Study On The Technique Of Imaging With Dual-band SWIR Linear InGaAs Detector

It’s well known that atmospheric correction plays an import role in getting the accutate water leaving radiance which is very crucial to the water color remote sensing. For the clean open seas which are normally case I water, two mature standard atmospheric correction algorithms have been put into operation. They are respectively NASA’s algorithm for SeaWiFS/MODIS and ESA’s algorithm for MERIS. The turbid near-coast waters which are normally case II water account for only a small part of the world’s ocean area, but they are closely related to people’s production and life. Due to the scattering of the case II water caused by the its complex components, the water leaving radiance of the near-infrared band is not zero and can not be omitted. In addition to that absorptive aerosols come into being over the coastal in consequence of human activity, mineral dust and gobi desert dust. The two standard atmospheric correction algorithms are based on two assumptions: ‘dark pixel’ and ‘weak-absorbing aerosols’. However, these two assumptions do not hold in coastal environments and the two standard atmospheric correction algorithms are no longer fit for case II water. Most of the China seas belong to case II water except for the South China Sea and the south part of the East China sea. Especially in the nearshore, the high turbidity is rare elsewhere in the world. Lacking of effective atmospheric correction algorithm for case II water, there are no valid water color products for these areas all the year round.Meanwhile water has more strong absorption in the SWIR band than in the nearinfrared band. So the water leaving radiance approximates to zero even in the turbid water. Moreover, by increasing the SWIR sensing channels we can discriminate between absorbing aerosols and non-absorbing aerosols from their different spectral responses. Therefore, people starts to explore new atmospheric correction method for case II water using dual SWIR bands.In order to improve the ocean color remote sensing application level of China, We put forword setting dual SWIR detection channels for case II water atmospheric correction on a wide band ocean color spectral imager. Aiming at the system requirements, this SWIR push-broom imaging system combines two sub modules to enlarge the total field of view. Each module adopts a 800 × 2 SWIR InGaAs FPA detector which is made by SITP with response from 0.9μm to 1.7μm. This imaging system achieves two detection channels with response from 1.232 to 1.252μm and 1.630 to 1.654μm via interference narrow band pass filter. According to the characteristics of the interference narrow band pass filter and the application requirements of large field of view, the image telecentric optical system is chosen.The major works and the innovation of this paper include:1. It’s the first time to set dual-band SWIR sensing channels onboard the water color remote sensing instrument for case II water atmospheric correction in China. The way of right use of the homemade dual-band SWIR InGaAs long linear array detectors for case II water atmospheric correction is explored.2. Based on the idea of integrated design, the SWIR imaging system handles with the timing driven and signal processing circuits of the two submodules in a unified framework. And this integrated design idea is expanded to the entire electronics system of the spectral imager project. All the 7 sensors belonging to the 3 imaging subsystems(VNIR SWIR and TIR) are driven and processed by a single high capacity FPGA chip. By implementing all the digital logic function and data storage function within a FPG A, the instrument is greatly enhanced in integration, reliability and flexibility;3. According to the performance requirements of high sensitivity and wide dynamic range, the electronic system for the SWIR detection channels is well designed. The detectors are provided with stable operating temperature,low noise bias power supply and low noise amplifier circuits to improve the system sensitivity. The requirements of different dynamic range on earth observation and on orbit calibration are met by adjusting the integration time, accumulative frequency and conversion gain via parameter injections;4. As a key member participated in the experiments in which the difficult problems such as “Ghost image”, “smear” and “different focal plane” were found. These problems are related to the detector and will have great negative impacts on the imaging quality of the system. It provides the basis for the improvement of the SWIR detector through the problems analysis and localization;5. Construted the test platform and the system was tested systemically. The results show that the performance of the dual band short wave infrared imaging system, such as MTF, sensitivity, spectral response and dynamic range, can satisfy requirements.