Jianlai Chen

A Frequency-Domain Imaging Algorithm for Highly Squinted SAR Mounted on Maneuvering Platforms With Nonlinear Trajectory

The imagery of highly squinted synthetic aperture radar mounted on maneuvering platforms with nonlinear trajectory is a challenging task due to the existence of acceleration and the cross-range-dependent range migration and Doppler parameters. In order to accommodate these issues, a frequency-domain imaging algorithm based on tandem two-step nonlinear chirp scaling (TNCS) with small aperture is proposed. For the cross-range-dependent range cell migration (RCM) caused by the linear range walk correction and acceleration, the first-step NCS is introduced to suppress this dependence and realize the unified RCM correction. Based on the differences between full-aperture and small-aperture data in the cross-range processing, the second-step NCS is introduced in frequency domain to equalize the cross-range-dependent Doppler parameters, for cross-range processing is more sensitive to the cross-range dependence than range processing. Furthermore, a novel geometric correction method based on inverse projection is utilized to eliminate the negative effects caused by the imaging processing. Simulation results and real data processing are presented to validate the proposed approach.

Design of Triple-Band Microstrip Filter with Transmission Zeros Using Open Stubs

In this paper, a novel triple-band microstrip filter with improved out-of-band suppression is presented. The theoretical design of triple-band filter is based on frequency transformation method, which is then realized by a group of triple-band resonators as the building block of the filter. Two open stubs with different lengths are loaded directly in the middle of two resonators to introduce a pair of transmission zeros, which achieves high out-of-band suppression filtering response with no need of cross-coupling. To validate the method, a practical triple-band filter with pass-bands at 3.3–3.4 GHz, 3.5–3.6 GHz, 3.7–3.8 GHz and transmission zeros at 3.1 GHz, 4 GHz is designed and fabricated.

A Parameter Optimization Model for Geosynchronous SAR Sensor in Aspects of Signal Bandwidth and Integration Time

Signal bandwidth and integration time are two significant parameters of a geosynchronous synthetic aperture radar (SAR) sensor. They directly determine the resolution characteristic of SAR imagery. Because their contributions to the ground impulse response width (IRW) curve (consists of -3-dB resolutions in every direction) are severely coupled, an analytical extraction method of the ground IRW curve is studied to analyze the coupling characteristic. Due to the coupling, the IRW curve is generally spatially variant and, thus, can degrade the quality of SAR imagery. To minimize the variation, the two parameters are optimized. However, the optimized signal bandwidth is found to be satellite position varied, which complicates the system design. To solve this problem, a parameter optimization model is built to obtain one optimal signal bandwidth as well as to decrease the variation.

A Two-Dimensional Beam-Steering Method to Simultaneously Consider Doppler Centroid and Ground Observation in GEOSAR

Due to Earths rotation and an elliptical satellite orbit, large Doppler centroids along satellite orbit inevitably occur in geosynchronous Earth orbit synthetic aperture radar (GEOSAR). Nonzero Doppler centroid causes a large range migration, which complicates the data acquisition and design of imaging algorithms. Thus, beam steering is used to decrease the centroid. At the same time, the ground observation of interest is prerequisite for applications. Thus, a unique two-dimensional (2-D) beam-steering method to simultaneously consider the reduction of Doppler centroid and ground observation for the GEOSAR is studied. The minimum-Doppler plane is proposed to minimize the centroid and to guarantee the beams that illuminate the area of interest. Subsequently, to achieve required ground coverage, beam directions determined by the minimum-Doppler plane are slightly adjusted. The method has been validated through the simulation of two types of orbits.

A TSVD-NCS Algorithm in Range-Doppler Domain for Geosynchronous Synthetic Aperture Radar

The ultralong synthetic aperture time and a very large scene cause severe 2-D spatial variation in geosynchronous synthetic aperture radar. The range variation was corrected using the range cell migration equalization and the modified chirp scaling function. The azimuth variation correction with the singular value decomposition and the azimuth nonlinear scaling was studied. The validity of the proposed imaging algorithm has been assessed. Satisfactory results were obtained in the removal of the azimuth variation, and the focusing of point targets from a synthetic aperture up to 1000 sand a scene of 150 km (azimuth) × 130 km (range).

