Scintillation Of Radio Signals Research Articles

Ionospheric irregularities and scintillations: a direct comparison of in situ density observations with ground-based L-band receivers

Ionospheric irregularities can affect satellite communication and navigation by causing scintillations of radio signals. The scintillations are routinely measured using ground-based networks of receivers. This study presents observations of ionospheric irregularities by Langmuir probes on the Swarm satellites. They are compared with amplitude scintillation events recorded by the Global Positioning System-Scintillation Network and Decision Aid (GPS-SCINDA) receiver installed in Mbarara (Lat: 0.6^{circ }hbox {S}, Lon: 30.8^{circ }hbox {E} , Mag. lat: 10.2^{circ }hbox {S}). The study covers the years from 2014 to 2018 when both data sets were available. It was found that the ground-based amplitude scintillations were enhanced when Swarm registered ionospheric irregularities for a large number of passes. The number of matching observations was greater for Swarm A and C which orbited at lower altitudes compared to Swarm B. However, some counterexamples, i.e., cases when in situ electron density fluctuations were not associated with any observed L-band amplitude scintillation and vice versa, were also found. Therefore, mismatches between observed irregularity structures and scintillations can occur just over a few minutes and within distances of a few tens of kilometers. The amplitude scintillation strength, characterized by the S4 index was estimated from the electron density data using the well-known phase screen model for weak scattering. The derived amplitude scintillation was on average lower for Swarm B than for A and C and less in accordance with the observed range. Irregularities at an altitude of about 450 km contribute strongly to scintillations in the L-band, while irregularities at about 510-km altitude contribute significantly less. We infer that in situ density fluctuations observed on passes over or near Mbarara may be used to indicate the risk that ionospheric radio wave scintillations occur at that site.

A Semi-Open Loop GNSS Carrier Tracking Algorithm for Monitoring Strong Equatorial Scintillation

Strong equatorial ionospheric scintillation of radio signals is often associated with simultaneous deep amplitude fading and rapid random carrier phase fluctuations. It poses a challenge for satellite navigation receiver carrier phase tracking loop operation. This paper presents a semi-open loop algorithm that utilizes the known position of a stationary receiver and satellite orbit information to estimate carrier phase variations during deep signal fading. Strong GPS scintillation data collected on Ascension Island in March 2013 are processed to reveal fast phase changes of half and full cycles over tens of milliseconds on all three GPS frequencies. Simulated signals are used to validate the algorithm and confirm that the fast phase changes are true scintillation effects rather than receiver processing artifacts. The algorithm performance is demonstrated through comparison with several closed-loop implementations. Potential errors associated with receiver clock bias and other environmental factors are also analyzed. A statistical summary of the concurrent fast phase changes and deep fades based on the processing results of the Ascension Island data is presented.

Daytime gigahertz scintillations near magnetic equator: relationship to blanketing sporadic E and gradient-drift instability

Observations made in non-equatorial regions appear to support the hypothesis that the daytime scintillation of radio signals at gigahertz (GHz) frequencies is produced by the gradient-drift instability (GDI) in the presence of a blanketing sporadic E (Esb) layer. However, the only evidence offered, thus far, to validate this notion, has been some observations of Esb in the vicinity of GHz scintillations. A more comprehensive evaluation requires information about electric field, together with the presence of a steep gradient, which is presumed to be that of Esb. In this regard, the region in the vicinity of the equatorial electrojet (EEJ) appears to be an ideal “laboratory” to conduct such experiments. The dominant driver of electron drift there is the same as that of the EEJ, the vertical polarization electric field, and indications are that the presence of Esb in that vicinity is controlled by a balance in horizontal transport of Esb, between the EEJ electric field and the neutral wind, as described in a model by Tsunoda (On blanketing sporadic E and polarization effects near the equatorial electrojet, 2008). In this paper, we present, for the first time, results from a comprehensive study of daytime GHz scintillations near the magnetic equator. The properties, derived from measurements, are shown, for the first time, to be consistent with a scenario in which Esb presence is dictated by the Tsunoda model, and the plasma-density irregularities responsible for GHz scintillations appear to be produced by the GDI.

