【作者】 白旭；

【导师】 张中兆；

【作者基本信息】 哈尔滨工业大学 ， 信息与通信工程， 2008， 博士

【摘要】 电视制导系统中,导引头采集的战场上的实时图像,经上行信道传送给控制中心,由操控人员根据获得的图像决定攻击目标,并由下行信道发送指令信息控制导弹的飞行方向,直至击中目标。由于无线信道带宽受限,图像信息的数据量巨大,在高的压缩比下图像的失真变得严重,影响制导操作人员对疑似目标的判断;同时由于系统应用在军事对抗领域,所以如何快速清晰的显示疑似目标以及如何保证信息传输过程中的安全是值得关注的问题。在此要求下,本文以基于小波变换的JPEG2000图像压缩标准为基础,根据电视制导系统的自身特点,结合JPEG2000标准中的感兴趣区域(ROI)编码技术、容错机制和码流结构,对快速定位攻击目标、提高图像显示实时性和信息保密通信三个方面内容进行了研究,解决了系统应用中的技术难题。首先,通过分析电视制导图像的特点,提出了一种改进的基于率失真优化的感兴趣区域编码算法。在JPEG2000压缩码流中,包括了来自每个子带的每个编码块的嵌入码流的贡献,当确定感兴趣区域后,在有些编码块中可能只包含很小的ROI区域,其他大部分都是背景区域,但是整个块内的系数都被赋予了和ROI区域一样高的优先级,对于无线信道带宽严重受限的可视制导系统来说,会明显降低ROI图像的质量。通过判读是否属于ROI掩模把ROI编码块中的背景区域和ROI分离开,采用一个系数调整两者的优先级,同时根据实际操作需要,对此系数的调整进行分级操作,新算法在基本不增加算法复杂度的情况下降低ROI编码块中背景区域的系数优先级,有好的重建ROI图像质量,达到在低信道速率下对疑似目标进行高质量显示的目的。其次,针对配备红外导引头导弹,提出了一种简单实用的基于灰度的提取感兴趣区域算法。因为目标区域的灰度总是高于背景的灰度,并且目标周围的背景往往具有比较均匀的分布。根据以上特点,把图像在空域分割成块,计算各图像块的灰度均值,通过排序找到最大值所在块,以最大值块作为中心扩展确定感兴趣区域,实现了对感兴趣区域的快速自动定位。此种算法产生的ROI掩模为矩形,不需要传输所有掩模信息,只要传输左上和右下两点值就可在收端得到ROI掩模信息。然后,基于JPEG2000的容错机制,结合信道采用的LDPC纠错码,在电视制导系统中采用了一种信源信道联合解码算法。信道解码采用对数和积迭代译码,每次迭代的结果送给JPEG2000解码器,解码器通过错误恢复机制发现每个编码块中的第一个错误比特,把每个码块在此比特前的信息反馈回信道解码,对每次迭代的信道码字的先验概率进行选择性修正更新,并提出了一种能实时应用的反馈因子确定方法。仿真结果表明,和分立解码方法相比,信源信道联合解码算法可以提高解码效率和接收信噪比。最后,对信息传输安全问题进行了研究。基于椭圆曲线公钥加密算法,提出了对电视制导指令和图像信息进行保护的方案。目前系统安全主要还是靠扩频通信系统来保障,单纯依靠扩频技术进行保密通信已经不再绝对安全,为此把椭圆曲线公钥加密算法引入电视制导系统。指令信息采用导弹的公钥进行全程加密,在导弹接收到信息后用导弹的私钥解密,保证了指令信息的安全;图像信息根据JPEG2000码流特点部分加密,从图像中提取部分信息和重要参数信息一起加密传输,加解密方式是指令加解密的逆过程。此种方案可以对信息发送的主体进行确认,防止了信息被截获和伪造。更多还原

【Abstract】 Video-guided missile is a kind of precision-guided weapons. It is a bomb with a video camera in the nose that can get the real-time target image, then transmit the image to control center through wireless up channel. Then the manipulators can select the target in which they interest in the image and send the commands to guide the bomb to attack it. For the limited wireless bandwidth, image which contains much data has to be compressed by high compression ratio; therefore, it is difficult for manipulators to confirm attacking targets. Because this system was applied in the military confrontation,the problem should deserve attention and consideration in video-guided missile system that how to display suspected target quickly and keep the communication security.In order to solve this problem, in this paper some method are proposed based on the region of interest coding, the error-resilience modes and the structure of code-stream in JPEG2000 standard after analyzing the characteristics of video-guided missile system.Firstly, an EBCOT-based ROI coding algorism was presented. The unit of EBCOT coding in JPEG2000 standard is block, some blocks only overlap with ROI mask in a small part, but all the coefficients in this block were assigned the same high priority. That will lead to the image of ROI decreasing sharply which is transmitted in limited wireless channel of video-guided missile system. The coefficients of background and ROI were distinguished in one block with ROI mask, and then adjusted their priority with a factor based on EBCOT. The adjustment was graded for practical applications. Manipulator can adjust the weight factor to improve the compression efficiency by yielding a good ROI and degrading the priority of background coefficient in the ROI code block. The proposed method show higher compression efficiency, lower complexity and good reconstructed ROI image quality in lower channel capacity.Secondly, aiming at infrared imaging guidance missile system, an automatic ROI extraction algorithm was proposed based on image gray. In the infrared image the gray value of object regions is always higher and the background which has uniform distribution. According to this feature IR image were segmented to many blocks in spatial information, calculated the mean of gray value to each block, find the block in which the maximal gray value is by sorting and then got the ROI through extending the block with maximal gray value. In this algorithm a rectangle ROI mask is got which only two points, the top left point and the bottom right point, can express, so just only the two points is transmitted. Therefore the proposed approach can fix on ROI rapidly and achieve good target searching performance.Thirdly, a joint source-channel decoding (JSCD) method was designed in video-guided missile system. JPEG2000 is used to perform source decoding with certain error-resilience modes; and a regular low-density parity-check (LDPC) code is selected as the channel error correcting code. For Sum-Product iterative channel decoder, each iterative results are send to source decoder; JPEG2000 decoder with error-resilience modes detects the first error bit in each block and feedbacks the information to channel decoder which update the log-likelihood ratio(LLR) of partial codeword bits. A novel feedback coefficient is applied in this system. Simulation results show that the JSCD scheme can accelerate the iterative procedure, and improve the channel SNR of communication.Finally, a novel scheme was proposed to protect the communication data from interception and falsification, the scheme was based on the Elliptic Curve Cryptosystem (ECC). Now the security of military radio communication were depends mainly on spread spectrum technology; but with the development of electronic product and computing capacity, it’s difficult to ensure the safety of information very well. In order to solve this problem, Elliptic Curve Public Cryptosystem is introduced into video-guided system. The system send the command data to the missile which is encrypted with the public key of missile and then the missile receives decrypts this command data with the private key of missile. For image information, combining with the structure of JPEG2000 code-stream, only some important parameters and partial extraction data were protected, the process of encrypting and decrypting were inverse to command data processing. The proposed method not only enhances the communication security but also confirm the identity information.更多还原