The time-varying plasma sheath surrounding a reentry spacecraft (or object) can influence the radar detection of the spacecraft seriously, causing detection error or even track loss. To study the influence of plasma sheath on radar detection, a time-varying plasma sheath model based on scattering matrix method is constructed. In the model, the surface media of reentry object is considered, together with some algorithm adjustments to avoid computational singularity, and the time-varying characteristics of plasma sheath are taken into account based on available measured data. By the model, the reflection coefficient of plasma sheath and the radar echo reflected by plasma-covered object are both calculated. And then a processing method of time-varying echo signal based on ambiguity function are put forward, by which the time delay and Doppler frequency of echo signal are analyzed. In the simulation, the rules of a series of time-varying parameters of plasma affecting the reflection coefficient are analyzed. Then, by ambiguity function analysis, the influence rules of plasma sheath on radar echo are revealed, accompanied by some qualitative and quantitative results. The proposed model, method, and simulation results are thought to be helpful for the radar detection of plasma-covered reentry objects.

Conventional sparsity-based direction of arrival (DOA) estimation algorithms suffer from performance degradation due to the offgrid problem. To address it, several research studies focused on onedimensional (1-D) off-grid problem have been conducted. However, due to the bias coupling between the azimuth and elevation directions, those works can’t be directly extended to the uniform rectangular array (URA). In this paper, we propose an effective sparsity-based two-dimensional (2-D) DOA estimation algorithm. The proposed algorithm provides the following benefits: a) we introduce a more accurate observation model where the bias coupling problem is eliminated and each grid bias can be compensated individually. b) besides the sparsity constraint, we engage additional energy constraint over the biases to improve the estimation performance. Due to the presence of multiple unknown parameters, an alternating descent method is adopted to solve the associated joint optimization problem. Finally, simulational results demonstrate that the proposed algorithm, compared with competitors, exhibits superior performance including high precision DOA estimation and low computational complexity.

In this paper, we propose an improved scattering-matrix method (ISMM) to deal with the propagation of electromagnetic (EM) wave in plasma. As a stratification-based method, the ISMM is based on the scattering-matrix method (SMM) and thus inherits the advantages of the SMM in accuracy and high efficiency. But different from the SMM, the ISMM improves the scattering matrices by adjusting the reference point for each layer interface, making the matrices more robust in calculation. This improvement enhances the robustness of the method in dealing with various media situations, especially for the case of a metal as the floor layer and that of extreme plasmas, which is a weakness for the SMM. After presenting the ISMM, we use it to analyze the EM wave reflected by a reentry plasma sheath, which is an important field but with few new results. With the study of various plasma parameters of the reentry plasma sheath, the ISMM is applied to reveal the characteristics of the sheath. In simulations, the validity of the ISMM is first demonstrated, and then the reflection, transmission, and absorption properties of the EM wave propagating in the plasma sheath are studied by the ISMM. The phenomena of ``sunken regions'' and ``phase jumps'' of the plasma sheath are revealed in simulations, which may cause energy loss and distortion of the EM wave.

A reentry object covered by a plasma sheath will produce two reflective signals in a radar echo, which seriously affects the detection of the reentry object by radar and results in positioning error and even tracking loss. On the basis of the scattering center model and the non-uniform plasma stratification model, the present paper establishes a radar scattering model of a reentry object covered by a plasma sheath. The model effectively retains the plasma distribution and electromagnetic wave propagation characteristics of the plasma sheath. Analysis of the signal components of radar echoes in an example shows to some extent that the non-mirror reflection signal is not generated by the plasma wake.

The majority of the existing work on designing nonuniform filter banks (NUFBs) cannot achieve linear-phase (LP) property, because of the high complexity associated with phase distribution. This correspondence proposes an idea of partial cosine modulation to obtain the LP property of NUFBs with rational sampling factors. It makes the efficient modulation technique possible to be used to design LP NUFBs. Except the separately designed lowpass and highpass analysis/synthesis filters, we obtain the bandpass by cosine modulation of several prototypes, worth of the name ¿partial cosine modulation.¿ By analyzing the phase issue of significant aliasing terms, we derive the matching conditions to make the required LP NUFB achievable. With these criteria being satisfied, the design problem becomes that of several prototypes as well as the lowpass and highpass filters, leading to a less design effort. By using the proposed method, near-perfect-reconstruction LP NUFBs can be obtained in a simple and efficient way as demonstrated by examples.