TanSat (Chinese Carbon Dioxide Observation Satellite Mission)
The TanSat (CarbonSat, Tan means "carbon" in Chinese) mission is the first minisatellite of China dedicated to the carbon dioxide (CO2) detection and monitoring. The project was proposed in the Chinese national program in 2010, and officially kicked off in January 2011. TanSat is funded by MOST (Ministry of Science and Technology) of China.
• A high-resolution Carbon Dioxide Spectrometer for measuring the near-infrared absorption by CO2
• CAPI (Cloud and Aerosol Polarimetry Imager) to compensate the CO2 measurement errors by high-resolution measurement of cloud and aerosol
• A spacecraft equipped with the two instruments, capable of performing scientific observations in multiple ways as mission required
• A ground segment which receives observation data and retrieves the atmosphere column-averaged CO2 dry air mole fraction (XCO2), and performs data validation by ground-based CO2 monitoring.
CAS (Chinese Academy of Sciences) undertakes the leading role of funding in the satellite development, with two scientific instruments designed and manufactured by CIOMP/CAS (Changchun Institute of Optics, Fine Mechanics and Physics/Chinese Academy of Sciences), located in Changchun, China. The satellite platform is developed by SIMIT (Shanghai Institute of Microsystems and Information Technology), which is also responsible for the overall satellite assembly, integration and testing. NSMC (National Satellite Meteorological Center) of CMA (China Meteorological Administration) is responsible for the ground segment and final data products. - The project includes also international partners: The University of Leicester and the University of Edinburgh, UK.
The main objective of the TanSat mission is to retrieve the atmosphere column-averaged CO2 dry air mole fraction (XCO2) with precisions of 1% (4 ppm) on national and global scales. The scientific goal of the project is to improve the understanding on the global CO2 distribution and its contribution to the climate change, and also to monitor the CO2 variation on seasonal time scales.
The following system requirements pertain to the TanSat mission:
1) The TanSat satellite shall fly in a sun-synchronous orbit at 13:30 hours, with a revisit period shorter than 16 days; the deviation of the equator crossing time shall be shorter than 15 minutes during 3 years.
2) The TanSat satellite shall be able to carry out nadir observations, sun-glint observations, and target observations.
3) The TanSat satellite shall be able to perform in-orbit spectrometric calibration and radiometric calibration for both instruments, Carbon Dioxide Spectrometer and CAPI, with an absolute accuracy of 5%, and a relative accuracy of 3%.
4) The localization accuracy of each instrument shall be better than one pixel.
5) The observation data shall be transmitted to the ground within eight hours.
6) The atmosphere column-averaged CO2 dry air mole fraction (XCO2) shall be retrieved with precisions of 1% (4 ppm).
7) A ground-based CO2 monitoring network shall be set up, with CO2 detection accuracy of 0.2 ppmv.
8) The TanSat satellite shall operate in orbit for at least three years.
Table 1: "Special Topic: Greenhouse Gas Observation From Space: Theory and Application," 7)
The TanSat minisatellite is designed and developed at SIMIT (Shanghai Institute of Microsystems and Information Technology). The configuration of TanSat is as shown in Figure 1. The payload is accommodated on the +Xs side of the platform, with the instrument boresight pointing to the +Zs. The solar arrays are on the ±Ys side of TanSat, with the solar cells always pointing to the –Xs after deployment. The overall volume of the TanSat satellite is about 150 cm (Ys) x 180 cm (Zs) x 185 cm (Xs). The launch mass of TanSat is about 620 kg (including 10 kg propellant), the design life is 3 years.
The pointing strategy of TanSat is rather complex due to the different requirements of the observation tasks. TanSat is designed to have eleven pointing modes as defined in Table 3. The pointing modes from No.1 to No.9 are designed with regard to the requirements of different observation tasks specified in the above. The Forward Nadir Pointing is also designed as a nominal pointing in the umbra times. The Solar Panel Pointing is dedicated to battery charging, especially in satellite safe mode. 10)
Figure 1: Configuration of the TanSat spacecraft (image credit: SIMIT)
Table 3: Summary of TanSat pointing modes
AOCS (Attitude and Orbit Control Subsystem): TanSat is a three-axis stabilized spacecraft. The architecture of the attitude control subsystem includes:
• Actuators: 4 reaction wheels, 4 torque-rods, 4 hydrazine thrusters
• Sensors: 3 sun sensors, 2 three-axial magnetometers, 2 star trackers, 2 gyros, 1 GPS receiver.
