Medium-Earth-orbit (MEO) synthetic aperture radar (SAR) could provide wide observable area and short revisit times while offering a moderate spatial resolution. The combination of maneuvering platform SAR receivers and MEO SAR transmitter has become a new research hotspot. However, the receiver often works in nonlinear trajectory with high maneuverability and highly squinted angle in practice. The curved trajectory of MEO SAR and the complex motion of the receiver would cause severe azimuth variance and range-Doppler coupling. The existing imaging algorithms mostly suffer from azimuth defocusing if applied to MEO/high maneuvering platform receiver bistatic SAR (MEO/HMP-BiSAR). To solve these problems, this article proposes a modified extended azimuth nonlinear chirp scaling (EANLCS) algorithm with a new perturbation function. A high-precision echo model is deduced for the modified EANLCS algorithm to describe the range history and echo phase. The modified EANLCS algorithm not only solves the problem of range cell migration and severe range-Doppler coupling, but it also equalizes the spatial variance of Doppler frequency modulation rates as well as high-order Doppler coefficients in MEO/HMP-BiSAR. The proposed algorithm markedly surpasses conventional algorithms in image quality. Simulations and real experiment are conducted to verify the effectiveness of the proposed algorithm.

A bistatic synthetic aperture radar (BiSAR) system with a Medium-Earth-Orbit (MEO) SAR transmitter and high-maneuvering receiver (MEO/HM-BiSAR) can achieve a wide swath and high resolution. However, due to the complex orbit characteristics and the nonlinear trajectory of the receiver, MEO/HM-BiSAR high-resolution imaging faces two major challenges. First, the complex geometric configuration of the BiSAR platforms is difficult to model accurately, and the ‘non-stop-go’ effects should also be considered. Second, non-negligible wavefront curvature caused by the nonlinear trajectories introduces residual phase errors. The existing spaceborne BiSAR imaging algorithms often suffer from image defocusing if applied to MEO/HM-BiSAR. To address these problems, a novel high-resolution imaging algorithm named MSSWCC (Modified Second-Order Space-Variant Wavefront Curvature Correction) is proposed. First, a high-precision range model is established based on an analysis of MEO SAR’s orbital characteristics and the receiver’s curved trajectory. Based on the echo model, the wavefront curvature error is then addressed by two-dimensional Taylor expansion to obtain the analytical expressions for the high-order phase errors. By analyzing the phase errors in the wavenumber domain, the compensation functions can be designed. The MSSWCC algorithm not only corrects the geometric distortion through reverse projection, but it also compensates for the second-order residual spatial-variant phase errors by the analytical expressions for the two-dimensional phase errors. It can achieve high-resolution imaging ability in large imaging scenes with low computational load. Simulations and real experiments validate the high-resolution imaging capabilities of the proposed MSSWCC algorithm in MEO/HM-BiSAR.

In order to design a charge-coupled device (CCD) driving circuit with a high frame rate and a large area array which can support the electronic image motion compensation function, first a basic driving circuit of the large-array CCD is reported in this paper. Next, electronic image motion compensation is realized by adding an image motion compensation timing generator and a main timing generator, Saa8103. The internal design structure of the image motion compensation generator is provided and is characterized as follows. The data are first sent by the system controller, the main timing generator and the image motion compensation timing generator. The generator, the vertical driver, the horizontal driver and the full-frame charge-coupled device chip are then composed. During the exposure period, the image motion compensation transfer clock is generated by the image motion compensation timing generator, and the charge-coupled device charge packet is driven to track the image movement after being amplified by the vertical and horizontal drivers to carry out electronic image motion compensation in the vertical direction. During the output period, the vertical and horizontal rotations generated by the main timing generator are carried out by the image motion compensation timing generator. The shift clock is transmitted to the vertical and horizontal drivers to drive the charge-coupled device to produce the output. The phase relationship of the four corners of the charge-coupled device is independently controlled by the image motion compensation timing generator, which supports not only the electronic image motion compensation but also the flexible selection of the output channel number and output mode. A field-programable gate array (FPGA)/complex programable logic device (CPLD) has been selected to be the timing generator of image motion compensation, and timing simulation is carried out. Lastly, the design of the driving circuit was verified by an indoor image motion compensation experiment, and a good compensation effect has been achieved. In this system, the charge-coupled device adopts a separate driving technology, and the charge transfer direction of each block can be established. Consequently, it is convenient to select the number of output channels and the output mode, which makes the camera suitable for different occasions, supports the charge-coupled device in terms of the output and electronic image motion compensation in various ways, and is capable of generating full-frame images. In order to meet the different requirements regarding the charge-coupled device frame frequency in different contexts, this proposed design overcomes the disadvantages of the complex structure and low reliability of traditional optical mechanical image motion compensation systems.

Ultra-Fast Silicon Detectors (UFSDs) are n-in-p silicon detectors that implement moderate gain (typically 5 to 25) using a thin highly doped p++ layer between the high resistivity p-bulk and the junction of the sensor. The presence of gain allows excellent time measurement for impinging minimum ionizing charged particles. An important design consideration is the sensor thickness, which has a strong impact on the achievable time resolution. We present the result of measurements for LGADs of thickness between 20 micro-m and 50 micro-m. The data are fit to a formula that captures the impact of both electronic jitter and Landau fluctuations on the time resolution. The data illustrate the importance of having a saturated electron drift velocity and a large signal-to-noise in order to achieve good time resolution. Sensors of 20 micro-m thickness offer the potential of 10 to 15 ps time resolution per measurement, a significant improvement over the value for the 50 micro-m sensors that have been typically used to date.