The Experimental Hyperspectral Imaging Microsatellites SPARK-01 and -02 Radiometric Calibration and Overall Performance

At 3:22 am UTC on 22 December 2016, two wide-swath push broom hyperspectral imaging microsatellites, SPARK-01 and -02, which were manufactured by the Shanghai Engineering Center for Microsatellites for experimental aims, were successfully launched at the Jiuquan satellite launch center by the CZ-2D rocket. SPARK-01 and -02 have spectral ranges of 400–1000 nm, a swath of ~100 km, a spatial resolution of 50 m and 2048 pixels along the cross-track direction. This report will give a comprehensive introduction to the radiometric performance of the satellites and data acquiring status. Due to the lack of on-board calibration device and less measurements before launch, the radiometric calibration coefficients were determined for these two satellites via a calibration experiment performed from the end of February to the beginning of March 2017 at the high-altitude, homogenous Dunhuang calibration site in the Gobi Desert in China. In-situ measurements, including ground reflectance, direct transmittance, diffuse-to-global irradiance ratio, and radiosonde vertical profile, were acquired. A unique relative calibration procedure was developed using actual satellite images. This procedure included dark current computation and non-uniform correction processes. The former was computed by averaging multiple lines of long strip imagery acquired over open oceans during nighttime, while the latter was computed using images using images acquired after the adjustment of the satellite yaw angle to 90 degree. This technique was shown to be suitable for large-swath satellite image relative calibration. After relative calibration, reflectance, irradiance, and improved irradiance-based methods were used to conduct absolute radiometric calibrations in order to predict the top-of-atmosphere (TOA) radiance. The calibration uncertainty is estimated to be less than 6%. Although these two experimental hyperspectral satellites have been decommissioned till April of 2017, a huge hyperspectral data, mostly over China and neighboring regions, have been acquired during half a year period. A number of dark current images and 90 degree yaw angle images were acquired to evaluate the relative stability of radiometric performance among the cross track pixels. Also, the relative differences of the response of detectors were also evaluated under different radiance levels by acquiring yaw 90 imageries over desert, common ground surface, snow/ice (i.e. Antarctic regions) and a comprehensive model for the relative radiometric correction of spark 01/02 was also refined. Furthermore, other radiometric performance was also evaluated, like signal-to-noise ratio, spectral wavelength shifts, bad pixels, etc