Training Resource Description
This video is part of training presented at the 12th ESA Training Course on Earth Observation, held in Latvia from 27 June to 1 July 2022.
It explains how Synthetic Aperture Radar (SAR) detects ships over the ocean by exploiting the strong radar reflections from metallic ship structures, particularly due to double-bounce scattering between the ship and the ocean surface. The radar’s incidence angle influences detection: higher angles enhance ship visibility by reducing ocean backscatter, while lower angles increase ocean clutter, making detection more challenging. SAR polarisation (HH preferred over VV) also affects the contrast between ships and the ocean.
Different SAR imaging modes (ScanSAR, Stripmap, Spotlight) affect resolution and thus the minimum detectable ship size. Studies show that higher incidence angles and shorter wavelengths (X-band) improve detection of smaller ships. Wind speed impacts detection probability by altering ocean backscatter, which affects contrast.
Challenges include ship movement causing positional shifts and image distortions (Doppler effects), spreading of ship signals into cross-shaped patterns, and false detections caused by azimuth ambiguities (‘ghosts’). To address these, detection workflows use algorithms such as Constant False Alarm Rate (CFAR), which adaptively set detection thresholds to distinguish ships from ocean clutter. Tools like SNAP provide implementations of these detection methods, allowing users to extract ship parameters such as position and size despite these complexities.
A basic understanding of SAR principles, such as radar backscatter and incidence angles, is helpful for following these detection techniques.