Xiaojiang Guo

Improved Channel Error Calibration Algorithm for Azimuth Multichannel SAR Systems

Multichannel synthetic aperture radar systems in azimuth can effectively suppress azimuth ambiguity and are promising in high-resolution wide-swath imaging. However, unavoidable channel errors will significantly degrade the performance of ambiguity suppression. Conventional subspace calibration methods usually estimate phase error via decomposing a Doppler-variant covariance matrix from one Doppler bin, and then average these errors estimated from several Doppler bins to improve the estimation accuracy, which will result in a large computational load. This letter presents an improved channel error calibration method, which works on the undersampled data of the individual azimuth channel. By a proposed matrix transformation method, the Doppler-variant covariance matrices will be transformed into a constant covariance matrix. Therefore, the improved calibration algorithm needs to estimate and decompose the new covariance matrix only once. The computation load could be greatly reduced. Moreover, the new covariance matrix can be estimated by training samples not only from range bins but also from Doppler bins, which will improve the estimation accuracy. Theoretical analysis and experiments based on simulations and measurements showed the high accuracy, efficiency, and robustness of the improved method, particularly in low signal-to-noise ratio.

Modified reconstruction algorithm based on space-time adaptive processing for multichannel synthetic aperture radar systems in azimuth

Abstract. A spectrum reconstruction algorithm based on space–time adaptive processing (STAP) can effectively suppress azimuth ambiguity for multichannel synthetic aperture radar (SAR) systems in azimuth. However, the traditional STAP-based reconstruction approach has to estimate the covariance matrix and calculate matrix inversion (MI) for each Doppler frequency bin, which will result in a very large computational load. In addition, the traditional STAP-based approach has to know the exact platform velocity, pulse repetition frequency, and array configuration. Errors involving these parameters will significantly degrade the performance of ambiguity suppression. A modified STAP-based approach to solve these problems is presented. The traditional array steering vectors and corresponding covariance matrices are Doppler-variant in the range-Doppler domain. After preprocessing by a proposed phase compensation method, they would be independent of Doppler bins. Therefore, the modified STAP-based approach needs to estimate the covariance matrix and calculate MI only once. The computation load could be greatly reduced. Moreover, by combining the reconstruction method and a proposed adaptive parameter estimation method, the modified method is able to successfully achieve multichannel SAR signal reconstruction and suppress azimuth ambiguity without knowing the above parameters. Theoretical analysis and experiments showed the simplicity and efficiency of the proposed methods.

Suppression of Azimuth Ambiguities of Strong Point-Like Targets for Multichannel SAR Systems

Multichannel synthetic aperture radar (SAR) signal reconstruction methods can effectively suppress azimuth ambiguities and achieve high-resolution wide-swath imaging. However, due to the characteristics of the practical antenna patterns, there exist non-bandlimited Doppler spectra, which will result in residual azimuth ambiguities, especially for strong targets. This letter presents a novel method for the suppression of the azimuth ambiguities of the strong point-like targets. First, we find out the positions of the strong point-like targets from the multichannel reconstructed SAR image. Then, we locate the ambiguous range history of each strong point-like target. Finally, the ambiguous components in the range history are filtered out by an orthogonal projection method. Therefore, the spectra of the strong point-like targets will be converted into the bandlimited spectra, and then, the azimuth ambiguities can be effectively suppressed by the conventional multichannel SAR signal reconstruction methods. Theoretical analysis and experiments demonstrate the feasibility of the proposed methods.

Unambiguous SAR imaging algorithm via spotlight mode for multichannel SAR systems

Azimuth multichannel synthetic aperture radar (SAR) systems are capable of high-resolution and wide-swath (HRWS) imaging. This paper presents a novel unambiguous SAR imaging algorithm via spotlight mode for multichannel SAR systems. Its derived that the least unambiguous pulse repetition frequency (PRF) is the function of azimuth width of the illuminated area. By constructing spotlight SAR signals via digital beamforming (DBF) from multiple azimuth receivers, the wide azimuth imaging area can be divided into multiple narrow azimuth areas. Therefore, the least unambiguous PRF for the generated spotlight SAR signals could be reduced and unambiguous SAR images can be obtained by spotlight SAR imaging algorithms. Theoretical analysis and experiments based on real measurements from an airborne multichannel SAR system showed the simplicity and efficiency of the proposed imaging method.

