OCO-2 (Orbiting Carbon Observatory-2)
OCO-2 is a NASA mission dedicated to studying atmospheric carbon dioxide. Carbon dioxide is the leading human-produced greenhouse gas driving changes in the Earth's climate. OCO-2 will provide a complete picture of human and natural carbon dioxide sources and "sinks," the places where the gas is pulled out of the atmosphere and stored. The aim is to map the global geographic distribution of these sources and sinks and study their changes over time. 1) 2) 3) 4) 5)
The OCO-2 mission is expected to provide answers to such questions as: "What controls atmospheric carbon dioxide (CO2)" ?
• Natural systems including the ocean and plants on land both absorb and emit carbon dioxide to the atmosphere
• Currently, these natural systems are absorbing about half of the carbon dioxide emitted by human activities
• These natural carbon dioxide "sinks" are limiting the rate of carbon dioxide buildup and its impact on the Earth's climate
• So far it is not known:
- Exactly where the carbon dioxide is being emitted and absorbed
- How much longer natural processes will continue to absorb the carbon dioxide that we emit in the presence of climate change.
Table 1: OCO-2 is a re-flight of the OCO-1 mission 6)
Sources and Sinks of Carbon Dioxide in the Earth System: 7)
Life as we know it would not exist without carbon. All living and once-living things (i.e., biomass) are based on carbon, the fourth most abundant element in our universe. Carbon, in many gaseous forms—e.g., CO2, carbon monoxide (CO), and methane (CH4)—can be released into the atmosphere or absorbed from the atmosphere by processes at the surface. The continual exchanges of carbon between the atmosphere, oceans, and terrestrial ecosystems define Earth's global carbon cycle. Carbon moves more quickly through some parts of the carbon cycle than others. For example, respiration (i.e., the conversion of carbon-containing molecules by biological systems into energy) is a rapid process compared to the longevity of trees, carbonate rocks, or fossil fuels.
Carbon dioxide is the most abundant carbon-bearing gas in Earth's atmosphere, and plays a special role in the carbon cycle. From an atmospheric perspective, sources emit or release carbon—primarily as CO2—into the atmosphere, while sinks remove CO2 from the atmosphere. Natural processes are affected by CO2, such that—collectively—CO2 emission and absorption are roughly balanced over time. Since the beginning of the industrial age, however, humans have disrupted this balance with increased use of carbon-containing compounds to provide energy for heat, light, and to meet our transportation needs and other industrial requirements.
For example, each time humans use coal or CH4 (also known as natural gas) to generate electricity, or drive a petroleum-powered car, or cut down a forest, or intentionally ignite a forest fire to clear land for agriculture, CO2 is released into the atmosphere. Unlike natural processes, these human activities absorb little or no CO2 in return and produce rapid increases in atmospheric CO2, currently adding approximately 36 billion tons of it each year. Fossil fuel combustion is the largest and most rapidly growing source of CO2 emission into the atmosphere, with global growth rates of 2.2% per year.
Since the turn of the century, the largest increases have occurred in the developing world, which is now responsible for 57% of all CO2 emissions. Changes in land use (e.g., clearing forests, which while growing act as repositories or sinks for carbon) are the second largest source—although this contribution is decreasing. In many instances, forests and other vegetated land areas previously harvested for wood or to grow crops will experience natural (or intentional) regrowth, called reforestation. This allows an area cleared for wood or crops multiple decades ago to act as a carbon sink again, removing CO2 from the atmosphere. However, not all such carbon sinks are replenished, and large-scale fluctuations in these reservoirs affect the global carbon cycle, ultimately impacting Earth's climate system.
Because CO2 reacts very slowly with other atmospheric gases and energy sources like solar ultraviolet radiation, most of the CO2 emitted today will remain in the atmosphere for several hundred years. As this long-lived gas mixes in Earth's atmosphere and is transported around the globe and throughout the carbon cycle, it will continue to impact our planet. Scientists need to understand the processes that are controlling the buildup of CO2 in Earth's atmosphere today so they can predict how fast CO2 will accumulate in the future.
To monitor the impact of CO2 emissions on the atmosphere, scientists rely on more than 150 ground-based stations around the world. These measurements show that CO2 has increased by more than 40%, from approximately 280 parts per million in volume (ppmv) to about 400 ppmv, since the beginning of the industrial age. In other words, 400 out of every one million air molecules is now a CO2 molecule. Half of this growth has occurred since 1980, and a quarter has occurred since 2001. The current CO2 abundance (2014) is now increasing by more than 2 parts per million (0.5%) each year.
Interestingly, however, this rapid buildup of CO2 accounts for less than half of the 36 billion tons of CO2 emitted into the atmosphere each year from fossil fuel use and other human activities. Processes at the surface are apparently absorbing the remainder. Measurements of the increasing acidity of seawater indicate that at least one quarter of the CO2 emitted by human activities is being absorbed by the ocean. The remaining quarter is presumably being absorbed by the land biosphere, but the identity, location, and processes controlling this sink are currently unknown. Scientists refer to this mystery as the "missing-carbon sink."
Despite decades of research that have steadily increased our understanding of the global carbon cycle, scientists still face tremendous challenges as they try to understand the processes controlling the increased rate of CO2 buildup in the atmosphere. For example, characterizing intense localized sources of CO2 associated with fossil fuel combustion is much easier than distinguishing and quantifying natural sources and sinks such as CO2 emitted from oceans, deforestation, and biomass burning. This is due in part to large gaps between ground-based instrument sites and thus limited availability of precise measurements over large portions of Earth's surface.
Figure 1: Schematic view of the carbon cycle sources and sinks (image credit: NASA)
Legend to Figure 1: Although natural and anthropogenic (i.e., human-generated) sources and sinks can be found almost anywhere in the world, human activities are "tipping the scale," causing the sources of carbon to "outweigh" the sinks. Such activities are contributing to a rise in atmospheric CO2, which impacts Earth's climate system. Note that this diagram is simply indicative, and does not include all known carbon sources and sinks.
Figure 2: Approximately half of the CO2 emissions from human activities stay in the atmosphere, while oceans and land sinks absorb the rest. Data from OCO-2 will help scientists better understand these sinks and their locations. Note that while there is substantial year-to-year variability, these per-centages reflect the long-term averages (image credit: NASA)
Satellite observations can provide the continuous, high spatial resolution, global observations of CO2 that are needed to help answer the question of where the carbon is going. The new observatory will dramatically increase the number of observations of carbon dioxide, collecting hundreds of thousands of measurements each day when the satellite flies over Earth's sunlit hemisphere. High-precision, detailed, near-global observations are needed to characterize carbon dioxide's distribution because the concentration of carbon dioxide varies by only a few percent throughout the year on regional to continental scales. Scientists will analyze the OCO-2 data, using computer models similar to those used to predict the weather, to locate and understand the sources and sinks of carbon dioxide.
Measurements from OCO-2 will also be used in conjunction with measurements from ground-based stations, aircraft, and other satellites operated by NASA and its partners. For example, OCO-2 data will be combined with measurements of water vapor and CH4—other strong greenhouse gases—from NASA's Aqua and Aura satellites and the Japanese GOSAT/Ibuki (Greenhouse Gases Observation Satellite) mission, to more fully understand the contribution of greenhouse gases to climate change. OCO-2 data will be supplemented with measurements of other atmospheric gases—such as tropospheric ozone and nitrogen dioxide—from NASA's Aura mission, to study the relationship between CO2 and other gases associated with air pollution. By combining Earth-observation data from multiple sources, scientists can view the Earth as one interconnected system, better understand how humans are contributing to climate change, and improve computer predictions of how climate will change in the future.
Table 2: Overview of NASA missions observing/contributing to the global Carbon Cycle measurements and Earth's Changing Climate (Ref. 7)
OCO-2 is based on the Orbiting Carbon Observatory (OCO) to the extent possible. OCO-2 will be a dedicated spacecraft to fly a single instrument comprised of three high resolution grating spectrometers. 8) 9)
The spacecraft is three-axis zero-momentum stabilized, using the LeoStar-2 bus of OSC [LeoStar-2 is of OCO, SORCE (Solar Radiation and Climate Experiment) and GALEX (Galaxy Evolution Explorer) heritage]. The bus houses and points the instrument, provides power, receives and processes commands from the ground, records, and downlinks the data collected by the instrument, and maintains its position with the EOS A-Train. The primary structure consists of a 2.12 m long hexagonal column that is 0.94 m wide.
