The GPS-based computerized ionospheric tomography (CIT) has the capacity to reconstruct the three-dimensional ionosphere (i.e., electron density distribution), making it one of the most important techniques for ionospheric observation. However, the CIT technology is unable of high vertical resolution due to the restricted viewing angle. Therefore, the precision of CIT cannot be substantially enhanced with GPS only. Aiming at this issue, this paper proposes a bi-iteration algorithm that integrates GPS, Phased Array L-band Synthetic Aperture Radar on board the Advanced Land Observing Satellite (ALOS PALSAR), and ionosonde data. The key is that the joint retrieval of PALSAR and ionosonde may offer high-precision one-dimensional electron density profile along the whole path, which can effectively enhance the authenticity of the iterative initial value. The corrected initial value is then fused into the process of CIT, which can finally improve the precision of vertical resolution after two iterations. Experimental verification demonstrates that due to the ability to obtain more realistic initial values, the reconstruction accuracy of the algorithm proposed in this paper is 51.4 percent and 45.1 percent higher than the accuracy with GPS alone and with GPS and ionosonde data, respectively. This indicates that the combination of these three kinds of data can effectively improve the precision of CIT.

The signal of spaceborne low-frequency full-polarization synthetic aperture radar (full-pol SAR) contains abundant ionospheric information. Phased Array L-band Synthetic Aperture Radar (PALSAR) working in the L-band has been verified as an emerging ionospheric sounding technology. Aiming for a future P-band SAR system, this paper investigates the ability of the P-band SAR system in ionospheric one-dimensional and two-dimensional detection. First, considering different systematic error levels, the total electron content (TEC) retrieval in L/P-band is studied by using three typical full-pol SAR data sets based on a circular polarization algorithm. Second, the TEC data retrieved by SAR are fused with the ionosonde, and the joint retrieval of ionospheric electron density is performed. Results show that the P-band TEC retrieval is approximately twice as accurate as the L-band retrieval under the same conditions, and possesses excellent robustness. In addition, the TEC obtained by L/P-band SAR can be used to correct the electron density of the topside on the ionosonde. Results also show that compared with the topside correction accuracy of L-band SAR, that of the P-band SAR is improved by more than 20%. SAR has natural high-resolution characteristics and the P-band signal contains more obvious ionospheric information than the L-band signal. Therefore, future spaceborne P-band SAR has many advantages in two-dimensional fine ionospheric observation and one-dimensional electron density retrieval.

The utilization of Global Positioning System (GPS) for three-dimensional ionospheric electron density reconstruction, i.e., computerized ionospheric tomography (CIT), provides significant importance in investigating the internal structure, variations, and disturbances within the ionosphere. However, the ill-posed problem caused by insufficient observational data or uneven distribution will rely heavily on the selection of initial values, which are typically derived from empirical models with low precision. Aiming at this issue, this paper uses the TEC obtained by the spaceborne full polarization synthetic aperture radar (PolSAR) to correct the Empirical Canadian High Arctic Ionospheric Model (E-CHAIM), thus improving the authenticity of the initial value. This study makes full use of the advantages of low frequency full PolSAR in ionospheric sounding, including high precision and resolution, as well as all-day and all-weather operation without a ground receiver. Therefore, the precision of GPS-based tomography can be enhanced, particularly for small-scale anomalies, and it is also simple and easy to achieve. Numerical and measured experiments using GPS, incoherent scatter radar, PolSAR, and E-CHAIM data in Alaska demonstrate that the reconstruction accuracy of the proposed CIT is significantly improved than that of the tomography results using only empirical model. In addition, the effects of PolSAR system errors and voxel size on CIT are analyzed to demonstrate the robustness of the method proposed in this paper.

The use of low-frequency spaceborne fully-polarized synthetic aperture radar (SAR), such as the ALOS PALSAR, to detect the structure of ionospheric has been widely validated in recent years. In terms of total electron content (TEC) retrieval, SAR exhibits significant advantages over GPS with regard to both accuracy and resolution at high-latitude. However, TEC cannot reflect the vertical structure of the ionosphere and still has limitations. On this basis, this letter presents a study into the application of fully-polarimetric PALSAR in computerized ionospheric tomography (CIT), which extends the horizontal ionospheric structure to the vertical, i.e., the three-dimensional (3-D) ionospheric electron density (IED) distribution. The reconstruction results from three groups of PALSAR measured data in Alaska are compared with the results from the nearby incoherent scatter radar (ISR). The accuracy has improved by 28.9%, 33.2%, and 47.6% respectively compared to the ionosphere model. This shows that the PALSAR has the ability to not only determine the horizontal structure of the ionosphere but also accurately reconstruct the 3-D IED, making it a promising approach for ionosphere sounding at high-latitude.

Finite-difference time-domain (FDTD) algorithm with a new method of plane wave excitation is used to investigate the RCS (Radar Cross Section) characteristics of targets over layered half space. Compare with the traditional excitation plane wave method, the calculation memory and time requirement is greatly decreased. The FDTD calculation is performed with a plane wave incidence, and the RCS of far field is obtained by extrapolating the currently calculated data on the output boundary. However, methods available for extrapolating have to evaluate the half space Green function. In this paper, a new method which avoids using the complex and time-consuming half space Green function is proposed. Numerical results show that this method is in good agreement with classic algorithm and it can be used in the fast calculation of scattering and radiation of targets over layered half space.