Prepared by CNSA Agenda Item: WG.3 CALIBRATION INFRASTRUCTURE AND TYPICAL APPLICATIONS OF CHINA LAND OBSERVATION SATELLITES Li Liu Executive summary (corresponding to ca ½ a page) This report introduces the calibration facilities for china land observation satellites, including both radiometric and geometric calibration sites. With these facilities, the typical applications are performed. 1
Calibration infrastructure and typical applications of China land observation satellites CGMS-42 CNSA-WP-06 1. CALIBRATION INFRASTRUCTURE 2.1 Dunhuang Dunhuang radiometric calibration field is located in the west of the Dunhuang City, Gansu Province. Its geographical coordinates is N40.04 ~ 40.28 and E94.17 ~ 94.5, from east to west about 60km, north and south about 40km. The site is alluvial fan of the party river where uniform area is 25km 2, with sparse vegetation distribution. The selected site is located in the center of the entire field area, with the area of 400m 2, and center altitude of 1229m. (a)gf-1 image of Dunhuang (b)actual view of Dunhuang Gobi Fig 1 Dunhuang radiometric calibration field Dunhuang radiometric calibration site is constructed with black, gray and white gravel between the 0.2~8.0cm. According to information provided by the Dunhuang meteorological, atmospheric aerosol type is between the desert and the continental type, but closer to the desert type. The main factor affecting the Dunhuang site is the total atmospheric dust opacity. In addition, industrial pollution of Dunhuang area is smaller, and fewer emissions of sulfuric acid and nitrogen hydride material have smaller impact on the atmosphere transmittance. Dunhuang radiometric calibration field has been approbated by international for the on-orbit absolute radiometric calibration in the near infrared remote sensor. Several calibration field campaigns have been conducted successfully to CBERS, FY, Beijing-1, HY-1, ZY-3, GF-1 and other pieces of Chinese satellites. 2
2.2 Qinghai Lake CGMS-42 CNSA-WP-06 Qinghai Lake, largest inland lake and saltwater lake in China, is located in the western region of Qinghai Province. It s area is 4473km 2, the perimeter of the lake is 360km, the maximum depth is 38m, with an average elevation 3.196km. The water surface temperature is evenly distributed which make it a natural source of infrared radiation target. The climate is cold semi-arid steppe climate, atmospheric is clean and dry. There is an island at the center of the lake. The calibration field is located at the east-southern of the island, 36 41'N~36 45'N,100 22'E~100 30'E. Because the water is so deep that small changes in surface temperature,, with the changing less than 1K. Fig 2 HJ-1B IRS image of Qinghai Lake Qinghai Lake field has the reflectance of 3% to 5% in visible band, and about 1% in the near-infrared band. It belongs to the geological stability of the plateau inland lake which has the property of high altitude, low aerosol particles, the continental aerosol type. The aerosol optical thickness of Qinghai Lake is about 0.1. Qinghai Lake is an ideal field to carry out calibration campaign of thermal infrared sensors. It has been widely used in on orbit absolute radiometric calibration of thermal infrared channels of domestic and foreign remote sensing satellites, such as HJ-1B, CEBERS, and FY series. 2.3 Etuokeqi grassland Etuokeqi grassland in Ordos City, Inner Mongolia is taken as SAR satellite calibration field in China. It has completed the measurement of backscatter unipolar of S and X band. Ground type is mainly made up of grasslands and sandy loam, which has lower scatter properties and relatively stable. Distance range and azimuth range is greater 3
than 100km and 30km respectively, with an area of 130km 40km. Noise ratio between the calibration reference and background clutter signal is less than 30dB. Topography of the site is less than 20 meters. Etuokeqi grassland is taken as the calibration field of HJ-1C. For the calibration of GF-3, the test of C-band scattering characteristics and comprehensive analysis of the applicability of the field are needed. 2.4 Songshan China (Songshan) satellite remote sensing calibration field is located in the northwest of Dengfeng City, Henan Province. The field is composed of radiation and geometric stationary target. For radiation calibration, there are fan- resolution target, blade edge target, a large area of gray target and point source matrix et al. For geometry calibration, there are two straight lines from the east to west, with the artificial signs in black and white diagonal cross pattern. The length of mainline is 45-50km, while the auxiliary line is 25km. In the middle of each signs is GPS mandatory for the flag. The size of signs is 2m 2m, 3m 3m and 5m 5m. Fig 3 GF-1 image of Songshan stationary target calibration field Songshan calibration field is the first stationary target of on orbit calibration in China. It has been widely used in on-orbit radiometric calibration, dynamic range, resolution and MTF testing of ZY-3, ZY-1 02C and GF-1. he actual application results show that the field can satisfy the needs of on orbit radiometric and geometric calibration of spaceborne sensors with high-resolution. 2. GF-1/WFV ON-ORBIT RADIOMETRIC CALIBRATION 4
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. 2.1 Site calibration Site calibration of GF-1/WFV is conducted in June 22, 2013, the 23rd and the 26 th, based on the reflectance-based method. In-situ measurement was taken at gypsum field, Dunhuang Gobi field and South lake. Surface reflectance data was measured by the SVC HR-1024 and ASD FR spectroradiometer at the times before and after 30 minutes when GF-1 cross over the field. CE318 was used to measure the atmospheric optical parameters. Taken the in-situ data as input, the radiate transfer model was used to calculate TOA radiance of each channel. Combined the DN of the field, the calibration coefficients can be calculated. the first channel will get WFV camera equivalent reflectivity, aerosol optical depth, Uncertainty analysis of the site calibration shows that the uncertainty of absolute radiometric calibration coefficients of GF-1/WFV based on the Dunduang field is 6.39%. 2.2 Cross-calibration 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. The MODIS is selected as a reference sensor in the cross-calibration with GF-1 WFV. We 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. 15 pairs of satellite images from July 2013 to February 2014 are selected to conduct the cross-calibration. 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 5
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. The average percent differences were consistent to within 7%. The longterm cross-calibration results reflected that WFV sensor is stable since launch. 6