LaST: Label-Free Self-Distillation Contrastive Learning With Transformer Architecture for Remote Sensing Image Scene Classification

The increase in self-supervised learning (SSL), especially contrastive learning, has enabled one to train deep neural network models with unlabeled data for remote sensing image (RSI) scene classification. Nevertheless, it still suffers from the following issues: 1) the performance of the contrastive learning method is significantly impacted by the hard negative sample (HNS) issue, since the RSI scenario is complex in semantics and rich in surface features; 2) the multiscale characteristic of RSI is missed in the existing contrastive learning methods; and 3) as the backbone of a deep learning model, especially in the case of limited annotation, a convolutional neural network (CNN) does not include the adequate receptive field of convolutional kernels to capture the broad contextual information of RSI. In this regard, we propose label-free self-distillation contrastive learning with a transformer architecture (LaST). We introduce the self-distillation contrastive learning mechanism to address the HNS issue. Specifically, the LaST architecture comprises two modules: scale alignment with a multicrop module and a long-range dependence capture backbone module. In the former, we present global-local crop and scale alignment to encourage local-to-global correspondence and acquire multiscale relations. Then, the distorted views are fed into a transformer as a backbone, which is good at capturing the long-range-dependent contextual information of the RSI while maintaining the spatial smoothness of the learned features. Experiments on public datasets show that in the downstream scene classification task, LaST improves the performance of the self-supervised trained model by a maximum of 2.18% compared to the HNS-impacted contrastive learning approaches, and only 1.5% of labeled data can achieve the performance of supervised training CNNs with 10% labeled data. Moreover, this letter supports the integration of a transformer architecture and self-supervised paradigms in RSI interpretation.