With the rapid development of deep learning, convolutional neural networks have achieved milestones in synthetic aperture radar (SAR) image object detection. However, object detection in SAR images is still a great challenge due to the difficulty in distinguishing targets from complex backgrounds. At the same time, most of the targets in SAR images are small and unevenly distributed, which makes it challenging to extract sufficient feature information. To solve these issues mentioned above, an efficient object detection network for SAR images based on Swin transformer and YOLOv7 is proposed in this article. First, we design a novel feature aggregation module Petty-specialized feature aggregation (PS-FPN) to enrich small targets’ semantic and spatial features while keeping the model lightweight. PS-FPN module uses the fusion of deep and shallow features by using cross-layer feature aggregation and single-branch feature aggregation to enhance the detection of small targets. Second, a novel attention mechanism strategy mix-attention is proposed to find more attention regions. Finally, we add one more prediction head to extract shallow features that effectively preserve small targets’ feature information. To verify the effectiveness of the proposed algorithm, extensive experiments are carried out on several challenging SAR image datasets. The results show that, compared with other state-of-the-art detectors, the proposed method can achieve significant performance based on lightweight detection.

Road extraction from remote sensing images is significant for urban planning, intelligent transportation, and vehicle navigation. However, it is challenging to automatically extract roads from remote sensing images because the scale difference of roads in remote sensing images varies greatly, and slender roads are difficult to identify. Moreover, the road in the image is often blocked by the shadows of trees and buildings, which results in discontinuous and incomplete extraction results. To solve the above problems, this paper proposes a multiscale feature encoding and long-range context-aware network (MECA-Net) for road extraction. MECA-Net adopts an encoder–decoder structure and contains two core modules. One is the multiscale feature encoding module, which aggregates multiscale road features to improve the recognition ability of slender roads. The other is the long-range context-aware module, which consists of the channel attention module and the strip pooling module, and is used to obtain sufficient long-range context information from the channel dimension and spatial dimension to alleviate road occlusion. Experimental results on the open DeepGlobe road dataset and Massachusetts road dataset indicate that the proposed MECA-Net outperforms the other eight mainstream networks, which verifies the effectiveness of the proposed method.

Optical remote sensing of aquatic vegetation in shallow water is an essential aid to ecosystem protection, but it is difficult because the spectral characteristics of the vegetation are sensitive to external features such as water background effects, atmospheric effects, and the structural properties of the canopy. A global sensitivity analysis of an aquatic vegetation radiative transfer model provides invaluable background for algorithm development for use in optical remote sensing. Here, we use the extended Fourier Amplitude Sensitivity Test (EFAST) method for the modelling. Four different cases were identified by subdividing the ranges of water depth and leaf area index (LAI) involved. The results indicate that the reflectance of emergent vegetation is affected mainly by the concentrations of chlorophyll a + b in leaves (Cab), leaf inclination distribution function parameter (LIDFa) and LAI. The parameter LAI is influential in sparse vegetation cases whereas Cab and LIDFa are influential in dense vegetation cases. Canopy reflectance for submerged vegetation is dominated by water parameters. Relatively, LAI and Cab are highly sensitive vegetation parameters. The analysis is extended to vegetation index as well, which takes the Sentinel-2A as the reference sensor. It shows that NDAVI (Normalized Difference Aquatic Vegetation Index) is suitable for retrieving LAI in all cases except deep-sparse for emergent vegetation, whereas NDVI (Normalized Difference Vegetation Index) would be better in the deep-sparse case. NDVI, NDAVI and WAVI (Water Adjusted Vegetation Index), respectively, are suitable for retrieving Cab, Car and LIDFa in dense cases. For submerged vegetation, the sensitivity of LAI to NDAVI is relatively high only in the shallow-sparse case. The adjustment factor L in SAVI and WAVI fails to suppress the sensitivity to water constituent parameters. The sensitivity of LAI and Cab to NDVI in deep cases is relatively higher than that to the other indices, which may provide clues for the construction of inversion algorithms in macrophyte remote sensing in the aquatic environment using spectral signatures in the visible and near infrared regions.

Compared with large scale ships, small scale ships occupy few pixels and have low contrast, so it poses a great challenge to detect multi-scale ships in SAR images. In order to improve the accuracy of multi-scale ship detection in SAR images, this paper designs a general multi-scale pyramid attention module (MPAM), which is a plug-and-play lightweight module that can adapt to many ship detection networks. In MPAM, a deep feature extraction sub-module (DFES) is first designed to use the multi-scale pyramid structure to divide the feature map into different levels, extracting rich features with resolution and semantic information for multi-scale ship detection. The channel multi-layer attention fusion sub-module (CMAFS) and spatial multi-layer attention fusion sub-module (SMAFS) are then designed to fuse the channel and spatial attention blocks on different level feature maps, which could better learn the dependent features from the channel and spatial dimensions, to enhance the feature representation. Finally, the fused feature map is input into the existing ship detection networks to obtain the detection result. Experiments on SAR datasets containing multi-scale ships show that the effectiveness of MPAM in improving the accuracy of the existing ship detection networks.

Dissolved Oxygen (DO) is an important water quality indicator, reflecting aquatic ecological health and influencing biological and biogeochemical cycles. Remote sensing of DO is exigent yet intricate due to its weak correlation with water optical properties. This study addresses this gap by integrating deep learning (DL) techniques with Chlorophyll-a (Chl-a), temperature, and temporal features. We employed a novel indirect approach to first estimate Chl-a and temperature using in-situ time series water quality data from 94 monitoring stations across the Hong Kong coast, covering five years (2019–2023), Sentinel-2 remote sensing data and normalized cyclic month features using machine learning algorithm eXtreme Gradient Boosting (XGB) and then utilized these predicted Chl-a and temperature values, along with normalized cyclic month features, to estimate DO leveraging DL technique two-dimensional convolutional neural network (2D CNN). XGB model achieved its accuracy in estimation of Chl-a (r = 0.78, RMSE = 3.16 μg/L, MAE = 2.31 μg/L) and temperature (r = 0.96, RMSE = 1.14 °C, MAE = 0.89 °C). 2D CNN yielded a promising DO estimation accuracy (r = 0.78, RMSE = 0.67 mg/L, MAE = 0.52 mg/L). This indirect DO estimation approach achieved 50% greater accuracy than direct methods and also showed 10% improvement in r, 8%–17% reduction in RMSE-MAE values over models trained using Chl-a and temperature only, highlighting the unique potential of XGB-2D CNN approach, incorporating cyclic month features. Inclusion of cyclic month features alongside Chl-a and temperature enabled the model to capture both seasonal variations and long-term trends, significantly enhancing the predictive performance of DO estimations. Consequently, monthly/seasonal mean products of Chl-a, temperature, and DO were generated for spatio-temporal analysis for practical use.