Instrument Design and Prelaunch Calibration of the High-Accuracy On-Board Calibrator (HAOC) on FengYun-3G

Abstract

Accurate, long-term, and continuous monitoring of Earth’s radiances is crucial for detecting subtle climate change signals. International expert teams are developing space-based reference systems to establish an SI-traceable radiometric benchmark in orbit. The High-Accuracy Onboard Calibrator (HAOC) is a pioneering prototype for China’s Space-Based Radiometric Benchmark (CSRB) initiative. This study details HAOC’s design, observation modes and theories, and prelaunch laboratory performance. Launched on April 16, 2023, aboard the FengYun-3G satellite, HAOC is a push-broom imaging spectrometer conceived as an in-orbit radiometric reference traceable to the International System of Units (SI). Spanning a spectral range of 400–1060 nm, it serves as a high-fidelity transfer calibrator for key Earth observation sensors, primarily the Medium Resolution Spectral Imager (MERSI) series on China’s FY-3 satellites. To circumvent the limitations of traditional aperture attenuators, HAOC pioneers a novel calibration approach using a motorized wheel with switchable NiCr thin-film attenuators. This unique architecture ensures exceptional optical stability and mitigates diffraction effects, facilitating the direct solar-based calibration with unprecedented accuracy. A comprehensive prelaunch characterization of HAOC’s spectral and radiometric properties confirmed its compliance with all design specifications, culminating in a combined laboratory radiometric uncertainty of 2.37% (k = 2). HAOC is now operating stably in orbit, generating substantial high-accuracy radiometric data of the Sun, Moon, and Earth. Validated, publicly available HAOC data from the National Satellite Meteorological Center will serve as a valuable resource for the international space-based climate research.