ACS (Attitude Control Subsystem): The ACS points the instrument for science and calibration observations and the body-mounted X-band antenna at the ground station for data downlink. Pitch, roll, and yaw are controlled by 4 reaction wheels. Three magnetic torque rods are used to de-spin the reaction wheels. OCO-2 uses the same types of Goodrich/Ithaco reaction wheels and torque rods that were used for OCO. However, the reaction wheels have been modified to address lifetime issues identified over the past decade.
Figure 3: Artist's rendition of the OCO-2 spacecraft (image credit: OSC, NASA)
The ACS of the OCO-2 spacecraft consists of star trackers, a MIMU (Miniature Inertial Measurement Unit), 13 sun sensors for safe mode control and a magnetometer for attitude determination and a reaction wheel assembly and magnetic torque rods for attitude actuation. The SED26 star trackers allow a precise attitude determination by acquiring images of the star-filled sky and comparing them with a catalog of stars and constellations using an onboard algorithm.
The system autonomously acquires attitude data in under three seconds and tracks up to ten stars simultaneously providing an attitude frame ten times/s. SED26 can operate with planets and the Moon in the field of view and uses a baffle to eliminate sunlight with sun exclusion angles of 25 or 30º. The star tackers can continue tracking acquired stars up to a spacecraft motion of 10°/s. Each star tracker unit weighs 3.5 kg and is 16 by 17 by 30 cm in size.
Three-axis angular measurement is provided by a MIMU (Miniature Inertial Measurement Unit) manufactured by Honeywell. The IMU (Inertial Measurement Unit) has extensive flight heritage and features a robust design using the GG1320 ring laser gyro that provides precise rotation measurements.
Figure 4: Illustration of the MIMU (image credit: Honeywell)
Attitude control is primarily provided by a RWA (Reaction Wheel Assembly). The RWA consists of four wheels to achieve three-axis control with built-in redundancy and a Wheel Drive Electronics Box that has one dedicated channel for each wheel. The reaction wheel assembly is a rotating inertial mass – when accelerating the wheel, the satellite body to which the wheels are directly attached will rotate to the opposite direction as a result of the introduced counter torque.
The spacecraft orbit is determined by a GPS receiver of General Dynamics Viceroy. Pointing information is provided by a star tracker (Sodern), a MIMU (Miniature Inertial Measurement Unit) of Honeywell, and a magnetometer of Goodrich.
EPS (Electrical Propulsion Subsystem): Two deployable solar panels supply ~900 W when illuminated at near normal incidence (use of GaAs solar cells). The solar panels charge an Eagle Picher 35 Ah NiH (Nickel-Hydrogen) battery that provides power during eclipse. OCO-2 uses the same battery model used by OCO.
OBC (On-Board Computer): The central electronics unit uses a RAD-6000 single-board flight computer of BAE to manage the attitude control, power, propulsion, and telecom systems, and the 128 Gbit solid-state recorder of Seaker that stores the science data collected by the instrument. The RAD6000 had to be modified to replace the SRAM (Static Random Access Memory) part, which was based on an obsolete, 16-chip MCM (Multi-Chip Module) with a newer SRAM part. - The CPU consists of 1.1 million transistors. It has an L1 cache memory of 8 kB and controls up to 128 MB of SRAM memory. The spacecraft control unit also includes 1 GB of Random Access Memory and 3 MB of non-volatile memory storing the flight software and command sequences without a loss of data in case of a power outage.
Figure 5: Spacecraft controller centered around the RAD-6000 (image credit: Lockheed Martin, BAE)
A propulsion subsystem is used for orbit maintenance (use of 45 kg of hydrazine). Once in orbit, it is being used to raise and adjust the orbit altitude and inclination as necessary to maintain the spacecraft's position in the A-Train. Finally, it is used to de-orbit the Observatory at the end of the mission.
RF communications: Both science and housekeeping data are usually returned to the ground at 150 Mbit/s using an L3 Communications X-band transmitter and a body-mounted X-band patch antenna. Spacecraft and instrument housekeeping data can also be returned by an S-band transmitter (2 Mbit/s) to a ground station or through a NASA TDRS (Tracking and Data Relay Satellite) system. The uplink is implemented with an S-band receiver and a pair of omni-directional antennas. OCO-2 will use S-band hardware from TAS (Thales Alenia Space) that meets the same performance requirements as OCO, but is based on a new, all-digital design. Science data is stored in a 96 Gbit solid state recorder that interfaces with the communications system to be able to downlink the acquired science data.
Figure 6: Photo of the S-band transceiver (image credit: TAS)
Figure 7: Illustration of OCO-2 elements (image credit: NASA)
Table 3: Overview of OCO-2 minisatellite parameters 10)
Figure 8: Photo of the OCO-2 spacecraft during integration and test at OSC (image credit: OSC)
• The OCO-2 mission underwent CDR (Critical Design Review) in August 2010 and key design point-C (KDP-C) in September 2010 (Ref. 1).
• The OCO-2 instrument will be delivered in the spring of 2012 (Ref. 41).
• On April 30, 2014, NASA's OCO-2 arrived at the VAFB launch site after traveling from Orbital Sciences Corp.'s Satellite Manufacturing Facility in Gilbert, AZ. The spacecraft now will undergo final tests and then be integrated on top of a ULA (United Launch Alliance) Delta-2 rocket in preparation for a planned July 1 launch. 11)
Figure 9: A truck convoy carrying NASA's Orbiting Carbon Observatory-2 spacecraft arrives at VAFB, CA (image credit: NASA)
Launch: The OCO-2 spacecraft was launched on July 2, 2014 (09:56:23 UTC) on a Delta-2 7320-10C vehicle from VAFB, CA. 12) 13) NASA contracted ULA (United Launch Services LLC) in July 2012. 14) 15) 16)
Note: Originally, NASA planned a launch of the OCO-2 mission for Feb. 2013 on a Taurus- XL 3110 vehicle of OSC (However, the configuration Taurus-XL and the OCO-1 spacecraft experienced already a launch failure on Feb. 24, 2009. This was followed by a launch failure of NASA's Glory spacecraft on a Taurus-XL 3110 vehicle on March 4, 2011). — Hence, in early 2012, NASA and OSC mutually agreed to terminate the existing launch contract for OCO-2. In early February 2012, NASA released a multi-mission request for launch service proposals that included the OCO-2 mission. Once NASA officials select a new rocket for OCO-2, it will involve a launch delay at least into the year 2014. 17) 18)
Orbit: Sun-synchronous orbit, altitude = 705 km, inclination = 98.2º, period = 98.8 min, repeat cycle of 16 days, equatorial crossing time at 13:30 hours on an ascending node.
OCO-2 will initially be launched into an orbit of 690 km altitude. The on-board propulsion system will then raise the orbit to 705 km. The OCO-2 spacecraft will become part of the "A-Train" (a loose formation of the Afternoon Constellation consisting of: Aqua, CloudSat, CALIPSO, Glory, PARASOL, Aura, and OCO-2) to correlate the OCO-2 data with data acquired by other instruments such as AIRS on Aqua. OCO-2 will fly ahead of the A-train, about 15 minutes before the Aqua spacecraft.
The A-Train formation flying coordination, data exchange management, and coordination of cooperative science campaigns is performed by the Afternoon Constellation Mission Operations Working Group (MOWG) composed of agencies contributing satellites to the A-Train. In early August 2014, the OCO-2 (Orbiting Carbon Observatory-2) is expected to join the A-Train.
OCO-2 science operations will begin about 45 days after launch. Scientists expect to begin archiving calibrated mission data in about six months and plan to release their first initial estimates of atmospheric carbon dioxide concentrations in early 2015 (Ref. 12).
