The Arctic region and the Antarctic region, as the two-polar regions of the earth, are sensitive to the global change to be the research focus. However, the existing earth observing system satellite data in the two-polar regions of the earth is not enough. The Moon is the unique natural satellite of the earth, which has advantages of global-scale coverage and long observation time. Therefore, the moon-based platform turns out to be a potential platform to comprehensively and continuously observe the Earth on a global scale, especially for the contrastive study of the Earth two-polar regions. Moreover, comparing to the limited life of the current satellites, the longevity of the moon is helpful to collect long-term time series data, which makes it possible to research long-term earth science phenomena in the two-polar regions. This paper comparatively analyzes the angular coverage performance of the two-polar regions of the earth observed from the moon-based platform. The observation angles of long-period 40 years through the geometry model of the moon-based platform from different sensor locations on the moon are calculated. The sensors are set on four potential sites on the moon--- the North Pole, the South Pole, the Sinus Iridum area and the Mare Nectaris area. When the two-polar regions of the earth are observed from four different locations on the moon, the different observation angular characteristics are obtained. This is helpful for the site selection of the moon-based platform.
A shadow extraction method for urban area is presented based on the hyperspherical color transform (HCT) fusion information distortion. We use the near-infrared band of WorldView-2 data to detect the shadow, because the near-infrared band as the long-wave band is more sensitive to shadow comparing to the short-wave band. In the hyperspherical color sharpening (HCS), n input bands are transformed from an n-dimensional Cartesian space to an n-dimensional hyperspherical color space to generate a single intensity component and n-1 angles, and then the intensity component is replaced with the adjusted panchromatic (Pan) image. After HCT, the information amount of the intensity is larger than that of the Pan band. When using the Pan to replace the intensity to get the fused multispectral (MS) image, the information amount is lost. To assess the information distortion of the fusion result, it is found that the shadow is sensitive to the difference index. Hence, the relative difference index is constructed to enhance the shadow information. More specifically, the relative difference index values are made high for shadow area while they are made low for non-shadow area. However, for the original MS image, the digital number values are low for the shadow area while they are high for non-shadow area. Then, by thresholding, the possible shadow area is separated from the non-shadow area. The experimental results show that this shadow extraction method is simple and accurate; not only the shadow of high building but also the little shadows of low trees and between buildings are all detected.
The principal component analysis (PCA) method is a popular fusion method used for its efficiency and high spatial resolution improvement. However, the spectral distortion is often found in PCA. In this paper, we propose an adaptive PCA method to enhance the spectral quality of the fused image. The amount of spatial details of the panchromatic (PAN) image injected into each band of the multi-spectral (MS) image is appropriately determined by a weighting matrix, which is defined by the edges of the PAN image, the edges of the MS image and the proportions between MS bands. In order to prove the effectiveness of the proposed method, the qualitative visual and quantitative analyses are introduced. The correlation coefficient (CC), the spectral discrepancy (SPD), and the spectral angle mapper (SAM) are used to measure the spectral quality of each fused band image. Q index is calculated to evaluate the global spectral quality of all the fused bands as a whole. The spatial quality is evaluated by the average gradient (AG) and the standard deviation (STD). Experimental results show that the proposed method improves the spectral quality very much comparing to the original PCA method while maintaining the high spatial quality of the original PCA.
Based on the two-step phase-shifting interference (PSI) technique in fractional Fourier transform (FRT) domain and random mixed encoding, we present a new scheme for double image encryption. In the proposed scheme, information of each primitive image is recorded in two intensity interference patterns of FRT spectra by PSI technique, from which an encrypted image for each primitive image can be digitally derived. Random mixed encoding is then employed to divide and recombine both encrypted images into a single synthetic encrypted image. During the mixed encoding process, repositioning operations based on shift-variance of FRT are performed on the encrypted images to realize the spatial separation of decoded results in the output plane. By inverse FRT with correct fractional order, any of the primitive images can be easily retrieved directly from the synthetic encoded image with the corresponding phase encoding key. Crosstalk effect due to the overlapping of decoded images is alleviated for their spatial separation. Computer simulation and experimental results are presented to verify the validity and efficiency of our scheme.
We present a hybrid configuration of joint transform correlator (JTC) and joint fractional transform correlator (JFTC) for encryption purpose. The original input is encoded in the joint fractional power spectrum distribution of JFTC. In our experimental arrangement, an additional random phase mask (master key) is holographically generated beforehand by a Mach–Zehnder interferometer with a JTC as the object arm. The fractional order of JFTC, together with the master key, can remarkably strengthen the safety level of encryption. Different from many previous digital-holography-based encryption schemes, the stability and alignment requirement for our system is not high, since the interferometric operation is only performed in the generation procedure of the master key. The advantages and feasibility of the proposed scheme have been verified by the experimental results. By combining with a multiplex technique, an application for multiple images encryption using the system is also given a detailed description.
In conventional interference-based optical encryption schemes, a potential cracker can retrieve partial information (silhouette) of the secret image using only one phase-only mask (POM). We resolve this drawback using a phase-blend operation and piecewise linear chaotic map (PWLCM) to further encode the POMs. One cannot recover a secret image visibly when inverse phase-blend operation and inverse chaotic permutation are not carried out with the correct decryption keys. Chaotic parameters of PWLCM, and random phase-angle function in the phase-blend operation enlarge the key space and improve the security of the proposed system greatly. Numerical simulations and optoelectronic experiments are performed to verify the effectiveness of the proposed scheme.
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