Solar spectral irradiance fluctuates periodically. MgII can describe solar activity and convert solar reference spectrum irradiance according to solar activity. In this paper, we first calculate the MgII, based on multiple solar spectra and construct a time-dependent MgII curve after considering the aggregation level of the curve. Then the error transfer formula of the MgII prediction value participating in the spectral time-varying calculation is derived. The requirement of absolute calibration accuracy is 3%, and the acceptable maximum prediction algorithm error need to below 2.24%. In the prediction algorithm, the Fourier first-class fitting is used for the principal components of the MgII curve, and the Fourier multi-class fitting is used for the high-frequency components. The order is determined by the fitting effect of the Fourier series at the end of the curve. Finally, MgII of the next 200 days and 3 years is predicted at 10 time points, and maximum average error between the true values and the predicted values is 1.863%(200 days) and 1.922%(3 years), which is less than the acceptable error of 2.24%. Predicting solar activity based on MgII is helpful for satellites to obtain accurate observation data.
In recent years, the form of site calibration has gradually evolved from manual measurement to automated mode. The National Satellite Meteorological Centre built up Dunhuang Site Automatic Observation Radiometric Calibration Operational System (DARCOS) in 2018, which achieved initial results. Ozone content is a parameter that has a relatively large influence on the visible light calibration of solar reflection channels. In automatic calibration systems, instruments for measuring ozone content are generally not involved, climate data is used instead. For the refined requirement of the algorithm of the DARCOS, in order to improve the forward accuracy near the Chappuis absorption band, by drawing on OMI ozone content products in 2013-2018, we performed seasonal analysis of Dunhuang ozone content data and built a yearly-collected data set model. On which it was found that in February, the monthly average of the concentration of the ozone column in the Dunhuang, fluctuated very obviously. Methods of correlation coefficient and machine learning random forest were applied respectively, to judge the correlation between meteorological parameters and the Ozone Content in February. The results: the ozone content in the Dunhuang region is negatively correlated with the temperature, average vapor pressure and minimum horizontal visibility.
Solar reflection band of typical in-orbit payload was calibrated by the use of the Dunhuang site automatic observation radiometric calibration operational system (DARCOS) in 2018. Several automatic observation instruments were installed at Dunhuang site. DARCOS integrated product generation, acquisition, archiving, publishing, retrieval, downloading, user hierarchical management and performance monitoring functions together. Automatic calibration algorithm for AQUAMODIS, JPSS1VIIRS, FY2GVISSR, FY3CVIRR and FY4AAGRI were developed. How to accurately measure the surface reflectance without human intervention and correct it to the payload viewing angle is an important subject needs to be solved. A surface reflectance algorithm based on surface radiance and total sky radiation was developed. The directionality calibration was performed using the micro-facet cosine linear kernel-driven (MICOKE) BRDF model. The reflectance was corrected by the site vicarious calibration campaign on August 7-9, 2018. The site correction factor exhibits two variations rules with increasing wavelength, which corresponds to the HIM detector design. The calibration results for 5 payloads between August 13 and December 31, 2018 were analyzed. Ratio of the apparent reflectance from space and ground base was used to judging the agreement. AQUAMODIS (1.02-1.15), JPSS1VIIRS (1.07-1.15), and FY3CVIRR(1.10 to 1.19) are in good agreement with the automatic observation products of the site. FY2GVISSR (1.25) and FY4AAGRI (1.07-1.31) show differences changing by time. As an efficient independent calibration method, automatic calibration could be an effective supplement to the in-orbit calibration.
