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摘要:
为了提高星上TDICCD相机成像系统的有效寿命，降低系统在轨运行状态变化后对成像质量的影响，提高星上系统可靠性，设计了星上实时辐射校正系统；通过旋转焦平面，完成星上实时辐射校正与处理，获得多片TDICCD通道间和像元间校正参数，提高相机成像系统的空间适应性和非均匀性指标。在FPGA内部完成校正参数实时存储与处理，并对参数的可靠性、稳定性进行优化。该方法能够实现相机在轨状态下的相机非均匀性校正参数的实时获取。测试结果表明：系统通道内的像元非均匀性能够提高到2.01%，能够实现星上实时辐射校正的功能，获得良好效果。
Abstract:In order to increase the lifetime of the TDICCD imaging system in space and decrease the impact on the imaging quality for a long-time working in orbit, a system of real-time radiation correction in space is designed. It generates real-time correction parameters of multi-TDICCD channels and pixels in-channel by rotating the focus plane before the calculation of the real-time calibration images, and improves the adaptability and PRNU (pixel response non-uniformity) values of the TDICCD mosaic camera imaging system. FPGA is used to calculate and save the parameters, and an optimization design is implemented to improve the system stability and reliability. This method can calculate the real-time PRNU correction parameters of the TDICCD mosaic camera in-orbit, and the PRNU value of TDICCD mosaic camera in-channel reaches 2.01% after real-time calibration. This method is potentially used in real-time radiation calibration, and has got a better result.
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Key words:
- TDICCD /
- real-time radiation correction /
- PRNU /
- memory control /
- FPGA
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Overview: As to the radiation calibration process of the TDICCD mosaic imaging system in space, it rotates the focus panel of the camera 90 degrees, and then takes photos to the ground radiation calibration field, getting the real-time calibration images of the TDICCD mosaic system. It corrects the TDICCD PRNU with 1-spot algorithm based on the real-time calibration images, and calculates the differences among the pixels and the channels. It verifies the performance of the real-time pixel correction algorithm in space, and enhances the specifications of the TDI-CCD mosaic system.
When rotating the camera focus panel, for all the pixels of TDICCD pointing to the same spot-scene group in the ground calibration field, it makes sure that the real-time radiation calibration could get better results if the ground calibration filed is of the same attribute in reflectivity. We analyse the control method of the imaging period, give the final calculation method by the use of the calibration imaging data, and finally describe the relationship between the calibration field range and the valid calibration imaging data positions. These descriptions give a detail design method of the real-time calibration correction system in space.
The proposed method can distinguish the strange pixels and the normal pixels in TDICCD mosaic system, which gives processing methods separately when using 1-spot algorithm to implement the TDICCD real-time calibration correction system. Considering the efficiency of the TDICCD pixel correction parameters got in ground calibration, a tactic is designed to enhance the reliability of the real-time pixel correction parameters; meanwhile, we design the diagram of the real-time calibration algorithm and the control flow in Xilinx FPGA, which gives a detail description of the pixel correction parameters storage and applications methods.
FPGA is used to calculate and save the parameters, and an optimization design is implemented to improve the system stability and reliability. We enforce the simulation experiments in lab with respect to the real-time radiation calibration algorithm, and give a comparison among different imaging calibration cases, such as ground calibration experiments, real-time calibration simulation experiments, and no calibration experiments. The results show that the real-time radiation correction algorithm could improve the performances of the PRNU in TDICCD mosaic system, and the PRNU of the TDICCD mosaic camera system in-channel reaches 2.01% after real-time calibration. This method is useful in real-time radiation calibration, and could get a better result in project.
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图 1 相机旋转90°实时辐射标定成像示意图。(a)正常成像模式；(b)实时标定模式
Figure 1. Diagram of camera real-time radiation calibration by rotating camera 90 degrees. (a) Normal imaging mode; (b) Real-time radiation calibration
图 2 积分级数64级下的实时标定数据有效时刻
Figure 2. The valid moment of the real-time radiation calibration image data below 64 integrated grades
图 3 一点法实现像元级参数星上实时计算的FPGA控制结构图
Figure 3. The structure diagram of real-time calculation to the pixel correction parameters in FPGA at 1-point correction mode
图 4 不同模式下的参数对比。(a)多片TDICCD地面标定下，多光谱(蓝)校正参数(灰度基底15)；(b)多片TDICCD实验室实时标定下，多光谱(蓝)校正参数表(灰度基底15)；(c)未标定前，单片TDICCD可见光谱段的响应均值曲线(8通道)；(d)实验室实时标定后，该片TDICCD可见光谱段的响应均值曲线(8通道)
Figure 4. Comparison of parameters at different modes. (a) Multi-spectral (blue) correction parameters of multi-TDICCD at ground-based radiation calibration mode; (b) Multi-spectral (blue) correction parameters of multi-TDICCD at lab-based real-time radiation calibration mode; (c) Average response curve of single TDICCD in PAN spectrum before calibration; (d) Average response curve of single TDICCD in PAN spectrum after calibration in lab
图 5 实验室实时辐射校正后的均匀性成像效果
Figure 5. Experimental imaging results of real-time radiation calibration in lab
表 1 靶场边长为a×Ncss+a×1时标定数据有效的像元位置
Table 1. The valid radiation calibration pixel place in 1 line image data mode
序号 图像行数 有效像元及位置 含义 1 第1行 第1个像元 第1个像元的有效灰度 2 第2行 第2个像元 第2个像元的有效灰度 …… …… …… …… 4 第n行 第n个像元 第n个像元的有效灰度 
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表 2 靶场边长为a×Ncss+a×M时标定数据有效的像元位置
Table 2. The valid radiation calibration pixel place in M lines image data mode
序号 图像行数 有效像元及位置 含义 1 第1行 第1个像元 第1个像元的有效灰度 2 第2行 第1~2个像元 第1~2个像元的有效灰度 3 …… …… …… 4 第M行 第1~M个像元 第1~M个像元的有效灰度 5 …… …… …… 6 第n行（n > M） 第n-M+1~n个像元 第n-M+1~n个像元的有效灰度 7 第n+1行 第nM+2~n个像元 第n-M+2~n个像元的有效灰度 8 …… …… …… 9 第n+M-1行 第n个像元 第n个像元的有效灰度
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表 3 EEPROM控制指令格式
Table 3. Instruction format of EEPROM Control
类型 Bit7 Bit6 Bit5 Bit4 Bit3 Bit2 Bit1 Bit0 读写指令 —— 有效：0
无效：1读EEPROM：00
写EEPROM：10
备用：11像元参数：0
通道参数：1固化区：0
预留区：1参数指令 —— 读：0
写：1谱段号000~111（8个谱段） 参数类型000~111（8个级数）
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表 4 不同辐射校正方法对PRNU的性能对比
Table 4. Comparison of the performances at different calibration methods to PRNU
标定方法 通道间PRNU/% 像元间PRNU/% 像元校正参数离散性 未标定 4.57 2.31 —— 地面辐射校正 2.76 1.89 0.011 标定灯标定 2.99 2.31 0.017 实时辐射校正 2.85 2.01 0.015
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