This is a general review of the existing climatological models of ionospheric radio scintillation for high and equatorial latitudes. Trans-ionospheric communication of radio waves from transmitter to user is affected by the ionosphere which is highly variable and dynamic in both time and space. Scintillation is the term given to irregular amplitude and phase fluctuations of the received signals and related to the electron density irregularities in the ionosphere. Key sources of ionospheric irregularities are plasma instabilities; every irregularities model is based on the theory of radio wave propagation in random media. It is important to understand scintillation phenomena and the approach of different theories. Therefore, we have briefly discussed the theories that are used to interpret ionospheric scintillation data. The global morphology of ionospheric scintillation is also discussed briefly. The most important (in our opinion) analytical and physical models of scintillation are reviewed here.

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A Review of Ionospheric Scintillation Models

[1] We develop a phase screen model called the Synthetic Aperture Radar (SAR) Scintillation Simulator (SAR-SS) for predicting the impacts of ionospheric scintillation on SAR image formation. SAR-SS consists of a phase screen generator and a propagator. The screen generator creates a 2-D random realization of spatial phase fluctuations resulting from the traversal of small-scale field-aligned irregularities in the ionosphere. It accounts for the motion of the radar platform, the drift of the ionospheric irregularities, and the oblique angle of propagation, all of which determine the scale sizes of the irregularities sampled by the radar beam. The propagator solves the 3-D parabolic wave equation using the split step technique to compute the ionospheric transfer function for two-way propagation. This ionospheric transfer function is used to modulate the SAR signal due to terrestrial features in order to assess the ionospheric impact on SAR image formation in the small target approximation. We compare simulated and observed PALSAR imagery over Brazil during disturbed ionospheric conditions. We demonstrate that SAR-SS can reproduce the field-aligned streaks in PALSAR imagery caused by irregularities in the equatorial ionosphere that have been observed by previous authors. The field-aligned streaks exhibited a dominant wavelength larger than the Fresnel break scale, which suggests that refractive scatter was dominant over diffraction as the physical mechanism responsible for the scintillation of the radar signal in this case. The spectral index of phase fluctuations in the screen was quite large (9.0), suggesting that these irregularities were possibly associated with bottomside sinudoidal irregularities rather than equatorial plasma bubbles.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2011RS004956

Simulating the impacts of ionospheric scintillation on L band SAR image formation

Scintillation caused by the electron density irregularities in the ionospheric plasma leads to rapid fluctuations in the amplitude and phase of the Global Navigation Satellite Systems (GNSS) signals. Ionospheric scintillation severely degrades the performance of the GNSS receiver in the signal acquisition, tracking, and positioning. By utilizing the GNSS signals, detecting and monitoring the scintillation effects to decrease the effect of the disturbing signals have gained importance, and machine learning-based algorithms have been started to be applied for the detection. In this paper, the performance of Support Vector Machines (SVM) for scintillation detection is discussed. The effect of the different kernel functions, namely, linear, Gaussian, and polynomial, on the performance of the SVM algorithm is analyzed. Performance is statistically assessed in terms of probabilities of detection and false alarm of the scintillation event. Real GNSS signals that are affected by significant phase and amplitude scintillation effect, collected at the South African Antarctic research base SANAE IV and Hanoi, Vietnam have been used in this study. This paper questions how to select a suitable kernel function by analyzing the data preparation, cross-validation, and experimental test stages of the SVM-based process for scintillation detection. It has been observed that the overall accuracy of fine Gaussian SVM outperforms the linear, which has the lowest complexity and running time. Moreover, the third-order polynomial kernel provides improved performance compared to linear, coarse, and medium Gaussian kernel SVMs, but it comes with a cost of increased complexity and running time.

https://www.mdpi.com/1424-8220/19/23/5219/pdf

The Impact of Different Kernel Functions on the Performance of Scintillation Detection Based on Support Vector Machines.

It is well known that many of the nighttime acquisitions of the L-band Advanced Land Observing Satellite (ALOS) Phased Array-type L-band Synthetic Aperture Radar (PALSAR) instrument over equatorial regions show significant distortions of the image amplitude information. These distortions have the form of amplitude stripes that are roughly aligned with the local geomagnetic field. While ionospheric scintillation has been identified as the source of these distortions, the exact nature of the induced artifacts on synthetic aperture radar (SAR) image quality and SAR signal phase has not yet been studied in sufficient detail. Hence, this paper provides a quantitative analysis of equatorial scintillation effects on SAR image quality and SAR phase. We have performed a statistical analysis of ALOS PALSAR images over equatorial regions to describe the observed distortions and relate them to ionospheric parameters. An ionospheric simulator was developed and validated that is capable of simulating ionospheric distortions based on ionospheric scintillation parameters. Using this simulator, we found that ionospheric scintillation in the equatorial zone can cause significant distortions of SAR image amplitudes, image focus, and SAR signal phase. We determined threshold ionospheric environmental conditions that lead to the formation of these image distortions. Based on these thresholds, we quantified the likelihood of occurrence of ionospheric distortions for the global equatorial belt and for L-band sensors ALOS PALSAR, ALOS-2 PALSAR-2, and NASA-ISRO SAR (NISAR).

The Influence of Equatorial Scintillation on L-Band SAR Image Quality and Phase

[1] We present the Radio Occultation Scintillation Simulator (ROSS), which uses the multiple phase screen method (MPS) to simulate the forward scatter of radio waves by irregularities in the equatorial ionosphere during radio occultation experiments. ROSS simulates propagation through equatorial plasma bubbles which are modeled as homogeneous electron density fluctuations modulated by a Chapman profile in altitude and a Gaussian window in the magnetic east-west direction. We adjust the parameters of the density model using electron density profiles derived from the ALTAIR incoherent scatter radar (9.4°N, 167.5°E, 4.3° north dip), and space-to-ground observations of scintillation using VHF and GPS receivers that are colocated with the radar. We compare the simulated occultation scintillation to observations of scintillation from the CORISS instrument onboard the C/NOFS satellite during a radio occultation occurring near ALTAIR on 21 April 2009. The ratio of MPS predicted S4 to CORISS observed S4 throughout the F region altitudes of 240–350 km ranged between 0.86 and 1.14.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010RS004591

Charles L. Rino

Papers

The Theory of Scintillation with Applications in Remote Sensing: Rino/The Theory