately 0.5, the rms value of phase and intensity fluctuations seems to be linearly correlated and approximately equal. For greater values of S4, there is no obvious correlation and measured values are greater for intensity than for phase. If we consider the case of GPS L1 scintillations, the typical value of S4 at equatorial regions is 0.3. Its occurrence is related to the season and the solar activity. It may reach a value of 0.5 with an occurrence 10% below and a value of 0.8 or even 1 in a few cases.

High latitude scintillations– Contrary to equatorial fluctuations, according to measurements performed in Alaska, polar fluctuations exhibit more phase than intensity fluctuations. The scintillation index is usually quite low. It seldom exceeds 0.2 and the same for the probability of occurrence which is very low in summer and in any case below the values obtained at equatorial regions. Results of measurements recently performed in Norway, seem to confirm the low values of S4. The phase fluctuations are correlated to amplitude fluctuations and the σφ values obtained are also quite low.

GISM model developed at IEEA uses the Multiple Phase Screen technique (MPS). It consists in a resolution of the Parabolic Equation (PE) for a medium divided into successive layers, each of them acting as a phase screen. It provides the statistical characteristics of the transmitted signals, in particular the scintillation index, the fade durations and the cumulative probability of the signal consequently allowing determination of the margins to be included in a budget link. Maps of the scintillation index S4 and of the phase standard deviation may also be obtained. Within the scope of the activities of COST 271 relevant to the problem of transionospheric propagation another model for scintillation on transionospheric links (such as employed for satellite navigation) has been developed in co-operation between the University of St. Petersburg, St. Petersburg, Russia and the School of Electronic and Electrical Engineering, the University of Leeds, Leeds, U.K. The Abdus Salam ICTP, Trieste, Italy also collaborated with both the teams providing the experimental data on scintillation, ideas for proper processing of the scintillation data and necessary expertise and data on the ionosphere modeling. The developed technique is based on a hybrid method and was first presented at the 27th General Assembly of URSI (Gherm et al., 2002b). It is an extension of the technique initially suggested in (Gherm et al., 2000). The extended method is a combination of the complex phase method and the technique of a random screen. For decades the theory of random phase screen was widely employed for interpretation of scintillation data. However, it is always the case that the parameters of the effective phase screen are chosen to better fit a given set of experimental data. The disadvantage of such a treatment is that the same parameters of the random screen cannot then be utilized for prediction of scintillation in different conditions of propagation. By contrast in the new hybrid method, the parameters of the random screen, appropriately introduced below the ionosphere, are not chosen empirically but are the results of a rigorous solution to the problem of propagation inside the ionosphere layer.

Detailed investigations in the scope of the complex phase method showed that, for the points of observation lying inside the ionosphere layer, fluctuations of the amplitude of a field at frequencies of the order of 1 GHz and higher are always well within the range of validity of the complex phase method. This is true even in the case of very large relative electron density fluctuations (up to 100%) and high values of the Total Electron Content (TEC). For smaller relative fluctuations and values of TEC, this is also true for lower frequencies. This means that propagation in the ionosphere layer for the frequencies mentioned may be rigorously described in the scope of the complex phase method. In turn, this means that at the specified frequencies, the regime of strong scintillation is not normally found inside the ionosphere layer, but may instead be found in the region below where the fields propagate down to the Earth’s surface. This then permits the complex phase method to be used to properly introduce the random screen below the ionosphere, and then to employ the rigorous relationships of the random screen theory to correctly convey the field down to the surface of the Earth. While propagating the fields between the bottom of the ionosphere and the Earth’s surface, the regime of strong scintillation may well be found. Comparisons of models and measurements are presented. The measurement techniques and detrending algorithms, the receiver transfer function and characterization and the effects of scintillations on positioning errors are also addressed in this paper.

