Published in IET Signal Processing Received on 26th February 2011 Revised on 15th March 2012 doi: 10.1049/iet-spr.2011.0354 Special Issue on Multi-Sensor Signal Processing for Defence: Detection, Localisation & Classification ISSN 1751-9675 Range Doppler and chirp scaling processing of synthetic aperture radar data using the fractional Fourier transform C. Clemente J.J. Soraghan Center for Excellence in Signal and Image Processing, Department of Electronic and Electrical Engineering, University of Strathclyde, Glasgow, Scotland E-mail: carmine.clemente@eee.strath.ac.uk Abstract: Synthetic aperture radar (SAR) systems are used to form high-resolution images from radar backscatter signals. The fractional Fourier transform (FrFT), which is a generalised form of the well-known Fourier transform, has opened up the possibility of a new range of potentially promising and useful applications that involve the use and detection of chirp signals that include pattern recognition and SAR imaging. In this study a time variant problem associated with the use of the FrFT for SAR processing is addressed and a new algorithm is presented that resolves this problem. Two new FrFT-based SAR processing algorithms are presented namely the FrRDA and the eFrCSA that are shown to improve the well-established range-Doppler and chirp-scaling algorithms for SAR processing. The performance of the algorithms are assessed using simulated and real Radarsat-1 data sets. The results confirm that the FrFT-based SAR processing methods provide enhanced resolution yielding both lower side lobes effects and improved target detection. 1 Introduction Synthetic aperture radar (SAR) is an imaging radar system for earth observation from satellite and airborne manned/ unmanned platforms. It is currently operational in recently launched polar-orbiting platforms such as TerraSAR-X, RadarSAT-2 and Cosmo SkyMed as well as in numerous other missions. Applications are diverse including disaster observation and management, mapping of renewable resources, geological mapping, snow/ice mapping and strategic surveillance of military sites. Moreover, the scientific community is more and more oriented to a wide range of applications where the first step is the production of a SAR image [1]. Obtaining optimum resolution from raw SAR data involves sophisticated signal processing techniques especially using high-resolution sensors (like TerraSAR-X and Cosmo-SkyMed) where the feature of the smaller scatterers increase is important. The range-Doppler algorithm (RDA) is a simple and efficient algorithm used to focus SAR data. It is a very flexible algorithm and widely used. This algorithm takes advantage of the approximate separability of processing in range and azimuth direction, allowed by the large difference in the time scales of the range and azimuth data. In addition, efficient range cell migration correction (RCMC) is performed in the so-called range-Doppler domain, which is the ideal reference plane to correct the hyperbolic behaviour of the target trajectories [1]. The RDA is actually one of the most used algorithms in both research and commercial fields because of its effectiveness and relative simplicity. RDA processors have been developed in the Gamma remote sensing modular SAR processor [2] and in the Radar Electronic Assembly (REA) processor [3]. In addition many research groups work with the RDA and the interest on this algorithm is growing especially in the field of the parallel SAR processing [4–6]. The chirp-scaling algorithm (CSA) was developed specifically to eliminate the interpolation step used for the RCMC [7, 8]. It is based on the scaling principle that applies a frequency modulation to a chirp-encoded signal to achieve a shift or scaling of the signal. In [9, 10] the fractional Fourier transform (FrFT) was applied to the SAR CSA [7] to form the fractional chirp- scaling algorithm (FrCSA). In the resulting FrCSA the FrFT was used in place of the Fourier transform to obtain a fractional representation of the received signal in both azimuth and range directions. Some good results in terms of enhanced resolution were reported; however, this algorithm did not take care of the shift variant problem introduced by the use of the FrFT. This approach used the design of a matched filter in the fractional domain in order to reduce the mismatch because of the approximations introduced in the signal spectrum analysis. However, in the FrCSA the shift variant property [11] of the FrFT introduced a modulation effect that cannot be removed from the received signal. It means that each echo in the fractional domain exhibits a different modulation because of its delay and thus a mismatch results when performing the matched filtering in the FrCSA. Using the FrFT for SAR appears similar to the SPECAN algorithm [1, 12]; however, the new proposed IET Signal Process., 2012, Vol. 6, Iss. 5, pp. 503–510 503 doi: 10.1049/iet-spr.2011.0354 & The Institution of Engineering and Technology 2012 www.ietdl.org