推扫式多挡增益光谱成像模拟及噪声分析

综合性遥感监测对载荷的灵敏度和动态范围都有较高的要求，为此提出一种基于逐像元多挡增益切换的光谱成像方法。不同于帧增益或者行列增益切换的成像方式，该方法利用4T-APS CMOS探测器无破坏读出的特点，可以进行像元级的增益优化。这种方式可以兼顾水体、植被、云层等多要素的成像需求，极大地提高了载荷的研制效益。其基本原理为探测器先进行全局曝光，然后根据多级积分电容的饱和判断，选取不饱和的最高增益信号以增益码加信号的形式下传。地面根据增益码对应的定标系数反推出像元真实辐射值。由于谱段多且为分段响应，为保证系统的定量化应用，建立多挡增益光谱成像模型并进行噪声分析具有重要意义。通过对噪声类型的分析，建立了多挡增益下的泊松-高斯噪声模型。基于该模型计算了像元受噪声影响以低增益读出的概率。结果表明，虽然噪声会影响读出增益的变化，但影响区间极小，入瞳辐亮度在5 mW·cm-2·μm-1·sr-1以内，信号比增益挡位辐亮度分界值小0.05 mW·cm-2·μm-1·sr-1即可保证正常读出的概率大于99.6%。随着信号增强，光子噪声增大，增益减小，影响区间扩大。根据多挡增益信噪比模型，分析得出光谱模式与合并通道模式下的信噪比变化。最后，利用宽波段成像光谱仪（WIS）数据作为入瞳辐亮度进行了四挡增益的推扫式光谱成像模拟，分析了多挡增益光谱图像的固有特点。根据噪声模型对中心波长为0.443 μm的光谱图像添加1~3σ的随机噪声，分析了噪声对地物目标所处增益的影响。结果显示，在满足信噪比指标的前提下，该系统的单挡动态范围为74 dB，总动态范围可达114 dB。该方法不但提高了水体等弱信号的信噪比，而且可以保证建筑、云层等亮目标不饱和。成像模拟及噪声分析不仅有利于该载荷的后续研制，也可以为同类光谱仪器的设计提供参考。

Simulation and Noise Analysis of Pushbroom Multi-Gain Spectral Imaging

Comprehensive remote sensing monitoring requires high sensitivity and dynamic range of the sensor, so a spectral imaging method based on a pixel by pixel gain switching is proposed. Unlike from the frame gain or row column gain switching imaging methods, this method can optimize the gain at pixel level by using the nondestructive readout characteristics of the 4T-APS CMOS detector. This method can take into account the imaging needs of water, vegetation, cloud and other factors and greatly improve the development efficiency of the payload. The basic principle is that the detector firstly carries out global exposure and then according to the saturation judgment of the multi-stage integral capacitance, selects the unsaturated highest gain signal to be transmitted down in the form of gain code plus signal. The real radiation value of the pixel is derived from the calibration coefficient of the gain code. Due to the multi spectral segment and piecewise response, it is important to establish the multi-gain spectral imaging model and analyze the noise to ensure the system’s quantitative application. Based on the analysis of noise types, the Poisson Gaussian noise model with multi-gain is established. Based on the model, the probability of low gain readout is calculated. The results show that although the noise will affect the change of the readout gain, the influence range is very small. When the radiance is within 5 mW·cm-2·μm-1·sr-1, the signal is less 0.05 mW·cm-2·μm-1·sr-1 than the value of the gain gear, so the probability of normal readout is greater than 99.6%. With the enhancement of the signal, the photon noise increases, the gain decreases, and the influence range expand. According to the multi-gain SNR model, the changes of SNR in spectral mode and combined channel mode are analyzed. Finally, the push broom spectral imaging simulation of four gains is carried out using the wideband imaging spectrometer (WIS) data as the entrance pupil radiance, and the inherent characteristics of the multi-gain spectral image are analyzed. Based on the noise model, add 1~3σ random noise to the spectral image with center wavelength of 0.443 μm, the influence of the noise on the gain of ground objects is analyzed. The results show that on the premise of meeting the SNR index, the single gain dynamic range of the system is 74 dB, and the total dynamic range is 114 dB. This method improves the SNR of weak signals such as water and ensures the unsaturation of bright targets such as buildings and clouds. Imaging simulation and noise analysis are conducive to the subsequent development of the sensors and provide a reference for the design of similar spectral instruments.