Background: The energy efficiency of electric storage water heaters is a concern for enterprises and customers because it is beneficial for energy saving. To decrease the workload of testing personnel and improve the testing efficiency, an automatic testing system for evaluating the energy efficiency performance of electric storage water heaters has been designed. Methods: The complete testing system includes a water supply subsystem, hot water outlet subsystem, hardware subsystem, and software subsystem. An application program was developed using Visual Basic to acquire the sensor data and transmit it in real time to a database server. The energy efficiency evaluating process was controlled by the application software,by combining the data acquisition technology, communication technology, and program logical control technology. Results: The designed system can run automatically and achieve reliable tests, two important parameters including the standing loss per 24 h and the hot water withdrawal rate were obtained. Conclusion: This system can be applied to evaluate the energy efficiency grades of electric storage water heaters.

Plasma sheath poses a serious challenge to inverse synthetic aperture radar (ISAR) imaging of hypersonic targets. This paper investigated the distribution characteristics of the electron density and velocity field in the plasma sheath surrounding the hypersonic target in various flight scenes. The incident depth and reflective surface of electromagnetic (EM) waves with X-band, Ku-band, and Ka-band can be determined based on the plasma frequency. We established the echo model coupled with the velocity field of the plasma sheath on the reflective surface and obtained one-dimensional range profiles and ISAR images of the hypersonic target in various flight scenes. The simulation results indicated that the non-uniform velocity field on the reflective surface induced displacement and diffusion in the one-dimensional range profile, resulting in ISAR image distortion. A changing flight scene and radar frequency can have an impact on imaging results. Peak Signal-to-Noise Ratio (PSNR) and Structural Similarity (SSIM) were utilized to assess the impact of plasma sheath on ISAR images. This study revealed the defocus mechanism of the ISAR image caused by the velocity field of the plasma sheath and provided a theoretical reference for the selection of radar frequency for hypersonic targets in various flight scenes.

The plasma sheath can induce radar signal modulation, causing not only ineffective target detection, but also defocusing in inverse synthetic aperture radar (ISAR) imaging. In this paper, through establishing radar echo models of the reentry object enveloped with time-varying plasma sheath, we simulated the defocusing of ISAR images in typical environment. Simulation results suggested that the ISAR defocusing is caused by false scatterings, upon which the false scatterings’ formation mechanism and distribution property are analyzed and studied. The range of false scattering correlates with the electron density fluctuation frequency. The combined value of the electron density fluctuation and the pulse repetition frequency jointly determines the Doppler of false scattering. Two measurement metrics including peak signal-to-noise ratio and structural similarity are used to evaluate the influence of ISAR imaging.

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Conventional compressive sensing (CS)-based imaging methods allow images to be reconstructed from a small amount of data, while they suffer from high computational burden even for a moderate scene. To address this problem, this paper presents a novel two-dimensional (2D) CS imaging algorithm for strip-map synthetic aperture radars (SARs) with zero squint angle. By introducing a 2D separable formulation to model the physical procedure of the SAR imaging, we separate the large measurement matrix into two small ones, and then the induced algorithm can deal with 2D signal directly instead of converting it into 1D vector. As a result, the computational load can be reduced significantly. Furthermore, thanks to its superior performance in maintaining contour information, the gradient space of the SAR image is exploited and the total variation (TV) constraint is incorporated to improve resolution performance. Due to the non-differentiable property of the TV regularizer, it is difficult to directly solve the induced TV regularization problem. To overcome this problem, an improved split Bregman method is presented by formulating the TV minimization problem into a sequence of unconstrained optimization problem and Bregman updates. It yields an accurate and simple solution. Finally, the synthesis and real experiment results demonstrate that the proposed algorithm remains competitive in terms of high resolution and high computational efficiency.