# Evaluation on Radiometric Capability of Chinese Optical Satellite Sensors
## 1 Introduction  
The stability and consistency of satellite sensors are critical for providing consistent data and products for the monitoring of Earth's resources and dynamics [1]. Remote sensing imagery is an effective tool for monitoring global environment and climate changes because of its long-term observations [2]. The quantitative applications using remote sensing are sensitive to the sensor's radiometric capability [3]. Discerning secular trends in geophysical processes is one goal of earth observation research, thus long-term data from remote sensing instruments with good performance is required [4]. Frequent calibration for on-orbit sensors is required. As [PERSON] [5] pointed out, radiometric calibration is a key radiometric characteristic required for understanding an instrument's performance. Sensors' radiometric capability can be expected to change during launch and on-orbit operations because of the space environmental varying and instrument aging [6]. In order to correct the changes, regular and reliable onboard absolute calibration are required to assure a sensor's radiometric capabilitywith good accuracy and stability. For those satellite sensors with a spaceborne calibration system, like Moderate Resolution Imaging Spectroradiometer (MODIS), spectral response shifts and bandwidth changes on-orbit can be tracked by an onboard calibrator. For some satellite sensors without onboard calibration calibrators, such as Advanced Very-High-Resolution Radiometer (AVHRR), they use vicarious calibration techniques once a month to provide full aperture calibrations with relatively high accuracy as an independent evaluation of the sensor performance [7]. However, most Chinese remote sensing satellite sensors lack onboard calibrators and radiometric calibration has been updated once a year based on a vicarious calibration procedure, which has a great influence on the application of the data. Since the polar orbiting meteorological satellite, the Feng Yun 1 (FY-1, the first satellite in FY series) launched on 9 July 1988, which signifies an unprecedented milestone in Chinese satellite remote sensing history, several series of Chinese optical satellites have been developed, like the China Brazil Earth Resource Satellite (CBERS), Huan Jing (HJ), Zi Yuan (ZY), Hai Yang (HY), and Gao Fen (GF). The FY series could effectively monitor typhoons, floods, forest and grassland fires, droughts, dust storms and other disasters, improve the ability of weather forecasting and climate change detection [8]. The HY series has been used to monitor the sea ice, sea surface temperature and wind field and predict catastrophic sea-states such as storm, tsunami, sea-ice, sea-fog and marine petroleum pollution [9]. The ZY-3 series is mainly used in cartography, DEM modeling and resources investigation [10]. The HJ series satellites provide important support for detection of surface water quality and atmospheric environment, and evaluation of environmental pollution or natural disasters [11]. In general, due to the continuous development of aerospace remote sensing, satellites have been successfully and widely applied in the fields of agriculture, forestry, land, water conservancy, urban and rural construction, environment, surveying and mapping, transportation, meteorology, ocean, earth science research, and so on. The continuity and consistency of Earth's surface measurements through time is the most crucial aspect in monitoring activities using these Earth-observing satellites. Due to the environmental factors of space and instruments aging, many of the changes will become apparent after decades or more of observation, so the sensors' attenuation should be updated in a timely fashion. However, unlike the MODIS, most Chinese remote sensing satellite sensors lack onboard calibrators. Radiometric updates through a vicarious calibration procedure occur once a year, as for the HJ and FY series, which greatly influences the application of data.