高分五号可见短波红外高光谱相机设计与研制

Development of visible and short-wave infrared hyperspectral imager onboard GF-5 satellite

本文介绍了高分五号(GF-5)卫星上的主载荷之一—可见短波红外高光谱相机AHSI (the Advanced Hyperspectral Imager)的基本结构、成像原理、系统组成、关键技术和系统性能。该相机是国际上首个采用改进型Offner结构凸面光栅分光的星载高光谱相机,具有60 km的幅宽、30 m的空间分辨率和5/10 nm的光谱分辨率,同时获取地表地物在400—2500 nm范围内330个谱段的空间、辐射与光谱信息,具有突出的地物探测和识别能力。相比国际上经典的高光谱相机Hyperion,该相机幅宽提高8倍,谱段数增加近百个,信噪比提升近4倍;与德国、日本、意大利、印度等国际上当前发展的高光谱相机比较,该相机在幅宽和光谱通道数等方面具有明显优势,其综合性能处于国际领先水平。

Hyperspectral imaging technology provides high spectral resolution information of objects on the Earth. Hyperion and Compact High-Resolution Imaging Spectrometer have been the main sources of spaceborne hyperspectral data in the past few decades. However, the quality and quantity of the data cannot totally meet the challenging requirements of various applications. In this study, we design and present a visible and short-wave infrared hyperspectral imager called the Advanced Hyperspectral Imager(AHSI), which is one of the six payloads on China’s GF-5 satellite launched on May 9, 2018. It is the first spaceborne hyperspectral sensor that utilizes convex grating spectrophotometry and an improved three concentric-mirror(Offner) configuration. We introduce the basic structure and imaging principle of AHSI.Ground object lights are reflected into an off-axis three-mirror telescope using pointing mirror. The lights are focused on a Field Of View(FOV) separator and then split into two parts. One part enters a visible/near-infrared(VNIR) spectrometer, and the other goes into a short-wave infrared(SWIR) spectrometer. Compound light is dispersed into a series of monochromatic light through convex grating and focused on a detector using a mirror with an improved Offner structure. A VNIR detector is a back-illuminated frame transfer chargecoupled device with a size of 2048×300 pixels. A SWIR detector is a HgCdTe focal plane cooled at 110 K, and has a size of 2048×512 pixels formed by four infrared focal plane arrays of the same size(512×512), with a staggering spatial arrangement. During calibration, sunlight is reflected into an optical system using a diffuse reflectance panel. The absolute radiation response of AHSI is calibrated with a solar diffuse reflection signal, and the degradation of the diffuse reflectance panel is corrected with a ratioing radiometer. The central wavelength and bandwidth are calibrated with onboard LED calibration component and solar atmospheric absorption profile, respectively(O2 adsorption peaks at 760 and 1260 nm).The main performance metrics of the AHSI are obtained and validated via well-designed on-orbit experiments. It has a spectral range of 400–2500 nm, with spectral resolutions of higher than 5 nm in the VNIR and 10 nm in the SWIR, respectively. The swath width is 60 km,and the spatial resolution is 30 m. Compared with Hyperion, which is a classical hyperspectral instrument, the imaging width of the AHSI is superior(eight times), the number of spectral bands has increased by hundreds, and the Signal-to-Noise Ratio(SNR) has improved by four times. Compared with the instruments recently developed/planned by Germany, Italy, Indian, and Japan, the AHSI has competitive performance and has evident advantages in terms of swath width and number of spectral channels. The AHSI has achieved the widest swath width and broadest spectral range to shorten the revisit time, improve the observation and monitoring efficiency, and refine the Earth’s observation.As one of the six main payloads on GF-5 satellite, the AHSI demonstrates the development of spaceborne hyperspectral imaging technology in China. It features a large FOV telescope, a low-distortion large flat-field fine spectrometer, a large-size infrared focal plane detector, a long-life large cooling capacity cryocooler, a high-precision calibration system, and a high-precision image compensation mechanism. The AHSI has outstanding capability of detecting and identifying ground objects, making it suitable to precision applications,such as ecological environment monitoring, land and resource survey, and oil/gas exploration.