Intensity Scintillation Index Research Articles

Intensity scintillation index and mean apparent radar cross section on monostatic and bistatic paths

We develop an expression herein for the intensity scintillation index on a two‐way (radar) path, in terms of the one‐way index and the correlation between scintillations produced on the uplink and downlink. The expression is appropriate for monostatic (fully correlated) and bistatic (totally or partially uncorrelated, or anticorrelated) paths whose links are statistically similar and obey Nakagami m statistics. A companion expression for the mean apparent radar cross section (RCS) in the presence of scintillation describes enhancement on monostatic paths and energy‐conserving depletion of mean apparent RCS on small‐angle bistatic paths. The companion expression, which does not depend upon Nakagami m statistics nor require statistical similarity, is consistent with more detailed calculations by previous authors. Special cases of both expressions are consistent with recent monostatic measurements.

Acoustic propagation experiments in a deep lake

Acoustic propagation experiments were conducted at Lake Pend Oreille, a deep (350 m) lake in northern Idaho. The goal of the experiments was to quantify the level and seasonal variability of transmission loss fluctuations of acoustic pulses propagated along wholly refracting paths in the lake. Continuous measurements of transmission loss at mid‐depth were recorded for approximately 1‐week periods during 5 months in 1988‐1989. Most measurements were made in the 10‐ to 40‐kHz range, using a fixed bottom‐moored projector and bottom‐moored hydrophones at ranges of 1 and 2 km. In May 1989, a series of temperature profiles was acquired concurrently with acoustic data over a 5‐day period. Acoustic transmission in this short‐range, high‐frequency regime is subject to weak scattering due to thermal microstructure. The mean temperature at mid‐depth is near 4°C with superimposed microstructure. During some periods, the temperature gradients were strong enough to cause several dB of short‐term acoustic fluctuations. The acoustic data have been processed to produce numerous received signal statistics including the mean, standard deviation and variance of the received signal level, intensity, logarithmic variance of the received intensity, and intensity scintillation index. This paper will present selected results from the experiments and provide a comparison with stochastic (weak scattering theory) and deterministic modeling approaches. [Work supported by DARPA.]

Cross‐correlation analysis and interpretation of spaced‐receiver measurements

Two algorithms which provide a statistical treatment to the estimation of the parameters of the cross‐correlation analysis of spaced‐receiver data are reviewed. Their results are compared using signals: (1) transmitted by a quasi‐stationary polar beacon and received at Goose Bay, Labrador (53.3°N, 60.3°W); and (2) transmitted by the orbiting Hilat satellite and received at Tromsø, Norway (69.7°N, 18.9°E). A good general agreement is displayed in this comparison. The former experiment indicates the possibility of extreme daily variations in the anisotropy of the ground diffraction pattern and in the true drift velocity of the in situ irregularities. The latter experiment displays geometrical enhancements in the intensity scintillation index S4, in the rms phase fluctuation σϕ and in the axial ratio of the ellipse which characterizes the anisotropy of the ground diffraction pattern, around the region of local L shell alignment of the ray paths. Increases of these parameters also are observed northward of Tromsø. These observations are thus consistent with a morphological model for anisotropy of high‐latitude nighttime F region irregularities proposed in the literature. Next, a possible dependence of the results of the spaced‐receiver measurements on the receiver baselines is discussed. It is argued that this mechanism could be responsible for the relatively small values of the anisotropy of the diffraction pattern obtained from the Hilat measurements at Tromsø. Finally, a procedure which combines a simple propagation model of the spaced‐receiver experiment with a nonlinear minimization algorithm is proposed to estimate the anisotropy of the in situ irregularities from that of the diffraction pattern.

Modeling and scientific application of scintillation results

Efforts continue to update and improve empirical models of the mean morphological behavior of ionospherically produced scintillation. While often intended for engineering application, these models also can be scientifically useful to the extent that they describe not only the signal fluctuations but also the plasma density irregularities that cause them. Such a model is contained in the computer code called WBMOD, which employs the phase screen propagation theory to calculate the power law spectral index and strength (at a fluctuation frequency of 1 Hz) of phase scintillation and the standard deviation of phase, for a user‐specified system. Explicit provision is made for an ionospheric outer scale, but the relevant phase spectrum is cut off by a system parameter in most applications. The intensity scintillation index S4 is calculated from weak‐scintillation theory and corrected for the saturating effect of multiple scatter on the basis of Rice statistics. The irregularity model in WBMOD describes three‐dimensionally anisotropic (sheetlike) irregularities on the nightside of the high‐latitude scintillation zone and axially symmetric (rodlike) irregularities elsewhere. The current status of WBMOD, which is undergoing continued refinement, is described in this paper for all latitude regimes. Plans for future development of its irregularity model are stated, and it is suggested that such models should now move in the direction of incorporating irregularity dynamics.

Geometrical control of the ratio of intensity and phase scintillation indices

Early observations of complex-signal scintillation revealed a sizeable difference in the ratio of the intensity scintillation index, S 4, to the phase scintillation index, σ φ , when measured at mid-latitude, auroral-zone, and equatorial stations. The differences observed between auroral and equatorial stations receiving a beacon signal from a polar-orbiting satellite have now been found to persist, for non-saturated values of S 4. The phase-screen theory for production of scintillation is employed in this paper to identify four factors that control the S 4 σ φ ratio, and the behavior of two of them is explored for models describing both sheetlike and rodlike plasma-density irregularities. The analysis permits rejection of the effects of static diffraction by field-aligned irregularities (whether rodlike or sheetlike) as the factor controlling the different behaviours observed. It is shown, on the other hand, that geometrical control of the effective outer scale imposed by detrend filtering in the presence of highly anisotropic irregularities can readily explain the observed behavior. Such effects arise in virtually all experiments and radio systems operating in the presence of a power-law spectrum with a very large outer scale, and they are important in controlling the relationship between phase and intensity scintillation.

A power law phase screen model for ionospheric scintillation: 1. Weak scatter

In this paper the weak scatter scintillation theory is reformulated to show explicitly the ramifications of an arbitrarily large ionospheric outer scale. The measured temporal phase spectrum, for example, is effectively truncated at a fixed frequency corresponding to the detrend time or the length of the data interval over which it is measured (whichever is smaller). As a consequence, the rms phase exhibits a complicated dependence on the relative irregularity drift velocity and the propagation geometry. This effect has not been included in previous analyses. By comparison, intensity scintillation data are intrinsically high‐pass filtered by the diffraction process. By taking advantage of this fact a simple closed form expression for the S4 intensity scintillation index has been derived. The theory is applied to representative data sets from the Wideband satellite. The interpretation of the ionospheric parameters deduced from the analysis is also discussed.