## 3 Results and Analysis
### Inversion Results  
In the process of matching the kernel coefficient, the matching precision is higher when the data sampling direction is more uniform in the spatial distribution of the hemisphere. Figure 8a shows the red band solar zenith angle, observation zenith angle, and relative azimuthal distribution for a cycle (8 May to 22 May 2012) of HJ-1 data, selecting a 3 \(\times\) 3 pixel window. The sampling direction of the data in the cycle is relatively uniform. To further determine the accuracy of the kernel coefficient matching, the kernel coefficient and the MOD43 BRDF kernel coefficient are used to create the direction reflectivity distribution. The solar zenith angle is set to 45\({}^{\circ}\) and the relative azimuth angle to the HJ-1 pixel relative azimuth angle. Figure 8b shows the red band inversion direction reflectivity for a cycle (8 May to 22 May 2012) of HJ-1 data and MOD43 BRDF data on 8 May 2012 in the farmland area of interest. The kernel coefficients of the matched nuclei and the MOD43 BRDF data of red band are 0.18559, 0, 0.0070382, and 0.209, 0.032, 0.049, respectively. The reflectivity of the inverted zenith direction is 0.1880 and 0.1832, respectively, whereas the MOD43 NBAR is 0.1828. Although the coefficients are different, the inversion of the reflectivity values is close. The inversion reflectivity of HJ-1 in the zenith direction is similar to that of MOD43 NBAR; it is proven that the inversion reflectivity result shows a small difference with the actual measurement result of MOD43. The results are within the error range. Figure 7: Four scatter plots of fusion reflectivity and observed reflectivity (x, observed; y, fusion): (a) the distribution of the red band points of forest; (b) the distribution of the NIR band points of forest; (c) the distribution of the red band points of farmland; (d) the distribution of the NIR band points of farmland. For the inversion results, i.e., the HJ-1 NBAR data, the MODIS NBAR product is used to compare with them, analyze their similarity and variance, and analyze the accuracy of the inversion results. In the forest and farmland areas, the average reflectivity data of HJ-1 NBAR and MOD43 NBAR zenith direction were obtained. Figure 9 shows the reflectivity of HJ-1 NBAR and MODIS NBAR time curves. It can be observed that the average reflectivity of HJ-1 NBAR is similar to that of MODIS average reflectivity, and the fluctuation range of reflectivity is almost the same. However, it can also be seen that the average reflectivity of HJ-1 NBAR fluctuates more violently. In the forest area of interest, the reflectivity of the NIR band significantly increased during the growing period, but it does not maintain a high value smoothly. There is a small fluctuation during the period, and the reflectivity curve of the red band fluctuates vigorously, and there are obvious anomalies. In the farmland area of interest, the trend of reflectivity is similar to that of MODIS, but there are smaller fluctuations before the growth period. The wet and dry conditions of the bare soil may be the reason. This result indicates that the average reflectivity of HJ-1 NBAR is more suitable to reflect the subtle changes in the surface. Similarly, we used the quantitative indicators to calculate the difference between the HJ-1 NBAR data and the measured values, and to analyze the accuracy of the inversion data. Figure 10 shows the scatter plots of the HJ-1 NBAR and MOD43 NBAR data. Table 4 shows the accuracy test result for the HJ-1 inversion data and MODIS inversion data. In the four scatter plots, the points are distributed on both sides of the y = x line. The MAPE values are within 20%, compared to the MOD43 NBAR, the mean absolute difference is small, and the predicted value is highly correlated with the actual observed value. By comparison, it can be seen that the HJ-1 30 m resolution data are more sensitive than the MOD43 500 m resolution data. In the case of the same fluctuation trend, the HJ-1 reflectivity exhibits smaller fluctuations.