From the viewpoint of earth system science, this paper discusses the observation capability of the second-generation of Chinese polar-orbiting, sun-synchronous operational meteorological satellite observation systems, Fengyun-3 (FY-3), based on the function and performance test results from the FY-3D satellite observation system in orbit. The FY-3 series of satellites have numerous remote sensing instruments and a wide range of imaging and sounding electromagnetic spectrometers onboard. These instruments can obtain reflectivity data for land surface, soil, vegetation, water body, snow cover, ocean color, and sea ice on earth’s surface over a wide spectral range, as well as information on the absorption and scattering radiative transfer of molecules and particles (clouds and aerosols) in earth’s atmosphere. All of these data can be used to retrieve physical and chemical information about the land, ocean, and atmosphere of the earth system. Comprehensive observation of the earth system by the FY-3 meteorological satellites is preliminarily realized.

The High-Spectral Infrared Atmospheric Sounder (HIRAS) is a Fourier transform spectrometer onboard the fourth polar-orbiting FengYun 3D satellite (FY-3D). The FY-3D HIRAS provides interferogram measurements of Earth view radiance spectra in three infrared spectral bands at 29 cross-track positions, each with a 2 x p 2 array of field of views (FOVs). The HIRAS level 1 radiance data cover the spectral bands from 650 to 1135 cm⁻¹ [long-wave (LW) band], 1210 to 1750 cm⁻¹ [mid-wave (MW) band], and 2155 to 2550 cm⁻¹ [short-wave (SW) band] with a spectral resolution of 0.625 cm⁻¹. The radiometric calibration algorithm and the methods of refining the nonlinearity (NL) and the polarization correction coefficients on orbit are summarized in this article. The NL correction coefficients are derived by minimizing the spread of the responsivity functions derived from the measurements of the internal calibration target with varying temperatures. The polarization correction coefficients are derived from the cold space observations and the routine Earth scene measurements. The radiometric accuracy is assessed by comparing the HIRAS measurements to the collocated Cross-track Infrared Sounder (CrIS) observations and radiance simulations. The results show that, compared to CrIS, the radiometric differences are about 0.3 and 0.7 K for the LW and MW bands, respectively, and 0.5 K for the CO absorption and window regions in the SW band. The consistency of the radiometric calibration among the four FOVs is estimated to be within 0.2 K for most of the spectral domain. Some remaining issues for the FY-3D HIRAS are also discussed.

Limited by the on-orbital calibration capability, scaling the measured radiance in accuracy and stability is challenging for the Earth observation satellites in the reflective solar bands (RSBs). Although the lunar calibration is a well-developed technique in the RSBs, limited work has been done for Chinese Earth observation satellites. To improve the on-orbital calibration performance, the advanced MEdium Resolution Spectral Imager (MERSI II), which is the primary payload of the fourth satellite of the Fengyun 3 Series (FY-3D), expands the space view angle of the imager in order to capture better lunar images. In this study, we propose an absolute radiometric calibration method based on the FY-3D/MERSI lunar observation data. A lunar irradiance model named ROLO/GIRO has been used together with the necessary data procedures, including dark current count estimation, single pixel irradiance calculation, and full disk lunar irradiance calculation. The calibration coefficients obtained by the lunar calibration are compared with the pre-launch laboratory calibration. The results show that the deviations between the two calibration procedures are in a reasonable range in general. However, a relatively high non-linear response was found in the low energy incidence for some detectors, which leads to the large deviation in the corresponding bands. This study explored an ideal and independent method to validate MERSI on-orbit radiometric performance. The lunar calibration practiced for MERSI also gave a valuable example that can be recommended to the other Chinese Earth observation satellites.

