In a significant stride for environmental monitoring and maritime applications, a team of researchers led by Jian Zeng from the China Centre for Resources Satellite Data and Application has developed a novel method to improve the accuracy and frequency of satellite data calibration. The study, published in the journal ‘Remote Sensing’ (translated from the original Chinese title), focuses on the Huanjing-2A/2B (HJ-2A/2B) satellites, which are equipped with advanced sensors capable of capturing high-resolution imagery of the Earth’s surface.
The HJ-2A/2B satellites, often referred to as China’s next-generation environmental monitoring satellites, carry four visible light wide-swath charge-coupled device (CCD) sensors. These sensors can acquire 16-meter multispectral imagery (16m-MSI) with a swath width of 800 kilometers, making them particularly useful for monitoring large areas, including vast oceanic regions. However, traditional calibration techniques have been limited by their frequency, which can hinder continuous monitoring efforts.
To address this challenge, Zeng and his team proposed a spectral-angle difference correction-based cross-calibration approach. This method uses the Landsat 8/9 Operational Land Imager (OLI) as a reference sensor to calibrate the HJ-2A/2B CCD sensors. The study utilized cloud-free image pairs of HJ-2A/2B CCD and Landsat 8/9 OLI, acquired simultaneously at the Dunhuang and Golmud calibration sites between 2021 and 2024. The researchers also incorporated atmospheric parameters from the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA5) dataset and historical ground-measured spectral reflectance data for cross-calibration.
“Our method improves both radiometric accuracy and temporal frequency,” said Zeng. “This is crucial for applications that require continuous and reliable data, such as maritime monitoring and environmental assessment.”
The methodology involved spatial matching and resampling of the image pairs, along with the identification of radiometrically stable homogeneous regions. To account for sensor viewing geometry differences, an observation-angle linear correction model was introduced. Spectral band adjustment factors (SBAFs) were also applied to correct for discrepancies in spectral response functions (SRFs) across sensors.
The results of the study demonstrated that the cross-calibration coefficients differed by less than 10% compared to vicarious calibration results from the China Centre for Resources Satellite Data and Application (CRESDA). Additionally, using Sentinel-2 MSI as the reference sensor, the cross-calibration coefficients were independently validated through cross-validation. The results indicated that the radiometrically corrected HJ-2A/2B 16m-MSI CCD data, based on these coefficients, exhibited improved radiometric consistency with Sentinel-2 MSI observations.
For the maritime industry, this advancement in satellite calibration technology holds significant commercial impacts and opportunities. Accurate and frequent satellite data is essential for various maritime applications, including vessel tracking, environmental monitoring, and resource management. Improved calibration methods can enhance the reliability of data used for these purposes, leading to better decision-making and operational efficiency.
“Further analysis shows that the cross-calibration method significantly enhances radiometric consistency across the HJ-2A/2B 16m-MSI CCD sensors,” Zeng added. “This is particularly beneficial for maritime sectors that rely on consistent and accurate data for their operations.”
The study also quantified the impact of atmospheric parameters and surface reflectance on calibration accuracy, with total uncertainty calculated. The proposed spectral-angle correction-based cross-calibration method not only improves calibration accuracy but also offers reliable technical support for long-term radiometric performance monitoring of the HJ-2A/2B 16m-MSI CCD sensors.
In summary, the research led by Jian Zeng and his team at the China Centre for Resources Satellite Data and Application represents a significant advancement in satellite calibration technology. The proposed method improves the accuracy and frequency of data calibration, which is crucial for maritime and environmental monitoring applications. The study, published in ‘Remote Sensing’, highlights the potential commercial impacts and opportunities for maritime sectors, emphasizing the importance of reliable and consistent data for operational efficiency and decision-making.