# Radiometric Cross-Calibration of GF-1 PMS Sensor with a New BRDF Model
## 1 Introduction  
Radiometric calibration is to convert the digital number (DN) value of a satellite sensor to a physical characterization, such as apparent radiance or apparent reflectance, in the visible and near-infrared sensors. Radiometric calibration is a critical step and the basis of the quantification of remote sensing application. The radiometric calibration not only characterizes the status and degradation with the time of the satellite sensors after launch, but also establishes the physical relationship between multi-source remote sensing data and expands the data source of quantitative remote sensing. Therefore, high precision radiometric calibration plays an important role in the development of quantitative remote sensing. The calibration techniques can be divided into prelaunch calibration and on-orbit (post-launch) calibration. There are five types of on-orbit calibration methods--inter-calibration [1; 2], solar/lunar calibration [3; 4], vicarious calibration [5; 6], scene calibration [7; 8; 9], and cross-calibration [10; 11; 12; 13; 14; 15; 16; 17; 18; 19]. Cross-calibration is an important and widely used on-orbit calibration method, which can be used for most optical sensors. The cross-calibration uses high-precision reference satellite data to achieve radiometric calibration of the sensors to be calibrated; thus, it can be used with high frequency and low cost. Furthermore, cross-calibration could be used for historical image. The research on cross-calibration is going on for more than 20 years. [PERSON] et al. described a methodology that used airborne hyperspectral imagery as reference data to calibrate multiple satellite sensors [11]. [PERSON] et al. used the Railroad Valley Playa test site in Nevada to cross-calibrate the Earth Observation-1 Advanced Land Imager (EO-1 ALI), Terra MODIS, and IKONOS with respect to the Landsat 7 Enhanced Thematic Mapper Plus (ETM+) [12]. [PERSON] et al. presented the results from the cross-calibration of the ETM+ and ALI image pairs using two approaches [13]. One approach was based on image statistics, and the other was based on vicarious calibration. [PERSON] et al. performed cross-calibrations between Landsat 5 TM and Landsat 7 ETM+ on multiple dates [14]. [PERSON] et al. presented the results of TM and ETM+ during the tandem-orbit configuration in 1999 [15]. Eight tandem image pairs were processed in cross-calibration. [PERSON] et al. evaluated the radiometric calibration agreement between MODIS and ETM+ over an African pseudo-invariant calibration site, Libya 4. The uncertainties of the BRDF, spectral response difference, and water vapor column were accounted for in this cross-calibration [16]. The cross-calibration method is also used for Chinese satellites by researchers, in virtue of its unique characteristics and precision of the results. GF-1 satellite is the first satellite of the Chinese high-resolution satellite constellation, launched on 26 April 2013, and equipped with two 2 m resolution panchromatic/8 m resolution Multispectral (PMS) cameras and four 16 m resolution wide-field-of-view (WFV) cameras. The China Centre for Resource Satellite Data and Application (CRESDA, the official operational management agency of GF-1 satellite) is responsible for the calibration of GF-1 using site calibration method and the publication of calibration coefficients. Since the launch, many researchers have invested substantial effort in the radiometric cross-calibration of GF-1 sensors. [PERSON] et al. (2014b) carried a cross calibration between GF-1 WFV sensor, Shijian 9A satellite (SJ-9A) PMS, and Terra MODIS sensor [17]. [PERSON] et al. (2015) cross calibrated the GF-1 satellite with Landsat 8 OLI sensor image [18]. [PERSON] et al. (2016) proposed a cross-calibration method to cross calibrate the GF-1 satellite WFV sensor, taking Landsat 8 OLI sensor as the reference [19]. The USGS spectral library was used for spectral band adjustment factors (SBAF) correction.