TanSat is an important satellite in the Chinese Earth Observation Program which is designed to measure global atmospheric CO ₂ concentrations from space. The first Chinese superhigh-resolution grating spectrometer for measuring atmospheric CO ₂ is aboard TanSat. This spectrometer is a suite of three grating spectrometers that make coincident measurements of reflected sunlight in the near-infrared CO ₂ band near 1.61 and 2.06 μm and in the molecular oxygen A-band (O ₂ A) at 0.76 μm. Their spectral resolving power (λ/Aλ) is ~19000, ~12800, and ~12250 in the O ₂ A, weak absorption band of molecular carbon dioxide band, and strong absorption of carbon dioxide band, respectively. This paper describes the laboratory radiometric calibration of the spectrometer suite, which consists of measurements of the dark current response, gain coefficients, and signal-to-noise ratio (SNR). The SNRs of each channel meet the mission requirements for the O ₂ A and weak CO ₂ band but slightly miss the requirements in a few channels in the strong CO ₂ band. The gain coefficients of the three bands have a negligible random error component and achieve very good stability. Most of the R-squared of gain coefficients model consist of five numbers of nine (e.g., 0.99999) after the decimal point, suggesting that the instrument has significant response linearity. The radiometric calibration results meet the requirements of an absolute calibration uncertainty of less than 5%.

TanSat is a key satellite mission in the Chinese Earth Observation program and is designed to measure the global atmospheric column-averaged dry-air CO₂ mole fraction by measuring the visible and near-infrared solar-reflected spectra. The first Chinese super-high-resolution grating spectrometer for measuring atmospheric CO₂ is aboard TanSat. This spectrometer is a suite incorporating three grating spectrometers that make coincident measurements of reflected sunlight in the near-infrared CO₂ band near 1.61 and 2.06 µm and in the molecular oxygen (O₂) A-band at 0.76 µm. The spectral resolving power (λ∕Δλ) values are ∼ 19 000, ∼ 12 800, and ∼ 12 250 in the O₂ A-band, and the weak and strong absorption bands of CO₂, respectively. This paper describes the prelaunch spectral calibration of the atmospheric carbon dioxide grating spectrometer aboard TanSat. Several critical aspects of the spectrometer, including the spectral resolution, spectral dispersion, and the instrument line shape function of each channel, which are directly related to producing the Level 1 products are evaluated in this paper. The instrument line shape function of the spectrometer is notably symmetrical and perfectly consistent across all channels in the three bands. The symmetry is better then 99.99 %, and the consistency in the worst case is better then 99.97 %, 99.98 %, and 99.98 % in the O₂ A, WCO₂, and SCO₂ bands, respectively. The resulting variations in the spectral calibrations and the radiometric response errors are negligible. The spectral resolution characterizations meet the mission requirements. The spectral dispersions have excellent consistency in the spatial dimension of each band, and there is good linearity in the spectral dimension of each band. The RMS errors of the fitting residuals are 0.9, 1, and 0.7 pm in the O₂ A-band, the WCO₂ band, and the SCO₂ band, respectively. Taken together, these results suggest that the spectral characterizations of the spectrometer aboard TanSat meet the mission requirements.

TanSat was successfully launched on December 22, 2016, and has been acquiring global measurements of CO₂ and O₂ spectral bands in reflected sunlight since early February 2017. The atmospheric carbon dioxide grating spectrometer (ACGS) is a spaceborne three-band grating hyperspectral spectrometer suite onboard TanSat. The ACGS is designed to measure high-spectral-resolution, coboresighted spectra of reflected sunlight within the molecular oxygen (O₂) A-band range from 0.758 to 0.778 μm and the weak and strong absorption bands of carbon dioxide (WCO₂ and SCO₂) ranging from 1.594 to 1.624 μm and from 2.042 to 2.082 μm, respectively. The spectral resolving power (λ/Δλ) of the ACGS is ~19,000, ~12,800 and ~12,250 in the O₂ A-band, WCO₂ band and SCO₂ band, respectively. The inflight radiometric calibration accuracy is better than 5%, which satisfies the required specification. The wavelength calibration accuracy of the O₂ A-band is ~0.19 pm, that of the WCO₂ band is ~0.27 pm, and that of the SCO₂ band is ~4.75 pm, all of which meet the 0.05 full-width at half-maximum (FWHM) requirement. The spectroscopic performance of the ACGS exceeds the mission requirements by a margin. The ACGS has noise levels that are comparable to or smaller than those observed during prelaunch testing, and the noise has remained stable in the three bands during inflight operations. The signal-to-noise ratio (SNR) levels of the three bands meet the specified requirements. As expected, the ACGS radiometric performance in the O₂ A, WCO₂, and SCO₂ bands was fairly good during its first 17 months inflight.