Figure 10: Artist's view of the OCO-2 spacecraft flying in the A-Train constellation with time they are separated when they fly (image credit: NASA/JPL)
Status of mission:
• December 13, 2016: A new NASA supercomputer project builds on the agency's satellite measurements of carbon dioxide and combines them with a sophisticated Earth system model to provide one of the most realistic views yet of how this critical greenhouse gas moves through the atmosphere. 19)
- Scientists have tracked the rising concentration of heat-trapping carbon dioxide for decades using ground-based sensors in a few places. A high-resolution visualization of the new combined data product provides an entirely different perspective. The visualization was generated by the Global Modeling and Assimilation Office at NASA's Goddard Space Flight Center, Greenbelt, Maryland, using data from the agency's OCO-2 (Orbiting Carbon Observatory-2) satellite, built and operated by NASA/JPL of Pasadena, CA.
- The 3-D visualization of Figure 11 reveals in startling detail the complex patterns in which carbon dioxide in the atmosphere increases, decreases and moves around the globe over the time period from September 2014 to September 2015.
Figure 11: Using observations from NASA's OCO-2 mission, scientists have developed a new model of carbon behavior in our atmosphere from Sept. 1, 2014, to Aug. 31, 2015. Such models can be used to better understand and predict where carbon dioxide concentrations could be especially high or low (image credit: NASA/GSFC, K. Mersmann, M. Radcliff, producers)
- Atmospheric carbon dioxide acts as Earth's thermostat. Rising concentrations of the greenhouse gas, due primarily to the burning of fossil fuels for energy, have driven Earth's current long-term warming trend. The visualization highlights the advances scientists are making in understanding the processes that control how much emitted carbon dioxide stays in the atmosphere and how long it stays there — questions that ultimately will determine Earth's future climate.
- Scientists know that nearly half of all human-caused emissions are absorbed by the land and ocean. The current understanding is that about 50% of emissions remain in the atmosphere, about 25% are absorbed by vegetation on the land, and about 25 % are absorbed by the ocean. However, those seemingly simple numbers leave scientists with critical and complex questions: Which ecosystems, especially on land, are absorbing what amounts of carbon dioxide? Perhaps most significantly, as emissions keep rising, will the land and the ocean continue this rate of absorption, or reach a point of saturation?
- The new dataset is a step toward answering those questions, explained Lesley Ott, a carbon cycle scientist at NASA/GSFC and a member of the OCO-2 science team. Scientists need to understand the processes driving the "carbon flux" — the exchange of carbon dioxide between the atmosphere, land and ocean, Ott said. "We can't measure the flux directly at high resolution across the entire globe," she said. "We are trying to build the tools needed to provide an accurate picture of what's happening in the atmosphere and translate that to an accurate picture of what's going on with the flux. There's still a long way to go, but this is a really important and necessary step in that chain of discoveries about carbon dioxide."
- OCO-2, launched in 2014, is NASA's first satellite designed specifically to measure atmospheric carbon dioxide at regional scales. "Since September of 2014, OCO-2 has been returning almost 100,000 carbon dioxide estimates over the globe each day. Modeling tools like those being developed by our colleagues in the Global Modeling and Assimilation Office are critical for analyzing and interpreting this high-resolution dataset," said David Crisp, OCO-2 science team leader at JPL.
- GMAO (Global Modeling and Assimilation Office) has previously included carbon dioxide in its GEOS Earth system model, which is used for all manner of atmospheric studies. This new product builds on that work by using the technique of data assimilation to combine the OCO-2 observations with the model. "Data assimilation is the process of blending model simulations with real-world measurements, with the precision, resolution and coverage needed to reflect our best understanding of the exchange of carbon dioxide between the surface and atmosphere," explained Brad Weir, a researcher based in the GMAO.
- The visualization showcases information about global carbon dioxide that has not been seen before in such detail: The rise and fall of carbon dioxide in the Northern Hemisphere throughout a year; the influence of continents, mountain ranges and ocean currents on weather patterns and therefore carbon dioxide movement; and the regional influence of highly active photosynthesis in places like the U.S. Corn Belt.
- While the finely detailed carbon dioxide fluctuations are eye-catching, they also remind GMAO chief Steven Pawson of the progress scientists are making with computer models of the Earth system. One future step will be to integrate a more complex biology module into the model to better target the questions of carbon dioxide absorption and release by forests and other land ecosystems.
- The results highlighted here demonstrate the value of NASA's unique capabilities in observing and modeling Earth. They also emphasize the collaboration among NASA centers and the value of powerful supercomputing. The assimilation was created using a model called the GEOS-5 (Goddard Earth Observing System Model-Version 5), which was run by the Discover supercomputer cluster at Goddard's NASA Center for Climate Simulation.
- "It's taken us many years to pull it all together," Pawson said. "The level of detail included in this dataset gives us a lot of optimism that our models and observations are beginning to give a coherent view of the carbon cycle."
• November 17, 2016: For decades, ground-based observatories have been measuring CO2, and those measurements have been steadily climbing. The atmospheric concentration of carbon dioxide now averages more than 400 parts per million, year-round, which is more than one third higher than CO2 levels before modern industrialization and fossil fuel use began. 20)
- Ground stations have provided a broad view of carbon in the atmosphere, and other models and estimates (such as economic data) have filled in some details. Even a few satellites have offered short-term or regional glimpses of CO2 patterns. But past efforts have been limited in various ways: by the inability to collect measurements over the oceans; by a lack of resolution or methodical measurement from space-based instruments; and by incomplete reporting by countries and companies monitoring the gas. Most of all, past measurements could not necessarily pinpoint the sources of carbon dioxide.
- Various studies and models have determined that humans release about 40 billion tons of carbon dioxide into the atmosphere each year. But where, exactly, are those emissions coming from today? A group of scientists from the FMI (Finnish Meteorological Institute) have used OCO-2 data to make satellite-based maps of human emissions of carbon dioxide. Those satellite observations match well with ground-based estimates.
- The maps on this page depict carbon dioxide anomalies in the atmosphere; that is, places where CO2 levels were higher than the normal fluctuations that occur with the seasons. They are based on work published in November 2016 by Janne Hakkarainen and colleagues at FMI (Ref. 22). The maps depict widespread carbon dioxide around major urban areas, as well as some smaller pockets of high emissions. The highest values in the study were observed over eastern China (Figure 12), with other hot spots in the eastern United States (Figure 13), Central Europe (Figure 14) and the Middle East (Figure 15).
- "OCO-2 can even detect smaller, isolated emitting areas like individual cities. It's a very powerful tool that gives new insight," said Hakkarainen, the atmospheric scientist at FMI who led the study. "One of the most interesting findings was to see a strong signal over Middle East that is not present in emission inventories—suggesting that the inventories might be incomplete over that area." (Note that the Middle East map does not show data east of Iran because calculations have not yet been made for those areas.)
Figure 15: Illustration of OCO-2 CO2 concentrations in the Middle East measured in the period 2014-2016 (image credit: NASA Earth Observatory, maps by Joshua Stevens, using OCO-2 anomaly data of Ref. 22)
• November 1, 2016: Scientists have produced the first global maps of human emissions of carbon dioxide ever made solely from satellite observations of the greenhouse gas. The maps, based on data from NASA's OCO-2 (Orbiting Carbon Observatory-2) satellite and generated with a new data-processing technique, agree well with inventories of known carbon dioxide emissions. 21)
- No satellite before OCO-2 was capable of measuring carbon dioxide in fine enough detail to allow researchers to create maps of human emissions from the satellite data alone. Instead, earlier maps also incorporated estimates from economic data and modeling results.
- The team of scientists from the FMI (Finnish Meteorological Institute), Helsinki, produced three main maps from OCO-2 data, each centered on one of Earth's highest-emitting regions: the eastern United States, central Europe and East Asia. The maps show widespread carbon dioxide across major urban areas and smaller pockets of high emissions. 22)
- "OCO-2 can even detect smaller, isolated emitting areas like individual cities," said research scientist Janne Hakkarainen, who led the study. "It's a very powerful tool that gives new insight."
