# Evaluation on Radiometric Capability of Chinese Optical Satellite Sensors
## 2 Materials and Methods
### Data  
As a typical representative of moderate-high resolution remote sensing sensors, the CCD cameras are VNIR imaging sensors onboard the Chinese environment and disaster monitoring and forecasting satellite constellation, which is called Huan Jing 1 in Chinese and abbreviated as HJ-1 (hereafter the CCD camera onboard HJ-1 satellite is written as HJ-1/CCD). The HJ shoulders heavy responsibility of environmental monitoring, disaster prevention and reduction in China, and has been in orbit for more than six years and remains in operation, despite a two-year design life. However, due to lack of an onboard calibration system to track HJ-1 sensors' on-orbit behavior throughout the life of the mission, vicarious calibration needs to be done every other time and the frequency is usually once a year and will be increased with the aging of instruments. Since the CCD cameras are not state-of-the-art instruments, the radiometric capability is not so stable that it does not change for the whole year. In order to obtain HJ-1/CCD data which are accurately and continuously calibrated, CRESDA performs the vicarious calibration measurements using the reflectance-based method at the Dunhuang calibration site, and releases the calibration coefficients once a year through its website at [[http://www.cresda.com](http://www.cresda.com)]([http://www.cresda.com](http://www.cresda.com)). The Feng Yun-3 satellite constellation, the second generation of Chinese polar-orbiting meteorological satellites with moderate-low resolution, is composed of three satellites: FY-3A, FY-3B, and FY-3C, respectively. FY-3A and FY-3C were launched on 27 May 2008 and 23 September 2013 by China National Satellite Meteorological Center (NASC), separately, in a sun-synchronous morning orbit with a local equator-crossing time of 10:30 a.m. in descending node, while FY-3B was launched on 5 November 2010 by NASC, in an afternoon orbit with an equator-crossing time of 1:30 p.m. in ascending node [15]. The MERSI and VIRR are the keystone payloads onboard FY-3. They are designed to enhance Chinese three-dimensional atmospheric sounding capability and global data acquisition capability. The MERSI images the Earth with 20 spectral bands covering from visible to long-wave infrared. The applications of MERSI include retrieval of true color image; monitoring cloud, ice, and snow coverage; monitoring ocean color; and detecting disasters such as flooding and wildfires [16]. The VIRR is a 10-channel VIS/IR multi-purpose imaging radiometer, with a spectral range from 0.43 \(\upmu\)m to 12.5 \(\upmu\)m. VIRR is the only instrument on the current FY-3 platform that can  
Figure 1: Location of the calibration site and a true color composite from Moderate Resolution Imaging Spectroradiometer (MODIS) imagery. provide high spatial resolution and narrow-band infrared window measurements, which are vital for cloud or fog detection and quantitative retrieval of cloud properties, surface temperature, and the earth radiation budget [17]. Annual field campaigns have been routinely carried out at China Radiometric Calibration Site (CRCS) since September 2008, and calibration coefficients is released or updated through its website at [[http://www.nsmc.cma.gov.cn](http://www.nsmc.cma.gov.cn)]([http://www.nsmc.cma.gov.cn](http://www.nsmc.cma.gov.cn)) [18]. The MODIS sensor was launched on 18 December 1999 aboard the Terra satellite and has been in operation for nearly 17 years, providing global data that are used to monitor long-term changes in the Earth system. The absolute calibration of MODIS reflective solar bands is based on the regular measurements from the Solar Diffuser and a Solar Diffuser Stability Monitor [19; 20; 21]. Another onboard calibrator, the Spectroradiometric Calibration Assembly, is used to track the VNIR band's spectral response shifts and bandwidth changes on-orbit [20]. MODIS also periodically views the moon through its space view port, which provides an additional point reference for the calibration [21].