Field Calibrations of On-Orbit Remote Sensors Based on CRCS Automatic Observing Systems for CAL/VAL

With the rapid development of remote sensing technology, a large number of satellites with a high-performance visible (VIS), near infrared (NIR) and thermal infrared (IR) detection capability have been launched into space. In China, the in-orbit field calibration is used to calibrate satellite instruments. Usually, this technique uses Gobi desert and plateau lake sites with a high altitude, a dry and clean atmosphere with few aerosol particulates, less human disturbance and uniform reflectance/temperature distribution and can reach high calibration accuracy. China has established two instrumented national radiometric calibration fields, called China Radiometric Calibration Sites (CRCS): a land surface site at Dunhuang Gobi and a water surface site at Lake Qinghai, for the absolute radiometric calibration of sensors. A number of instruments are deployed at these sites to gain accurate knowledge about atmospheric and earth surface conditions, which are needed to calibrate satellite instruments.

The CRCS Dunhuang site is located in the Gobi Desert in northwest China, about 35 km west of Dunhuang City, Gansu Province. Covering approximately 30 km × 30 km, the entire site is formed on a stable alluvial fan of the Danghe River and its surface consists of cemented gravel without vegetation. This site was chosen as one CRCS site due to its extremely homogeneous surface conditions. The center area (600 m × 600 m) of the site is designed for high spatial resolution visible/near-infrared (VIS/NIR) sensors such as the China-Brazil Earth Resources Satellite (CBERS) and Gaofen Satellite (GF) series. The extended large area (20 km × 20 km) is used for low spatial resolution sensors such as the Multichannel Visible and Infrared Scanning Radiometer (MVISR), Visible and Infrared Radiometer (VIRR), and Medium Resolution Spectral Imager (MERSI) onboard the Fengyun-1 and 3 (FY-1/3) series of polar-orbiting satellites. It is also used for the field calibration of the VIS/NIR channels on Chinese geostationary weather satellites (FY-2/FY-4 series). The field calibration for the FY series of satellites has been conducted operationally since 2001 for only the VIS/NIR channels. Due to the lack of onboard VIS/NIR calibrators, the in-orbit field calibration based on the CRCS Dunhuang site is still the primary method for China’s satellite sensors’ VIS/NIR channels, such as the FY series satellites, Haiyang (HY) series of Ocean Satellites, Disaster and Environmental Monitoring Satellites (HJ) and CBERS series satellites. In recent years, the CRCS Dunhuang site has also been used for FY satellite IR channel field radiometric calibration.

Lake Qinghai, the largest saline lake in China, is located in the northeast of the Tibet Plateau and is the most important CRCS site for infrared sensors. The results from several sets of field experiments from 1994 to 2015 at Lake Qinghai indicated that the natural conditions, such as water surface radiative characteristics and local atmospheric conditions were suitable for the in-flight absolute radiometric calibration of the thermal channels of earth observation remote sensors. The lake’s total area is 4473 km² and its perimeter is 360 km. The shape of the lake is like an egg; it is 109 km long from east to west and 65 km wide from north to south. The average depth of the lake is 19 m, with a maximum depth of 32.8 m. The elevation of the water surface is 3196 m above sea level, and the volume of the lake is 105 Gm³. Lake Qinghai has a plateau continental climate with plentiful sunlight. Its climate has four distinct seasons. It has a windy spring, a short dry and hot summer, a clear and cool autumn and a long wet and freezing winter. The shortest and the longest recorded periods of ice cover on the lake are 76 days and 138 days, respectively. The depth of the ice layer is 40 cm on average and 90 cm at maximum. The water surface temperature tends to be spatially uniform, with a variation of less than 1 K. It is naturally a good infrared radiant point. There is a hilly island in the middle of the lake and the satellite-ground synchronous observation area on the lake is located in the southeast of the island.

In order to obtain the continuous, automatic and operational field observing data of CRSC ground features, upgrading and updating of the surface and atmospheric parameters observing systems were carried out at CRCS Dunhuang and Lake Qinghai, include construction field observing station at CRCS Duhuang Site and upgrading buoys at Lake Qinghai. The construction of Dunhuang field observing station include house, observing filed, instrument platforms, data transmission systems, power supply, tower crane, road, safeguard facilities and so on. Many new designed and developed instruments for the new field station had been tested during CRCS2014 to CRCS2016 field campaigns. Especially, a field observing instruments’ onsite calibration lab, including a clean room with optical tables and integrating sphere radiation source was constructed at the CRCS Dunhuang automatic observing station.

During CRCS2015 and CRCS2016, a set of method evaluations to the field calibration based on automatic field observing data were carried out. Three basic field observing instruments: Automated Test site Radiometer (ATR), VIS-SWIR spectra-radiometer (VSSR) and High-precision solar radiometer (HSR) were deployed at CRCS Dunhuang automatic observing station to obtain the continuous, automatic and operational field observing data from June 2014 to now. The complete flow scheme of field calibration base on automatic observing data was designed and implemented. The field calibration method based on ATR, HSR and VSSR long-term automatic observing data was used to TERRA/AQUA MODIS and the calibration results were compared with the MODIS onboard calibration. The preliminary evaluation results of automatic field calibration showed that 70 days from Sep. to Nov. 2016, 7 times field calibrations were carried out, averaged 3 times per month. The field calibration averaged uncertainties were around 2% for most channels. This field observing station will be an open field test and exchange platform for sharing of observing data, research and infrastructure, promote exchanges and cooperation between the relevant disciplines and units.