Multimodal Bilinear Fusion Network With Second-Order Attention-Based Channel Selection for Land Cover Classification

As two different tools for earth observation, the optical and synthetic aperture radar (SAR) images can provide complementary information of the same land types for better land cover classification. However, because of the different imaging mechanisms of optical and SAR images, how to efficiently exploit the complementary information becomes an interesting and challenging problem. In this article, we propose a novel multimodal bilinear fusion network (MBFNet), which is used to fuse the optical and SAR features for land cover classification. The MBFNet consists of three components: the feature extractor, the second-order attention-based channel selection module (SACSM), and the bilinear fusion module. First, in order to avoid the network parameters tempting to ingratiate dominant modality, the pseudo-siamese convolutional neural network (CNN) is taken as the feature extractor to extract deep semantic feature maps of optical and SAR images, respectively. Then, the SACSM is embedded into each stream, and the fine channel-attention maps with second-order statistics are obtained by bilinear integrating the global average-pooling and global max-pooling information. The SACSM can not only automatically highlight the important channels of feature maps to improve the representation power of networks, but also uses the channel selection mechanism to reconfigure compact feature maps with better discrimination. Finally, the bilinear pooling is used as the feature-level fusion method, which establishes the second-order association between two compact feature maps of the optical and SAR streams to obtain the low-dimension bilinear fusion features for land cover classification. Experimental results on three broad coregistered optical and SAR datasets demonstrate that our method achieves more effective land cover classification performance than the state-of-the-art methods.