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GMTI and GMTIm in FMCW SAR

Abstract This chapter mainly discusses the GMTI and GMTIm algorithms in FMCW SAR. The nonlinearity correction and GMTI algorithms in FMCW SAR are studied. Then, the differences between a FMCW SAR and a pulse SAR are analyzed, and additional phase error of FMCW SAR is calculated. Furthermore, a nonlinearity correction algorithm based on Homomorphic Deconvolution is proposed, and an along-track moving target indication algorithm is presented. By using the additional phase error, the GMTI performance of FMCW SAR is improved.

Introduction

Miniaturization is a developing tendency of the SAR system. A novel FMCW SAR system with small size, light weight, low power-cost, and high range resolution is developed by applying FMCW technology into SAR system [1]. Since the concept of FMCW SAR was proposed by London University in 1988, U.K. [2], Japan, Netherlands, U.S., German and China have all devoted to this domain. The FMCW SAR system from TUDelft University [3], MiSAR from German, MicroSAR and NanoSAR from the U.S. are all practical FMCW SAR systems. The advantages of FMCW SAR make it highly suitable for small platforms, such as a UAV, and make it useful in short-range military surveillance and target indication. The low-cost character also promises it a broad application prospect in civil area. Therefore, FMCW SAR is a hotspot in SAR system research.

In FMCW SAR systems, the radar transmits chirp signals during the entire PRI, and the deramping operation is used in the received signal to reduce the burden of A/D [4]. However, the unique transmit and receive operations make the signal model of FMCW SAR different from that of a pulse SAR. The stop-and-go approximation is not valid in FMCW SAR, and the RVP is induced by the deramping operation [5].

In practical applications, the continuous transmit and receive also induced the crosstalk of the transmitted and received signals. This problem is also solved by separated transmitter and receiver, thus is not discussed in this chapter [6]. The

© Springer Nature Singapore Pte Ltd. 2017 71

J. Yang, Study on Ground Moving Target Indication and Imaging Technique of Airborne SAR, Springer Theses, DOI 10.1007/978-981-10-3075-8_5

large time-bandwidth product signal of FMCW SAR cannot maintain strictly linear, the existence of nonlinearity in transmitted signal severely limits the performance of FMCW SAR imaging. After deramping operation, the transmitted nonlinearity is overlapped with the received nonlinearity in beat signal, making the nonlinearity problem more complicated, especially for large distance [7]. Therefore, nonlinearity must be corrected either in signal production stage or during imaging process.

The small size and light weight characters make FMCW SAR high suitable for military applications, such as missile-borne seeker. Therefore, the research on the GMTI and GMTIm algorithms in FMCW SAR are important. The principle of FMCW SAR makes its signal echo different from that of a pulse SAR. This chapter establishes the echo model of a moving target in FMCW SAR, and analyzes the echo phase errors in detail. These phase errors are compensated in traditional FMCW SAR GMTI and GMTIm algorithms as interference, while a novel algorithm is proposed to indicate along-track moving targets based on the additional phase error.

This chapter mainly focuses on the nonlinearity correction, GMTI and GMTIm of FMCW SAR. Section 5.2 analyzes the echo model of FMCW SAR. In Sect. 5.3, the nonlinearity problem is analyzed, and a correction algorithm based on homomorphic deconvolution is proposed. An along-track moving target indication algorithm is presented in Sect. 5.4. In Sect. 5.5, we address the conclusion.

 
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