Fusion of VNIR Optical and C-Band Polarimetric SAR Satellite Data for Accurate Detection of Temporal Changes in Vegetated Areas

In this paper, we propose a processing chain jointly employing Sentinel-1 and Sentinel-2 data, aiming to monitor changes in the status of the vegetation cover by integrating the four 10 m visible and near-infrared (VNIR) bands with the three red-edge (RE) bands of Sentinel-2. The latter approximately span the gap between red and NIR bands (700 nm–800 nm), with bandwidths of 15/20 nm and 20 m pixel spacing. The RE bands are sharpened to 10 m, following the hypersharpening protocol, which holds, unlike pansharpening, when the sharpening band is not unique. The resulting 10 m fusion product may be integrated with polarimetric features calculated from the Interferometric Wide (IW) Ground Range Detected (GRD) product of Sentinel-1, available at 10 m pixel spacing, before the fused data are analyzed for change detection. A key point of the proposed scheme is that the fusion of optical and synthetic aperture radar (SAR) data is accomplished at level of change, through modulation of the optical change feature, namely the difference in normalized area over (reflectance) curve (NAOC), calculated from the sharpened RE bands, by the polarimetric SAR change feature, achieved as the temporal ratio of polarimetric features, where the latter is the pixel ratio between the co-polar and the cross-polar channels. Hyper-sharpening of Sentinel-2 RE bands, calculation of NAOC and modulation-based integration of Sentinel-1 polarimetric change features are applied to multitemporal datasets acquired before and after a fire event, over Mount Serra, in Italy. The optical change feature captures variations in the content of chlorophyll. The polarimetric SAR temporal change feature describes depolarization effects and changes in volumetric scattering of canopies. Their fusion shows an increased ability to highlight changes in vegetation status. In a performance comparison achieved by means of receiver operating characteristic (ROC) curves, the proposed change feature-based fusion approach surpasses a traditional area-based approach and the normalized burned ratio (NBR) index, which is widespread in the detection of burnt vegetation

Large-Scale Monitoring of New Built-Up Areas from Joint Use of Sentinel-1/2 Images

Building detection can be achieved manually by human experts from optical multispectral (MS) images. However, the time required to perform manual labeling and the landscape variability due to seasonal changes make this process very time consuming and almost unfeasible for large scale monitoring

A segmentation-based approach to SAR change detection and mapping

2noreservedThe potentials of SAR sensors in change detection applications have been recently strengthened by the high spa- tial resolution and the short revisit time provided by the new generation SAR-based missions, such as COSMO- SkyMed, TerraSAR-X, and RadarSat 3. Classical pixel-based change detection methods exploit first-order statis- tics variations in multitemporal acquisitions. Higher-order statistics may improve the reliability of the results, while plain object-based change detection are rarely applied to SAR images due to the low signal-to-noise ratio which characterizes 1-look VHR SAR image products. The paper presents a hybrid approach considering both a pixel-based selection of likely-changed pixels and a segmentation-driven step based on the assumption that structural changes correspond to some clusters in a multiscale amplitude/texture representation. Experiments on simulated and true SAR image pairs demonstrate the advantages of the proposed approach.mixedGarzelli, Andrea; Zoppetti, ClaudiaGarzelli, Andrea; Zoppetti, Claudi

Detail-preserving change detection from amplitude SAR images

This paper presents a modified version of the information-theoretic feature in [1] for change detection in multitemporal synthetic aperture radar (SAR) imagery. The proposed method is capable of capturing small-area structural changes between the two images, by maintaining at the same time the capability of rejecting statistical changes due to speckle patterns or co-registration inaccuracies. This improvement is obtained by bootstrapping the original procedure by means of an adaptive preliminary selection of potentially changed pixels driven by the logarithm of the pixel ratio. Experimental results have been carried out on true SAR images acquired by the COSMO-SkyMed constellation

Optimizing SAR change detection based on log-ratio features

The paper investigates how to optimize the performances of unsupervised log-ratio based change detection algorithms for two-date 1-look amplitude SAR images. The usual approach of pre-processing the SAR images at different dates with state-of-the-art despeckling filters is critically discussed. Those adaptive filters are very efficient, also for the challenging case of 1-look images, for speckle reduction of single-date image data and then for providing reliable classification, detection, or parameter estimation results. However, they are not able to ease the discrimination of statistical from structural changes in 1-look SAR images for which reliable point-target detection is nearly impractical. A simple, yet very effective, multiscale method for change detection and automatic change mapping is proposed and tested on simulated 1-look SAR images. The adopted pre-processing is based on guided image filtering with different window sizes. It improves the detection of changed regions without introducing any geometrical constraint and significantly reduces the false alarm rate. Experimental tests on simulated SAR images and Spotlight COSMO-SkyMed images demonstrate the advantages of the proposed algorithm

