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One small drive from the US embassy in Beijing last month may yet prove to be a giant leap for cooperation between the American space agency Nasa and China’s space programme.

In mid-July, a US embassy vehicle drove into the guarded compound of the Chinese Academy of Science’s Institute of Atmospheric Physics in central Beijing, carrying Dr Michael Freilich, the director of Nasa’s earth science division, Christopher Blackerby, the agency’s Pacific Rim representative, and other US government staff.

Once inside they spent hours with their Chinese counterparts in a closed-door meeting on TanSat, a Chinese satellite to be launched later this year.

In an attempt to thaw the icy relationship between the two countries in space, Beijing had made a major concession several years ago, offering management of TanSat, China’s first carbon-sniffing satellite, to Nasa so that it could be incorporated into one of its constellations of earth observation satellites. That would have given scientists around the world a new tool to monitor greenhouse gas emissions and discover the mechanisms driving climate change.

To the disappointment of the research community, the arrangement did not work out, and the small delegation led by Freilich was in Beijing to discuss with Chinese scientists what crumbs of teamwork – such as data exchanges and information sharing – could be salvaged.

But even that could be illegal.

Five years ago, the US Congress passed a law banning Nasa from using federal funds to “develop, design, plan, promulgate, implement, or execute a bilateral policy, programme, order, or contract of any kind to participate, collaborate, or coordinate bilaterally in any way with China or any Chinese-owned company”.

Nasa has not responded to South China Morning Post queries about the Beijing meeting and no information about the delegation’s visit to Beijing has been revealed on the agency’s website.

The issue was “sensitive to both sides”, said Professor Liu Yi, the lead scientist of the TanSat project, who confirmed the meeting with the American visitors but declined to reveal details of their discussions.

While the details remain unclear, some space experts view the stillborn TanSat deal as an encouraging sign that the “cold wall” separating China and the US in space is beginning to melt.

Many in the West have long regarded China’s state-funded science programmes as accomplices in the theft of intellectual property through reverse engineering of overseas technology. But in recent years, through costly projects such as TanSat, China has sought to portray itself as a worthy collaborator, equipped with cutting-edge technology. At the same time, the US government has also been taking baby steps to shed or bypass some decades-old China-exclusion policies.

TanSat, with tan being Chinese for carbon, is not the world’s first carbon satellite, but it could be the most precise, especially for monitoring greenhouse gas emissions in China, thanks to specially built ground facilities designed to improve the accuracy of data collection. The project has been funded by the Ministry of Science and Technology as part of China’s answer to international criticism of its role as the world’s largest emitter of carbon dioxide. TanSat’s scientific instruments, built by Liu’s team at the Chinese Academy of Sciences using home-grown technology, will be able to sniff out the generation, dispersal and sinking of carbon dioxide in the atmosphere with unprecedented precision.

Two years ago, Nasa launched a similar satellite, the Orbiting Carbon Observatory 2 (OCO-2), which became the sixth satellite in the “Afternoon Train” (A-Train) constellation of earth observation satellites from Japan, Europe, Canada and the US. They are placed only a few minutes apart from each other in a sun-synchronous orbit at an altitude of 705km and cross the equator together at a location that varies each day at around 1.30pm solar time, hence the name, allowing researchers to collect multidimensional data on water, clouds, winds and trace chemicals at a single location at almost the same time.

If TanSat had joined the A-Train, a scientist involved in the Chinese project said, it would have given scientists a wealth of data to study the carbon cycle, something he likened to a three-dimensional view as opposed to a two-dimensional one.

Many factors could have led to the failure of the proposed satellite arrangement. On the US side, in addition to possible legal complications, there would have been technical challenges in coordinating TanSat’s orbit with the other A-Train satellites. On the Chinese side, some people expressed concern about giving Americans first-hand data on the location and intensity of carbon emission sources in China, which could be used to estimate the level of economic activity, the scientist said, requesting anonymity due to the political sensitivity of the issue.

But the Sino-US attempt at collaborate has been seen as a positive sign. A mainland space expert said it may, in part, have been made possible by last year’s retirement of former US congressman Frank Wolf, who initiated numerous China-exclusion laws after his election to the House of Representatives in 1981, including the one impacting on Nasa in 2011.

“With Wolf gone, Nasa might feel a bit more comfortable about re-engaging China,” the space expert said, pointing out that previous visits to China by Nasa’s chief had come under fire from Wolf.

In 1980s, China and the US enjoyed a honeymoon on many areas, including space, with Chinese rockets repeatedly providing low-cost satellite-launch services for American companies. But the collaboration stopped with the crushing of the Tiananmen Square pro-democracy protest in 1989 and the imposition of US sanctions.

In the US, fear of Chinese theft of American technology then took hold. “”We don’t want to give them the opportunity to take advantage of our technology, and we have nothing to gain from dealing with them,” Wolf said in 2011.

But the trend towards collaboration in space between the two countries was “inevitable”, said Professor Zong Qiugang, an astrophysicist at Peking University’s Institute of Space Physics and Applied Technology who has worked with Nasa, the European and Japanese space agencies and the China National Space Administration.

He said China was planning and launching more scientific research missions, such as a dark matter probe and a large telescope mounted on a space station, and researchers in both countries were pushing their governments to end the freeze, while also coming up with smart methods to overcome political barriers.

“If a Chinese and an American scientist want to meet, they can invite a European or Japanese scientist to sit in on the meeting,” he said. “The bilateral ban can then be sidestepped.”

Though the space race between the US and Soviet Union during the cold war had spurred many breakthroughs, including mankind’s first steps on the moon, the heavy military involvement in the programmes and their high cost had proven unsustainable, Zong said, adding that a joint mission to Mars could break the ice between China and the US.

2020 will be the biggest year ever for Mars exploration, with China, the US, Russia, Europe, Japan, India and the United Arab Emirates all planning to send probes to the red planet.

“A race to Mars will be a waste of taxpayers’ money,” Zong said. “If we are going to Mars, to send the first human visitors there and bring them back, it will go beyond the capability of any single nation.”

By 2020 the world would be different from today, and China would have a space station, a fleet of powerful rockets and an even bigger economy to support a Mars mission.

“Politicians should sit down and work out a plan, a joint China-US mission will open a new chapter in the history of human space exploration,” Zong said.

Quelle: South China Morning Post

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Background zu TanSat:

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.

The project includes 4 research topics: ^{1) 2) 3) 4) 5) 6)}

• 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⁾

Table 2: Team of the TanSat project ^8) 9)

Spacecraft: 

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 500 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

Launch: A launch of the TanSat (CarbonSat) satellite is scheduled for December 2016 on a long March 2D vehicle from JSLC (Jiuquan Satellite Launch Center), China (Ref. 13). ¹²⁾

• 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. ¹³⁾

- 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: 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)

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 6).

Figure 6: 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 6, CarbonSpec is composed of a pointing subsystem, a telescope subsystem, a beam splitter subsystem, a diffraction grating spectrometer subsystem, and an imaging subsystem. Figure 7 and Table4 show the optical schematics and specifications of the Spectrometer, respectively.

Figure 7: 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 8: Illustration of the pointing subsystem (image credit: CIOMP)

Figure 9: 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 10 shows the structure of the VNIR and SWIR and the lens structure. Table 5 gives the specification of the CAPI instrument.

Figure 10: Illustration of the CAPI structure (image credit: CIOMP)