Global warming is a major problem, for which carbon dioxide (CO2) is the main greenhouse gas involved in heating the troposphere. However, the poor availability of global CO2 measurements makes it difficult to estimate CO2 emissions accurately. Satellite measurements would be very helpful for understanding the global CO2 flux distribution if the CO2 column-averaged dry-air mole fraction (XCO2) could be measured with a precision of 1–2 ppm (Baker et al., 2010). The Greenhouse Gases Observing Satellite (GOSAT) (Yokota et al., 2009; Yoshida et al., 2013; Kuze et al., 2014) was launched in 2009, followed by the Orbiting Carbon Observatory 2 (OCO-2) (Eldering et al., 2016; Crisp et al., 2017; Bösch et al., 2011) in 2014. Tansat, a Chinese Earth observation satellite dedicated to monitoring CO2, was launched in December 2016 and is the third satellite capable of monitoring greenhouse gases by hyper-spectral nearinfrared/shortwave infrared (NIR/ SWIR) measurement. The TanSat mission was supported by the Ministry of Science and Technology of China, the Chinese Academy of Sciences, and the China Meteorological Administration. TanSat is an agile, sun-synchronous satellite that operates in three observation modes—namely, the nadir, sun-glint, and target modes. The line of sight tracks the principal plain in nadir mode and the glint in sun-glint mode, which increases the incident signal level and guarantees high performance of the charge-coupled device (Liu et al., 2013a; Cai et al., 2014). The Atmospheric Carbon dioxide Grating Spectroradiometer (ACGS) was designed to measure near-infrared/shortwave infrared backscattered sunlight in the molecular oxygen A-band (0.76 μm) and two CO2 bands (1.61 and 2.06 μm) (Wang et al., 2014; Li et al., 2017; Zhang et al., 2017). The Cloud and Aerosol Polarization Imager (CAPI) measures in ultraviolet, visible, and NIR regions to improve the information on aerosol optical properties and the cloud mask for the CDS measurements (Chen et al., 2017a, 2017b; Wang et al., 2017).

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First Global Carbon Dioxide Maps Produced from TanSat Measurements

The Chinese global carbon dioxide monitoring satellite (TanSat) was launched successfully in December 2016 and has completed its on-orbit tests and calibration. TanSat aims to measure the atmospheric column-averaged dry air mole fractions of carbon dioxide (XCO2) with a precision of 4 ppm at the regional scale, and in addition, to derive global and regional CO2 fluxes. Progress towards these objectives is reviewed and the first scientific results from TanSat measurements are presented. TanSat on-orbit tests indicate that the Atmospheric Carbon dioxide Grating Spectrometer is in normal working status and is beginning to produce L1B products. The preliminary TanSat XCO2 products have been retrieved by an algorithm and compared to NASA Orbiting Carbon Observatory-2 (OCO-2) measurements during an overlapping observation period. Furthermore, the XCO2 retrievals have been validated against eight ground-site measurement datasets from the Total Carbon Column Observing Network, for which the preliminary conclusion is that TanSat has met the precision design requirement, with an average bias of 2.11 ppm. The first scientific observations are presented, namely, the seasonal distributions of XCO2 over land on a global scale.

The TanSat mission: preliminary global observations

The accuracy of atmospheric trace gas retrievals depends directly on the accuracy of the molecular absorption model used within the retrieval algorithm. For remote sensing of well-mixed gases, such as carbon dioxide (CO 2 ), where the atmospheric variability is small compared to the background, the quality of the molecular absorption model is key. Recent updates to the 1.6 µm and 2.06 µm CO 2 absorption model used within the Orbiting Carbon Observatory (OCO-2) algorithm are described and validated. A set of 164 atmospheric spectra from the Total Carbon Column Observing Network (TCCON) is used to compare three models, both previous and current versions of absorption coefficient tables (largely derived from recent multispectrum fitting analyses targeted specifically at these bands) as well as a recent model constructed to use the HITRAN 2012 compilation. Both spectral residuals and retrieved column-averaged CO 2 mixing ratios (XCO 2 ) are included in the comparison. Absorption coefficients based on the updated multispectrum fitting analyses provide residuals comparable to or smaller than either the previous version of the multispectrum fits or the HITRAN 2012-based model. For the 2.06 µm band the updated model finds noticeably lower residuals for low water content cases. It is found that apart from a scaling factor the prior and updated absorption models result in similar retrieved values of XCO 2 for the 2.06 µm band and a slightly different airmass dependence for the 1.6 µm band.

