SITE SELECTION AND EVALUATION OF CHINA'S NEW RADIOMETRIC CALIBRATION TEST SITES

The ideal radiometric calibration site is the basis for achieving on-orbit radiometric calibration. At present, China mainly uses the Dunhuang national test site for on-orbit radiometric calibration. However, the Dunhuang national test site has a shortage of ground objects and surface damage, and it is urgent to select and evaluate the new calibration sites. By analyzing the calibration site selection requirements, three new radiometric calibration sites were selected in Dunhuang and Golmud. Obtaining high-resolution satellite image data of each calibration site and evaluating the spatial uniformity of the site on different spatial scales. The radiometric stability of the sites is evaluated using the long-term sequence of Landsat8 image data from 2013 to 2019. Atmospheric and surface tests were carried out at various sites in August 2019, and atmospheric information and ground spectral data were obtained to evaluate the spectral characteristics of the sites. The surface directional characteristics of the sites are evaluated by using MODIS images at different angles in sunny weather on adjacent dates. Obtaining MOD05 water vapor products from 2013 to 2019 of each calibration site and conducting monthly statistics. The number of sunny days at each calibration site is counted and the drought conditions of the site are evaluated. The results show that the three new radiometric calibration sites have uniform and stable surface characteristics and atmospheric characteristics, and are suitable for on-orbit radiometric calibration of satellite sensors, which provides a reference for the location and evaluation of future calibration sites.


Introduction
With the in-depth application of different series of satellites such as meteorological satellites, resource satellites, marine satellites, environmental satellites and mapping satellites in various industries, the demand for quantitative remote sensing is becoming more and more urgent. Radiometric calibration is the premise of quantitative remote sensing, and its accuracy directly affects the reliability and accuracy of quantitative application of remote sensing data [1] . Radiometric calibration not only characterizes the state of satellite sensors after launch and degradation over time, but also establishes the physical relationship between multi-source remote sensing data and expands the quantitative remote sensing data source [2] . Therefore, high-precision radiometric calibration plays an important role in the development of quantitative remote sensing, and the ideal radiometric calibration site is the basis for achieving on-orbit radiometric calibration.
At present, China mainly uses the Dunhuang national test site for on-orbit radiometric calibration of satellite sensors. The Dunhuang national test site has the advantages of flat ground, single type of land, large area, and clean atmosphere. Since the 1990s, many sensors have been successfully calibrated on-orbit and recognized internationally. It is one of the recommended on-orbit calibration sites [3] . However, the type of the Dunhuang national test site is too single to be able to carry out multi-point calibration and authenticity test, and the test site is affected by natural weather such as sandstorms and cloud rain, and the number of calibrations is limited [4] . In addition, with the development of ground synchronization tests in the Dunhuang national test site for many years, the surface damage has become increasingly serious, and the surrounding human activities have gradually increased. It is urgent to establish new calibration sites to meet the increasing demand for on-orbit calibration and authenticity test in China. Therefore, the site selection work was carried out in Dunhuang and Golmud. In the Dunhuang area, the Dunhuang new test site was selected, and in the Golmud area, the Dazaohuo east and the Dazaohuo west were selected. In August 2019, atmospheric and surface tests were carried out on each site, and the characteristics of the sites were evaluated using ground-measured data and historical satellite images.
The first section of this paper gives an overview of the current status and existing problems of the calibration sites; The second section introduces the site selection principles and site characteristics evaluation indexes; the third section introduces the radiometric calibration sites; the fourth section carries out the analysis of the evaluation indexes; the final section gives the relevant conclusions.