- Human emissions of carbon dioxide have grown at a significant rate since the Industrial Revolution, and the greenhouse gas lingers in the atmosphere for a century or more. This means that recent human output is only a tiny part of the total carbon dioxide that OCO-2 records as it looks down toward Earth's surface. "Currently, the background level of carbon dioxide in the atmosphere is about 400 parts per million, and human emissions within the past year may add only something like three parts per million to that total," said Hakkarainen. The data-processing challenge, he noted, was to isolate the signature of the recent emissions from the total amount.
- The team's new data-processing technique accounts for seasonal changes in carbon dioxide, the result of plant growth and dormancy, as well as the background carbon dioxide level. To be sure their method was correct, they compared the results with measurements of nitrogen dioxide — another gas emitted from fossil fuel combustion — from the OMI (Ozone Monitoring Instrument), a Dutch-Finnish instrument on NASA's Aura satellite. OMI and OCO-2 are both in the A-Train satellite constellation, so the two measurements cover the same area of Earth and are separated in time by only 15 minutes. The two measurements correlated well, giving the researchers confidence that their new technique produced reliable results.
- Coauthor Johanna Tamminen, head of the atmospheric remote sensing group at the Finnish Meteorological Institute, noted that with its comparison of OCO-2 and OMI data, "The research demonstrates the possibility of analyzing joint satellite observations of carbon dioxide and other gases related to combustion processes to draw out information about the emissions sources."
- OCO-2 Deputy Project Scientist Annmarie Eldering of NASA's Jet Propulsion Laboratory, Pasadena, California, said, "We are very pleased to see this research group make use of the OCO-2 data. Their analysis is a great demonstration of discovery with this new dataset." Eldering was not involved in the study.
• June 13, 2016: Scientists warn that the global warming target will be overshot within two decades, as annual concentrations of CO2 set to pass 400 parts per million (ppm) in 2016. Atmospheric concentrations of CO2 will shatter the symbolic barrier of 400 ppm this year and will not fall below it in our lifetimes, according to a new Met Office study. 23) 24)
Figure 16: The UK Met Office forecasts 2016 will see annual CO2 concentrations breach 400 parts per million. To keep below global warming of 2ºC - the ‘safe' level - concentrations must be kept below 450ppm image credit: OCO-2 /NASA/JPL, Caltech)
- Carbon dioxide measurements at the Mauna Loa observatory in Hawaii are forecast to soar by a record 3.1ppm this year – up from an annual average of 2.1ppm – due in large part to the cyclical El Niño weather event in the Pacific, according to Ref. 24). The surge in CO2 levels will be larger than during the last big El Niño in 1997/98, because man-made emissions have increased by 25% since then, boosting the phenomenon's strength.
- The UK Met Office also attributes around a fifth of the current El Niño's severity to forest fires, which were started by humans and exacerbated by drought. The paper's lead author, Richard Betts of the Met's Hadley Centre and Exeter University, said the fact that the 400ppm threshold had been breached a year earlier than expected carried a warning for the future. "Once you have passed that barrier, it takes a long time for CO2 to be removed from the atmosphere by natural processes," he said. "Even if we cut emissions, we wouldn't see concentrations coming down for a long time, so we have said goodbye to measurements below 400ppm at Mauna Loa."
- The leap across the 400ppm watershed at the Hawaiian observatory will not change any climate change fundamentals. Rather, it marks a psychological rubicon, and reminder of the clock ticking down on global warming.
- The UN's IPCC (Intergovernmental Panel on Climate Change) says that CO2 concentrations must be stabilized at 450ppm to have a fair chance of avoiding global warming above 2ºC, which could carry catastrophic consequences. — Doing that that will require a 40-70% emissions cut by 2050, compared to 2010 levels, and zero emissions by the end of the century.
- Despite the Paris agreement last December and a boost in renewable energy that has at least temporarily checked the growth in global emissions, the world is on track to substantially overshoot the target. "We could be passing above 450ppm in roughly 20 years," Betts said. "If we start to reduce our global emissions now, we could delay that moment but it is still looking like a challenge to stay below 450ppm. If we carry on as we are going, we could pass 450ppm even sooner than 20 years, according to the IPCC scenarios."
• October 29, 2015: Armed with a full annual cycle of data, OCO-2 scientists are now beginning to study the net sources of carbon dioxide as well as their "sinks" — places in the Earth system that store carbon, such as the ocean and plants on land. This information will help scientists better understand the natural processes currently absorbing more than half the carbon dioxide emitted into the atmosphere by human activities. This is a key to understanding how Earth's climate may change in the future as greenhouse gas concentrations increase. 25)
- OCO-2 began routine science operations in September 2014. "We can already clearly see patterns of seasonal change and variations in carbon dioxide around the globe," said Annmarie Eldering, OCO-2 deputy project scientist at NASA/JPL in Pasadena, California. "Far more subtle features are expected to emerge over time."
- The first year of data from the mission reveals a portrait of a dynamic, living planet. Between mid-May and mid-July 2015, OCO-2 saw a dramatic reduction in the abundance of atmospheric carbon dioxide across the northern hemisphere, as plants on land sprang to life and began rapidly absorbing carbon dioxide from the air to form new leaves, stems and roots. During this intense, two-month period, known as the "spring drawdown," OCO-2 measurements show the concentration of atmospheric carbon dioxide over much of the northern hemisphere decreased by two to three percent. That's 8 to 12 parts per million out of the global average background concentration of 400 parts per million.
- "That's a big but expected change," said Eldering. "This is the first time we've ever had the opportunity to observe the spring drawdown across the entire northern hemisphere with this kind of spatial resolution, seeing changes from week to week."
- Also as expected, OCO-2 data show increased concentrations of carbon dioxide associated with human activities. Higher carbon dioxide levels of several parts per million are seen in regions where fossil fuels are being consumed by large power plants or megacities. Enhanced levels are also seen in the Amazon, Central Africa and Indonesia, where forests are being cleared and burned to create fields for agricultural use.
- Researchers Abhishek Chatterjee of the Global Modeling and Assimilation Office at NASA's Goddard Space Flight Center, Greenbelt, Maryland; and Michelle Gierach and Dave Schimel of JPL are investigating a strong correlation observed between atmospheric carbon dioxide over the Pacific Ocean and the current El Nino. Fluctuations in carbon dioxide appear to be strongly linked with warmer sea surface temperatures. OCO-2's unprecedented density of measurements is giving researchers a unique data set to understand and separate the roles that sea surface temperatures, winds, regional emissions and other variables may be playing in the carbon dioxide concentrations.
- Through most of OCO-2's first year in space, the mission team was busy calibrating its science instrument, learning how to process its massive amount of data, and delivering data products to NASA's GES-DISC (Goddard Earth Sciences -Data and Information Services Center) in Greenbelt, Maryland, for distribution to the world's science community. Scientists are comparing OCO-2 data to ground-based measurements to validate the satellite data and tie it to internationally accepted standards for accuracy and precision.
- Routine delivery of OCO-2 data — calibrated spectra of reflected sunlight that reveal the fingerprints of carbon dioxide — began in late 2014, while estimates of carbon dioxide derived from cloud-free OCO-2 observations have been delivered since March 2015. Recently, the OCO-2 team reprocessed the OCO-2 data set to incorporate improvements in instrument calibration and correct other known issues with the original data release.
Figure 17: Global average carbon dioxide concentrations as seen by NASA's Orbiting Carbon Observatory-2 mission, June 1-15, 2015 (image credit: NASA/JPL)
Legend to Figure 17: OCO-2 measures carbon dioxide from the top of Earth's atmosphere to its surface. Higher carbon dioxide concentrations are in red, with lower concentrations in yellows and greens. Scientists poring over data from OCO-2 mission are seeing patterns emerge as they seek answers to questions about atmospheric carbon dioxide. - Among the most striking features visible in the first year of OCO-2 data is the increase in carbon dioxide in the northern hemisphere during winter, when trees are not removing carbon dioxide, followed by its decrease in spring, as trees start to grow and remove carbon dioxide from the atmosphere (Ref. 25).