Influence of Radio Wave Propagation on the Properties of Solar Microwave Bursts

Scintillation of radio signals passing through the solar corona is considered. An expression describing the dynamic spectrum of these scintillations on the basis of multibeam propagation of radio waves is derived. Properties of the analytically calculated spectrum are shown to coincide with zebra-structure properties of solar radio bursts. It is determined that the time profile of the scintillations caused by multibeam propagation may appear as impulses of emission or absorption or may have a sawtooth form. It is concluded that assuming specific emission source features is not the only way to explain the zebra structure, since the effect of multibeam propagation of radio waves through the solar corona and interplanetary space yields a simple explanation of the phenomenon discussed.

Estimating the amplitude scintillation index from sparsely sampled phase screen data

Phase screen techniques are commonly used to model scintillation of radio signals passing through a disturbed ionosphere, but observational phase or in situ density from both archival and real‐time sources is often sampled at rates well below the resolution desired for input to such models. Previous phase screen resolution criteria do not address the computation of the amplitude scintillation index (S4), which is widely used in scintillation monitoring systems, for practical applications that rely on coarsely sampled phase data. We investigate the accuracy of S4 estimates from sparsely populated one‐dimensional phase screen models through systematic removal of samples from high‐resolution discrete phase screen models in novel numerical experiments. We also provide analytic approximations for the weak‐scatter case and study the effects of antialias filtering. This “subsampling” analysis shows that 2–5 samples per Fresnel radius (rF) are usually sufficient to compute S4 to within 90% of its true value, depending on the form of the phase spectral density function (SDF) and the strength of scattering. We make initial application of the subsampling techniques to data‐based phase screens for studying equatorial ionospheric scintillation with generally satisfactory results. Finally, we outline the use of the techniques developed in this paper to other practical problems, including SDF determination and computing irradiance patterns.

The netlander ionosphere and geodesy experiment

The NEtlander Ionosphere and Geodesy Experiment (NEIGE) of the Netlander Mission to Mars has two series of scientific objectives: (1) to determine Mars orientation parameters in order to obtain information about the interior of Mars and about the seasonal mass exchange between atmosphere and ice caps; and (2) to determine the total electron content (TEC) and the scintillation of radio signals in order to study the large- and small-scale structure of the ionosphere of Mars. These two sets of information will be derived from measurements of amplitudes and Doppler shifts of radio links at UHF and X-band between the Netlander microstations on the Mars surface and an orbiter and between this orbiter and the Earth (at X-band).

Galileo through the Sun's streamers

The solar wind has two distinct components, slow and fast. Where on the surface of the Sun to they originate?. In January, the Sun occulted our line of sight to the Galileo satellite on its tour of Jupiter's moons, and so provided a remarkable bonus for Solar physicists. The scintillation of radio signals from Galileo, as they passed through the Sun's atmosphere, shows that the slow wind comes from equatorial 'streamers' in the corona, and the fast wind comes from the whole solar surface — not just the polar regions, as had been thought.

Analysis of electromagnetic wave scattering by ionospheric irregularities

A quasi‐particle approach is used to analyze the effect of bottomside sinusoidal irregularities on the propagation of beacon satellite signals through the ionosphere. In this work, the radio wave is treated as a distribution of quasi‐particles described by a Wigner distribution function, and the irregularities are modeled as a two‐dimensional density modulation on an uniform background plasma. The collision of quasi‐particles with the irregularities modifies the quasi‐particle distribution and gives rise to the wave‐scattering phenomenon. The multiple scattering process is generally considered in this deterministic analysis of radio wave scattering off the ionospheric density irregularities. The analysis shows that this two‐dimensional density grating effectively modulates the intensity of the beacon satellite signals. This spatial modulation of the wave intensity is converted into time modulation due to the drift of the ionospheric irregularities, which then results in the scintillation of the beacon satellite signals. The modulation is due mainly to the transverse (to the wave propagation) grating. The results of the analysis agree well with the existing experimental data on the scintillation of Atmosphere Explorer satellites signal (Basu et al., 1986). The model is further based on to predict the scintillation of radio signals for their long distance propagation. For a fixed scintillation index (S4) the predicted propagation distance (d) versus the wave frequency (ƒ) follows a power law, namely, d∝ƒK, where k ≈ 2.