Calibration site selection principles and site characteristics evaluation indexes
The selection of the radiometric calibration site is the key link in the on-orbit calibration, which directly affects its accuracy. In order to reduce the uncertainty of each link in the on-orbit calibration process, it is necessary to consider the surface characteristics and atmospheric characteristics of the site [5] . Site surface characteristics include spatial uniformity, radiometric stability, spectral characteristics and surface directional characteristics; site atmospheric characteristics include site water vapor content, the number of sunny days, and so on. The site selection principle of the radiometric calibration site and the site characteristics evaluation indicators are as follows: (1) Spatial uniformity The test site with good spatial uniformity can not only meet the needs of different spatial resolution satellite calibration, but also reduce the influence of poor registration; it can also reduce the mixed pixel error caused by ground measurement instruments. The spatial uniformity of the site selects the high-resolution satellite full-color image of the test area, and the uniformity is evaluated by the coefficient of variation of the site gray value.
(2) Radiometric stability The surface of the test site should be as uniform as possible, usually requiring no vegetation cover, such as desert and Gobi. Using the time series Landsat8 satellite imagery, the apparent reflectance of different phases is calculated to achieve radiometric stability evaluation.
(3) Spectral characteristics The reflectance spectral curve of the calibration test site is smooth with no obvious peaks and troughs. This can reduce the spectral response matching error of the sensor and facilitate the cross calibration of the satellite sensor.
The measured spectra of different test sites in the same time period were compared to evaluate the spectral characteristics.  The calibration test site should select arid areas, which can not only reduce the change of surface reflectance caused by rainfall, but also have more sunny days to test. Using the time series water vapor content of MODIS, the monthly average water vapor content is obtained; the historical weather query is used to count the number of sunny days at each site.

Calibration sites introduction
(1) Dunhuang national test site Since the end of the 1990s, the Dunhuang national test site has been used for calibration tests of Chinese satellites. It is located 15km west of Dunhuang City, and the site is large, flat and homogeneous. The coordinates are 40.093°N and 94.394°E, and the altitude is 1253m.
There are very few disturbances and low aerosol loads from the clouds, no vegetation on the ground, composed of sand and small black stones [6] . In addition, the surface reflectance spectrum is relatively stable within the dynamic range of the optical satellite sensor. Therefore, the Dunhuang national test site has been widely used for the calibration of GF-1/PMS, WFV, CBERS-02/CCD, FY-3A/MERIS, HJ-1A, 1B/CCD and other sensors [7] .
(2) Dunhuang new test site As there are solar power stations near the Dunhuang national test site, this will have an impact on the calibration tests. In this paper, Landsat8 image data is used to determine a most uniform area of 5km×5km in   Table 1.   Table 2.   In Figure 3, the abscissa is the number of days    Table 3 -Table 6.  It can be seen from Table 3 and Table 4 that the relative differences between back scattering and vertical scattering in each band in Table 3 Table 3 are 18.30%, 6.95%, 3.54%, 4.61%, while the relative differences in Table 4 Figure 2), resulting in an abnormal apparent reflectance.
From the forward scattering and vertical scattering in Table 3 to Table 6, it can be seen that the relative difference in apparent reflectance of the blue band, green band, red band and near infrared band are 18.30%~19.05%, 6.95%~8.31%, 3.10%~6.09%, 4.50% ~7.41%. The relative difference in apparent reflectance of the blue band is large, and BRDF correction is required; the apparent reflectance difference of the green band, the red band, and the near infrared band is less than 8.5%. The relative differences in apparent reflectance of the four bands in each site have the same trend, the blue band is the largest, the red band is the smallest, and the Lambertian properties of each site are similar.

Site water vapor content
Using the time series of MODIS water vapor content products from 2013 to 2019, the variation of water vapor content was calculated to achieve quantitative evaluation of water vapor content at each site. The results are shown in Figure 5 (a-d) and Table 7.  It can be seen from Figure 5 that in the different months, the monthly average water vapor content of each site increased first and then decreased, with the highest content in July and August, the lowest content in January  Figure 6 (a-b) and Table 8 -Table 9.  It can be seen from Figure 6  It can be seen from  In summary, the newly selected three radiometric calibration sites are superior to the Dunhuang national test site in terms of spatial uniformity and radiometric stability; each site has similar spectral characteristics and surface directional characteristics, requiring BRDF correction.
The Golmud area is drier than the Dunhuang area, and the number of sunny days in the Dunhuang area is higher than in the Golmud area. The newly selected three sites meet the requirements of the radiometric calibration site and are suitable for on-orbit radiometric calibration of satellite sensors, which provides a reference for the selection and evaluation of future calibration sites.