Zheng-Wen Xu

Potential Effects of the Ionosphere on Space-Based SAR Imaging

There has been a considerable interest in the use of lower frequency (VHF/UHF) space-based synthetic aperture radar (SAR) for realizing the foliage and ground penetration. The phase perturbation, signal distortion and imaging resolution degradation by the ionosphere will be particularly severe, however the model is not yet well established and still needs to be further studied. In this paper, on the basis of possible improvements for the model proposed by Ishimaru and others, potential ionospheric effects on SAR imaging are evaluated. First, for analyzing azimuthal resolution, we apply the fourth moment recently obtained in general case of strong fluctuation regimes, which is expected to give results for wider conditions. The Gaussian approximation was used in the previous model; however it is only valid in the fully saturated regimes. Second, for analyzing image shift and distortion, besides group delay, the higher-order dispersion is considered. Third, for discussing range resolution degraded due to pulse broadening, besides the dispersion, the multiple scattering of ionospheric turbulence is studied. Fourth, the Faraday rotation effect is analyzed. Numerical simulations are shown using ionospheric turbulence spectrum and TEC inferred from the International Reference Ionosphere (IRI) and satellite beacon observations.

On the vertical drift of ionospheric F layer during disturbance time: Results from ionosondes

The morphology of low-latitude ionosphere is greatly affected by the zonal electric field, especially at disturbance time. In this study, historical data from two low-latitude ionosondes located at Haikou and Chongqing, China, are used to study the disturbance vertical drift properties of ionospheric F layer during the initial and main phases of 50 intense storms. The disturbance drift from an empirical model is used as an indicator to denote the E x B disturbance magnitude. It is found that the drifts both at the base and at the peak height are increased in magnitude when the disturbance becomes stronger. For the same disturbance, ionospheric vertical drifts at the base height are comparable with those at the peak height over Haikou, but they are larger than the latter over Chongqing, both drifts being smaller than those at the magnetic equator. The drifts are larger in daytime than in nighttime, but no rule is found on their seasonal dependence. This indicates that low-latitude storm time ionospheric vertical drift of the ionospheric layer exhibits both consistent and individual properties at different locations.

TEC retrieval from spaceborne SAR data and its applications

It is well known that the spaceborne synthetic aperture radar (SAR) at VHF-UHF band can be seriously affected by the ionosphere. Thus, the geophysical information of the ionosphere will be embedded in the low-frequency SAR echoes after they transverse the ionosphere. Correspondingly, the total electron content (TEC), a typical ionospheric information parameter, can be retrieved from the spaceborne SAR data. However, the existing dual-band techniques for TEC retrieval usually do not include consideration of multiple scattering effects caused by turbulent ionosphere, which plays an important role in the total path delay of signal under the strong fluctuation regimes. The result of TEC retrieval is therefore inaccurate and not applicable. Aiming at this issue, first, this paper analyzes the effects of regular background and the irregularity of electron density on SAR at L-band, and the theoretical formulation is given. Then, a triband path delay technique of TEC retrieval based on the SAR data is proposed. By using three path delays corresponding to three specific frequencies within the signal bandwidth, this technique can remove the errors of multiple scattering due to the irregularity, and a high accuracy resolution of TEC value therefore can be obtained. Meanwhile, the sensitivity of this technique is analyzed. Finally, compared with traditional dual-band technique, the numerical simulations show that the correction of SAR imaging based on triband technique is improved significantly. In addition, the resolution of reconstruction imaging using computerized ionospheric tomography performs significantly better based on the triband technique.

Forecasting the local ionospheric foF2 parameter 1 hour ahead during disturbed geomagnetic conditions

[1]xa0Using the support vector machine (SVM), an empirical local ionospheric forecasting model over Lanzhou (ELIFMOL) has been developed to predict the critical frequency of the F2 layer (foF2) during disturbed geomagnetic conditions. This study focuses on the reliable prediction of foF2 during geomagnetic storms, which is important for practical applications as well as for further understanding of the storm dynamics. In this paper, we investigate whether foF2 during disturbed geomagnetic conditions at a single station can be well predicted one hour ahead by using some inputs to an SVM network, such as the latest foF2 observations, hourly quiet time foF2 (foF2QT), time, and the hourly time-weighted accumulation series derived from the geomagnetic index (ap(τ)). The input observation data cover the period of January 1958 to December 2000 at Lanzhou (36.1°N, 104.0°E) in China. The foF2 forecasted by the ELIFMOL are compared with the monthly median values and with those by the STORMMEDIAN model, the persistence model, and the STORMfoF2QT model during geomagnetic storms occurring from 2001 to 2006. As for the data sets used in this paper, the results show that the performance of ELIFMOL is better than that of the latter models.