Low‐frequency modulation of plasmas and soft electron precipitation near the dayside cusp

On December 2, 1983, the HILAT satellite trajectory was approximately aligned with the noon‐midnight meridian while crossing the northern polar cusp/cleft region. During that time we observed (1) an increase in scintillations of radio signals, (2) strongly variable fluxes of precipitating low‐energy electrons, (3) gradients in the thermal ion density, (4) fluctuations in the east‐west component of the ion drift velocities superimposed on the noon sector convection pattern, and (5) field‐aligned currents. In the region corresponding to the highest level of radio scintillations we also detected fluctuations in electron fluxes on a time scale of half a second superimposed on a low‐energy inverted‐V event. The analysis of these measurements, which provide information on both large‐ and small‐scale dynamics of the dayside cusp, leads to the following conclusions: (1) the large‐scale convection pattern is characteristic for the northern hemisphere high‐latitude ionosphere when the interplanetary magnetic field has negative Y and Z components; (2) the most intense scintillations and the cusp field‐aligned currents occur on recently opened magnetic field lines; (3) plasma created by increased fluxes of precipitating low‐energy electrons convects antisunward with velocity of ∼0.5 km/s; (4) enhanced chemical reactions due to the Joule heating associated with large electric fields maximize at the equatorward edge of the cusp, leading to the net depletion in total electron content and plasma density measurements; (5) plasma irregularities throughout the F region are formed due to a combined effect of structured soft electron precipitation and current‐convective instabilities; and (6) if the subsecond fluctuations of the ion drift measurements seen during the inverted‐V event are due to static variations in the ambient electric field, there are potential drops of a few tens of volts between the altitude of the satellite (∼830 km) and the lower ionosphere.

Flayer scintillations and the aurora

Defense Meteorological Satellite Program photographs of the aurora are correlated with the scintillation of radio signals from both low-altitude and synchronous satellites. Measurements from several stations ranging from subauroral to auroral locations show that the irregularities producing the scintillation exist both equatorward and poleward of the optical aurora. The scintillations are seen to peak when the satellite path is through the aurora, the values falling off with distance from the aurora. Factors which allow for interpretation of the data under two different irregularity formations are applied to the median curves of the variation of scintillation with distance from the aurora for each station. The dominating irregularities for meter wavelength scintillation appear to be elongated and elliptical. To illustrate the extension of the F layer irregularities equatorward of the optical aurora, the magnetic storm of April 18, 1974, is examined. Different mechanisms for the irregularity production, such as heat flow and low-energy electron precipitation, are suggested.

Measurement of Irregularity Heights by the Spaced Receiver Technique

The scintillation of radio signals from an artificial earth satellite has been analyzed by spaced receiver techniques to obtain the heights of the electron density irregularities responsible for scintillations observed at the ground. We report such data taken near the maximum of the present solar cycle. These results are also compared with results of data taken near the minimum phase of the solar cycle. Eighty‐four passes of BE‐B extending from January 1968 to July 1968 were reduced for height values. The heights were computed by measuring the time delays in the ground pattern by graphical cross‐correlation techniques. These delays were then substituted into a height equation for a spherical earth and ionosphere. The spherical formula was developed so that height values for sub‐ionospheric latitude from 36° to 50°N could be measured. The diurnal behavior and the latitude dependence of irregularity height are studied and compared with results obtained during the solar minimum.

