Zhihua He

Echo-Domain Phase Synchronization Algorithm for Bistatic SAR in Alternating Bistatic/Ping–Pong Mode

When bistatic synthetic aperture radar (SAR) works in alternating bistatic/ping-pong mode, it can perform phase referencing without a synchronization link between different satellites. The key to this is to extract a compensation phase from the bistatic echoes, including oscillator phase differences. An echo-domain phase synchronization algorithm using the correlation of bistatic echoes is proposed. Because of its larger equivalent synchronization frequency, the echo-domain algorithm has higher phase synchronization accuracy than the algorithm in the image domain. Experiments using simulated bistatic echoes with real single-stripmap SAR echo validate the proposed echo-domain phase synchronization algorithm.

Real-Time Raw-Signal Simulation Algorithm for InSAR Hardware-in-the-Loop Simulation Applications

To address the new requirements for InSAR raw-signal simulation for hardware-in-the-loop simulation application, a real-time raw-signal simulation algorithm with high accuracy is proposed. The raw signal is expressed as the convolution of the transmitting signal and the scene modulation signal (SMS), and the convolution is then implemented using the fast Fourier transform. The SMS is calculated by the position approximation in the time domain, and an interpolation technique is used to maintain the accuracy of the fast approximation algorithm. The SMS is filtered to the radar sampling frequency for real-time implementation. The approximation error of the fast algorithm is analyzed quantitatively to obtain the minimum interpolation factor. Ground-based hardware-in-the-loop simulation results are provided to validate the algorithm.

Generalized Chirp Scaling Combined with Baseband Azimuth Scaling Algorithm for Large Bandwidth Sliding Spotlight SAR Imaging

This paper presents an efficient and precise imaging algorithm for the large bandwidth sliding spotlight synthetic aperture radar (SAR). The existing sub-aperture processing method based on the baseband azimuth scaling (BAS) algorithm cannot cope with the high order phase coupling along the range and azimuth dimensions. This coupling problem causes defocusing along the range and azimuth dimensions. This paper proposes a generalized chirp scaling (GCS)-BAS processing algorithm, which is based on the GCS algorithm. It successfully mitigates the deep focus along the range dimension of a sub-aperture of the large bandwidth sliding spotlight SAR, as well as high order phase coupling along the range and azimuth dimensions. Additionally, the azimuth focusing can be achieved by this azimuth scaling method. Simulation results demonstrate the ability of the GCS-BAS algorithm to process the large bandwidth sliding spotlight SAR data. It is proven that great improvements of the focus depth and imaging accuracy are obtained via the GCS-BAS algorithm.

Phase synchronization processing method for alternating bistatic mode in distributed SAR

In the distributed synthetic aperture radar (SAR), the alternating bistatic mode can perform phase reference without a synchronization link between two satellites compared with the pulsed alternate synchronization method. The key of the phase synchronization processing is to extract the oscillator phase differences from the bistatic echoes. A signal model of phase synchronization in the alternating bistatic mode is presented. The phase synchronization processing method is then studied. To reduce the phase errors introduced by SAR imaging, a sub-aperture processing method is proposed. To generalize the sub-aperture processing method, an echo-domain processing method using correlation of bistatic echoes is proposed. Finally, the residual phase errors of the both proposed processing methods are analyzed. Simulation experiments validate the proposed phase synchronization processing method and its phase error analysis results.

Spaceborne SAR raw signal simulation of ocean scene

According to fractal ocean surface model, electromagnetic scattering model under Kirchhoff approximation and the raw signal simulation procedure of dynamic scene based on time domain, spaceborne synthetic aperture radar (SAR) raw signal of ocean scene is generated. The SAR images obtained from the echo of both simple cosine wave and complex fractal ocean surface are in accordance with the tilt modulation and the velocity bunching theoretically, and also with the statistical properties of real ocean SAR images, which validate the simulation procedure. The raw signal could be the input data of studying along-track InSAR (ATI) for ocean current measurements.