• Feb. 2015: (OCO-2 is NASA's first satellite designed to collect the measurements needed to estimate the column-averaged carbon dioxide (CO2) dry air mole fraction, XCO2, with the sensitivity, accuracy, and resolution needed to characterize the CO2 sources and sinks on regional scales over the globe. OCO-2 was successfully launched from Vandenberg Air Force Base in California on July 2, 2014 and joined the 705 km Afternoon Constellation (A-Train) on August 3, 2014. The three-channel imaging grating spectrometer was then cooled to its operating temperatures and a comprehensive series of characterization and calibration activities were initiated. Since early October 2014, the observatory has been routinely collecting almost 1 million soundings over the sunlit hemisphere each day. Early cloud screening results indicate that 15-30% of these measurements may be sufficiently cloud free to yield precise estimates of XCO2. Initial deliveries of calibrated, geo-located OCO-2 spectra to the NASA Goddard Earth Science Data and Information Services Center (GES DISC) began on December 30, 2014. Preliminary estimates of XCO2 retrieved from these data are currently being validated against observations from the TCCON (Total Carbon Column Observing Network) and other standards. Routine deliveries XCO2 and other products, including surface pressure and chlorophyll fluorescence, to the GES DISC are expected to begin before the end of March, 2015. 26)
• Dec. 18, 2014: The first global maps of atmospheric carbon dioxide from OCO-2 demonstrate its performance and promise, showing elevated carbon dioxide concentrations across the Southern Hemisphere from springtime biomass burning. Preliminary analysis shows these signals are largely driven by the seasonal burning of savannas and forests. 27) 28) 29)
Figure 18: Global atmospheric CO2 concentration as recorded by NASA's OCO-2 mission (image credit: NASA/JPL-Caltech)
Legend to Figure 18: Global atmospheric carbon dioxide concentrations from Oct. 1 through Nov. 11, as recorded by NASA's Orbiting Carbon Observatory-2. Carbon dioxide concentrations are highest above northern Australia, southern Africa and eastern Brazil. Preliminary analysis of the African data shows the high levels there are largely driven by the burning of savannas and forests. Elevated carbon dioxide can also be seen above industrialized Northern Hemisphere regions in China, Europe and North America.
- The early OCO-2 data hint at some potential surprises to come. The agreement between OCO-2 and models based on existing carbon dioxide data is remarkably good, but there are some interesting differences. Some of the differences may be due to systematic errors in the measurements, and the team is currently in the process of nailing these down. But some of the differences are likely due to gaps in the current knowledge of carbon sources in certain regions — gaps that OCO-2 will help fill in. Through photosynthesis, plants remove carbon dioxide from the air and use sunlight to synthesize the carbon into food. Plants end up re-emitting about one percent of the sunlight at longer wavelengths. Using one of OCO-2's three spectrometer instruments, scientists can measure the re-emitted light, known as solar-induced chlorophyll fluorescence (SIF). This measurement complements OCO-2's carbon dioxide data with information on when and where plants are drawing carbon from the atmosphere.
Carbon dioxide in the atmosphere has no distinguishing features to show what its source was. Elevated carbon dioxide over a region could have a natural cause — for example, a drought that reduces plant growth — or a human cause.
Legend to Figure 19: The map shows solar-induced fluorescence, a plant process that occurs during photosynthesis, from August through October 2014 as measured by NASA's Orbiting Carbon Observatory-2. This period is springtime in the Southern Hemisphere and fall in the Northern Hemisphere. Photosynthesis is highest over the tropical forests of the Southern Hemisphere but still occurs in much of the U.S. Grain Belt. The northern forests have shut down for the winter.
• OCO-2 began routine operations on 6 September 2014.
• August 11, 2014: Just over a month after launch, the OCO-2 spacecraft has maneuvered into its final operating orbit and produced its first science data, confirming the health of its science instrument. - Through the month of July, a series of propulsive burns was executed to maneuver the observatory into its final 705 km, near-polar orbit at the head of the international Afternoon Constellation, or "A-Train," of Earth-observing satellites. It arrived there on Aug. 3. Operations are now being conducted with the observatory in an orbit that crosses the equator at 13:36 hours local time. 32)
- With OCO-2 in its final orbit, mission controllers began cooling the observatory's three-spectrometer instrument to its operating temperatures. The spectrometer's optical components must be cooled to minus 6ºC to bring them into focus and limit the amount of heat they radiate. The instrument's detectors must be even cooler, minus 153ºC, to maximize their sensitivity.
- With the instrument's optical system and detectors near their stable operating temperatures, the OCO-2 team collected "first light" test data on Aug. 6 as the observatory flew over central Papua New Guinea. The data were transmitted from OCO-2 to a ground station in Alaska, then to NASA's Goddard Space Flight Center in Greenbelt, Maryland, for initial decoding, and then to NASA's Jet Propulsion Laboratory in Pasadena, California, for further processing. The test provided the OCO-2 team with its first opportunity to see whether the instrument had reached orbit with the same performance it had demonstrated before launch.
- Over the next several weeks, the OCO-2 team will conduct a series of calibration activities to characterize fully the performance of the instrument and observatory. In parallel, OCO-2 will routinely record and return up to 1 million science observations each day. These data will be used initially to test the ground processing system and verify its products. The team will begin delivering calibrated OCO-2 spectra data to NASA's Goddard Earth Sciences Data and Information Services Center for distribution to the global science community and other interested parties before the end of the year. The team will also deliver estimates of carbon dioxide to that same center for distribution in early 2015.
Sensor complement: (OCO-2 instrument)
The OCO-2 instrument is almost a copy of the OCO instrument, designed and developed by HSSS (Hamilton Sundstand Sensor Systems), Pomona, CA. The only changes being made to the instrument are associated with parts obsolescence or mitigation of known performance issues (Table 4). 33) 34) 35) 36)
The objective of the OCO-2 spectrometers is to measure sunlight reflected off the Earth's surface. The rays of sunlight that enter the spectrometers will pass through the atmosphere twice, once as they travel from the Sun to the Earth, and then again as they travel from the Earth's surface to the OCO-2 instrument. Carbon dioxide and molecular oxygen molecules in the atmosphere absorb light energy at very specific colors or wavelengths. Thus, the light that reaches the OCO-2 instrument will display diminished amounts of energy at those characteristic wavelengths. The OCO-2 instrument employs a diffraction grating to separate the inbound light energy into a spectrum of multiple component colors. The reflection gratings used in the OCO-2 spectrometers consist of a very regularly-spaced series of grooves that lie on a very flat surface. The back of a compact disc is an everyday example of a diffraction grating. 37) 38)
The characteristic spectral pattern for CO2 can alternate from transparent to opaque over very small variations in wavelength. The OCO-2 instrument must be able to detect these dramatic changes, and specify the wavelengths where these variations take place. Thus, the grooves in the instrument diffraction grating will be very finely tuned to spread the light spectrum into a large number of very narrow wavelength bands or colors. Indeed, the OCO-2 instrument design incorporates 17,500 different colors to cover the entire wavelength range that can be seen by the human eye. A digital camera covers the same wavelength range using just three colors.
The OCO-2 experiment requires the measurement of three relatively small bands of electromagnetic radiation, where the spectral wavelength ranges of these three critical bands are widely separated. To accomplish this task economically, OCO-2 employs three spectrometers instead of one. Each spectrometer will measure light in one specific region of the spectrum. The focal plane associated with each spectrometer is designed to detect very fine differences in wavelength within each of these spectral ranges.
OCO-2 measurements must be very accurate. To eliminate energy from other sources that would generate measurement errors, the light detectors for each camera must remain very cold. To ensure that the detectors remain sufficiently cold, the OCO-2 instrument design will include a cryocooler, which is a refrigeration device. The cryocooler keeps the detector temperature at or near -120° C. 39) 40)
Table 4: Changes planned for the OCO-2 instrument
Figure 20: Illustration of the OCO-2 instrument (image credit: HSSS/JPL)
As for OCO, the OCO-2 instrument consists of three, co-bore-sited, long-slit, imaging grating spectrometers optimized for the O2 A-band at 0.765 µm and the CO2 bands at 1.61 and 2.06 µm (Figure 22). The 3 spectrometers use similar optical designs and are integrated into a common structure to improve system rigidity and thermal stability. They share a common housing and a common Cassegrain telescope (Ref. 3).
