Multitemporal cross-calibration of GF-1 WFV and Terra MODIS reflective solar bands

The Gao Fen-1(GF-1) satellite with WFV sensors onboard was launched on April 26, 2013, as part of Gao Fen earth observing system. The Terra Moderate Resolution Imaging Spectroradiometer (MODIS) has been operational for more than a decade. With its high accuracy onboard calibration system, the data from MODIS has become a critical component of numerous applications. Cross-calibration between the sensors is critical to bring the measurements from different sensors to a common radiometric scale. Because of the differences in the spectral reflectance function (SRF), the measured physical quantities can be significantly different while observing the same target. This paper focuses on evaluation the radiometric calibration agreement between GF-1/WFV and Terra/MODIS using the near-simultaneous and cloud-free image pairs over Dun huang pseudo-invariant calibration site in the visible and near-infrared spectral range. The SRF differences of the analogous WFV and MODIS spectral bands provide the opportunity to explore, understand, quantify, and compensate for the measurement differences between these two sensors. Assuming that the ground target is spectrally and temporally stable, a typical reflectance spectrum over the Dun huang site obtained by in-situ measurements was used to compute spectral band adjustment factors (SBAF) for the cross-calibration. No BRDF correction was applied to reduce the seasonal oscillations since the analysis were restricted to only near-nadir images. The cross-calibration was initially performed by comparing the top-of-atmosphere (TOA) reflectance between the two sensors. WFV band 4 has the presence of the water vapor and oxygen absorption features which is absent from the corresponding MODIS band 2 which made the WFV measured TOA reflectance lower than the MODIS's. Overall, the average percent differences were consistent to within 7%. The long-term cross-calibration results reflected that WFV sensor is stable since launch. Although the application of the techniques presented in this paper was focused on comparing near-nadir observations from WFV and MODIS reflective solar bands, it can be easily extended and applied to evaluate the differences between any other similar sensors.