Objective

2

Synthetic aperture radar (SAR) is an important means of earth observation considering its all-weather

day-and-night

and penetrating imaging capabilities. SAR has been extensively used in battlefield detection and intelligence acquisition. SAR is a kind of electromagnetic wave coherent imaging system. A SAR image not only has variability but also has a strong speckle noise

which leads to considerable difficulties in target recognition of a SAR image. A manual interpretation of numerous SAR image data is difficult given the diversity of SAR image acquisition methods. A SAR automatic target recognition can effectively improve the utilization efficiency of SAR image data. However

the current SAR image target recognition algorithm has two main problems. First

the characteristics of target recognition

such as edge

corner

contour

texture

and other low-level features

are not representative. Second

in the traditional SAR image target recognition method

an effective filtering algorithm is crucial

but the filtering process is time-consuming. A convolutional neural network model is presented in this study to solve the problems of time-consuming filtering process and low recognition accuracy in the SAR target recognition.

Method

2

First

a network structure of the feature extraction part was specifically designed for the characteristics of SAR images

which are slightly different from optical images. We must design a reasonable network structure for the characteristics of SAR images. First

a SAR image that reflects a target radar echo intensity is a gray image because the feature information is less in a SAR image than in an optical image. Second

speckle noise inevitably exists in the SAR image. Third

the pixel size of the target is small because of the resolution limitation of the SAR image. Owing to the characteristics of SAR images

the convolutional neural network applied to SAR image target recognition must use a small convolution kernel and an appropriate convolution layer number. The feature extraction part of the proposed convolutional neural network model consists of four convolutional layers

four nonlinear layers

and two pooling layers. Second

an L2 norm was introduced to the cost function to improve the anti-noise and generalization performances of the model. Theoretical deduction shows the means by which the L2 norm enhances the noise immunity and generalization performance of the model. Third

Dropout reduced the computational complexity of the network and improved the generalization performance of the model. Dropout is a regularization technique for the reduction of overfitting in neural networks by preventing complex co-adaptations in training data. Dropout is an efficient technique for conducting model averaging with neural networks. Finally

the influence of filtering on the convergence speed and accuracy of the network was investigated. In the traditional SAR image target recognition method

the effective filtering algorithm is crucial

but the filtering process is time-consuming.

Result

2

Experimental data were obtained from the United States Moving and Stationary Target Acquisition and Recognition database. Experimental results of 10 types of target recognition showed that the overall recognition rate (including the variant) of the improved convolutional neural network increased from 93.76% to 98.10%. The improved feature extraction network structure extracts effective target features

thus improving the accuracy of the model. The accuracy of target variant recognition in SAR images had also been considerably improved. Notably

L2 regularization and Dropout enhanced the generalization performance of the model. Different sets of comparative experiments were set up to illustrate the effectiveness of improving and optimizing the network structure. The accuracy rate decreased from 98.10% to 97.06% when the first layer uses a 9×9 convolution kernel instead of two cascaded 5×5 convolution kernels. The accuracy of network identification increases from 94.91% to 96.19% when using L2 regularization

thereby indicating that L2 regularization can effectively improve the accuracy of network identification. Dropout increases the fluctuation range of the recognition rate

thus increasing the recognition accuracy to the highest level. Noise suppression experiments on the convolutional neural network were conducted to analyze the effects of three filtering methods

namely

Lee

bilateral

and Gamma MAP (Maximum A Posteriori)

on the training process and results of the model. The experiments verified that the feature extraction process of the convolutional neural network can suppress the speckle noise of the SAR image and can save time during the filtering process. The filtering process consumes additional time

does not improve the convergence speed of convolutional neural network training

and decreases the recognition accuracy because it may filter out effective target recognition features

such as target texture

thus resulting in a decrease in recognition accuracy.

Conclusion

2

The convolutional neural network model proposed in this study improves the accuracy and generalization of the network

does not require a time-consuming filtering process

and is an effective method for target recognition of SAR images.