The telescope consists of an 11 cm aperture, as well as a primary and a secondary mirror. The relay optics assembly includes fold mirrors, dichroic beam splitters, band isolation filters and re-imaging mirrors. Each spectrometer consists of a slit, a two-lens collimator, a grating, and a two-lens camera. Each of the three spectrometers has essentially an identical layout. Minor differences among the spectrometers, such as the coatings, the lenses and the gratings, account for the different bandpasses that are characteristic of each channel. The focal ratios of the instrument optics will range from f/1.6 to f/1.9.
To implement an optically fast, high-spectral-resolution measurement system, the OCO-2 instrument combines refractive and reflective optical techniques. Since the light in the common telescope and relay optics assembly will not separate into the three distinct wavelength bands, these instrument subsystems use primarily reflective optics. On the other hand, the extremely narrow channel bandpasses make potential chromatic aberrations in the spectrometers negligible, which enable the use of refractive optics.
The light path is illustrated in Figures 21 and 22. Light entering the telescope is focused at a field stop and then re-collimated before entering a relay optics assembly. There, it is directed to one of the three spectrometers by a dichroic beam splitter, and then transmitted through a narrowband pre-disperser filter. The pre-disperser filter for each spectral range transmits light with wavelengths within ~±1% of the central wavelength of the CO2 or O2 band of interest and rejects the rest. The light is then refocused on the spectrometer slits by a reverse Newtonian telescope.
Figure 21: Illustration of OCO grating spectrometers and scheme of optical system (image credit: JPL/HSSS)
Each spectrometer slit is about 3 mm long and about 25 µm wide. These long, narrow slits are aligned to produce co-bore-sited fields of view that are ~0.0001 radians wide by ~0.0146 radians long. Because the diffraction gratings efficiently disperse only the light that is polarized in the direction parallel to the slit, a polarizer was included in front of the slit to reject the unwanted polarization before it enters the spectrometer, where it could contribute to the scattered light background.
Once the light traverses a spectrometer slit, it is collimated by a 2-element refractive collimator, dispersed by a gold-coated, reflective planar holographic diffraction grating, and then focused by a 2-element camera lens on a 2-dimensional FPA (Focal Plane Array), after traversing a second, narrowband filter. The narrowband filter just above the FPA is cooled to ~180 K to reject thermal emission from the instrument.
Following the OCO design, the spectral range and resolving power of each channel includes the complete molecular absorption band as well as some nearby continuum to provide constraints on the optical properties of the surface and aerosols as well as absorbing gases. To meet these requirements, the O2 A-band channel covers 0.758 to 0.772 µm with a resolving power of > 17,000, while the 1.61 and 2.06 µm CO2 channel cover 1.594 to 1.619 µm and 2.042 to 2.082 µm, respectively with a resolving power > 20,000.
The spectrometer optics project a 2-dimensional image of a spectrum on 1024 by 1024 pixel FPA with 18 µm pixels (Figure 23). The grating disperses the 1024-pixel wide spectrum in the direction perpendicular to the long axis of the slit. The full-width at half maximum (FWHM) of the slit image on the FPA is sampled by 2 to 3 pixels in the direction of dispersion. The length of the slit limits spatial field of view to only ~190 pixels in the dimension orthogonal to the direction of dispersion. Science measurements are restricted to the center ~160 of these 190 pixels.
For normal science operations, the FPAs are continuously read out at 3 Hz. To reduce the downlink data rate and increase the signal to noise ratio, ~20 adjacent pixels in the FPA dimension parallel to the slit (i.e. the "spatial direction" in Figure 23) are summed on board to produce up to 8 spatially-averaged spectra along the slit. The along-slit angular field of view of each of these spatially-averaged "super-pixels: is ~1.8 mrad (0.1º or ~1.3 km at nadir from a 705 km orbit). The angular width of the narrow dimension of the slit is only 0.14 mrad, but the focus of the entrance telescope was purposely blurred to increase the effective full width at half maximum of each slit to ~0.6 mrad to simplify the boresight alignment among the 3 spectrometer slits.
Figure 23: The illumination and readout scheme used for the OCO-2 FPAs (image credit: NASA/JPL)
In addition to the 8 spatially-binned, 1024-element spectra, each spectrometer also returns 4 to 20 spectral samples without on-board spatial binning to provide the full along-slit spatial resolution. Each of these full-resolution "color stripes" covers a 220 pixel wide region of the FPA that includes the full length of the slit (190 pixels) as well as a few pixels beyond the ends of the slit (Figure 23). These full-spatial-resolution color stripes are used to detect spatial variability within each of the spatially summed super pixels and to quantify the thermal emission and scattered light within the instrument.
For the OCO instrument, the entrance slits for the 3 spectrometers were carefully co-aligned during the optical bench assembly to ensure that all 3 spectrometers would share a common bore site. After the instrument vibration test, an optical component in the 1.61 µm CO2 channel shifted, introducing a ~70 arcsec shift in the bore site of that channel. The root cause of the misalignment was traced to a specific step in the optical bench assembly process. While it was not possible to correct this misalignment for the OCO instrument, a second vibration test was performed to ensure that no further movement would occur, and the science algorithms were modified to accommodate the pointing offset.
For the OCO-2 instrument, the optical bench assembly process has been modified to avoid this problem. This modification will be verified by performing a "seating vibration" followed by an alignment test prior to full optical bench integration.
The OCO instrument used Teledyne mercury cadmium telluride (HgCdTe) FPAs in the 1.61 and 2.06 µm CO2 channels and a silicon, HyViSiTM FPA in the O2 A-band channel. All 3 FPAs used Teledyne HAWAII-1RGTM read-out integrated circuits, so that a common design could be used for their control and readout electronics design.
New FPAs are needed for the OCO-2 instrument for two reasons.
- First, there were not enough high quality spare HgCdTe FPAs from the OCO instrument, to provide flight and flight spare FPAs for the CO2 channels in the OCO-2 instrument.
- Second, there was a strong desire to mitigate the residual image artifacts discovered in the HyViSi FPA during the OCO pre-flight instrument testing. The substrate-removed HgCdTe HAWAII-1RG FPA's from Teledyne, like those recently flight qualified for the Hubble Space Telescope Wide Field Camera-3 (WF3), could address both of these issues. These FPA's use the same electrical, thermal, and mechanical interfaces as those on the OCO instrument, minimizing the design changes needed for their accommodation. They also have slightly lower read noise than those used for the OCO instrument. In addition, because these FPAs are sensitive to the wavelengths sampled by the A-band as well as those sampled by the CO2 channels, it might be possible to use these FPA's in all 3 channels. This both reduces risk and provides an approach for mitigating the HyViSi residual image issues, because the HgCdTe FPAs show no evidence of this problem.
Because of their higher dark currents, the HgCdTe FPAs in the two CO2 channels on OCO were maintained below 120 K, while silicon FPA in the O2 A-band channel was cooled to < 180 K. For the OCO-2 instrument, the cryolinks to the FPA's have been redesigned to maintain all three FPA's at < 120 K. This change preserves the option of using substrate-removed HgCdTe FPA's in all three channels. It may also facilitate the use of an existing HyViSi FPA in the A-band channel. Recent laboratory tests show that operating the A-Band HyViSi FPA at 120 K, rather than 180 K reduces the amplitude of the residual image anomaly to almost undetectable levels.
New cryocooler: To cool its FPAs, the OCO instrument, used a pulse tube cryocooler of NGST (Northrup Grumman Space Technology) that was thermally coupled to an external radiator though variable conductance heat pipes. This cryocooler was the flight spare from the EOS Aura TES (Tropospheric Emission Spectrometer) project, and the last of its kind.
A different cryocooler was therefore needed for the OCO-2 instrument. A single-stage version of the NGST pulse tube cryocooler used by the NOAA ABI (Advanced Baseline Imager) projected for GOES-R (Geostationary Operational Environmental Satellite – R) was adopted to minimize the changes to the instrument's thermal and electrical interfaces. This cryocooler, referred to as CSS (Cryogenic Subsystem), is slightly smaller and more efficient than the one used by the OCO instrument, but did require changes in the cryocooler electronics and the heat pipes.
