## 3 Field Measurements and Data Processing
### Spectral and Radiometric Calibrations  
The complex environment experienced during the long-term unattended in situ measurements will degrade the performance of the spectrometer, optical fibers, and cosine-corrected foreoptics. Thus, regular calibration needs to be carried out to maintain the quality of the observed spectra, especially for the SIF retrieval. This is particularly relevant to the precise location of the narrow absorption band and to the absolute irradiance and radiance. Spectral calibration is regularly carried out by the SIFSpec manufacturer using the method described by the spectrometer manufacturer (Ocean Optis, Inc., Dunedin, FL, USA).  
Absolute radiometric calibration is regularly conducted to maintain the performance of the instruments. In general, the calibration is conducted before the instrument installed for every growth period for cropland site and once a year for the forest site. To obtain the precise radiometric coefficients, the laboratory radiometric calibration method is used for this system. As with traditional laboratory calibration, an integrating sphere, Labsphere XTH2000C (Labsphere Inc., North Sutton, NH, USA), is used as the standard light at the calibration laboratory of the Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences. The 20 inch diameter integrating sphere has an 8 inch diameter port, which is coated with SpectraflectR white coating, giving near Lambertian properties and resulting in a better spectral reflectance of 0.99 in the range from 400 nm to 1100 nm and a better radiance uniformity of 98%. The broadband unpolarized light covering the spectral range from 350 nm to 2400 nm is generated with a wide radiant brightness range, which varies from 0 to 7000 cd/m\({}^{2}\). The radiant temperature ranges from 3299 to 6000 K.  
The optical fiber containing the cosine-corrected foreoptic and bare fiber are simultaneously placed at the entrance aperture of the integrating sphere, as shown in Figure 3. Once the IT is optimized by confirming that the maximum value measured across all of the bands exceeds 80% of the saturated value, five measurements are made in sequence using the manual observation mode. Next, the DCs for the corresponding IT are recorded by opening the shutter of the light channel. After that, the calibration coefficients \(K(\lambda)\) for the upward and downward channels are calculated using Equation (2):  
\[\mathrm{K}(\lambda)=\frac{L_{light}(\lambda)\cdot IT}{(DN(\lambda)-DC(\lambda) )}, \tag{2}\]  
where \(DN(\lambda)\), \(DC(\lambda)\), and \(IT\) are the raw digital count values, dark current, and integration time, respectively, corresponding to each measurement. \(L_{light}\) is the radiance of the calibration light source.  
The results of the calibration of one system at the DM site, conducted in the laboratory in two different years, are displayed in Figure 4. It can be observed that the calibration factors are almost the same for these two years, which indicates the overall stability of the instrument. However, the calibration factor for the downward channel in 2019 is slightly different from that in 2018. This can be partly explained by the degradation of the cosine-corrected foreoptic owing to the long-term exposure to the field environment. It also indicates the necessity for annual radiometric calibration and maintenance.