High accuracy absorption coefficients for the Orbiting Carbon Observatory-2 (OCO-2) mission: Validation of updated carbon dioxide cross-sections using atmospheric spectra

TanSat is the 1st Chinese carbon dioxide (CO2) measurement satellite, launched in 2016. In this study, the University of Leicester Full Physics (UoL-FP) algorithm is implemented for TanSat nadir mode XCO2 retrievals. We develop a spectrum correction method to reduce the retrieval errors by the online fitting of an 8th order Fourier series. The spectrum-correction model and its a priori parameters are developed by analyzing the solar calibration measurement. This correction provides a significant improvement to the O2 A band retrieval. Accordingly, we extend the previous TanSat single CO2 weak band retrieval to a combined O2 A and CO2 weak band retrieval. A Genetic Algorithm (GA) has been applied to determine the threshold values of post-screening filters. In total, 18.3% of the retrieved data is identified as high quality compared to the original measurements. The same quality control parameters have been used in a footprint independent multiple linear regression bias correction due to the strong correlation with the XCO2 retrieval error. Twenty sites of the Total Column Carbon Observing Network (TCCON) have been selected to validate our new approach for the TanSat XCO2 retrieval. We show that our new approach produces a significant improvement on the XCO2 retrieval accuracy and precision when compared to TCCON with an average bias and RMSE of -0.08 ppm and 1.47 ppm, respectively. The methods used in this study can help to improve the XCO2 retrieval from TanSat and subsequently the Level-2 data production, and hence will be applied in the TanSat operational XCO2 processing.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2020JD032794

Toward High Precision XCO2 Retrievals From TanSat Observations: Retrieval Improvement and Validation Against TCCON Measurements.

Compact hyperspectral imaging spectrometers with a wide field of view (FoV) have significant application value. However, the aberration field of such imaging spectrometers is sensitive, and varies for different wavelengths when reducing the spectrometer volume. It is difficult to explain the variation in the aberration field using traditional aberration theory. In this study, we extend the vector aberration theory (VAT) to the Offner imaging spectrometer. We deduce the expression of the aberration field decenter vector of the Offner spectrometer based on the real ray-trace method. Furthermore, we derive the expression of the third-order vector aberration of the system. Subsequently, we explain some common phenomena in the Offner imaging spectrometer. This new analysis method can provide useful guidance for designing a compact wide FoV Offner imaging spectrometer. With this new insight, we designed a compact wide FoV Offner imaging spectrometer with a freeform tertiary mirror. Compared to conventional spectrometers with the same specifications, the total length of the spectrometer decreased by 37%, and the volume by 75%. After the tolerance analysis, the freeform optics satisfied the existing machining technology. The analysis method presented in this paper furthers the designer’s understanding of the aberration field of the Offner imaging spectrometer. The method is significant in the design of a compact wide FoV Offner imaging spectrometer with freeform optics.

Analysis method of the Offner hyperspectral imaging spectrometer based on vector aberration theory.

