Aiming at the current status that the reconstruction quality of spectral image is yet to be enhanced, an improved two-step iterative shrinkage/threshold (TwIST) algorithm is proposed in this paper. Firstly, according to the coded aperture spectral imaging principle, a mathematical model for spectral image reconstruction of coded aperture spectral imaging system based on compressed sensing is established. Then, taking the spatial smooth transition characteristics of spectral image as a priori knowledge, two improvements are proposed based on the traditional TwIST algorithm, selecting the total variation regular constraint terms and denoising the updated terms in each iteration. Finally, in order to verify the improved algorithm, the reconstructed spectral images are simulated. It shows the reconstructed spectral images retain the spatial details well, and the reconstructed spectral curve is in good agreement with the original spectral curve, indicating that the improved algorithm is effective in the high-precision reconstruction of spatial information and spectral information of the target.
Aiming at the current status that the on-orbit imaging accuracy is low and the application-oriented imaging system is not perfect, the detailed simulation of entire space-based imaging chains of space object is carried out in this paper. Firstly, the influence factors on entire space-based imaging chains of space object are analyzed, including the physical parameters of the object, the performance parameters of the detector, and the orbit parameters of the sun, object and detector. Then, a model for entire space-based imaging chains of space object is built. The model consists of radiation transmission based on the bidirectional reflectance distribution function (BRDF) and image degradation based on the modulation transfer function (MTF). Finally, the grayscale images of the International Space Station (ISS) are simulated. It shows the blurring effect caused by each link of entire chains can be truly, accurately and completely reflected.
In order to enhance the capability of space-based surveillance, the detailed modeling for visible imaging characteristics of space object is described in this paper. Firstly, a space-based imaging detection model is built based on the scattering visible radiation from space object. The model consists of radiation transmission based on the bidirectional reflectance distribution function (BRDF) and grayscale transformation based on the 256 levels. Then, according to the position of the sun, object and detector, the imaging conditions such as imaging angle and size are analyzed. Finally, the grayscale images of the HuanJing-1 satellite are simulated. It shows the grayscales for the different regions of the object appear great difference, indicating that the space-based detector needs a larger dynamic range.
KEYWORDS: Satellites, Bidirectional reflectance transmission function, Sun, Digital signal processing, Surveillance, Mathematical modeling, Sensors, Dye sensitized solar cells, Solar cells, Electromagnetic scattering theory
In order to enhance the capability of space-based surveillance, the detailed non-resolved space object characterization with brightness data is described in this paper. Firstly, according to the optical scattering theory, mathematical model for brightness characteristics of space object is established with the bidirectional reflectance distribution function (BRDF) by region classification and grid division. Then, brightness of typical geosynchronous satellites is simulated. Influences of the shape, size and status on brightness are analyzed. Characterization with brightness data is proposed. It shows the shape, size and status of the object can be deduced with brightness data over a range of time-space periods. Finally, the several special fields of non-resolved space object characterization are discussed
In order to verify the effect of stray light elimination design, the detailed stray light analysis of one modified large
aperture optical telescope using TracePro is described in this paper. Firstly, the sources of stray light in optical telescope
and the influence of stray light on optical telescope are introduced. Then, the principle of stray light analysis using
TracePro is presented. The solid model, surface properties and light paths of the modified large aperture optical telescope
are determined. Ray splitting and importance sampling are adopted to ensure the calculation accuracy and reduce the
time consumption. The Point Source Normalized Irradiance Transmittance (PSNIT) curve of the system is plotted. It
shows the PSNITs are less than 10-12 when off-axis angles are larger than 30°, which satisfies the requirement of the
system. Finally, the several special fields of stray light control are discussed.
Temporally and Spatially Modulated Fourier Transform Imaging Spectrometer (TSMFTIS) is a new imaging
spectrometer without moving mirror and slit. Through scanning, it can acquire sequential images superposed with
interference fringes. The interferogram can be acquired by orderly arranging the extracted interference information of the
same spatial point from the sequential images, and the spectrum can be recovered by using FFT. Therefore, the attitude
of the bearing platform will affect the images so as to reduce the accuracy of the recovered spectrums. Since current
attitude measurement accuracy can not meet the needs of error correction, in this paper, the image registration method is
applied to acquire the accurate translations for the future correction between two sequential images. The single-step DFT
registration method is applied to register the selected window areas away from the null optical path difference position in
sequential images. That is full utilizing of common information meanwhile reducing impact of interference fringes and
improving registration accuracy and efficiency. In the simulation experiment, a common large remote sensing image is
used as ground object. The Fourier shift principle is applied to acquire simulation scanning images with sub-pixel
displacement. Artificial spectral data cube produced with the RGB values of each image is utilized as the input data of
the TSMFTIS, and sequential images superposed with interference fringes are acquired. Registration according to the
method mentioned above is performed and the results are compared with the accurate values. It shows that the method is
feasible and can achieve sub-pixel level accuracy.
An accurate modeling method is presented for the optical scattering characteristics of space target. Bidirectional
reflection distribution function (BRDF) can effectively describe spatial and spectral scattering characteristics of target's
surface material. According to background radiation environment and surface appearance of space target, mathematical
models on optical scattering characteristics of space target in visible and near infrared band are established by
introducing BRDF. The calculation formulas of irradiance on entrance pupil of detector caused by reflected background
radiation from target surface and magnitude of target are put forward. By setting target body rectangular coordinate system,
the relative positions of background radiation sources, detector and target are determined in terms of coordinate
conversion, and the "visual surfaces" of target to detector are defined by the vector coordinate method. Energy
distribution on entrance pupil of detector and magnitude characteristics are calculated in detail by the given geometry
dimension and surface physical parameters of the typical space target, and simulation results are demonstrated. The
calculation results indicate that the irradiance of the solar sail has the same order as that of the target body, 10-6W/m2 ,
and the optical scattering characteristics of target are related to the geometrical shape, surface material of target, as well
as with the real-time positions of the sun, the earth and the target. Simulation results prove the correctness of modeling
method with BRDF.
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