# On-Orbit Absolute Radiometric Calibration and Validation of ZY3-02 Satellite Multispectral Sensor
## 3 Methodology
### Synchronous Measurement of Atmospheric Parameters  
Aerosol parameters were collected at the time of ZY3-02 satellite overpass. An automatic sun photometer (CIMEL CE318) was used to measure the aerosol optical depth (AOD) and the total columnar water vapor (CWV). The AOD and CWV were retrieved by the AERONET data process center from the CE318 sun photometer observed data at the Baotou site, which joined the Aerosol Robotic Network (AERONET) in September 2013. Figure 8 shows the measurement of atmospheric parameters of the AERONET Baotou site. The AOD at 550 nm is estimated from the Langley algorithm-derived AOD at the 440, 670, 870, and 1020 nm channels with the Angstrom empirical equation, as follows:  
\[\tau_{\mathrm{a}}(\lambda)=\beta\cdot\lambda^{-\alpha} \tag{1}\]  
where \(\tau_{\mathrm{a}}(\lambda)\) is AOD at the wavelength \(\lambda\), \(\beta\) is [PERSON]'s turbidity coefficient, and \(\alpha\) is the [PERSON] exponent; \(\alpha\) and \(\beta\) are independent of the wavelength, and they can  
Figure 6: Spectral reflectance measurement data of gray-scale permanent targets.  
Figure 7: Spectral reflectance measurement data of calibration tarps.  
be calculated by fitting the AOD at the 440, 670, 870, and 1020 nm channels. Table 3 shows the synchronous measurement of atmospheric parameters at the time of ZY3-02 satellite overpass.  
The vertical atmospheric profiles of temperature, humidity, and pressure were collected with a radiosonde balloon at the times of the ZY3-02 satellite overpass. In fact, one of the most important sources of systematic error in the reflectance-based vicarious calibration approach is caused by the assumption of aerosol properties including size distribution, refractive index, etc., which determine the microphysical status of aerosol particles affecting the complex interaction between light and aerosol particles [21]. In remote-sensing applications, the aerosol properties are assumed empirically as several types, resulting in a lot of error in radiometric calibrations. However, AERONET directly produced aerosol properties specifically for this scenario, minimizing the error. The simulation of the radiative transfer model shows that the top of atmosphere spectral radiance is highly sensitive with different aerosol properties in a high-aerosol optical depth condition. In other words, the assumption of aerosol properties would bring a large error in predicting the top of atmosphere spectral radiance [22; 23]. In order to reduce the uncertainty caused by the aerosol properties assumption, we used the inversion products of the AERONET Baotou site. AERONET collaboration provides the aerosol inversion products (such as size distribution, refractive index, phase function, and asymmetry factor) of the Baotou site. The aerosol inversion products from AERONET collaboration would be taken as the input of a radiative transfer model to calculate the band-specific TOA radiance in this study, to overcome the abovementioned problems of the reflectance-based approach.  
Figure 8: The measurement of atmospheric parameter of AERONET Baotou site.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Atmospheric Parameters** & **Synchronous Measurement** \\ \hline AOD @ 550 nm & 0.1276 \\ \hline CWV & 0.8763 g/cm\({}^{2}\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Synchronous measurement of atmospheric parameters.