Cubic Phase Distortion and Irregular Degradation on SAR Imaging Due to the Ionosphere

The interest in the use of spaceborne synthetic aperture radar (SAR) for collecting earth bio/geophysical information and detecting foliage-obscured targets has been increased. However, the signals are inevitably affected by the ionosphere, particularly at very high frequency and ultrahigh frequency. Thus, it is crucial to understand the potential effects of the ionosphere on SAR systems. In this paper, three possible contributions are made to analyze these effects. First, for analyzing range resolution, in addition to linear and quadratic phase errors due to the background ionosphere, the cubic phase error is considered. The expected mean electron density inferred from the International Reference Ionosphere is used. Second, for analyzing azimuthal resolution, the effects of ionospheric irregularities are evaluated under the conditions of oblique incidence and anisotropic irregularities. The model is presented on the basis of the multiple phase screen (MPS) method. Compared with the previous model, the MPS method can give results in good agreement with both weak and strong scattering theories. Finally, based on the theory of moment equation, range resolution degradation caused by the multiple scattering is also studied in the case of anisotropic irregularities. By using the range Doppler algorithm, a number of degraded point responses due to these ionospheric effects are shown, and then, some evaluation results are listed.

Effects of Anisotropic Ionospheric Irregularities on Space-Borne SAR Imaging

This paper discusses the degradation of space-borne synthetic aperture radar (SAR) imaging due to anisotropic ionospheric irregularities. In the case of the oblique incidence and anisotropy of ionospheric irregularities, two types of potential ionospheric effects on SAR imaging are derived from an improved transverse correlation function. First, in the range direction, the image shift due to both multiple scattering and dispersion of irregularities is considered. In addition, pulse broadening due to multiple scattering of the irregularities is also studied, which leads to degradation of the range resolution. Second, in the azimuthal direction, the decorrelation distance is obtained by the second moment of the generalized ambiguity function. Using the range Doppler algorithm (RDA), several degraded point target responses due to the ionospheric irregularities are evaluated. The simulation results show that the adverse effects of ionospheric irregularities on SAR imaging are worsened at lower frequencies and stronger ionospheric fluctuations.

Diurnal specification of the ionospheric f0F2 parameter using a support vector machine

[1]xa0This paper proposes a method for forecasting the ionospheric critical frequency, f0F2, up to 5 h ahead using the support vector machine (SVM) approach. The inputs to the SVM network are the universal time; day of the year; a 2 month running mean sunspot number (R2); a 3 day running mean of the 3 h planetary magnetic ap index, the solar zenith angle; the present value f0F2(t) and ten previously observed values f0F2(t − i), where i = 1, 2, 3, 4, 19, 20, 21, 22, 23, 24; and the six derivatives of previous 30 day running means of f0F2fmF2(t − j), where j = 19, 20, 21, 22, 23, 24. The output is the predicted f0F2 up to 5 h ahead. The network is trained using the ionospheric sounding data from seven Chinese stations, i.e., Guangzhou, Haikou, Chongqing, Beijing, Lanzhou, Changchun, and Manzhouli stations at solar maximum and minimum. In order to test the predictive ability, the SVM was verified with different data from the training data. The quality of the proposed model prediction is evaluated by comparison with corresponding predictions from the persistence reference, the autocorrelation and the neural network (NN) models. By using data from seven Chinese stations, it is shown that the performance of the SVM model is superior to that of the autocorrelation and persistence models, and that it is comparable to that of the NN model.

Simulation of ionospheric effects on SAR imaging with noise at P-band

In the past, most of the spaceborne synthetic aperture radars (SAR) operating at C-band or higher frequencies, and the ionospheric effects are negligible in these bands. Recently, the use of lower frequencies for measuring the foliage and ground penetration has been an increasing interest. However, the ionosphere is a highly dispersive medium, and can cause group delay and pulse broadening when the carrier frequency is low, such as P-band. Thus the range resolution in SAR imaging is degraded due to the ionospheric effects. And the irregularity of the ionosphere also can reduce the azimuthal resolution. Furthermore, the spaceborne SAR receiver would generate noise, which affects the quality of the echoes so as to SAR imaging. This paper presents both the analytical study and numerical simulation to investigate the ionospheric effects on SAR imaging with Gaussian white noise at P-band.

The Qujing incoherent scatter radar: system description and preliminary measurements

The Qujing incoherent scatter radar (QJISR), the first one in China with the geographic location (25.6°N, 103.8°E), was brought into operation since the spring of 2014. The QJISR was a mono-static pulsed radar working in the operating frequency 500xa0MHz, the peak power 2 Megawatt, and a 29-m steerable parabolic dish. This paper mainly presents the basic configuration and implementation of QJISR, including the antenna, transmitter, receiver, signal processing, and data analysis. Some preliminary observation results are also reported including the raw echo, power spectra, and its ionospheric parameters: electron density, electron temperature, ion temperature, and drift velocity.

Second-order statistics of electromagnetic pulse propagation through the turbulent ionosphere

The characteristics of the signal fluctuations and the channel model are described by several important coherence parameters, which are determined by a two-position, two-frequency and two-time mutual coherence function of the received signals after propagating through random media. The paper discusses these second-order statistical quantities by using an analytic solution to the mutual coherence function for a plane wave recently obtained by iteration of an integral equation. Afterwards, the power impulse response, spectra and delay-Doppler scattering function are also derived and discussed.