Improved beat frequency estimation in the MLBF Doppler ambiguity resolver

Abstract

Among the current Doppler ambiguity resolvers, the Multi-Look Beat frequency (MLBF) algorithm proves to be the most reliable one, especially in high contrast areas. The existing MLBF algorithm uses FFTs to measure the central frequency of the beat signal but the estimation accuracy is limited by quantization errors. This paper proposes an improved method of estimating the beat frequency in the MLBF algorithm that is based on phase increments. In our work, we examined five established frequency estimators and found that the Iterative Linear Prediction (ILP) method has the best performance. The experimental results on RADARSAT-1 data show that the new MLBF algorithm using ILP can obtain the correct ambiguity number in a higher percentage of blocks and that the RMS error of the results is less than half that of the existing method. I. INTRODUCTION In high quality SAR data processing, the estimation of the Doppler centroid frequency is an essential procedure for good image focus. Due to the fact that the azimuth data are sampled by the PRF, the Doppler centroid estimate is observed in two parts: the baseband Doppler centroid and the Doppler ambiguity. In the estimation of the baseband part, algorithms such as the Spectral fit and Average Cross Correlation methods can give reliable estimates in most cases (1). A number of algorithms have been developed to find the Doppler ambiguity number, such as Look Misregistration (2), Multiple PRF (3), Wavelength Diversity (WDA) (4), Multi-look Cross Correlation (MLCC) and Multi-look Beat frequency (MLBF) (5) algorithms. However, the accuracy and robustness of the Doppler ambiguity estimate still needs to be improved to satisfy the current high quality SAR processing requirements. The Multi-look Beat frequency (MLBF) algorithm proposed in 1996 (6) takes advantage of the differences between the azimuth frequency of two range looks to estimate the Doppler centroid. It has good performance in medium and high contrast areas. It also avoids estimating the offset frequency due to the antenna characteristics, as required in the WDA and MLCC algorithms. However, because the existing MLBF algorithm uses FFTs to estimate the central frequency of the beat signal, the estimate accuracy is affected by quantization errors, which are related to the FFT length. In addition, the algorithm using FFTs cannot be applied directly to burst mode data, such as ScanSAR data (6). In this paper, an improved beat frequency estimation method is presented that uses frequency estimators based on phase increments of the beat signal. Experimental results with RADARSAT-1 data show that it has a significantly better performance than the existing method of estimating the beat frequency. II. THE EXISTING MLBF ALGORITHM A. The principle of the beat frequency The MLBF algorithm is based on the fact that the Doppler centroid frequency can be derived from the azimuth frequency difference of radars operating at two different center frequencies. In this algorithm, the range compressed signal, s(η), is divided into two range looks, s1(η) and s2(η). Then, by multiplying (beating) the signal of one look with the conjugate of the other look, a beat signal results for a point target: