Active-passive Hybrid Detection Of Short-distance Noncooperative Space Targets

With the rapid development of aerospace technology in recently years, many countries pay more attention to the space-based surveillance system because of its several advantages such as being free from atmospheric disturbance, flexible observation time and capacity for observation to small & medium targets at short distance.Simplex passive imaging system can be used to measure direction and luminance of space targets, but it is unable to measure target's distance and velocity exactly. Accordingly this kind of system can't satisfy the requirement of space-based surveillance of space targets very well.Under the background of research of short-distance early warning and defense technology of space system, this dissertation put forwards a means of active-passive hybird detection technique with low-light level visible imaging system and solid state coherent ladar, which is used for detection, positoning and automatic target recognition (ATR) of short-distance noncooperative space targets. The works in this dissertation are also exploratory researches for this kind of satellite-borne hybird detection system that will be developed in the future.At first, the optical environment of near-earth space was studied and several primary space visible-light illuminants were picked out. It comes to the conclusion that the domain of visible illuminance in near-earth space is 1179W/m2~0.0011W/m2. Modeling of optical characteristics of small spatial object with shapes of plane, sphere, cylinder and cone was carried out, and the dependence of the irradiance on objects at a certain distance to the azimuth angles, phase angles of illuminant and detectors were also described. Under the same optical conditions, the emulation of the modeling shows that the brightest object for detector is plane, sphere and cylinder are darker, and cone is the darkest. To verify our modeling, experiments were carried out in dark room to simulate the optical detecting in space. Static and dynamic objects were captured by ICCD camera, by analyzing and processing the images, the correctness of the models and the feasibility of using low-light imaging system to trace and survey the spatial objects were proved.Then, based on foregoing research, the mission requirement of active-passive hybrid detection of space targets is programmed and the informations of target required by the mission was discussed. Combine the movement characteristics of detection system and space targets, an eary and feasible method for space-based monostatic orbit determination is put forward. After that the hybird detection system is devided into modules, and the structure, pattern and workflow of each module was also studied out.According to the requirement of target imaging, staring imaging pattern was employed in imaging system. CCD, ICCD, EBCCD, EMCCD were compared and the signal to noise ratio (SNR) of these devices were derivated and used to evaluate their performance in the low-illuminance space target observation. The conclusion show that EMCCD is the ideal device for space-based observation, and e2v's CCD97BI was used in design procedure of scanning and staring imaging subsystems as examples. Two off-axis TMA optical systems were designed for each of imaging subsystems and the structure parameters of them were also presented. Then the integration, search pattern, observation pattern and operation attitude of the imaging system were discussed.According to the requirement of distance and velocity measurement, ladar was designed to work in bistable pulse coherent detection pattern. LD-pumped mode-locked and Q-switched Nd:YAG laser was employed as seed source and Yb-doped double-clad optical fiber was employed as pulse power amplifier. Optical phased array (OPA) were employed as laser transmitter. By joining multiple OPAs, the main lobe beamwidth can be reduced and the pointing accuracy can be improved. Optical phased array devices can be classified into transmission type and reflection type generally. Based on the model of blazed phase grating, the scanning performances, including scanning diffraction-efficiency and scanning angle-accuracy, of two type devices were studied. Additionally the influence of laser linewidth to the scanning angle-accuracy was calculated. Laser receiver was designed to share the same aperture with staring imaging subsystem.According to the requirement of automatic target recognition, image fusion technology was employed to do target classification. Luminance image, laser intensity image, laser range image and their noise mechanisms were studied, and their simulated images of targets were presented. Several image denoising algorithm were compared in image preprocessing emulation, and a new method-"gradient demension quantification" was put forward to enhance and quantify the uneven feature of target surface. While singular value vectors of target images were used as target characteristic, the decision-level fusion of active-passive images were studied based on the gray correlation annlysis. The results of emulations revealed the effectiveness of this means.