To continuously monitor, identify, and classify a wide range of areas all day and satisfy the hyperspectral remote sensing requirements in disaster reduction, environment, agriculture, forestry, marine, and resource areas, the authors participated in a pre-research program for full spectrum hyperspectral detection in geostationary orbit. As part of the program, the authors designed a cryogenic infrared spectrometer working at the diffraction limit. Such spectrometer complied with prism dispersion, exhibiting a 120 mm long slit, 2.5-5 µm band range, and 50 nm minimum spectral resolution. The spectrometer should overtake a temperature variation of 143 K for its assembly temperature at 293 K and the working temperature at 150 K. Low-temperature invar and carbon fiber were adopted as the framework material. The spectrometer was composed of two reflective Zerodur mirrors and one CaF2 Fery prism. Compensation mounts were developed for the reflective mirrors, while a spring-loaded autocentering cryogenic lens mount was designed for a CaF2 prism. CaF2 material exhibits a large linear expansion coefficient, making its mount difficult to design. The alignment requirements of the system were described, and the calculations that ensure the lenses undergo both appropriate stresses and temperature differences were presented. Structural thermal optical performance analysis was also conducted to assess the degradation in optical performance caused by temperature variation to verify the overall optomechanical design.

Optomechanical design and simulation of a cryogenic infrared spectrometer.

Hyperspectral imaging spectrometers with a wide field of view (FoV) have significant application values. However, enhancing the FoV will increase the volume of the imaging spectrometer and reduce the imaging quality, so a wide-FoV spectrometer system is difficult to design. Based on the theory of off-axis astigmatism, we present a method that includes a "prism box," "partial anastigmatism," and a partial differential equation to solve the parameters of a freeform surface. In this method, a compact wide-FoV imaging spectrometer with a freeform surface is designed. The spectrometer is an Offner structure with two curved prisms as the dispersion elements. The primary mirror and tertiary mirror of the Offner spectrometer are an aspheric surface and a freeform surface, respectively, to correct the off-axis aberration of a wide FoV. The ratio of the slit length to the total length of the spectrometer is close to 0.4. In comparison to conventional spectrometers of the same specifications, the total length of the spectrometer is reduced by 40% and the volume by 70%. The compact imaging spectrometer has potential application in the field of space remote sensing. In addition, the design method of the spectrometer provides a reference for the design of other optical systems with freeform surfaces.

Design of a compact hyperspectral imaging spectrometer with a freeform surface based on anastigmatism

Carbon Observing Satellite (Tan-Sat) is the first satellite of China designed to monitor column-averaged atmospheric carbon dioxide (XCO2) by detecting gas absorption spectra of the solar shortwave infrared radiation reflected from the Earth’s surface and atmosphere. Two instruments are accommodated on Tan-Sat: the high resolution hyperspectral sensor for carbon observation grating spectrometer (HRHS-GS) and the cloud and aerosol polarimetric imager (CAPI). HRHS-GS will provide the space-based measurements of CO2 on a scale and with the accuracy and precision to quantify terrestrial sources and sinks of CO2. CAPI is used to identify the contamination by optically thick clouds and to minimize the impact of scattering by aerosol. These two instruments work together to collect global column CO2 concentrations with correction for cloud and aerosol contamination. The instrument design of HRHS-GS is presented. Ocean reflectivity and the incident radiation of the instrument for transverse electric and transverse magnetic polarizations in glint mode are discussed. The changes to glint mode operation are described. The spectral characteristics of HRHS-GS were determined through the laser-based spectral calibration. The onboard spectral calibration method based on spectrum matching is introduced. The availability was verified, satisfying the onboard spectral calibration accuracy requirement of better than Δλ/10 (Δλ is spectral resolution).

Development and characterization of Carbon Observing Satellite

This article first reviews the main characteristics of the High-Resolution Hyperspectral Sensor for carbon observation Grating Spectrometer (HRHS-GS) and discusses the impact of spectral resolution...

Yuquan Zheng

Papers

Optomechanical design and simulation of a cryogenic infrared spectrometer.

Design of a compact hyperspectral imaging spectrometer with a freeform surface based on anastigmatism

Analysis method of the Offner hyperspectral imaging spectrometer based on vector aberration theory.

Development and characterization of Carbon Observing Satellite

Laboratory spectral calibration of TanSat and the influence of multiplex merging of pixels