Vicarious Calibration for the AHSI Instrument of Gaofen-5 With Reference to the CRCS Dunhuang Test Site

The visible-shortwave infrared Advanced Hyperspectral Imager (AHSI) is a payload onboard the Gaofen-5 satellite, which is China’s first hyperspectral satellite and is part of the Chinese High-Resolution Earth Observation System. As a supplement to the onboard radiometric calibration of the AHSI instrument, vicarious calibration is also required, which is independent of the instrument-based calibration. In this article, a reflectance-based vicarious calibration approach is presented, which takes surface reflectance data, aerosol data, and atmospheric water vapor data into account. The Dunhuang test site, which is one of the China Radiometric Calibration Sites (CRCS) for the vicarious calibration of spaceborne sensors, possesses stable, uniform, and measurable surface objects, so it was chosen as the radiation source to replace the laboratory and onboard calibrators. A Spectra Vista Corporation (SVC) spectral radiometer and a CE318 sun photometer were utilized for the measurement of the surface reflectance and the condition of the aerosol, respectively. The radiance at the entrance pupil at the top of atmosphere was then obtained through the MODerate resolution atmospheric TRANsmission (MODTRAN) atmospheric transmission model. The surface reflectance was obtained using the Fast Line-of-sight Atmospheric Analysis of Hypercubes (FLAASH) atmospheric model for validation. The results show that, with regard to the calibration coefficients, the calibrated AHSI instrument presents a stable radiometric performance among different land-cover types. The ratios on all the bands are between 0.8 and 1.2 and are consistent with the reflectance data from the Dunhuang test site. The ${R} ^{{2}}$ values are all greater than 0.95 and the spectral angle is all less than 2°. The standard deviations of the ratios are less than 3% for each chosen band, which proves that the calibrated data have a high consistency with the in situ measurements. When compared with Landsat 8 and Sentinel-2, the mean errors of the surface reflectance are all under 0.06, which further demonstrates that the calibrated reflectance has a high accuracy.

I. Introduction

With the advent of hyperspectral imagery, which is designed to identify and analyze the physicochemical characteristics of land cover, the importance of absolute radiometric calibration has come to the fore. The Gaofen-5 (GF-5) satellite is an important part of the China High-resolution Earth Observation System (CHEOS). GF-5 carries two optical imaging sensors, along with an atmospheric infrared ultraspectral sounder, a greenhouse gas monitoring instrument, an environmental trace gas monitoring instrument, and a directional polarization camera [1]. The visible-shortwave infrared (SWIR) Advanced Hyperspectral Imager (AHSI), which was designed by the Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, China, is one of the main payloads onboard the GF-5 satellite. The AHSI instrument was developed covering 330 spectral bands to characterize the solar reflective regime from 400 to 2500 nm, with a narrow swath width of approximately 60 km. The spectral resolution is about 5 nm for the visible and near-infrared (VNIR) region from 400 to 1000 nm and about 10 nm for the SWIR region from 1000 to 2500 nm. The spatial resolution is around 30 m [2].