The multiple scattering of waves in irregular media. II. Spatial autocorrelation functions

When a wave passes through a large thickness of a non-absorbing medium containing weak random irregularities of refractive index, large amplitude and phase fluctuations of the wave field can develop. In a previous paper it was shown how to calculate the probability distributions and average values of these fluctuations. An essential feature of the treatment was that it took into account waves which have been scattered many times (multiple scatter). The present paper considers the spatial autocorrelation functions of the intensity and phase fluctuations in a plane parallel to the wave front of the incident wave for conditions of multiple scatter. These autocorrelation functions are important since they are used in studying the scintillation of radio signals from stellar sources, and yield information about the scattering medium causing the scintillations. The autocorrelation functions of the field quantities depend on the moment matrix of a four-dimensional joint probability distribution. The moments, which are elements of the matrix, are certain average values of the scattered field. A set of integro-differential equations is formulated for the moments and solved analytically for some special cases. General solutions for the moments are obtained by numerical integration, in the case where the irregularities of refractive index have a Gaussian autocorrelation function. Curves for the spatial autocorrelation functions of intensity and phase in this case are given for different distances in the medium.

Possible Relation of a Specific Ionospheric Event to Simultaneous Meteorological Data

DURING the night of October 31/November 1, 1967, a sudden change in the structure of amplitude scintillations of radio signals from the S-66 BEC satellite was observed at Fort Monmouth. Observations made at 41, 40 and 20 MHz are indicated in Fig. 1, which relates to orbit 12239; the satellite was passing in a west–east direction near 40° N latitude. The unusual event was manifested by (1) a train of strong and frequent amplitude scintillations which was interrupted for a short period and resumed 60 s later and by (2) a similar interruption showing also a scintillation-free zone during 44 s of the following orbit, 12240. The polarization of the signals displayed corresponding variations. Usually such short and sudden interruptions of strong amplitude scintillations are not observed with satellite signals. The geographic location of the subionospheric tracks from 350 km altitude of these two passages is shown by the dashed lines in Fig. 2. The start and end of the scintillation-free intervals, as obtained from amplitude recordings, are given in Table 1. The related total integrated electron content was obtained from measurements of a polarization follower having sampling times of 80 µs and automatic digitized printout for each time second. The electron content curves show a familiar oscillatory pattern1 which characterizes the observation of severe amplitude scintillations. During the “no-scintillations” periods shown in Table 1, the electron content followed a smooth curve.

The effects of field-aligned ionospheric irregularities on satellite scintillations

Observations of scintillations of radio signals from the satellites BEB and BEC made at Adelaide have been analysed to obtain information about the ionospheric irregularities which cause the scintillations. Observations of the radio diffraction pattern over the ground, made with three spaced receivers, indicate ionospheric irregularities aligned with their longer dimension along the Earth's magnetic field, and with axial ratios in the range 2–20. Observations of increases of scintillation depth when the direction of the radio ray is close to that of the magnetic field show effects in qualitative agreement with theory, but much larger in magnitude than would be expected on the basis of the above values of axial ratio. It is suggested that this result may indicate the presence of a new type of weakly ionized irregularities with very large values of axial ratio. However, further theoretical development appears to be necessary before a completely satisfactory model of such irregularities, consistent with all the observations, can be given.

Wave Propagation in a Random Medium With Parabolic Background

Wave propagation in a random medium with inhomogeneous background is studied. The medium is assumed to he charactenzed by a dielectric permittivity ε0(x)+ε1(x) where ε0(x) is a function of space and ε,(x) is a random function of position. With ionospheric propagation application in mind, the background medium is taken as an mhomogeneous plasma region with electron density varying as a function of z. A slab of random irregularities is assumed to be imbedded in the plasma. fluctuations and correlatiOn functions of spherical waves propagating through this medium are calculated using W.K.B. and pertubation techniques. They are compared with the results obtained for the case where the background medium is homogeneous. It is found that the corrections to the fluctuations due to the inhomogeneous background are maximum when the irregularity slab is at the peak of the electron density profile. From this result, it is believed that scintillations of radio signals from satellites are more easily observable when the irregularities are near the peak of the ionosphere electron density profile.