AOCS exploits these sensors and actuators and is capable of attitude determination and stabilized mission pointing specified as follows:
• Attitude pointing accuracy: ≤ 0.1º
• Attitude measurement accuracy: ≤ 0.03º
• Pointing stability: ≤ 0.001º/s.
EPS (Electric Power Subsystem): The power subsystem manages the generation, storage and distribution of the electrical power needed by the spacecraft subsystems. By different combinations of observation tasks during one orbit, there are many different cases of power consumption and generation for the onboard battery. Based on analysis of all possible combinations, the TanSat is equipped with solar panels of 10 m2 in size, which ensures an end of life power generation of 1790 W. A high performance Li-battery is selected for power storage, with an overall capacity of 80 Ah. The nominal bus voltage is 28 V.
OBDH (OnBoard Data Handling) subsystem: TanSat has a centralized high performance data handling system, with a TSC695F CPU for central command control. The onboard computer controls and commands the activities of all the other satellite subsystems, and fulfils functions including flight dynamics control, AOCS data processing, payload data management, telemetry control, power and thermal control, and so on. The onboard computer communicates with the payload computer and other subsystems through the CAN bus. Figure 2 shows the EM (Engineering Model) of the TanSat onboard computer.
Figure 2: Engineering Model of the OBC (image credit: SIMIT)
Propulsion subsystem: TanSat features a propulsion subsystem to maintain the nominal orbit during the satellite lifetime, especially in terms of the orbit height and the equator crossing local time. Four 1N hydrazine thrusters are placed on the –Xs panel of the platform. A 20 liter capacity tank is selected with 10 kg propellant.
TT&C (Tracking, Telemetry and Telecommand) subsystem: An S-band TM/TC link provides the control and telemetry between TanSat and the ground station for satellite monitoring and control. The subsystem provides bidirectional and full-duplex communication, with an uplink data rate of 2 k bit/s and a downlink data rate of 8.192 kbit/s.
The onboard storage and communication subsystem stores the observation data transmitted through an LVDS (Low Voltage Differential Signaling) interface by the payload, and transmits data to the ground stations via an X-band link with a data rate of 64Mbit/s. The capacity of onboard storage is 128 Gbit. Other than an X-band antenna along the +Zs axis, the satellite uses a second tilted antenna to increase the on-board-to-ground communication time during Sun-glint observations, as shown in Figure .
Figure 3: Schematic view of the X-band antennas (image credit: SIMIT)
Figure 4: Photo of the TanSat Chinese Carbon Dioxide Observation Satellite (image credit: NRSCC, ESA) 11)
TanSat project status/milestones:
• Dec. 2014: planned CDR (Critical Design Review)
• July 2014: Electromechanical integration
• June 2013: Start of Phase C
• March 2013: PDR (Preliminary Design Review)
• Sept. 2011: SRR (Science Requirement Review)
• Feb. 2012: Kick-off of project
China is the third country after Japan and the United States to monitor greenhouse gases through its own satellite.
• March 2016: According to correspondence with Prof. Liu Yi of IAP/CAS, there were several occasions for discussions with NASA Management to integrate the TanSat mission into the NASA-managed international A-Train constellation of Earth-observing satellites. It would make sense to have the TanSat mission of China and the OCO-2 (Orbiting Carbon Observatory-2) mission of NASA in the same constellation. However, after discussions with Michael Freilich, Earth Science Director of NASA, who attended a TanSat workshop at IAP/CAS, Beijing, in Sept. 2015, the TanSat project decided to drop the A-Train option, due to the complicated requirements and operational procedures for all participants in the A-Train. 15)
- Results and outlook: The TanSat project has the possibility to cooperate with the participants in the A-Train, since the orbits of the A-Train and the TanSat mission are very close to each other.
Orbit: Sun-synchronous orbit, altitude of ~ 700 km, inclination = 98.2º, LTAN (Local Time on Ascending Node) = 13:30 hours. The revisit period is 16 days.
Figure 5: Artist's rendition of the deployed TanSat spacecraft (image credit: TanSat collaboration)
Secondary payloads: The launch vehicle carried also a high-resolution microsatellite and two spectrum nanosatellites for agricultural and forestry monitoring (Ref. 12).
