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SMAC稳定性测试光源选用内置卤钨灯的积分球光源,即卤钨灯产生的辐射能量经过积分球内壁的匀光涂层多次漫反射后,形成的均匀面光源。测试原理图如图1所示,因红外探测器的制冷对工作温度有较高要求,故将SMAC放置于真空低温设备内,令其通过光学窗口观测光源,在一定时间内保持光源、SMAC状态不变,采集SMAC观测数据并评价数据稳定性误差。
Instrument Main characteristics Integrating sphere Sphere diameter: 1200 mm
Exit port diameter: 400 mm
Light source/number: Tungsten halogen lamp/16Cryogenic vacuum tank Temperature control range: −175 to 100 ℃
Temperature control accuracy: ± 2 ℃
Vacuum degree: 10−3 Pa to normal pressureStandard detector Stability error:0.4%@1380 nm Table 1. Information of instruments for stability test
卤钨灯积分球光源预热稳定后,SMAC对光源进行1 h的持续观测。以0.5 min为间隔统计SMAC数据的最值、均值,计算稳定性误差,表达式为:
式中:
$ {E_{stability}} $ 为SMAC稳定性误差;$ {d_{\max }} $ 、$ {d_{\min }} $ 、$ {d_{average}} $ 分别为SMAC稳定性观测数据最大值、最小值、均值。SMAC卷云识别波段(1380 nm)的稳定性测试曲线(见图3)呈现周期性的、较大幅度的波动,稳定性误差高达4.08%。SMAC卷云识别波段数据的波动,可能来源于以下两个方面:(1)卤钨灯光源自身的稳定性误差;(2)水汽波动影响,即卤钨灯至真空低温设备光学窗口的辐射传输路径上,水汽含量的波动使其对1380 nm波段辐射能量的吸收程度出现波动,进而使SMAC卷云识别波段接受到的能量出现波动。
其中,卤钨灯光源稳定性与电源供电稳定性有关,表现为当电流波动时,光源辐射光谱随之整体上升或下降。SMAC的870 nm通道为窗区通道,即不受传输路径上水汽波动影响,其同时段观测数据能够直观地反映光源稳定性。由图3可以看出,870 nm通道观测数据较为平稳,稳定性误差仅0.37%,表明卤钨灯输出稳定,卷云识别波段的大幅度周期性波动与卤钨灯稳定性无关。
另一方面,辐射传输路径上的水汽对1380 nm波段的辐射存在强吸收,空气中水汽含量的变化直接影响SMAC所接收到的辐射量,而周期性的波动与洁净实验室内的空气循环有关。为此,可采取以下两个措施排除水汽波动对测试数据的干扰:(1)控制辐射传输路径上的水汽波动幅度,在根源上抑制其对1380 nm波段辐射的干扰作用;(2)设置同波段标准探测器,对1380 nm处的辐射水平进行同步监测,间接校正水汽波动影响。
Stability measurement method of cirrus cloud diagnosing band for synchronous monitoring atmospheric corrector
doi: 10.3788/IRLA20210647
- Received Date: 2021-12-20
- Rev Recd Date: 2022-01-20
- Publish Date: 2022-07-05
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Key words:
- remote sensing /
- atmospheric correction /
- stability measurement /
- water vapor absorption band
Abstract: Synchronous Monitoring Atmospheric Corrector (SMAC), a support for the main camera, was one of the mayor facilities of high resolution and multi-mode imaging satellite. SMAC was designed to achieve atmospheric correction for high spatial resolution images based on radiative transfer model by getting synchronous parameters of aerosols and water vapor. SMAC had a special channel, where the radiance could be mostly absorbed by water vapor, for remote sensing of cirrus cloud. This channel was sensitive to the fluctuation of water vapor in the lab while stability measurement, thus the real performance of this channel could be covered up. The influence of water vapor fluctuation on stability test data was suppressed by controlling water vapor and setting synchronous monitoring detector. The results show that the stability error of SMAC’s channel for cirrus cloud detection decreased from 4.08% to 0.23%. It indicates that the stability measurement method of cirrus cloud diagnosing band for SMAC is reasonable and effective, which could reflect the stability performance of the product, and has important guidance significance for stability measurement of similar instruments.