| 题名 | 基于TDI探测器的航空相机像移模型及调光算法研究 |
| 作者 | 赵嘉鑫 |
| 学位类别 | 博士 |
| 答辩日期 | 2015-05 |
| 授予单位 | 中国科学院大学 |
| 导师 | 张涛 |
| 关键词 | 时间延迟积分 航空相机 像移 调制传递函数 调光算法 |
| 其他题名 | Image Smear Modelling and Exposure Algorithm of Aerial Camera with TDI Sensor |
| 学位专业 | 光学工程 |
| 中文摘要 | 像移是航空相机成像技术的一个基本问题，探讨如何准确地建立像移模型对提高航空相机成像能力具有十分重要的理论研究与应用价值。从刚体运动学的角度出发，像移的来源可以概括为两个部分：相机在三维空间中6个自由度的刚体运动以及TDI探测器中电荷累加器在像平面二维空间的线性运动。其中，前者包括载机飞行引起的前向像移和姿态变化引起的角运动像移，后者包括电荷转移离散性引起的TDI固有像移。目前虽然已经有很多关于像移模型的研究成果，但它们都是在不同程度约束条件下建立的简化模型，具有一定的局限性，已无法适应当前航空相机的快速发展。 在航空相机中，影响像移量的调光参数包括TDI级数、曝光时间和Binning模式。如果仅以像移最小作为调整调光参数的唯一条件，那么在提高调制传递函数的同时，很可能会导致信噪比的降低，总的图像质量并不一定提升。为保证图像质量，需要优化航空相机的调光参数，使图像灰度、调制传递函数和信噪比均满足指标要求。如何针对不同类型的TDI探测器和不同结构的航空相机建立像移模型并描述它们的调制传递函数？如何计算最优的调光参数？本文针对这两个问题进行了深入研究。 首先，论文研究了CCD以及CMOS的像素结构，对比分析了不同快门类型的TDI-CCD和TDI-CMOS的电荷转移原理。通过积分采样等数学方法，建立了Multi-line CMOS的电荷转移模型和调制传递函数模型。仿真和成像实验结果表明，当曝光时间小于 行转移周期时（ 为相位数），Multi-line CMOS的调制传递函数优于TDI-CCD。 其次，本文对两种基于TDI型探测器的航空相机类型（全景式航空相机和推扫式航空相机）建立了像移模型。在建模过程中，根据成像机构特点和模型复杂度，对两种相机分别采用了两种建模方法，其一，根据全景式结构中扫描角和前向补偿角远大于载机平稳飞行时姿态角，简化了坐标变换矩阵的构成，并利用微分法推导出全景式航空相机的欧拉描述解析式。与拉格朗日描述相比，欧拉描述是基于探测器像平面固定网格的描述方法，在计算过程中网格保持与像素位置一致，更准确的描述了整个像平面的速度场。其二，利用差商法推导出推扫式航空相机的像移模型，在建模基础上进一步讨论了像移误差对于定位精度的影响，分析结果表明，在双像空间前方交会的条件下，以质心作为定位算子、以中心曝光时刻作为POS的时间戳可达到最高的定位精度。 最后，本文从图像质量的角度出发，提出了航空相机的调光最优化问题。与仅以图像平均灰度值作为目标函数的现有调光算法不同，本文以TDI级数、模拟增益、曝光时间、Binning模式作为变量，由平均灰度值、调制传递函数、信噪比构成最优化问题中的目标函数，并利用光滑重构的思想，将其转换为含参数的光滑方程组后通过牛顿法进行数值求解，数值实验证明了算法的有效性。 |
| 英文摘要 | Image smear is one of the basic problems in aerial remote sensing technique. Discussion on how to accurately model image smear has very important value in both theoretical research and application for improving the performance of aerial camera imagery. From the perspective of rigid body kinematics, image smear can be summarized as two parts, i.e., camera’s 6-DOF rigid-body motion in the three-dimensional coordinate system and TDI adder’s linear motion in two-dimensional image coordinate system. The former includes the image smear caused by aircraft forward flying and camera gimbal rotation, whereas the latter includes that introduced by TDI discrete charge transfer. At present, the research on image smear modelling, most of which is based on simplified models under different constraint with limitations, still needs to improve. Several parameters, including TDI stages, exposure time and binning mode, could impact image smear. However, if we choose the minimum of image smear as the only object function of exposure algorithm, the SNR of image could decline correspondingly, which might leads to image quality degradation. Consequently, the object function of optimum exposure algorithm should take image gray level, MTF model and SNR model into account. How to build image motion model for different TDI sensors and aerial cameras? How to solve the exposure optimization problem? We discuss these issues in this dissertation, the details are as follow: First, we study the structure and the scheme of TDI-CCD, TDI-CMOS with different shutter. With the assumption of the optical system being linear filter and sensor pixel being integral sampler, models for the above sensors are developed, especially the MTF theoretical model for Multi-line CMOS. Simulation result shows that the MTF of Multi-line CMOS is superior to TDI-CCD when the exposure time is within 1/2φ row transfer period (φ refers to clock phase number). Secondly, image smear models of panoramic aerial camera and pushbroom aerial camera are developed respectively. Due to the diversity of these two types of aerial camera, different mathematical methods are used. For panoramic camera, we simplify the transform matrix and derive the Eulerian description by using kinematics and differential method. In this case, comparing to Lagrangian description, the image smear is given by functions in terms of fiexed grid, which is more accurate and intuitive. The results of computer simulation and experiment indicate that forward image motion and scan image motion could not be fully compensated with scan-mirror head and TDI sensor. For pushbroom camera, we develop the model by using difference quotient method and first order approximation. In addition, we discuss geolocation error caused by image smear. Simulation results show that when centroid is used as image location operator and middle exposure pulse used as POS event trigger, geolocation accuracy could reach maximum. Finally, from the perspective of image quality, the concept of exposure optimization is proposed. Instead of using average image DN as the only object function, MTF model and SNR model, which are in terms of TDI stages number, analog gain, exposure time and binning mode, are used to reformulate object function. Since the equalities and inequalities and not easy to solve, we use equivalent smoothing function to reconstruct the problem and solve it with Newton algorithm. Numerical experiments prove the validity of the algorithm. |
| 公开日期 | 2015-12-24 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.ciomp.ac.cn/handle/181722/48961]  |
| 专题 | 长春光学精密机械与物理研究所_中科院长春光机所知识产出 |
| 推荐引用方式 GB/T 7714 | 赵嘉鑫. 基于TDI探测器的航空相机像移模型及调光算法研究[D]. 中国科学院大学. 2015. |
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