Figure 6: Initially proposed orbit of the TanSat spacecraft in the international A-Train (Afternoon Constellation) of Earth-observing satellites, managed by NASA (image credit: IAP/CAS)
• April 13, 2018: TanSat has produced its first global carbon dioxide maps. TanSat was launched by a collaborative team of researchers in China, and these maps are the first steps for the satellite to provide global carbon dioxide measurements for future climate change research. The researchers published the maps, based on data collected in April and July 2017, in the latest edition of the journal Advances in Atmospheric Sciences, a Springer journal. 16) 17)
After TanSat was launched in December 2016, in-orbit and calibration tests were completed in the summer of 2017, and the performance of the instrument has since been evaluated in test sessions. TanSat XCO2 retrieval algorithm was developed based on IAPCAS (Institute of Atmospheric Physics Carbon dioxide retrieval Algorithm for Satellite) remote sensing. IAPCAS also informed the development of ATANGO (Application of TanSat XCO2 Retrieval Algorithm in GOSAT Observations). Its retrieval accuracy and precision have been validated by Total Carbon Column Observing Network (TCCON) measurements, and the retrieval product has been applied in estimations of carbon flux inversion in China.
The retrieval relies on mathematical and physical models to approach XCO2 from hyperspectral measurements restored in the L1B data. The result is the best estimate after comparison between the simulated satellite-received spectrum and measurements. The first global XCO2 maps based on TanSat measurements show the global distribution over land in April and July 2017 (Figure 7).
Based on the maps, a seasonal decrease in CO2 concentration from spring to summer in the Northern Hemisphere is obvious, and results from a change in the rate of photosynthesis. This effect is also reflected in the XCO2 gradient between the Northern Hemisphere and Southern Hemisphere shown in Figure 7a. Emission hotspots due to anthropogenic activity, such as industrial activity and fossil fuel combustion, are clearly evident in eastern China, the eastern United States, and Europe, as reflected by the relatively high levels of XCO2.
Figure 7: Global XCO2 maps produced from TanSat in nadir mode in (a) April and (b) July 2017. The colored marks indicate the XCO2 values and the color scale bar is shown at the bottom of each figure (image credit: TanSat Team)
Outlook: There are still gaps in the TanSat measurements between the footprints of each orbit, and these missing measurements also continue to impact on the carbon flux inversion estimation.18) The gaps can be filled by using both OCO-2 (Orbiting Carbon Observatory-2) mission of NASA and TanSat XCO2 measurements because the footprint tracks are almost parallel and interlaced, such that OCO-2 provides an additional measurement track between two TanSat tracks. This improves the spatial coverage significantly when compared with the use of a single satellite (i.e., either OCO-2 or TanSat). Accuracy and precision of XCO2 data are essential for the joint application of OCO-2 and TanSat data. Hence, research focusing on the validation of satellite in-orbit calibration and retrieval algorithms is required to evaluate their precision and reduce the bias associated with their use.
Sensor complement: (CarbonSpec, CAPI)
TanSat is equipped with two instruments: Carbon Dioxide Spectrometer and CAPI. To enhance the system efficiency and reliability, the two instruments are integrated into a common structure and electronics device, sharing one common electrical box (Figure 8).
Figure 8: Illustration of the two instruments into a common structure (image credit: CIOMP)
CarbonSpec (Carbon Dioxide Spectrometer):
CarbonSpec, also referred to as CDS (Carbon Dioxide Spectrometer) is a high-resolution grating spectrometer dedicated to CO2 detection by measuring the near-infrared absorption of CO2 at 1.61 µm and at 2.06 µm, and the molecular oxygen (O2) A-band in reflected sunlight at 0.76 µm. The resolving power in the A-band is near 21,000, while that in the CO2 bands is near 12,000. The footprint size is ~2 km x 2 km and the swath is 20 km wide at nadir.
As shown in Figure 8, CarbonSpec is composed of a pointing subsystem, a telescope subsystem, a beam splitter subsystem, a diffraction grating spectrometer subsystem, and an imaging subsystem. Figure 9 and Table4 show the optical schematics and specifications of the Spectrometer, respectively.
Figure 9: Schematic view of CarbonSpec instrument (image credit: CIOMP)
Table 4: Specification of the CarbonSpec observation parameters
The pointing subsystem is a special design of the CarbonSpec. It includes a pointer mirror, one side of which directly reflects the light from the ground to the telescope subsystem, and the other side of the mirror can diffusely reflect the incoming light for radiation calibration of the instrument. The pointer mirror is fixed on a one-dimensional rotation device which can rotate from 0 to 360º. By adjusting the rotation angle, the spectrometer is able to detect the target located from -30º to 10º in cross-track.