Medium Resolution Spectral Imager (MERSI) is the key imaging sensor on board Fengyun-3 (FY-3), the second generation polar-orbiting meteorological satellites in China, currently operating on both FY-3A, FY-3B and FY-3C satellites. It has 20 spectral bands, including 19 reflective solar bands (RSBs) with center wavelengths from 0.41μm to 2.1μm and 1 thermal emissive band (TEB) with center wavelength 12μm, making observations at two spatial resolutions: 250 m (bands 1-5) and 1km (bands 6-20). The FY-3C has been launched in 23, Sept., 2013. The MERSI doesn't carry on-board calibration standards. To obtain RSBs radiometric responses, pre-launched field radiometric calibration test which is called Solar Radiation Based Calibration(SRBC) was taken in Dali in 27, Feb. to 2, Mar., 2013. For the SRBC measurement which the sun was the source of irradiance, MERSI viewed the reflected solar irradiance from a set of the sixteen reference spectral on panels with different reflective level. The uniformity, reflectivity and BRDF (Bidirectional reflection of distribution function) of sixteen reference panels were tested in advance. There are two kinds of calibration coefficient generation methods used in SRBC. One is similar as the Sea-WiFS pre-launch calibration method by Langley calibration. Besides this, we use a portable spectrometers produced by Analytical Spectral Devices inc. (ASD inc.) named FieldSpec 3 to measure the absolutely reflected radiance simultaneously. The calibrated spectrometers measured radiance could be as the reference radiance and the the calibration coefficient of the MERSI can be calculated. We called this method Calibration Based on Reference Instrument(CBRI). The results of these two methods are comparable. The CBRI results are less then 6% difference with Langley calibration method in most channels except water-vapor channels and channel 15. An non-linear feature of the most FY-3C/MERSI detectors was found for the first time. This phenomenon is even more obvious for the water-vapor channels. The second order coefficient determined by pre-launched calibration is quite useful to improve the on-obit calibration accuracy.
BRDF has numerous applications in on-orbit satellites vicarious calibration. The 2013 Dunhuang Gobi surface directional reflectance measurements experiment were held during Aug. 20 to Aug. 28. In order to match the spatial resolution (0.25-1.25km) of meteorological satellites, 3*3 sample points were selected covering the 10*10km area. All the data were measured during (3 hours before and after) the noon without taking into account the large sun zenith angle because of the lack of the satellite passing through. Totally 9 groups of directional reflectance (DREF) were measured by the use of ASD (350-2500nm), standard reference board and a portable DREF measurement system. At each point, DREF were measured by different observation zenith angle (0, 20, 40 and 60 degree) and azimuth angle (0, 45, 90, 135, 180, 225, 270, 315 and 360 degree) in 30 minutes. Different BRDF models were selected such as Walthall, Sine Walthall, Hapke, Roujean and Ross-Li. The model coefficients were derived corresponding to the observed data. The relative differences (RD) of the models with respect to the measured values were calculated. The accuracy of MCD43 products in the Julian day of 233 and 241 were also validated. Results showed that Ross-Li model had the smallest RD. The RD between the DREF from MCD43 products and the measured values were 10.26%(233) and 8.96% (241)@550nm, respectively.
After January 13, 2012, FY-2F had successfully launched, the total number of the in orbit operating FengYun-2 geostationary meteorological satellites reached three. For accurate and efficient application of multi-satellite observation data, the study of the multi-satellites normalization of the visible detector was urgent. The method required to be non-rely on the in orbit calibration. So as to validate the calibration results before and after the launch; calculate day updating surface bidirectional reflectance distribution function (BRDF); at the same time track the long-term decay phenomenon of the detector's linearity and responsivity. By research of the typical BRDF model, the normalization method was designed. Which could effectively solute the interference of surface directional reflectance characteristics, non-rely on visible detector in orbit calibration. That was the Median Vertical Plane (MVP) method. The MVP method was based on the symmetry of principal plane, which were the directional reflective properties of the general surface targets. Two geostationary satellites were taken as the endpoint of a segment, targets on the intersecting line of the segment's MVP and the earth surface could be used as a normalization reference target (NRT). Observation on the NRT by two satellites at the moment the sun passing through the MVP brought the same observation zenith, solar zenith, and opposite relative direction angle. At that time, the linear regression coefficients of the satellite output data were the required normalization coefficients. The normalization coefficients between FY-2D, FY-2E and FY-2F were calculated, and the self-test method of the normalized results was designed and realized. The results showed the differences of the responsivity between satellites could up to 10.1%(FY-2E to FY-2F); the differences of the output reflectance calculated by the broadcast calibration look-up table could up to 21.1%(FY-2D to FY-2F); the differences of the output reflectance from FY-2D and FY-2E calculated by the site experiment results reduced to 2.9%(13.6% when using the broadcast table). The normalized relative error was also calculated by the self-test method, which was less than 0.2%.