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

A multifrequency (ten spectral lines between VHF and S band) coherent radio beacon is presently transmitting continuously from a 1000-km, high-inclination orbit for the purpose of characterizing the transionospheric communication channel. Its high phase-reference frequency (2891 MHz) permits direct observation of complex-signal scintillation, and its very stable, sun-synchronous orbit allows repeated pre-midnight observations at low latitudes and near-midnight observations at auroral latitudes. We present here early results of the observations; salient points include the following. First, most of the data are consistent with phase-screen modeling of the production of ionospheric scintillation, including an ƒ−2 frequency dependence for phase variance. Second, propagation theories invoking weak, single scatter seldom are adequate, because even moderate intensity scintillation usually is accompanied by phase fluctuations comparable to or greater than a radian. Third, under conditions producing GHz scintillation (near the geomagnetic equator), lower frequencies show marked diffraction effects, including breakdown of the simple ƒ−2 behavior of phase variance and loss of signal coherenceacross a band as narrow as 11.5 MHz at UHF.

Early results from the DNA Wideband satellite experiment—Complex‐signal scintillation

The retarding potential analyzer aboard Ogo 6 has provided high-resolution observations of the ion concentration along the satellite path. Changes in ion concentration as small as 0.03% and at times as small as 0.01% could be measured. Spatial resolution varied from 35 to 380 m. Samples of data have been analyzed to determine the spectral properties of the F region irregularities observed. The most common frequency spectrum observed suggests that the responsible irregularities result from the turbulent dissipation of larger irregularities. At the equator, the larger irregularities are probably produced by convective electric fields. At high latitudes, electric fields may also be involved, but other factors such as precipitating particles may contribute to, or be primarily responsible for, the production of large irregularities. Examples of other types of spectra associated with wavelike irregularities and with 'ground glass' (high-frequency noise) irregularities are also shown.

In situ measurements of the spectral characteristics of F region ionospheric irregularities

An analytical/numerical technique is presented that provides a solution to the parabolic wave equation and allows for direct calculation of the received time-domain signal after propagation of a wide-bandwidth waveform through a turbulent ionized medium. The results are applicable to the case of satellite communication or to radar observation through a frequency-selective transionospheric propagation channel. A number of examples of the application of multiple phase-screen (MPS) techniques are given that enhance intuitive understanding of wave propagation. Numerical predictions of scattering through a striated barium cloud are compared to experimental observations. Good agreement is also shown between MPS calculations and theoretical results for cases in both weak and strong scattering regimes.

Multiple phase-screen calculation of the temporal behavior of stochastic waves

In this survey, we fully review almost all potential ionospheric effects on the performance of space-based radar systems (SBRs), which operate in the ambient ionosphere environment; in particular, we review the use of space-based synthetic aperture radar systems (SARs) for imaging. There are two families of effects involved. One is the effects of the background ionosphere (non-turbulent ionosphere), such as dispersion, group delay, refraction, Faraday rotation, and phase shift. The other is the effects due to ionospheric irregularities, such as refractive index fluctuation, phase perturbation, angle-of-arrival fluctuation, pulse broadening, clutter, and amplitude scintillation. These effects adversely affect SAR imaging in several respects, such as by causing image shift in the range, and degradations of the range resolution, azimuthal resolution, and/or the resolution in height (elevation). We also review ionospheric irregularity characteristics and descriptions, propagation channel statistics, ...

A survey of ionospheric effects on space-based radar

IN September 1970, the Mount Longonot satellite telecommunication station, situated in the Rift Valley thirty miles north-west of Nairobi, was put into commercial operation by the East African External Telecommunication Company. Telephone and telegraph links are maintained with five other stations by means of the Intelsat III-F4 satellite, which is in quasi-geostationary orbit over the Indian Ocean at 62.5° E longitude. Fig. 1 shows the positions of the satellite and the ground stations and the carrier frequencies used are. given in Table 1.

Scintillation Fading of Signals in the SHF Band

FOUR stations were set up in southern and eastern Africa (Table 1) to observe variations of total electron content during the eclipse of the Sun on June 30, 1973. Each station was equipped with a conventional Faraday rotation recording system consisting of a receiver and a mechanically rotating aerial. We recorded the amplitude fading of the 137 MHz signals from the geostationary satellite, Intelsat IIF3, located at approximately 14°W. The four stations were operating for a total period of 10 d, including June 30, the day of the eclipse.

R. F. Kelleher

Papers

Scintillation Fading of Signals in the SHF Band

Faraday rotation studies in Africa during the solar eclipse of June 30, 1973