New Ship Detection Method using Polarimetric TerraSAR-X Data in the EU FP7 DOLPHIN Framework
Lehner, Susanne; Velotto, Domenico
German Aerospace Center - DLR, GERMANY
Target at sea monitoring finds its utmost importance in global monitoring applications, e.g. offshore platform localization and integrity in case of extreme weather event, and environment and security, e.g. automatic ship detection and tracking to contrast illegal activity like pollution, piracy, human traffic etc. A non-cooperative and efficient monitoring tool able to operate 24/7 is therefore preferable.
Metallic targets over the ocean surface, being man-made structures, are usually responsible of a mixture of basic scattering mechanisms: single-bounce returns from surfaces perpendicular to the radar illumination; double-bounce returns due to the dihedral formed by vertical structure over the sea surface; multiple-bounce returns. Synthetic Aperture Radar (SAR) observation of target at sea relies on the fact that the signal return of such objects is usually stronger than the one from the surrounding sea surface. This means that the Normalized Radar Cross Section (NRCS) is quite large and therefore targets can be visually recognized as bright spots over SAR images.
The information content provided by the NRCS collected by a single-polarization SAR (the Horizontal-Horizontal polarization, i.e. HH, is assessed to perform better respect to the Vertical-Vertical, i.e. VV one) is in general quite poor to efficiently observe metallic targets at the sea. The increased interest towards polarimetric SAR (PolSAR) data to fill the lack of information has driven the scientific community to conceive more effective techniques, e.g. Cloude-Pottier decomposition parameters, coherent target decomposition (CTD). Along with this interest, new SAR missions equipped with high resolution X-band sensors, i.e. TerraSAR-X (TS-X), Tandem-X (TD-X) and COSMO-SkyMed (CSK), have been launched recently. As far as target at sea detection is concerned, high resolution SAR images are preferred to medium- or low-resolution SAR images especially if small targets and coastal zones monitoring are of interest. TS-X and TD-X are equipped with coherent SAR sensors capable of acquire dual-pol images (with the following combinations: HH/HV, VH/VV and HH/VV) at the expense of a smaller swath. The coverage of any target monitoring tool is of course of great importance as well as for SAR-based target detection system. To justify the smaller swath of PolSAR data, the benefits of using such data in application like target at sea observation needs to be demonstrated.
In this study we exploit a simple physical property, known as reflection symmetry, that differentiates the objects in the observed scene, i.e. sea surface and metallic targets.
Sea surface being a natural target is expected to follow the reflection symmetry law while targets at sea being man-made structures are expected to break this law. To measure the departure from the reflection symmetry, the magnitude of the correlation between the combinations of cross-polarized channels (HH/HV and VH/VV) has been used. First the reflection symmetry properties are verified against actual SAR images, and then a sensitivity study is performed to show the potential of the proposed method. Direct comparison with classical single-polarization method, demonstrated a great improvement in the Target to Clutter Ratio (TCR), hence suggesting that the correlation between cross-polarized channels improve the detection of targets at sea. As outcome of sensitivity study a simple but very effective filter, that acts as clutter suppression while enhancing targets at sea is developed and a detection method is tuned in order to detect metallic targets at sea in dual-polarimetric X-band SAR data with low false positives.
Experiments undertaken over X-band Single look Slant range Complex (SSC) TerraSAR-X SAR data confirm the soundness of the approach. Peculiarity of X-band SAR data like, internal wave, rain cells, etc., that are often cause of false positives have been taken into account. The filter is tested on both HH/HV and VV/VH dual-polarimetric combinations and the outputs are verified through available ground truth measurements, i.e. Automatic Identification System (AIS) data, oil rigs location and nautical chart.