OBSERVATIONS of the scintillation of radio waves from stars and artificial satellites have become one of the most important tools in the investigation of Ionospheric irregularities. Due to the good results of this method the number of observatories using this means is increasing, and at the same time the observations have lost their local meaning. The remarkable longitude and latitude effects, tides etc., call for the need for analysing and comparing the results of a number of observatories in a standard way for our picture of the ionosphere and its changes to become more comprehensive. In analysing scintillation records, the methods of stochastic processes are being used more frequently. On the other hand an autocorrelation function is not a commonly used function and moreover it is of little or no use when the scintillations are only roughly to be compared with other geophysical quantities. When this is the case, most papers use only simple indices as a measure of scintillation depth or rate. There are several different measures in use, chosen in different ways, but they are as a rule certain simple functions of the scintillation distribution moments. In order to compare the results of observations they must all be expressed either by using the same index, or, if different indices are used, it must be possible to convert easily from one to another.

The relations between the most common measures of scintillation depth have been discussed by BRIWS and PARKIN (1963) who have come to some very interesting semi-empirical conclusions. The purpose of this communication is firstly to draw attention to the little known fact that the desired probability distribution of scintillation, which could enable us to perform any necessary conversion between different measures, has been in reality at our disposal, and secondly, to find the relations between several indices and to con&n the conclusions found by BRICWS and PARKIN (1963).

New propagation measurement campaigns have been completed or are in progress, providing new data for the evaluation of link degradations on satellite links. New propagation models and prediction techniques are available, covering the traditional propagation effects along with several new areas. New satellite applications have been thrust into the forefront of the satellite communications industry, requiring new approaches for the evaluation of propagation effects. The proliferation of new and competing applications in the frequency bands allocated to space communications has increased the importance and priority of understanding spectrum sharing and interference mitigation. Propagation conditions are a critical component of a viable sharing and interference process. Each of these areas is discussed further below. Propagation data has also been developed from other sources including terrestrial links, tracking beacons, and from direct measurement of information bearing signals. For example, land mobile propagation data in the 1.5 GHz region was obtained in the Eastern U.S. from MARECS-B2 and in Australia from ETS-V and INMARSAT.

Propagation research has resulted in the development and publication of propagation prediction models in several new areas. These include:

  • Tropospheric Scintillation
  • Cloud Attenuation and Scintillation
  • Ice Depolarization
  • Wet Surface Effects
  • Combined Effects
  • In addition, extensive modeling updates and revisions have been developed for the traditional propagation factors such as
  • Rain Attenuation
  • Atmospheric Gaseous Attenuation
  • Ionospheric Scintillation
  • Frequency Scaling
  • Worst Month, and
  • Site Diversity

The operating frequency of the space link is the critical factor in determining the type and severity of impairments introduced by the Earth’s atmosphere. A radiowave will propagate from the Earth's surface to outer space provided its frequency is high enough to penetrate the ionosphere, which is the ionized region extending from about 50 km to roughly 2000 km above the surface. Regions (or layers) in the ionosphere, designated D, E, and F, in order of increasing altitude, act as reflectors or absorbers to radiowaves at frequencies below about 30 MHz, and space communications is not feasible. As the frequency is increased, the reflection properties of the E and F layers are reduced and the signal can penetrate the ionosphere. Radiowaves above about 30 MHz will propagate through the ionosphere, however, the properties of the wave could be modified or degraded to varying degrees depending on frequency, geographic location, and time of day. Ionospheric effects tend to become less significant as the frequency of the wave increases, and above about 3 GHz the ionosphere is essentially transparent to space communications, with some notable exceptions which will be discussed later. Space communications transmissions will proceed unimpeded as the frequency of transmission is increased up to frequencies where the gaseous constituents of the troposphere, the region from Earth’s surface up to 10-20 km in altitude, primarily oxygen and water vapor, will absorb energy from the radiowave. At certain specific absorption bandswhere the radiowave and gaseous interaction are particularly intense, space communications are severely limited. It is in the atmospheric windowsbetween absorption bands that practical earth-space communications have developed, and it is in these windows that we will focus our attention in our study of radiowave propagation effects.

Up to this writing there are experiments and further enhancements being done by scientist and experts on the field to further provide a more reliable connectivity in areas that were found to have greater scintillation effects.