Low-light remote sensing compensates for the limitations of optical remote sensing in low-light environments, such as during nighttime or early morning. To utilize low-light band Earth observation data for quantitative remote sensing, accurate on-orbit radiometric calibration is essential. However, the medium resolution spectral imaging-low light imaging spectrometer (MERSI-LL) onboard China’s first early morning orbit satellite, FengYun-3E satellite (FY-3E), faces challenges due to solar stray light interference and the lack of onboard calibration. By analyzing FY-3E/MERSI-LL observation data, it is found that the Antarctic umbra region (polar night, devoid of solar radiation) in winter provides an opportunity for calibration, as the influence of stray light is minimized. Cross-calibrating FY-3E’s low-light band using data from the umbra region and comparing it with other low-light remote sensing instruments can help avoid stray light interference during the calibration process.

In this paper, we focus on Dome C, a stable ice-covered area in the Antarctic umbra region, as the calibration target. National Oceanic and Atmospheric Administration NOAA-20’s visible infrared imaging radiometer suite (VIIRS) and day/night band (DNB) channel is used as the reference for cross-calibration of FY-3E’s low-light band. Cross-observation data from VIIRS and MERSI-LL are analyzed. The varying levels of moonlight during different polar night days provide a broad dynamic range for the low-light band’s brightness. A lunar irradiance model is used to calculate the lunar radiation under different moon phases over multiple days. Spectral corrections based on the spectral response function and angular corrections based on the lunar model are applied to the VIIRS data to derive the reference radiance for the MERSI-LL low-light band. Calibration coefficients are then obtained by performing linear regression between the reference radiance and the digital number (DN) values of MERSI-LL.

The cross-calibration method proves effective for on-orbit radiometric calibration of the FY-3E low-light band, with a linear correlation coefficient exceeding 0.98 (Fig. 6). By using observations from different lunar phases, the dynamic range of the observed radiance for a single target is extended (Fig. 7). An error analysis is conducted for both the operational calibration results and the cross-calibration results. Using the 2021 cross-calibration coefficients as the calibration coefficients for the subsequent five lunar cycles, and comparing the differences between the cross-calibration results and the operational calibration results with respect to the VIIRS reference, the cross-calibration results demonstrate a smaller root mean square error (RMSE), closer to the 1∶1 line, with an average deviation of less than 10%, compared to the regression results of the operational calibration (Fig. 9). The relative deviations of the operationally calibrated irradiance and cross-calibrated irradiance relative to VIIRS for different lunar cycles are calculated separately according to Eq. (11), the average relative deviations of the cross-calibration results are smaller than those of the operational calibration results for each month (Table 5). From a time-series perspective, in July 2021, when the cross-calibration was performed, assuming no initial decay of MERSI-LL, by May 2022, the instrument had experienced a 3.64% decay, and by July 2023, the instrument had undergone a 9.96% decay, necessitating regular updates of the calibration coefficients.

In this paper, we propose a method for in-orbit cross-calibration of the FY-3E low-light band using observations from the multi-lunar-phase Antarctic umbra region. By leveraging the high calibration accuracy of NOAA-20 VIIRS/DNB as a reference, this method cross-calibrates the FY-3E MERSI-LL low-light band and monitors on-orbit calibration deviations. Through careful selection of cross-matched data, angular corrections based on the lunar model, and spectral corrections based on the spectral response function, the method achieves absolute radiometric calibration of MERSI-LL in orbit. The obtained calibration coefficients are comparable to the official operational calibration results. Compared to the official operational calibration, the cross-calibration method aligns more closely with VIIRS results, with an average relative deviation of less than 10%. The method can be applied to calibrate the FY-3E MERSI-LL low-light band, which is affected by stray light, assess calibration accuracy, and track the long-term stability of the instrument’s radiometric response.