Lijia Huang

Focusing of Medium-Earth-Orbit SAR With Advanced Nonlinear Chirp Scaling Algorithm

The signal processing of the medium-Earth-orbit synthetic aperture radar (SAR) is more challenging than that of the current low-Earth-orbit SAR because the imaging geometry is more complicated, and the range and azimuth variances are more severe. This paper deals with these imaging problems in three aspects. First, an advanced hyperbolic range equation (AHRE) is proposed for the first time, which is more precise for a spaceborne SAR than the conventional hyperbolic range equation (CHRE). Second, the point target spectrum based on the AHRE is analytically derived, which is useful for developing efficient SAR processing algorithms. Third, the well-known nonlinear chirp scaling (NLCS) algorithm is modified according to this new spectrum, and the so-called AHRE-based advanced NLCS (A-NLCS) algorithm is established. The simulation results validate the correctness of our method for L-band SAR systems at altitudes from 1000 to 10000 km with an azimuth resolution around 3 m. It is also shown that the A-NLCS algorithm has better performance than the CHRE-based algorithms in longer integration time cases. Therefore, we recommend the A-NLCS algorithm for a spaceborne SAR with a lower frequency, finer resolution, and higher satellite altitude.

Medium-Earth-Orbit SAR Focusing Using Range Doppler Algorithm With Integrated Two-Step Azimuth Perturbation

Existing low-Earth-orbit synthetic aperture radar (SAR) algorithms generally assume that the data are azimuth invariant. However, this assumption does not hold for the medium-Earth-orbit (MEO) SAR systems due to the significantly longer azimuth integration time and complex imaging geometries. As a result, the MEO SAR data cannot be processed accurately and efficiently using the existing algorithms. To solve this problem, this letter proposes a two-step azimuth perturbation (AP) method that uses the first-step AP to remove the bulk azimuth variance at the range processing stage and the second-step AP to remove the residual variance at the azimuth processing stage. As an example, an improved range Doppler algorithm with the integrated two-step AP is discussed in this letter. Simulations of an L-band MEO SAR with 5-m resolution at 10 000-km orbit height are used to demonstrate the validity and accuracy of this algorithm.

A general two-dimensional spectrum based on polynomial range model for medium-earth-orbit Synthetic Aperture Radar signal processing

The kth-order polynomial range model (PRM) for Medium-Earth-Orbit (MEO) Synthetic Aperture Radar (SAR) signal processing is proposed, analyzed, and verified in this paper. The coefficients of the PRM are calculated according to the relative state vectors between the satellite and the given target on earth, which implies power series expansion around the beam center crossing time. In addition, the relevant two-dimensional spectrum is deduced out as well by using the principle of stationary phase (POSP) and the series reversion approximation. The accuracy of this spectrum is flexible and limited both by the order of the PRM and the order of the spectrum in an expanded form. Therefore, the PRM-based spectrum is general for MEO SAR with any azimuth resolution at any orbit height.

Ground resolution analysis based on gradient method in geosynchronous SAR

In geosynchronous synthetic aperture radar (SAR), due to the complex relative motion between the satellite and the target, and the effect of the curvature on the earth surface, the conventional imaging geometry for ground resolution analysis should be innovated. In this paper, the imaging geometry is advanced by adding a pitch angle. Then, based on the new geometry, the range resolution and the Doppler gradient resolution in the ground plane are deduced using gradient method. Finally, the resolution analysis is verified by simulation results.

Medium-Earth-orbit SAR imaging based on keystone transform and azimuth perturbation

Due to the significant azimuth variance property in medium-Earth-orbit (MEO) synthetic aperture radar (SAR) echo, it is difficult for the conventional SAR algorithms to achieve a good compromise between accuracy and efficiency. A novel algorithm based on Keystone transform (KT) and azimuth perturbation (AP) is introduced in this paper to handle this problem. The function of KT is to correct the range walk and thus to mitigate the azimuth variance effect on range processing. The function of AP is to equalize the Doppler histories in each range gate and thus to mitigate the azimuth variance effect on azimuth compressing. Simulation results of an L-band MEO SAR with 5 m resolution at 10,000 km altitude demonstrate the capability of our algorithm.

An imaging algorithm based on keystone transform for one-stationary bistatic SAR of spotlight mode

This article proposes an imaging algorithm based on Keystone Transform for bistatic SAR with a stationary receiver. It can efficiently be applied to high-resolution spotlight mode, and can directly be process the bistatic SAR data which have been ranged compressed by the synchronization reference pulses. Both simulation and experimental results validate the good performance of this algorithm.

A High-Resolution SAR Focusing Experiment Based on GF-3 Staring Data

Spotlight synthetic aperture radar (SAR) is a proven technique, which can provide high-resolution images as compared to those produced by traditional stripmap SAR. This paper addresses a high-resolution SAR focusing experiment based on Gaofen-3 satellite (GF-3) staring data with about 55 cm azimuth resolution and 240 MHz range bandwidth. In staring spotlight (ST) mode, the antenna always illuminates the same scene on the ground, which can extend the synthetic aperture. Based on a two-step processing algorithm, some special aspects such as curved-orbit model error correction, stop-and-go correction, and antenna pattern demodulation must be considered in image focusing. We provide detailed descriptions of all these aspects and put forward corresponding solutions. Using these suggested methods directly in an imaging module without any modification for other data processing software can make the most of the existing ground data processor. Finally, actual data acquired in GF-3 ST mode is used to validate these methodologies, and a well-focused, high-resolution image is obtained as a result of this focusing experiment.

SAR interferometrie phase filtering based on wavelet transform and local frequency estimation

A novel approach combining the local frequency estimation with wavelet transform is presented to reduce interferometric phase noise for InSAR. First, the maximum likelihood estimator is used to obtain the frequency range of the noisy interferogram. Then, the wavelet transform is employed to obtain the wavelet coefficients of the real and imaginary parts of the complex interferogram. For the wavelet coefficients within the estimated frequency range and that out of the range, the NeighShrink and VisuShrink methods are employed respectively to shrink them. As a result, the noise can be effectively filtered without the loss of detailed information of the interferogram based on the advantages of the two shrinkage methods. The performance of noise reduction and fringe preservation is verified by the experiments with real interferogram.