The phenomenon of scintillation of radio waves propagating through the ionosphere is reviewed in this paper. The emphasis is on propagational aspects, including both theoretical and experimental results. The review opens with a discussion of the motivation for stochastic formulation of the problem. Based on measurements from in-situ, radar, and propagation experiments, ionospheric irregularities ate found to be characterized, in general, by a power-law spectrum. While earlier measurements indicated a spectral index of about 4, there is recent evidence showing that the index may vary with the strength of the irregularity and possibly a two-component spectrum may exist with different spectral indices for large and small structures. Several scintillation theories including the Phase Screen, Rytov, and Parabolic Equation Method (PEM) are discussed next. Statistical parameters of the signal such as the average signal, scintillation index, rms phase fluctuations, correlation functions, power spectra, distributions, etc., are investigated. Effects of multiple scattering are discussed. Experimental results concerning irregularity structures and signal statics are presented. These results are compared with theoretical predictions. The agreements are shown to be satisfactory in a large measure. Next, the temporal behavior of a transionospheric radio signal is studied in terms of a two-frequency mutual coherence function and the temporal moments. Results including numerical simulations are discussed. Finally, some future efforts in ionospheric scintillation studies in the areas of transionospheric communication and space- and geophysics are recommended.

Radio wave scintillations in the ionosphere

In May 1974 a new era in satellite radio beacon studies of the ionosphere opened with the ATS-6 Radio Beacon Experiment. The history of radio beacon studies up to that time is reviewed briefly and the particular features of the ATS-6 beacon are discussed together with the basic theory required to interpret the measurements. The main emphasis is on the ATS-6 beacon experiment but other beacon data are discussed which provide the necessary background. The diurnal and seasonal variations of the total electron content and the plasmaspheric content are presented for the U.S.A. and Europe. In winter the plasmaspheric content over the Western Hemisphere maximizes at night while in Europe and the Pacific it appears to peak near noon. This is thought to be caused by flow of plasma from the local and conjugate ionospheres. Night maxima of total electron content are found showing that they do not arise from depletions of the plasmaspheric content. The plasmaspheric content is highly sensitive to solarterrestrial disturbance, it reaches a minimum on the third day of a storm and may take between 10 and 20 days of partial filling and emptying to recover. Traveling disturbances in U.S.A., Europe, and India show similarities of speeds but not of direction. Beacon observations of micropulsations in total content, tropospheric fluctuations and Fresnel diffraction by intense ionospheric irregularities are discussed together with radio wave scintillations and some applications of beacon radio data to communications and navigation.

Recent progress in satellite radio beacon studies with particular emphasis on the ATS-6 radio beacon experiment

A simple model is proposed for simulating equatorial transionospheric radio wave scintillation. The model can be used to test Global Positioning System phase tracking loops for scintillation robustness because it captures the scintillation properties that affect such loops. In the model, scintillation amplitude is assumed to follow a Rice distribution, and the spectrum of the rapidly-varying component of complex scintillation is assumed to follow that of a low-pass second-order Butterworth filter. These assumptions are justified, and the model validated, by comparison with phase-screen-generated and empirical scintillation data in realistic tracking loop tests. The model can be mechanized as a scintillation simulator that expects only two input parameters: the scintillation index S 4 and the decorrelation time tau0. Hardware-in-the-loop tests show how the model can be used to test the scintillation robustness of any compatible GPS receiver.

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Simulating Ionosphere-Induced Scintillation for Testing GPS Receiver Phase Tracking Loops

The theoretical framework and experimental methodology used to interpret observations of ionospheric scintillations in terms of geophysical processes are reviewed and recent experimental observations of ionospheric scintillations are discussed in this paper. During the past 15 years significant progress has been made in several areas. In particular, significant advances have been made in theoretical understanding of the strong scintillation regime and the effects of short-term temporal variations of the scintillation producing irregularities on observations made with spaced-receiver geometries in both weak and strong scintillations. This improved understanding of the scintillation process has significantly increased the utility of the technique particularly in the equatorial latitudes where geometrical effects are least important.

Deducing turbulence parameters from transionospheric scintillation measurements

Amplitude scintillation spectra and statistics obtained simultaneously at VHF (257.55 MHz), L band (1541.5 MHz), and C band (3945.5 MHz) from the MARISAT communications satellite (15°W) are presented. The measurements were made at Ascension Island (7°58′S, 14°25′W, 30°S DIP) over a high elevation propagation path within a few degrees of the magnetic meridian plane. The multi-frequency data and the unique propagation geometry enable us to study the structures of the equatorial ionospheric irregularities closely. Numerical simulation is used to model a specific event. Comparisons between the modeling results and the observations indicate that the data are consistent with the recently measured (in situ) two-component power law irregularity spectrum with a shallow large-scale regime and a steeper small-scale regime, and a spectral break at about 1-km scale size.

Observations and modeling of multi-frequency VHF and GHz scintillations in the equatorial region

Simultaneous multifrequency scintillation data observed at a mid-latitude station of transmissions from the ATS 6 radio beacon experiments are used to find the power spectra of amplitude scintillation. With the frequency spacing of 40, 140, and 360 MHz, weak and strong scintillations are often observed. Single-scatter scintillation theory is applied to interpret the weak scintillation data, and the theory correctly predicts the spectral shape. For strong scintillations the spectra no longer show a distinct break frequency at which the spectra begin to roll off. Because of decorrelation by multiple scattering there is also broadening at the high-frequency portion of the spectrum, even though the slope of the high-frequency asymptote remains about the same as that in the weak scintillation case. Some of these features are compared with predictions based on the multiple-scatter scintillation theory.

R. Umeki

Papers

Multifrequency spectra of ionospheric amplitude scintillations