Figure 10: Illustration of the pointing subsystem (image credit: CIOMP)
Figure 11: Schematic of the CDS (Carbon Dioxide Spectrometer) and CAPI assembly (image credit: TanSat collaboration, Ref. 9)
CAPI (Cloud and Aerosol Polarimetry Imager):
The CAPI instrument is a wide FOV (Field of View) moderate resolution imaging spectrometer with polarization channels, used to compensate errors which are caused by clouds and aerosols based on observation in the following spectral bands:
- Ultraviolet: 0.38 µm
- Visible: 0.67 µm
- Near infrared: 0.87, 1.375 and 1.64 µm
Other than the cloud and aerosol detection in the various wavelength bands, CAPI is designed to obtain polarization observation data at 0.67 µm and 1.64 µm in three angles, so as to enhance the retrieval accuracy of the clouds and of aerosols.
CAPI uses six lenses to realize the cloud and aerosol detection in nine channels at 5 spectrum bands. Lens 1 (0.38 µm), lens 2 (0.87 µm and 0.67 µm @ 0º), and lens 3 (0.67 µm @ 60º&120º) constitute the VNIR (Visible Near Infrared) spectrum. Lens 4 (1.375 µm and 1.64 µm), lens 5 (1.64 µm @ 60º), and lens 6 (1.64 µm @ 120º) constitute the SWIR Short Wave Infrared) spectrum.
Figure 12: Illustration of the CAPI structure (image credit: CIOMP)
Table 5: Specification of the CAPI spectral parameters
The CAPI instrument will record images in 5 spectral channels (0.38, 0.67, 0.87, 1.375, and 1.64 µm) with a spatial resolution of 0.5 km over a 400 km wide swath. The 0.67 and 1.64 µm channels sample 3 independent polarization angles. Soundings recorded by the spectrometer will be used to retrieve XCO2, while data from CAPI will be used to correct cloud and aerosol interference. The target accuracy of the CO2 measurements is 1 to 4 ppm on regional scales (500 km x 500 km) and monthly time scales. The current plan is to validate these results against a comprehensive, multi-site ground based measurement network in China as well as other internationally-recognized standards.
Cloud detection is an essential preprocessing step for retrieving carbon dioxide from satellite observations of reflected sunlight. During the pre-launch study of TanSat, a cloud-screening scheme was presented for CAPI (Cloud and Aerosol Polarization Imager), which only performs measurements in five channels located in the visible to near-infrared regions of the spectrum. The scheme for CAPI, based on previous cloud-screening algorithms, defines a method to regroup individual threshold tests for each pixel in a scene according to the derived clear confidence level. This scheme is proven to be more effective for sensors with few channels. 19)
The work relies upon the radiance data from the VIRR (Visible and Infrared Radiometer) onboard the Chinese FY-3A (FengYun-3A) polar-orbiting meteorological satellite, which uses four wavebands similar to that of CAPI and can serve as a proxy for its measurements. The scheme has been applied to a number of the VIRR scenes over four target areas (desert, snow, ocean, forest) for all seasons. To assess the screening results, comparisons against the cloud-screening product from MODIS are made. The evaluation suggests that the proposed scheme inherits the advantages of schemes described in previous publications and shows improved cloud-screening results. A seasonal analysis reveals that this scheme provides better performance during warmer seasons, except for observations over oceans, where results are much better in colder seasons.
Both instruments collect observation data in pushbroom mode. To meet the mission requirements, the TanSat spacecraft carries out different observation tasks in different scanning configurations. As shown in Figure 13, TanSat is able to perform the following 5 observation modes:
• Nadir mode: observation over land
• Sun-glint mode: observation over the ocean
- Sun glint track
- Principle plane track
• Target mode: observation validation
- Surface target track
- Multi angles for one target.
Figure 13: TanSat nadir observation mode (image credit: TanSat Team)
Figure 14: TanSat sun glint observation mode (image credit: TanSat Team)
Figure 15: TanSat target observation mode (image credit: TanSat Team)
Figure 16: Payload reference frame (image credit: SIMIT, Ref. 10)
• Spectral calibration accuracy: superior to 1/10 FWHM
• Radiometric calibration accuracy: 3%(relative), 5%(absolute) (also for CAPI)
• OBC (OnBoard Calibrator):
- CO2 Spectrometer : LED + solar calibration
- CAPI : LED +lunar + solar calibration.
Nadir observation: In the nadir observation mode, the satellite observes the sub-satellite track by receiving the sunlight reflected of Earth's surface, as shown in Figure 17. In this mode, the satellite can obtain the most of the high-resolution data. It is the nominal mode for CO2 detection of the land surface, since the reflection ratio of land is relatively high compared to the ocean surface.