Isolation improvement method for frequency modulated continuous wave SAR

Frequency modulated continuous wave (FMCW) synthetic aperture radar (SAR) is promising in earth observation with compact, lightweight, cost-effective and high resolution advantages. However, conventional FMCW SAR systems do not apply to high-resolution wide-swath (HRWS) imaging due to the high requirement of the isolation between the transmitter and the receiver. This paper presents a novel isolation improvement technique for FMCW SAR. Firstly, the isolation requirement versus slant range for FMCW SAR is analyzed. Then, a notch design method of the antenna pattern is presented to suppress the leakage from the transmitter into the receiver. At last, simulations are performed to demonstrate the validity of the proposed technique. Its proved that the proposed method can effectively improve the isolation between the transmitter and the receiver.

Array error estimation method for multichannel synthetic aperture radar systems in azimuth

Abstract. Multichannel synthetic aperture radar systems are usually employed to suppress azimuth ambiguity and realize high-resolution wide-swath imaging. However, unavoidable array errors will significantly degrade the performance of ambiguity suppression and imaging quality. This paper presents an array error estimation method based on cross correlation. First, unambiguous Doppler spectra are obtained by selecting a short length of range profiles from strong targets. Then, array errors are estimated by a proposed cross-correlation method. Finally, a preprocessing method to improve the estimation accuracy is proposed. The proposed method takes full advantage of the training samples from strong targets and estimates array errors by the coherent integration technique, which improves the estimation accuracy and robustness. Theoretical analysis and experiments based on simulations and measurements showed the validity of the proposed method, especially in low signal-to-noise ratios.

Azimuth-Variant Phase Error Calibration Technique for Multichannel SAR Systems

Multiple azimuth channels are usually employed to overcome the inherent limitation between high resolution and wide swath in synthetic aperture radar systems. However, unavoidable channel phase errors will significantly degrade the performance of ambiguity suppression. Conventional calibration methods usually regard these phase errors as constants during the whole observation time and ignore the azimuth-variant phase errors, which may not totally suppress azimuth ambiguities especially for very strong targets. This letter presents an azimuth-variant phase error calibration technique. The proposed technique first selects a strong point-like target as the calibration source. Then, the azimuth-variant phase errors can be estimated by comparing the phase of the calibration source and the corresponding steering vector in the range-compressed signals. Besides, a preprocessing method is presented to improve the calibration accuracy when the selected calibration source is affected by noise or interferences. Theoretical analysis and experiments demonstrate the feasibility of the proposed technique.

Transmitter leakage canceling for LFMCW SAR

Linear frequency modulated (LFM) continuous wave (CW) synthetic aperture radar (SAR) is promising in airborne earth observation with compact, lightweight, cost-effective and high resolution advantages. For CW imaging radars, however, weak targets may be submerged by sidelobes of strong range interferences including transmitter leakage and nadir signal, especially when the slant range is far. This paper firstly discusses how to suppress transmitter leakage and nadir signal suppression for LFMCW SAR by choosing appropriate pulse repetition interval (PRI) and digital sampling frequency. Although the transmitter leakage could be weakened by an appropriate PRI and finite impulse response (FIR) filter, the residual leakage is so strong that its sidelobes will still submerge some weak targets. Then, a novel transmitter leakage canceling method based on orthogonal projection is derived, which could effectively reduce the sidelobe level of transmitter leakage. Theoretical analysis and experiments on a real unmanned LFMCW SAR system showed the efficiency of the proposed method.

An automatic RCMC technique based on BFGS method

Here automatic range cell migration correction (RCMC) is studied. Classic RCMC in Range-Doppler Algorithm (RDA) is realized by sinc interpolation so that exact instantaneous slant range distance between antenna phase center (APC) and scatterers is necessary. However, the later element above could not be satisfied especially in airborne SAR. Inspired by improved global method used for ISAR range alignment, we propose an automatic RCMC technique with Broyden-Fletcher-Goldfard-Shano(BFGS). This technique creates N variables (time shifts) to adjust range migration curve to make cost function minimum. Exact instantaneous slant range distance is not required while computation efficiency is a little lower than that of classic RCMC. Simulation validates this idea from the view of three isolate scatterers. It also proves that BFGS-RCMC has phase-keep property in azimuth direction.