The size of low-latitude ionospheric irregularities determined from observations of discrete sources of different angular diameters

Several methods have been used to measure the horizontal dimensions of ionospheric irregularities that cause the scintillation of radio signals propagating through the terrestrial ionosphere. A frequently used technique is to measure the size of the shadow pattern on the ground with spaced receivers using a very strong radio source (usually Cassiopeia A or Cygnus A) or a satellite as the source of radio frequency energy. The technique described in this paper is to utilize the differences in the angular diameter of various discrete radio sources to determine irregularity size. The phenomenon, by which a radio source of small angular diameter scintillates while one of large angular diameter does not, is directly analogous to the optical portion of the electromagnetic spectrum where stars scintillate but planets do not.

The scintillation of radio signals from the Discoverer 36 satellite

The results of measurements on “scintillation” observed on 20 Mc/s radio signals from satellite Discoverer 36 are presented. The observations cover geomagnetic latitudes 44°–67° N and show a steady increase in the scintillation intensity with latitude. A sharp increase in scintillation intensity occurs at a latitude a few degrees to the equatorward side of the auroral zone. There is a close correspondence between the incidence of scintillation and spread- F and it is concluded that the same irregularities in the F-layer (above about 250 km) are responsible for the two phenomena. Data are presented on the diurnal variation in the lower latitude limit of scintillation and its variation with magnetic activity.

Polarization measurements during scintillation of radio signals from satellites

Introduction. Satellite recordings made with a dipole do not display the regular Faraday effect during periods of strong scintillation [Yeh and Swenson, 1959]. The recent development of a low-loss electrostatic goniometer which works at frequencies up to the VHF range [Gschwendtner and Swenson, 1963] has made it possible to investigate the state of polarization of a satellite radio signal under conditions of scintillation. During weak scintillation the polarization angle and axial ratio were found to have only their regular variations.

Scintillation of radio signals from satellites

Radio signals on a frequency of 20 Mc/s from a satellite in a polar orbit over a period of one month were recorded at two sites in. southeastern Australia to locate regions causing scintillation of signals. Very little scintillation was present during daytime transits between 1200 and 1800 h local time, but between 1900 and 2300 h local time it was observed consistently. The indications are that between latitudes 20°S and 40°S the scintillation was patchy, arising in regions a few hundred miles in north-south extent and more than 1000 miles in east-west extent. A marked increase in the incidence of scintillation with increasing latitude began at 38°S and between 40°S and 50°S, the region responsible for scintillation here being apparently more continuous in both time and latitude. Heights of the scintillation region deduced by triangulation from the two sites ranged from 200 to 350 km. It is shown that, although this method of observation imposes some limitation on the accuracy of these values, much of the scintillation observed must have originated within this height range.

Frequency correlation of line-of-sight signal scintillations

The frequency autocorrelation of amplitude and phase scintillations of radio signals propagated over turbulent line-of-sight paths is calculated theoretically. The single scattering (Born) approximation to the electromagnetic response of the turbulent irregularities is used, limiting the results to small amplitude and phase variations. However, the results are valid for both Fresnel (near zone) and Fraunhofer (far zone) scattering. The calculations are made for an arbitrary model of the turbulent irregularities by using the spectrum method, which postpones the specialization of the analysis to a particular turbulence model until the wave-propagation aspects of the problem are completed. It is shown that the signal variations on adjacent carrier frequencies have high correlation for frequency separations comparable to the carrier frequency for all propagation conditions, indicating that the "medium bandwidth" for line-of-sight paths is very large. This also means that there should be negligible pulse distortion for high-speed data links or high resolution radars operating on line-of-sight paths. These predictions are compared with the few available experimental results.