Weigen Huang

Study of the propagation direction of the internal waves in the South China Sea using satellite images

Internal wave propagation carries considerable vertical shear which can lead to turbulence and mixing. Based on the analysis of more than 2 500 synthetic aperture radar (SAR) and optical satellite images, the internal wave propagation in the whole South China Sea was investigated systematically. The results show that (1) in the northeastern South China Sea, most internal waves propagate westward from the Luzon Strait and are diffracted by coral reefs near the Dongsha Islands. Some impinge onto the shelf and a few are reflected; (2) in the northwestern South China Sea, most internal waves are generated at the shelf and propagate northwestward or westward to the coast; (3) in the western South China Sea, most internal waves propagate westward to the Vietnamese coast, except a few propagate southward to the deep sea; and (4) in the southern South China Sea, most internal waves propagate southwestward to the coast. Some propagate southeastward to the coast of Kalimantan Island, and a few propagate southeastward because of the influence of the Mekong River.

Coastally trapped atmospheric gravity waves on SAR, AVHRR and MODIS images

Alternative dark–bright patterns on two ENVISAT Advanced Synthetic Aperture Radar (ASAR) images of the east coast of the Korean Peninsula acquired on 18 and 19 May 2004 are interpreted as atmospheric gravity waves (AGWs) on the basis of simultaneous multi‐satellite observations and atmospheric gravity wave theory. The AGWs appeared in the form of a wave packet containing several waves located between 50 and 200 km offshore. The wavelengths were ranging from 13 to 20 km. The lengths of AGW crests were from 20 to 150 km. An NOAA‐17 pass was received about 30 min after the ASAR pass. Its channel 4 infrared (IR) image clearly shows wave‐like moisture patterns. However, the sea surface temperature (SST) image derived after applying nonlinear calibration and split‐window atmospheric correction shows no wave patterns. A daytime true‐colour MODIS image taken about 14 h later still shows a cloud band of same AGWs, indicating the lifespan of the standing AGWs can be over half a day. Although oceanic internal waves (IWs) may also cause similar wave patterns imaged by spaceborne SAR as they modulate the ocean surface roughness, we provide evidence to eliminate this possibility in this case. The characteristics of satellite observed AGWs are in good agreement with these simulated by a linear coastal AGW model.

Comparison of ship detection algorithms in spaceborne SAR imagery

The algorithms discussed in this paper are three Constant False Alarm Rate (CFAR) models, which include the Probabilistic Neural Network(PNN) model, the K-Gamma model and the double parameters model. The SAR data utilized in the paper include ERS-2, ENVISAT and Radarsat SAR data. The data are applied in ship detection experiments and the results of ship detection of three models are compared. The results show that the PNN models applicability is the best .The performance of PNN model in ERS and ENVISAT SAR data is better than the K-Gamma model. The K-Gamma model can only do well in Radarsat SAR data. The double parameters model can fit local distribution of SAR image in the sea.

An improved CFAR model for ship detection in SAR imagery

This paper presents an improved constant false alarm rate (CFAR) model for ship detection in synthetic aperture radar (SAR) imagery. The model includes the probabilistic neural networks, CFAR technique, golden section method and area growth method. It is compared with other ship detection methods. The results show that the improved CFAR model performs well

A Feature-Based Approach for Automatic Registration of NOAA AVHRR Images

In this paper, a feature-based approach for automatic registration of NOAA AVHRR images is presented. The proposed approach makes use of the correlations between regions of the reference and sensed images both in feature and image spaces. The approach is summarized as follows: (i) selection of potential control points in the reference image, automatically. Distinctive feature points with stronger edge energy than most of other pixels in reference image are selected as potential control points,(ii) Finding for corresponding control points in the sensed image. The positions of corresponding control points in the sensed image are computed by means of feature matching using region similarity measures.(iii) Mapping function estimation and transformation of the sensed image. Experimental results demonstrate that the proposed approach is capable of registering NOAA AVHRR images with high accuracy.

Nonlinear internal wave amplitude remote sensing from SAR imagery

The amplitude of internal waves is very difficult to retrieve from satellite. In this paper, a method is given to estimate the amplitude of nonlinear internal waves from synthetic aperture radar (SAR) imagery. It is assumed that the observed groups of nonlinear internal wave packets on SAR imagery are generated by local semidiurnal tides. The mean distance between the leading crest of two successive wave packets has been used to derive the group velocity of the nonlinear internal waves. The amplitude of nonlinear internal waves has been calculated from a model which consists of the KdV equation, action balance equation and Bragg scattering model. Case studies in China Seas show reasonable results.

Fractal characterization of Ikonos imagery

An IKONOS image was used to examine the spatial complexity of the band spectra of remote sensing. Triangular prism method and double blanket method were applied to calculate the fractal dimensions of all bands for each land cover type. The results show the image texture characteristics of the images by bands and land cover types. As the conclusions, the fractal dimensions of all bands of each land type range between 2.0 and 3.0. But different type sub-images show different complexities, thus different dimensions. Dimension values reflect spectral characters and spatial characters of sub-images. Thus, it is believed that the measurement and analysis of land cover can be more effectively and efficiently realized using fractal characterization of high spatial and spectral resolution remote sensing data.

Satellite observation of a red tide in the East China Sea during 2005

Red tides occurred frequently in the East China Sea in recent years. A red tide dominated by Prorocendrum donghaiense and Karenia mikimotoi happened in the Zhejiang Coastal Waters in late-May 2005. EOS MODIS water color data were utilized in extracting the red tide information. Sea surface temperature data derived from NOAA AVHRR and EOS MODIS were also analyzed to understand the possible formation mechanism of this red tide. The relationships between the red tide and related oceanographic features were discussed based on image data. The results indicate that the red tide was associated with several oceanographic processes, such as coastal front. This study showed that the combination of remote sensing data of water color and sea surface temperature can be useful in studying and understanding the oceanography of red tides.

Raster to vector conversion of classified remote sensing image

An approach to vectorization of classified remote sensing image is presented based on the characteristics of raster data. This approach consists of three digitizing steps during the raster to vector convening: pre-processing, processing and postprocessing. First, all image pixels are categorized to some different types according to the characteristics of raster data. Then, polygons are generated by tracing these typed image pixels, with topological attribute built-in. Finally, the vector data are post-processed to reduce the redundancy and stored to GIS’s based file. Compared to traditional methods this approach creates topological information of vector data automatically during converting. It has high performance, and could be implemented step by step. The result vector data could be applied and used directly in the geographical information system. An example is used to illustrate this approach with classified Landsat TM image. Keywords-Raster to Vector Conversion; Topology; Remote Sensing Imag; Geographical Information System

Optimal SAR parameters for ship detection

SIR-C L-band and C-band images have been used to study the optimal synthetic aperture radar (SAR) parameters for ship detection. The study is based on the radar backscattering from ships and their surrounding sea areas. The results have shown that L band is better than C band for ship detection. HV polarization is the best one among classical polarizations (HH, HV, VV and VH). L band with the circular polarization is the best choice for ship detection with SAR imagery. KeywordsSAR; ship detection; optimal parameters.