Band selection is an effective method to reduce redundancy in a hyperspectral image (HSI) without compromising the original contents. Popular band selection methods usually use strong assumptions, such as linear or nonlinear assumptions with simple predefined Kernel functions, to model the correlations between bands. However, this kind of strong assumption may not valid in the real environment due to the complex interactions between bands. In this letter, we treat hyperspectral band selection as a spectral reconstruction task. By assuming that an HSI can be sparsely reconstructed from a few informative bands, we propose an attention-based autoencoder to model the underlying nonlinear interdependencies between bands. The proposed model consists of two parts: an attention module and an autoencoder. The attention module is used to produce the attention mask which selects the most informative bands for every pixel. The autoencoder uses these informative bands to reconstruct the raw HSI. The final band selection is conducted via clustering column vectors of the attention mask and exploring the most representative band for each cluster. Different from most of the existing band selection methods, the proposed method directly learns global nonlinear correlations between bands without strong assumptions. The proposed model is easy to implement and all the parameters can be jointly optimized using the stochastic gradient descend algorithm. Experiments on three open public data sets show that the proposed method offers the promising results.

GF-(Gaofen- means high resolution in Chinese) satellite launched in 2015 is the first geosynchronous orbit remote sensing satellite in China. To evaluate the on-orbit modulation transfer function (MTF) of the space-borne panchromatic camera in GF-4, a modified pulse target is proposed. This new type target is laid in the uniform low-reflection background region whose center region is a high-reflection square with the size of 3 ~4 ground sampled distance (GSD), and then two low-reflection rectangle stripes are extended along the center square toward both ends with the length over 4 GSD. In consideration of the staring imaging mechanism of GF-4, the target image sequences are accessed with relative random distance taken by the space-borne camera in a short period to reduce accidental error of one sampling and enhance the accuracy of estimation. Based on the multi-sampling calibrating images, maximum a posteriori estimation model and gradient descent solution with grid searching method are used to fit the pulse response function (PRF) when taking the target center positions as references. MTF is then calculated from PRF via Fourier transformation. Numerical simulation results reveal our method can keep the accuracy stable despite of different kinds of noise. Actual calibration result using this method shows that on-orbit MTF of GF-4 camera at Nyquist frequency is 0.1473.

The subpixel dynamic imaging technique of a beam splitter is one of the most effective super-resolution imaging methods. Aiming to create a linear time delay integration charge coupled device (TDI–CCD) subpixel imaging system based on the optical assembly method, its modulation transfer function (MTF) is analyzed based on the spatial over-sampling theory. Firstly, Fourier transformation of the sampling point is used to describe the frequency domain characteristics of TDI–CCD, which transform a unit cell of the spatial sampling lattice into a bandwidth cell in the spatial–frequency domain. Considering the effects of velocity mismatch and misalignment, the best subpixel staggering position of the linear TDI–CCD pair is given. Moreover, according to the analysis of the MTF of super-resolution reconstruction results from multiple subpixel images with random spatial offsets, the condition of sampling in the limitation of the enhancement of MTF is obtained. The numerical simulation and real experimental analysis reveal results that are consistent with the theoretical model.

Vertical greening serves as one of the important forms in enriching urban landscape. In this paper, the writer investigated the liana landscape in eight parks of Xi’ an and the matching types of several landscape elements (architecture, gallery, trellis, courtyard, flower rack, porch, highway, railway), discussed the common lianas in this city, and evaluated the landscape effect of lianas with different landscape elements by using scenic beauty method. How to better use lianas for landscaping plays a significant role in vertical greening landscape construction of Xi’ an City.