Figure 17: Schematic view of nadir and sun-glint observations (image credit: CIOMP)
For TanSat, there are two kinds of nadir observation:
1) Nominal nadir observation - scanning the ground surface with the normal of instrument CCD lines orienting along the flight direction, forward or backward. Due to the constraint of satellite design, the satellite needs to quickly slew 180º around the yaw axis while flying over the sub-solar point to keep the sunlight on the side of the solar panel. This slew imposes the most strict maneuver requirement for the satellite: 180º in five minutes (around yaw axis).
2) Principle-plane nadir observation - scanning the ground targets with the instrument boresight within 5º of the principal plane, which is a plane defined with respect to the instrument aperture, the surface target, and the sun. While the satellite carries out the principle-plane nadir observation as it flies from the south to the north, it slowly slews for about 160º around the yaw axis.
To achieve a good SNR of the observation data, the surface solar zenith angle in both nadir modes shall be < 80º. Every orbit the satellite observes for ~46 minutes in the sunlit phase. Considering the seasonal changes, TanSat is always able to detect surface targets ranging from latitude -51º to 56º, and from -82º to 79º during summer or winter.
Sun-glint observation: In sun-glint observation mode, the satellite observes the sun glint point of the Earth, with the instrument boresight within 5º of the principal plane. As the satellite observes the sun-glint spot in orbit, the spacecraft attitudes changes due to the relative target move and the principle plane following: changing around -60º~+60º about the pitch axis, changing approximately for 160º about the yaw axis, while the roll angle is kept to about 20°. The maximum slew rate during the observation is about 0.2°/s about the yaw axis, which occurs while the satellite passes the sub-solar point.
As shown in Figure 17, the sun-glint observation mode is generally used to observe over the ocean surface, where the reflection ratio is relatively lower than on land. The surface solar zenith angle of the target glint spot is constrained to smaller than 70° for better SNR. With in sun-glint observation mode, TanSat is always able to detect the surface targets ranging from latitude -43° to 46°, and from -86° to 79° during summer or winter.
Target observation: In target observation mode, the satellite observes a target point or area on Earth's surface from different view angles ranging from -60° to 60°. Targets can be distributed over the area from -30° ~ 10° across the ground track. The observation for a ground target point or area can last for 8-10 minutes while the satellite flies over the target. The maximum slew rate during the observation is around 0.6°/s about the pitch axis.
Target observation is useful for CO2 detection over hot-spots. It can also be used to observe ground CO2 monitoring stations, so that the spaceborne observation data can be compared and validated by the ground-based data.
Sun observation: In sun observation mode, the boresight of the CO2 spectrometer is pointed toward the sun, for instrument spectrometric and radiometric calibration. There are two kinds of sun observation modes:
- Direct sun observation with the reflection side of the pointer mirror of the CO2 spectrometer
- Diffusion sun observation with the diffusive side of the mirror.
In direct sun observation mode, the satellite points its +Xs axis to the sun and maneuvers sinusoidally about 1° so as to acquire a full scan of the whole sun for radiometric calibration.
In the diffusion sun observation mode, the satellite points its +Zs axis toward the sun with 15° bias on the pitch axis so that the sunlight doesn't enter into the field of view of the CAPI instrument. The satellite carries out the diffusion sun observation while flying from the dark side of the orbit to the sunlit side. The spectrometric calibration of the CarbonSpec instrument is conducted first by observing in the sunlit phase through the atmosphere, this is followed with the radiometric calibration by observing the sun directly.
Moon observation: In moon observation mode, the satellite points the CAPI instrument toward the full moon, with a small sinusoidal periodic maneuver of the pitch axis; this permits spectrometric and radiometric instrument calibration.
The TanSat ground segment consists of three systems (Figure 18):
1) The ground data management system is based on the ground segment of the FY (Feng Yun) satellite series of China, regarding the monitoring and control of the spacecraft, including the science data reception and processing/archiving functions.
2) The CO2 ground monitoring network, for detecting and monitoring CO2 with six ground-based instruments, distributed over the mainland of China. The ground observation data will be compared to the spaceborne data for validation.
3) CO2 retrieval subsystem: for XCO2 retrieval based on the observation data, and for the provision of CO2 data products.
Figure 18: Overview of the TanSat system architecture (image credit: SIMIT, CIOMP, NSMC)
Figure 19: Ground-based monitoring network of TanSat (image credit: TanSat collaboration) 20)
Figure 20: Surface CO2 validation stations (image credit: TanSat collaboration)
Figure 21: Ground satellite receiving stations (image credit: TanSat collaboration)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).