Application of Fractional Fourier Transform to Moving Target Indication via Along-Track Interferometry
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Application of Fractional Fourier Transform to Moving Target Indication via Along-Track Interferometry Shen Chiu Radar System Section, Defence R&D Canada (DRDC), 3701 Carling Avenue, Ottawa, ON, Canada K1A 0Z4 Email: [email protected] Received 21 June 2004; Revised 4 March 2005 A relatively unknown yet powerful technique, the so-called fractional Fourier transform (FrFT), is applied to SAR along-track interferometry (SAR-ATI) in order to estimate moving target parameters. By mapping a target’s signal onto a fractional Fourier axis, the FrFT permits a constant-velocity target to be focused in the fractional Fourier domain thereby affording orders of magnitude improvement in SCR. Moving target velocity and position parameters are derived and expressed in terms of an optimum fractional angle α and a measured fractional Fourier position u p , allowing a target to be accurately repositioned and its velocity components computed without actually forming an SAR image. The new estimation algorithm is compared with the matched filter bank approach, showing some of the advantages of the FrFT method. The proposed technique is applied to the data acquired by the two-aperture CV580 airborne radar system configured in its along-track mode. Results show that the method is effective in estimating target velocity and position parameters. Keywords and phrases: fractional Fourier transform, along-track interferometry, parameter estimation.
1.
INTRODUCTION
Canada’s RADARSAT-2 commercial SAR satellite, to be launched in spring 2006, will have an experimental mode (called MODEX for moving object detection experiment) that will allow the full antenna to be broken into two subapertures with two parallel receivers to define two independent data channels [1]. These two subapertures, arranged to lie along the flight path, record two echoes (the dual-receive mode), one from each wing for every pulse transmitted at the full antenna. The two apertures enable one to detect targets with nonzero radial velocity by providing essentially two identical views of the observed scene but at slightly different times. In addition to the dual-receive mode of operation, RADARSAT-2 will also support an alternating-transmit mode where pulses are transmitted alternately from each wing and received alternately on each wing. This mode allows greater separation of the two-way phase centers in the along-track direction and also the possibility of generating a third phase center for three-aperture ground moving target indication (GMTI). This mode of operation is currently being investigated in preparation for RADARSAT-2 MODEX demonstration but is not examined in this paper, since it has been recognized that a two-aperture approach is suboptimum [2, 3]. It can be shown that a moving target with a slant range velocity vr causes a differential phase shift ϕATI = 4πvr τ/λ
(τ is the time between two observations and λ is wavelength), which may be detected by interferometric combination of the signals from a two-channel along-track SAR system [4]. Moving target signa
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