Generalized Nonlinear Chirp Scaling Algorithm for High-Resolution Highly Squint SAR Imaging

This paper presents a modified approach for high-resolution, highly squint synthetic aperture radar (SAR) data processing. Several nonlinear chirp scaling (NLCS) algorithms have been proposed to solve the azimuth variance of the frequency modulation rates that are caused by the linear range walk correction (LRWC). However, the azimuth depth of focusing (ADOF) is not handled well by these algorithms. The generalized nonlinear chirp scaling (GNLCS) algorithm that is proposed in this paper uses the method of series reverse (MSR) to improve the ADOF and focusing precision. It also introduces a high order processing kernel to avoid the range block processing. Simulation results show that the GNLCS algorithm can enlarge the ADOF and focusing precision for high-resolution highly squint SAR data.

Spaceborne AT-InSAR raw signal simulation of dynamic ocean scene

The interpretation of Synthetic Aperture Radar (SAR) images of dynamic ocean scene is extremely complex, owing to the comprehensive mechanisms of ocean SAR imaging. Along-track Interferometric SAR (AT-InSAR) technique provides us a convenient tool to cope with these problems. But the lack of in-situ data of AT-InSAR is one of the most restricted factors for the further study of this technique. Simulation tools, if appropriately based on sound models, can provide extremely fruitful results to improve the comprehension of the physics and to interpret the SAR images. This paper outlines a spaceborne AT-InSAR raw signal simulator for dynamic ocean scene in time domain. Firstly, we adopt the fractal model to construct the large scale wave of the ocean scene, and then the scene is divided into facets (of approximately half the size of a resolution cell) and each facet is given a dynamic behaviour under certain constraint. Secondly, the RCS corresponding to each frame of the dynamic sea surface is calculated with Kirchhoff Approximation method. At last, the spaceborne AT-InSAR raw signal simulation flow, considering the actual procedure of real satellite moving in the obit, is established. In the dynamic ocean scene raw signal simulation flow, the scene of ocean surface and corresponding RCS was updated at each azimuth slow time. In this procedure, the raw signal can reflect the system error, and it will be quite helpful for the SAR system design. And the simulator is also validated by the accordance of the simulation results and the theoretical results. We adopted the MPI (Message Passing Interface) and parallel computing techniques to implement the fast simulation.

GEO-UAV bistatic circular synthetic aperture radar: Concepts and technologies

This paper proposed an innovative bistatic radar system, named as GEO-UAV bistatic circular synthetic aperture radar (BCSAR). In GEO-UAV BCSAR, the geostationary satellite serves as the transmitter, while the receiver is mounted on the UAV platform moving along a circular trajectory. Combining the technical advantages of GEO SAR and that of UAV platform, GEO-UAV BCSAR benefits from low-cost, high operational flexibility and 3-D imaging ability. This paper introduced its system concept, and discussed its imaging ability and technical challenges. It is shown that the proposed GEO-UAV BCSAR could be able to provide a potential alternative for future remote sensing missions.

Ananlysis and compensation of vibration error of high frequency synthetic aperture radar

Synthetic Aperture Radar (SAR) system of high frequency, high resolution and high frame rate is one of the hot research areas. At present, the analysis of vibration error influence mainly embarks from the theoretical derivation, and simulated in level model. In order to accurately analyze the influence of high frequency vibration to high frequency SAR, it is necessary to simulate in signal level. Due to the short wave length of high frequency SAR, impact of high frequency vibration which is almost negligible to the conventional SAR of microwave band must be finely processed. Phase Adjustment by Contrast Enhancement (PACE) is a better SAR imaging autofocusing algorithm in the case of low Signal Noise Ratio (SNR), which can precisely estimate high frequency phase errors. In actual implementation, interpolation method is an effective means to improve the efficiency of the algorithm.

Phase synchronization method for distributed spaceborne fmcw SAR system

Phase synchronization is a key problem which has to be carefully considered in the distributed radar systems. In this letter, the use of a continuous duplex phase synchronization method for the distributed frequency modulated continuous wave synthetic aperture radar system is investigated. The bistatic raw signal model containing the phase difference due to the spatially separated oscillators is introduced. The continuous duplex phase synchronization procedure is proposed later and the synchronization method performance is predicted.