Figure 24: Photo of the pulse tube cryocooler (image credit: NASA/JPL)
Figure 25: Photo of CSS integrated with OBA (Optical Bench Assembly), image credit: NASA/JPL
Table 5: Overview of OCO-2 instrument parameters
For normal science operations, the spacecraft bus orients the instrument to collect science data in Nadir, Glint, and Target modes. The varies modes optimize the sensitivity and accuracy of the observations for specific applications.
1) Nadir mode: the instrument views the ground directly below the spacecraft (local nadir). Observations are made whenever the solar zenith angle is < 85º.
2) Glint mode: the instrument views the location where sunlight is directly reflected on the Earth's surface (the OCO-2 is pointed towards the bright spot where solar radiation is specularly reflected from the surface). The glint mode enhances the instrument's ability to acquire highly accurate measurements, particularly over the ocean. Also, the Glint measurements over the ocean should provide much higher SNR values. Glint soundings are being collected at all latitudes - whenever the local solar zenith angle is < 75º. OCO-2 switches from Nadir to Glint modes on alternate 16-day global ground-track repeat cycles so that the entire Earth is mapped in each mode on roughly monthly time scales.
3) Target mode: the instrument views a specified surface target (calibration site) continuously as the satellite passes overhead (max pass duration up to 9 minutes). The Target mode provides the capability to collect a large number of measurements over sites where alternative ground-based and airborne instruments also measure atmospheric CO2.
- The instrument collects up to 12000 soundings during a single 9-minute overpass at surface observation zenith angles between 0 and 75º
- A small oscillation can be superimposed on pointing to scan the spectrometer bore site across the target as the observatory flies overhead, imaging a 15 x 30 km area: ideal for mapping point sources within cities.
Figure 26: Schematic view of the observation modes (image credit: NASA/JPL, Ref. 41)
Naturally, the size of the footprint increases when observations are made in Glint or in Target modes (that are different from nadir).
For OCO, the nominal plan was to switch from Nadir to Glint observations on alternate 16-day global ground-track repeat cycles so that the entire Earth is mapped in each mode every 32 days. A similar approach has been adopted for OCO-2. Comparisons between Nadir and Glint observations will provide opportunities to identify and correct for biases introduced by the viewing geometry. Target observation will be acquired over an OCO-2 validation site roughly once each day.
The same data sampling rate is used for Nadir, Glint, and Target observations. In each mode, the instrument can collect up to 8 soundings over its 0.8º wide swath every 0.333 s. For nadir observations from a 705 km orbit, traveling at ~7 km/second, the 0.333 s frame rate yields surface footprints with down-track dimensions < 2.25 km. The cross-track dimension of the swath depends on the orientation of the slit with respect to the orbit path, which changes as the spacecraft travels from south to north along its orbit track.
Near the polar terminators, when the spectrometer slit is oriented perpendicular to the orbit track, the cross-track swath for nadir observations is ~10.5 km wide. At the sub-solar latitude, where the spectrometer slit is almost perpendicular to the orbit track, the cross-track dimension of the swath is limited to the projected width of the slit, which is only about 0.1 km wide at nadir.
Figure 27: Variation of OCO-2 footprint size and orientation during an orbit (image credit: NASA/JPL)
Pre-flight calibration: The instrument consists of three imaging spectrometers, one for each band (Figure 28) . The radiometric calibration process requires characterizing both the dark current level and gain coefficients of each instrumental channel. Light is directed into the spectrometers through a common telescope and a series of beam splitters and reimagers. Just before the incoming light enters each spectrometer, a linear polarizer selects the polarization vector parallel to the entrance slit. Each spectrometer works in the first order and uses a flat holographic grating. 42)
At each spectrometer's focus, an area array collects the spectrum. As is typical in imaging spectrometers, one dimension measures the field angles along the slit, and the other dimension measures the different wavelengths.
Figure 28: Schematic view of the spectrometer optics chain as well as the integrating sphere for the radiometric ground test. Lamp D is external to the integrating sphere, and its brightness is controlled by an adjustable slit (image credit: NASA/JPL)
Calibration approach: The OCO-2 instrument focal planes will record the brightness of the incident spectral radiances as raw data numbers (DN). Data numbers are measures without units. For the OCO-2 mission, data numbers that represent spectral radiances range from 0 to 216. The OCO-2 Ground Data System will be responsible for the conversion of these data numbers into wavelength-dependent measurements that are expressed in meaningful physical units. The physical units that the OCO-2 mission will use for spectral radiance are photons per square meter per steradian per second. Radiometric calibration is the process that collects and applies the parameters needed to convert the instrument output into physical measures. The radiance measure generated by the calibration process will be the critical component of the OCO-2 Level 1B Product. 43) 44)
A dedicated team will oversee the radiometric calibration process for the entire OCO-2 mission. This team will maintain the algorithms and update the parameters required to generate accurate radiances. Ongoing calibration exercises during the space operations of OCO-2 will ensure, that the mission obtains bias-free radiance measurements. Accurate radiance measures are crucial to retrieve XCO2 with the precision needed to determine the geographic distribution of CO2 sources and sinks. This aspect of the OCO-2 mission is vital as CO2 sources and sinks must be inferred from small (<2%) spatial variations in XCO2. OCO-2 will have a precision of <0.3% (1 ppm), thus allowing for the quantification of CO2 sources and sinks.
The calibration team will characterize the instrument on the ground before Observatory launch. The characterization exercise will yield the initial set of parameters required to convert instrument data numbers into incident radiances. Once the instrument begins to operate in flight, its behavior will change due exposure to the space environment. For the remainder of the mission, the calibration team will use on-board calibration capabilities to track instrument behavior, and modify the calibration parameters to ensure accurate assessment of instrument measure.
On orbit calibration: The OCO-2 instrument employs an OBC (On Board Calibrator) to detect changes in the instrument gain and wavelength response. While the spacecraft flies over the dark side of the Earth, the instrument will automatically collect calibration data using the OBC. Unlike the OCO-2 science data, clear sky conditions will not be required to acquire this data. The mission will regularly perform four types of calibration using the OBC. Each calibration generates a unique data collection, which include:
• Cal_solar data: To collect these data, the instrument will deploy an attenuation screen in front of the telescope. The Observatory will point the instrument telescope at the Sun while the instrument line of sight is above the Earth's atmosphere. The radiances and the wavelengths of the spectral lines in the solar spectrum are well established. Thus, radiances recorded in the Cal_solar data will provide a means to calibrate the absolute instrument response as well as relative instrument response among the three OCO-2 spectrometers. The wavelengths where radiances appear in the Cal_solar data will also provide a means to calibrate the spectral wavelength associated with each spectral sample.
• Cal_limb data: These data are an extension of the Cal_solar data. The Observatory will acquire both data sets in sequence. Acquisition of the Cal_solar data will immediately precede the Cal_limb data. The instrument attenuation screen will remain deployed and the instrument telescope will continue to view the Sun. However, as the Observatory orbit progresses, the instrument's line of sight will pass through the Earth's atmosphere. Thus, Cal_limb spectra will contain both Solar absorption lines as well as absorption lines that are characteristic of the atmosphere's chemical content.
Figure 29: Overview of the OBC (highlighted blue circle) that overlays the instrument telescope/collimator assembly. (image credit: NASA/JPL)
• Cal_dark data: The mission will employ two means to collect these data. Either the instrument will view the dark ocean at night, or the instrument will apply a cover to the viewing telescope. Cal_dark data will specify the focal plane response for a totally dark scene. Thus, these values will specify the "zero point" on the radiance scale. Once a calibration is applied, measurements that are equivalent to the "zero point" will indicate no incident light.
Cal_dark data will be collected at two points on the night side of the orbit. One set of Cal_dark data will always collected at the same relative location of the Observatory orbit relative to the day-night terminator. These data monitor long-term drift of the zero point. A second set of Cal_dark data will be collected at different locations over the night side of the orbit. These data will monitor shifts in the zero-point offset that are associated with changes in instrument or spacecraft temperature.
