2. Data Set and Method
2.1. Data Overview
2.2. DCC Target Extraction
2.3. BRDF Correction
2.4. Spectral Matching
2.5. Evaluation Index
3. Results and Discussion
- The attenuation is very small for Aqua/MODIS within 3 years although slight attenuation does exist. As can be seen from Figure 4, the downward trend of the DCC reflectance fitting line of MODIS in 3 years is almost invisible. The total attenuation rate of Aqua/MODIS is only 1.13%, and the annual average attenuation rate is 0.50%. This also shows the radiometric performance of Aqua/MODIS is stable and can be used as reference sensor.
- The radiometric performance of FY-4A/AGRI obviously attenuates. As we can see in Table 2, the total attenuation rate of FY-4A/AGRI over a 3-year period is 9.11%, with an annual average attenuation rate of 4.00%. In addition, a significant downward trend can be seen from Figure 4. It is worth noting that the downward trend is not stable, but fluctuating, which may be caused by instability of radiometric performance or change of on-orbit calibration coefficient.
- The TOA reflectance of FY-4A/AGRI in DCC targets is much lower than that of Aqua/MODIS. The average value of DCC pixels of Aqua/MODIS is about 0.95, while that of FY-4A/AGRI is about 0.75, −24.99% relative deviation to Aqua/MODIS. This situation is partly due to differences in spectral response between FY-4A/AGRI and Aqua/MODIS. As can be seen from Figure 2, the band range of Aqua/MODIS is between 620 and 670 nm, while the band range of FY-4A/AGRI is between 550 nm and 750 nm, wider than that of Aqua/MODIS. As we all know, the influence of atmosphere on radiation is mainly caused by scattering. When the wavelength is longer, the effect of scattering on radiation decreases, and the effect of atmospheric absorption on radiation increases, resulting in the decrease in TOA reflectance in the VNIR band. The most likely reason that causes the large relative deviation between FY-4A/AGRI and Aqua/MODIS is the calibration coefficient used in VNIR channel. This is happening on other Fengyun series satellites as well. The radiometric performance evaluation results of FY2D, FY2E and FY2F based on MODIS using simultaneous nadir observation (SNO) method also show that the reflectance is much lower than that of MODIS in VNIR band, and this deviation could be reduce though cross radiometric calibration .
- The TOA reflectance of FY-4A/AGRI fluctuates obviously compared with Aqua/MODIS. Table 2 shows the stability index of FY-4A/AGRI is 0.04, larger than that of MODIS 0.02. As can be seen from Figure 4, the TOA reflectance of Aqua/MODIS fluctuates in a small range near the trend line, while the fluctuation of FY-4A/AGRI is more obvious. This fluctuation has a certain regularity (rising in January, falling in April, rising in July and falling in October). The reason for this phenomenon may be the unstable state of the satellite during the earth shadow period in mid-March–April and mid-September–October every year .
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
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|Channel||Wavelength Range (um)||Spatial Resolution (km)||Main Uses|
|VNIR-1||0.45–0.49||1||Small particle aerosol, true color synthesis|
|VNIR-2||0.55–0.75||0.5–1||Vegetation, image navigation and registration, star observation|
|VNIR-3||0.75–0.90||1||Vegetation, aerosol on water surface|
|Sensor||Relative Deviation (%)||Total Attenuation Rate (%)||Annual Average Attenuation Rate (%)||Stability Index|
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