Efficient nonparametric extraction of urban footprint from VHR SAR images

Very-High-Resolution Synthetic Aperture Radar (VHR-SAR) images are of particular interest to characterize and monitor urban areas at a global scale. While automatic urban footprint extraction from SAR images can be theoretically performed at very high spatial and temporal resolutions, in practice it requires a huge amount of processing time and memory resources for a global coverage. The paper presents a fast procedure for 1m-resolution Spotlight mode scene analysis which shows limited memory requirements and robustness to one-look speckle disturbance. The proposed method adopts a multiscale approach, with a radiometrically scalable scheme. Experimental results with objective assessment on Spotlight Cosmo-SkyMED images are presented and validated on reference urban classes provided by the Copernicus Urban Atlas 2012 of the European Space Agency

Geometrically Accurate Change Mapping From VHR SAR Images

The paper presents an efficient unsupervised change mapping algorithm for 1-look amplitude SAR images. The method exploits the complementary characteristics of two change features and proposes a morphological non-parametric map combination to produce the final change map. The method is able to preserve the geometry of the changed regions without increasing the overall false alarm rate PFA . This is made possible by morphologically combining the binary change maps obtained from a low-PFA high-order statistics (CKLD) change feature and a detail-preserving multiscale ratio detector. A new thresholding method is also proposed for the CKLD feature. Experimental tests, performed on simulated 1-look SAR images, visually and objectively demonstrate the advantages of the proposed algorithm

Urban footprint from VHR SAR images: Toward a fully operational procedure

Very-High-Resolution Synthetic Aperture Radar (VHR-SAR) images are of particular interest to characterize and monitor urban areas at a global scale. While automatic urban footprint extraction from SAR images can be theoretically performed at very high spatial and temporal resolutions, in practice it requires a huge amount of processing time and memory resources for a global coverage. The paper presents a fast procedure for 1m-resolution spotlight mode scene analysis which shows limited memory requirements and robustness to onelook speckle disturbance. The proposed method adopts a multiscale approach, with a radiometrically scalable scheme, and exploits theoretic electromagnetic scattering models of buildings. Experimental results with objective assessment on spotlight Cosmo-SkyMED images will be presented and validated on reference urban classes provided by the Copernicus Urban Atlas 2012

A fast and reliable change detection feature from bi-temporal amplitude SAR images

In this paper, we propose a change detection feature for an amplitude SAR image pair, based on both information theoretic (IT) assumptions and a CFAR criterion derived from the probabilistic model of the ratio image. In particular, the proposed method aims to introduce two main improvements with respect to the previous IT-based approaches. The first goal is to find a strategy to adaptively quantize the 2-D scatterplot instead of applying clustering. This is carried out by performing a preliminary partition of the image pixels according to a constant false alarm rate criterion that is based on the probabilistic model of the ratio image. The second goal is to test the proposed method in order to assess reliable performances in case of severe speckle noise and in case of small percentage of change within the scene. Therefore, experimental results have been carried out with simulated changes applied to synthetically-generated 1-look SAR images produced from an optical remote sensing image. True Cosmo-SkyMed SAR images have been also considered on a damage assessment scenario

Fusion of VNIR Optical and C-Band Polarimetric SAR Satellite Data for Accurate Detection of Temporal Changes in Vegetated Areas

In this paper, we propose a processing chain jointly employing Sentinel-1 and Sentinel-2 data, aiming to monitor changes in the status of the vegetation cover by integrating the four 10 m visible and near-infrared (VNIR) bands with the three red-edge (RE) bands of Sentinel-2. The latter approximately span the gap between red and NIR bands (700 nm–800 nm), with bandwidths of 15/20 nm and 20 m pixel spacing. The RE bands are sharpened to 10 m, following the hyper-sharpening protocol, which holds, unlike pansharpening, when the sharpening band is not unique. The resulting 10 m fusion product may be integrated with polarimetric features calculated from the Interferometric Wide (IW) Ground Range Detected (GRD) product of Sentinel-1, available at 10 m pixel spacing, before the fused data are analyzed for change detection. A key point of the proposed scheme is that the fusion of optical and synthetic aperture radar (SAR) data is accomplished at level of change, through modulation of the optical change feature, namely the difference in normalized area over (reflectance) curve (NAOC), calculated from the sharpened RE bands, by the polarimetric SAR change feature, achieved as the temporal ratio of polarimetric features, where the latter is the pixel ratio between the co-polar and the cross-polar channels. Hyper-sharpening of Sentinel-2 RE bands, calculation of NAOC and modulation-based integration of Sentinel-1 polarimetric change features are applied to multitemporal datasets acquired before and after a fire event, over Mount Serra, in Italy. The optical change feature captures variations in the content of chlorophyll. The polarimetric SAR temporal change feature describes depolarization effects and changes in volumetric scattering of canopies. Their fusion shows an increased ability to highlight changes in vegetation status. In a performance comparison achieved by means of receiver operating characteristic (ROC) curves, the proposed change feature-based fusion approach surpasses a traditional area-based approach and the normalized burned ratio (NBR) index, which is widespread in the detection of burnt vegetation