Model-Based Pseudo-Quad-Pol Reconstruction from Compact Polarimetry and Its Application to Oil-Spill Observation

Hindawi Publishing Corporation Journal of Sensors Volume 2015, Article ID 734848, 8 pages http://dx.doi.org/10.1155/2015/734848 Research Article Model-Based Pseudo-Quad-Pol Reconstruction from Compact Polarimetry and Its Application to Oil-Spill Observation Junjun Yin,1,2 Wooil Moon,1 and Jian Yang2 1 Geophysics, Faculty of Environment, Earth and Resources, University of Manitoba, Room 328, Wallace Building, Winnipeg, MB, Canada R3T 2N2 2 Department of Electronic Engineering, Tsinghua University, Room 8-302, Rohm Building, Beijing 100084, China Correspondence should be addressed to Junjun Yin; Received 30 December 2014; Revised 16 March 2015; Accepted 15 April 2015 Academic Editor: Mike McShane Copyright © 2015 Junjun Yin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Compact polarimetry is an effective imaging mode for wide area observation, especially for the open ocean. In this study, we propose a new method for pseudo-quad-polarization reconstruction from compact polarimetry based on the three-component decomposition. By using the decomposed powers, the reconstruction model is established as a power-weighted model. Further, the phase of the copolarized correlation is taken into consideration. The phase of double-bounce scattering is closer to 𝜋 than to 0, while the phase of surface scattering is closer to 0 than to 𝜋. By considering the negative (double-bounce reflection) and positive (surface reflection) copolarized correlation, the reconstruction model for full polarimetry has a good consistency with the real polarimetric SAR data. 𝐿-band ALOS/PALSAR-1 fully polarimetric data acquired on August 27, 2006, over an oil-spill area are used for demonstration. Reconstruction performance is evaluated with a set of typical polarimetric oil-spill indicators. Quantitative comparison is given. Results show that the proposed model-based method is of great potential for oil-spill observation. 1. Introduction Crude oil and petroleum products pollution has severe impact on the marine environment. It results in large scale damage to local ecosystem, presenting potential enormous harm to deep ocean and coastal fisheries, wildlife, and regeneration. Rapid increase in oil-spill pollution is primarily due to the increased human ocean activities, which increase the risk of oil-spillage from both ship/oil-platform/pipeline accidents and routine ship operations like tank washing and engine effluent discharge. One of the mostly used instruments for ocean surveillance is synthetic aperture radar (SAR), which has the all-day and all-weather imaging capability and is sensitive to the ocean surface capillary-gravity waves [1–3]. Polarimetric SAR (Pol-SAR) offers multichannel polarimetric information, and the fully or quad polarimetric (quadpol) SAR system allows the complete backscattering characterization for scatterers. However, the fully polarimetric imaging mode suffers from system complexity, data volume, and the limited imaging coverage compared to SAR systems which use a single polarization for transmission [4]. In 2005, a polarimetric imaging concept was proposed and generally well known as compact polarimetry (CP) [5]. At present, both the Indian RISAT-1 and JAXA ALOS/PALSAR-2 can provide the CP mode. In the future, the CP mode will be prepared for launches of other Earth Observation (EO) satellites, for example, SAOCOM-1 and Radarsat Constellation Mission (RCM). The compact SAR data can be processed in two manners: the first one is to reconstruct pseudo-quad-polarization data from compact polarimetry, and then many quad-pol methods can be applied to the reconstructed data [5–9] for various applications; the second one is to extract target scattering information directly from compact data [4, 10, 11]. In this study, we focus on the reconstruction method. In the multipolarization reconstruction, two assumptions are very essential. One is the well-known reflection symmetry assumption, and the other is the polarization state extrapolation model, that is, the reconstruction model. There mainly exist five reconstruction methods in the literature. 2 Studies in [5–8] are based on reflection symmetry to estimate the pseudocovariance matrix. The main difference between these methods is that they adopt different reconstruction model parameter 𝑁. 𝑁 is determined by either theoretical assumptions or empirical tests. In [9], we developed a method which can be applied to the nonreflection case based on the four-component decomposition. However, the fourcomponent decomposition is not applied well to describe the ocean surface where reflection symmetry always holds for most sea state conditions. Methods proposed in [7] by Collins et al. and in [8] by Li et al. are designed for ocean target detection. However, both methods are all empirical methods which need the prior fully polarimetric SAR data to fit the model parameter 𝑁. The nonlinear regression is usually used to determine the best curve fitting parameters. The difference between these two methods is that Li’s model parameter 𝑁 needs updating when performing the iteration, while Collins’s model parameter 𝑁 is a constant only relating to the incidence angle. Moreover, in [7], Collins et al. proposed to use a negative exponential curve to estimate 𝑁 for the 𝐶-band RADARSAT-2 data, while in [8], Collins’s model parameter is fitted with a polynomial function for the 𝐿-band UAVSAR data. This implies that, for different SAR sensors, imaging geometry, and sea conditions, the fitting curves would probably like to vary greatly. This is not beneficial for practical applications. If the selected objective curve is not a best fit of the acquired data, then reconstruction performance can be expected to deteriorate. In this paper, a model-based reconstruction method is proposed to extract the quad-pol information from compact polarimetry for oil-spill observation. By assuming a three-component decomposition for backscatter of the ocean surface, the model parameter 𝑁 is estimated based on the decomposed scattering powers. 𝐿-band ALOS/PALSAR-1 fully polarimetric data are used for demonstration. 𝐿-band polarimetric SAR data, especially the satellite data, are not widely investigated for oil-spill detection due to its long wavelength. Since 𝐿-band polarimetric SAR satellites are in operation (e.g., ALOS/PALSAR-2) and to be planned for future missions, it is necessary to explore the performance of 𝐿-band compact polarimetric data for oil-spill observation. Performance of the proposed method is evaluated in terms of a set of polarimetric indicators which are widely used for oil-spill observation in the literature [1]. The organization of the rest of this paper is as follows. In Section 2, we briefly introduce the 𝐿-band test data. In Section 3, the proposed reconstruction method is (...truncated)