Systematic analyses of challenges and solutions in geosynchronous synthetic aperture radar

As an emerging technique to obtain remotely sensed data, geosynchronous synthetic aperture radar (GEOSAR) is still at the infant stage for the development of the basic theory although some prediction indicates that the first GEOSAR satellite would be in orbit around 2020. Before the launching, a series of challenges in GEOSAR must be addressed and the feasible solutions should be carried out. Thus, systematic analyses of challenges and solutions in GEOSAR are proposed here. In this paper, the GEOSAR has been studied through analyses of bi-static imaging geometry, signal model, Doppler characteristics, Doppler bandwidth and synthetic aperture time, and resolution evaluation of the GEOSAR imagery, in which both challenges and solutions are included.

Complex images of a point charge in rectangular conducting planes

The exact images of a point charge in rectangular conducting planes are discretely located on an infinite plane, making it difficult to compute the electrostatic field rapidly. In the paper, a complex image theory for the electrostatic problem in rectangular conducting planes is investigated. The research shows that only a few complex images are used to replace the countless exact images, but the error is less than 0.2% when they are utilized to compute the electrostatic potential functions, and the execution time is greatly reduced. >

A 2-D Space-Variant Motion Estimation and Compensation Method for Ultrahigh-Resolution Airborne Stepped-Frequency SAR With Long Integration Time

For the ultrahigh-resolution airborne stepped-frequency synthetic aperture radar, very large synthetic bandwidth and very long integration time may lead to a 2-D space-variant (SV) motion error when the aircraft flies off the ideally straight trajectory due to the atmospheric turbulence. This new type of error complicates the motion estimation and motion compensation (MOCO). For the motion estimation, we present a jointly 2-D SV motion error estimation method to simultaneously consider the range-variant motion error and the azimuth-variant motion error. For the MOCO, we propose a 2-D SV-MOCO method. The method is implemented through three processing steps: 1) two-step MOCO for the space-invariant motion error and the range-variant phase error; 2) range block-based chirp-z transform (CZT) for the range-variant envelope error; and 3) range block division for the range-dependent azimuth-variant phase error based on the azimuth subaperture method. Finally, processing of simulated data and real data validates the proposed methods.

A novel beam direction determination method for minimizing Doppler centroid in GEO SAR

Due to the effects of the earths rotation and the satellites elliptical orbit, the Doppler centroid varies along the orbit in geosynchronous earth orbit synthetic aperture radar (GEO SAR). With an ultrahigh orbit height, the beam may illuminate outside the earth surface with a rotation angle large than 9 degrees. Therefore, the usual attitude steering methods to minimize the Doppler centroid in low earth orbit SAR (LEO SAR) may not be suitable for GEO SAR. Considering the above two effects of GEO SAR and the beam illuminating restriction, a beam determination method to minimize Doppler centroid in GEO SAR is proposed in this paper. Guaranteeing that the beam not illuminate outside the earth surface, the proposed method can drastically decrease the Doppler centroid and the equivalent squint angle.

A subaperture imaging scheme for wide azimuth beam airborne SAR based on modified RMA with motion compensation

Airborne SAR imaging processing needs to estimate motion error to compensate non-ideal trajectory. In this paper, a subaperture imaging scheme for wide azimuth beam airborne SAR systems is proposed. First, the motion error is estimated from the subaperture data and the modified range migration algorithm is applied to obtain the coarse focused image, whose azimuth resolution corresponds to the subaperture Doppler bandwidth. The subaperture image is then projected into a fine grid image, whose coordinates is defined by the imaging geometry. As the subaperture data stream is coming, the azimuth resolution of the grid image will become higher and higher. Finally, the fine image with the azimuth resolution corresponding to the full aperture data can be obtained. Since the motion error estimation is based on the sub-aperture data, the imaging processing is suitable for real-time SAR imaging.