The output of the FY-3B satellite's medium resolution spectral imager (MERSI) visible on-board calibration (VOC) degrades with time, which causes concerns regarding its reliability in absolute radiometric calibration. The users must distinguish between the uncertainties determined by the VOC system's radiometric output and the MERSI detectors since this will lead to a detailed temporal evolution comprehension of the MERSI system and VOC radiometric characteristics. Additionally, this can ensure the remote sensing data are fully calibrated and utilized in the observed target study. We aim to investigate the output variation of the MERSI VOC system and have made special efforts to extract the information variations of VOC radiometric performance. The annual degradation rates which are defined as the percentage difference between the results of the first and last measurements of each year are employed to evaluate the VOC radiometric performance. The results are evaluated against the trap detector monitoring to further validate the proposed proceeding approach.

Based on the characteristics of the satellite orbit and the structures of the MERSI VOC, we introduce a novel methodology to assess changes in the VOC system's radiometric output, with a particular focus on analyzing the relationship between the sunlight calibration opportunities and the angles of solar zenith and solar azimuth. Then we screen out the sunlight-based calibration data from multiple light sources (interior lamps, sunlight, space view background) calibration data. The analysis is to provide perspectives on the comparative radiometric performance of MERSI. Meanwhile, the majority band response seems to follow a somewhat downward trend. Subsequently, the performed relative response characterization step employs an exponential function created via least-squares fitting of the VOC data. High-quality MODIS data are leveraged to develop a top-of-atmosphere (TOA) bidirectional reflectance distribution function (BRDF) model and thus enhance the study precision. The time series of TOA reflectance acquired by BRDF model fitting is compared with that measured by MODIS, with the time series of BRDF modeling residuals analyzed. This model is consequently utilized in cross-calibration processing with nearly 10 years' worth of on-orbit data from MERSI. Cross-calibration processes include spectral matching between the two distinct sensors, viewing geometry correcting, and spectral interpolation. Additionally, the TOA reflectance is further converted to calibration coefficients using a calibration equation with zenith angle, azimuth angle, digital counts, and earth-sun distance. This is to comprehensively evaluate MERSI's absolute radiometric performance, and the relative and absolute radiometric characteristics of MERSI are standardized based on the initial regression point. This standardization treats the normalized difference as an indicator of the decay in VOC radiometric performance.

Recent studies using analysis of the MERSI sensor response to the Libya4 pseudo-invariant site and cross-calibration with MODIS show that the FY-3B MERSI has not deteriorated as much as the sunlight-based calibration trend has suggested. The comparison of the above lifetime trends and the relative and absolute radiometric characteristics of MERSI produce a distinction estimate in the calibrations of consecutive FY-3B MERSI pairs. We conclude that the degradation effects of the VOC radiometric performance can explain the observed differences. The results illustrate that the degradation rates of VOC radiometric performance are wavelength-dependent, with an initially higher rate gradually decreasing over the years and eventually stabilizing. Notably, in the early mission stages, the shortwave outputs (below 500 nm) exhibit a substantial degradation, reaching up to 49.51%. Conversely, the decay rates at longer wavelengths (800-1000 nm) are relatively modest, remaining within 26%. In the later stages of the satellite's mission life, the decay rates for most wavelengths are approximately 0.64%, except for 412 nm, which experiences a higher rate at approximately 1.91%. For further validating the employed proceeding approach, we make a comparison of the decay in VOC radiometric performance calculated by us with that monitored by the trap detector. Since we cannot determine how the data amount that passes through the filter changes while in orbit, the radiometric performance of VOC is all normalized by the first measurement value. The results indicate that the maximum percent differences observed throughout the instrument's lifetime remain below 15% at 470 nm and 14% at 65 nm.

A general procedure is developed and implemented to provide users with the ability to characterize the decay rate in the VOC system's radiometric output. The results demonstrate that the maximum annual decay rates (ADRs) of the short-wave output (<500 nm) range from 46%-50%, while the longer wavelengths (800-10000 nm) reveal relatively smaller changes of approximately 26%. The current procedure implementation leads to further comprehension of changes in the VOC system output. The adopted novel methodology serves as a valuable reference for extending analogous endeavors that aim at conducting on-orbit absolute radiometric calibration for other sensors.