• Cal_lamp data. To collect these data, the instrument will turn on one of three small light bulbs. Light from the bulb will illuminate a reflector. The reflector will diffuse the light to produce a uniform field that is directed into the instrument telescope. Since the spatial and spectral distribution from these bulbs is uniform and well known, Cal_lamp data will provide the "flat fields" that are used to define the relative radiometric response for each detector on the focal plane.
• Vicarious Calibration (VC) will employ precise in situ measurements collected at the Earth's surface to estimate the solar radiation field at the top of the Earth's atmosphere. The calibration team will compare these data with measurements acquired by the Observatory. The comparisons will yield a correction table. Application of the correction table will force OCO-2 measurements to conform to the Vicarious Calibration experiment. This adjustment will provide both an absolute and channel-relative calibration for OCO-2 data products.
• "Flat fielding" will employ Earth scene statistics which will be collected from sets of Nadir Mode data. These statistics will provide a means to verify the sample-relative-gain coefficients generated by the On Board Calibrator. A consistent decrease in the radiance for one sample, as compared to a neighbor sample at the same wavelength in the same spectrum, will indicate an error in the calibration process. When systematic effects are detected, the team will apply the Earth scene statistics to update the characterization of the On Board Calibrator.
• Cal_doppler observations will view the Sun over one entire day side of the orbit. These data will provide measurements of the solar spectrum over the full range of Doppler shifts that the Observatory encounters. These data will also provide a means to apply Doppler corrections to instrument wavelength measurements.
Figure 30: Routine and special calibration observations ensure on-orbit performance (image credit: NASA/JPL)
To ensure accuracy, the spaceborne CO2 estimates are validated through comparisons with near-simultaneous measurements of CO2 acquired by ground-based Fourier Transform Spectrometers in TCCON (Total Carbon Column Observing Network). This network currently includes over a dozen stations, distributed over a range of latitudes spanning Lauder, New Zealand and Ny Alesund, Norway, and is continuing to grow. To relate TCCON measurements to the WMO CO2 standard, aircraft observations have been collected over several stations, using the same in situ CO2 measurement approaches used to define that standard. OCO-2 will target a TCCON site as often as once each day, acquiring thousands of measurements as it flies overhead. These measurements will be analyzed to reduce biases below 0.1% (0.3 ppm) at these sites. The spaceborne CO2 estimates will be further validated through comparisons with CO2 and surface pressure measurements from ground based sites with the aid of data assimilation models to provide a more complete global assessment of measurement accuracy (Ref. 3).
In May 2004 a new approach for studying greenhouse gases in our atmosphere came from an unlikely source: a lone trailer in Park Falls, WI, USA. That site became the first station of the TCCON, a ground-based network of instruments providing measurements and data to help better understand the sources and sinks of carbon dioxide (CO2) and methane (CH4) to and from Earth's atmosphere. Now, a decade after the first site became operational, TCCON has expanded and provides important information about regional and global atmospheric levels of carbon-containing gases from many stations, worldwide (Figure 31). 45)
Figure 31: TCCON has expanded rapidly over the last decade and data have been obtained from 22 locations (red dots) spread around the globe. Blue squares indicate future stations (image credit: Caltech)
Each of the TCCON stations accommodates a FTS (Fourier Transform Spectrometer) that provides precise measurements of the amount of direct sunlight absorbed by atmospheric gases. At each site, the FTS produces a spectrum of sunlight; from that spectrum, researchers determine the abundance of CO2, CH4, carbon monoxide (CO), and other gases in the atmospheric column extending from the surface of the Earth to the top of the atmosphere. In the absence of clouds, one measurement is made approximately every two minutes.
Data from the individual stations provide information about regional carbon sources and carbon sinks. Furthermore, by combining the data from all the stations, researchers can monitor carbon as it is exchanged—"circulates"—between the atmosphere, the land, and the ocean, explains atmospheric chemist Paul Wennberg at Caltech, who is the elected chair of TCCON.
The TCCON is a partnership arrangement. Although the TCCON stations are scattered around the globe and are overseen by numerous investigators, every partner has agreed on what instruments are used and how they are operated; everyone is using a common analysis software so that the measurements are comparable across the whole network.
Originally, data from each of these stations were intended to help validate measurements obtained from NASA's OCO (Orbiting Carbon Observatory) satellite, which failed upon launch in 2009 due to a faulty fairing separation. OCO and TCCON [were to] provide a new type of data—a type of CO2 measure that had never been used before, called the column average mixing ratio. Measurements from TCCON provide the precise column average mixing ratio of CO2 at discrete locations around the world, and OCO would have provided a similar measurement from space; comparing the two at coincident times and locations were to provide an important evaluation of the satellite data.
Despite the loss of OCO, TCCON continued to expand in recognition of its importance in carbon cycle science and for validation of other remote sensing projects. TCCON provided the very first key observations regarding column average data, long before there were spaceborne estimates.
Table 6: TCCON station locations, lead investigators, and institutions
Ten Years of Data: Discoveries and Contributions: Over the years, studies using data from TCCON stations have revealed new information about the sources and sinks of CO2 and CH4. These include the discovery of elevated CH4 emissions from Los Angeles, CA, and Four Corners, NM, as well as regional enhancements of CO2 from fossil fuel emissions. Furthermore, TCCON has provided key observations on how uptake of CO2 by the boreal forest—northern forests that span the range from Alaska to Siberia—depends on surface temperature. More broadly, data from TCCON are also being used to evaluate large-scale carbon models and improve global estimates of the sources and sinks of CO2 and CH4 (Figure 32). Understanding the interactions between climate and carbon dynamics is critical for predicting future levels of atmospheric CO2.
The network's ability to collect very precise data has also proved to be very useful for validating the European Space Agency's SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY), which flew on Envisat, launched in 2002, and was the first instrument to yield global measurements of CO2 and CH4 from space. John Burrows, PI of SCIAMACHY remarks, that the creation of TCCON filled a key missing element in the observational system required to meet the challenge [of quantifying] greenhouse gases. In fact, the combination of the SCIAMACHY and TCCON datasets became a milestone in remote sensing, revealing important carbon sources and sinks in Europe, North America, and Siberia. The unprecedented combination of ground-based and spaceborne measurements helped to underscore the importance of wet-land sources of CH4 and the impact of increased CH4 from fracking and oil fields. TCCON has pioneered a key element of the ground segment measurements required to provide the evidence base for policy making for the next 100 years.
More recently, TCCON data have been the core of the validation effort for CO2 and CH4 measurements from the Japanese GOSAT (Greenhouse Gases Observing Satellite) that was launched in January 2009. Osamu Uchino of JAXA says that TCCON has been and will [continue to] be a key [player] in the GOSAT product validation, and together, both TCCON and GOSAT data are contributing significantly to carbon-cycle science.
Figure 32: [Top] Observations of CO2 from TCCON stations have shown that over the past decade, the column mole fraction of CO2 (XCO2) has increased by more than 20 parts per million (ppm). In fact, this past winter (2013-14) all sites in the Northern Hemisphere exceeded 400 ppm. [Bottom] TCCON observations indicate the CH4 concentrations have also increased substantially since 2006–07.
With the launch of OCO-2, TCCON is now on-track to fulfill its initial purpose—but the network has already proven itself to be a success, and it is due in large part to the network's inherent spirit of collaboration.
Figure 33: Expected sampling for a single 16-day ground track repeat cycle, where OCO-2 is collecting glint observations (image credit: NASA, Ref. 15)
The OCO ground system is designed around communicating with the observatory once per day for downlink passes. The majority of the on orbit operations repeat in 16 day cycles (oscillating between glint and nadir, orbits). The target observations happen daily, and require daily uplinks. These targets are chosen by the validation team the day prior and communicated to the Orbital Mission Operations Team. Following these target selections, the appropriate commands are sent to the spacecraft. The high volume of science data returned requires a highly automated science & data processing system with a high level of coordination between the science team at JPL and the Operations teams at OSC & GSFC. The OCO-2 ground system is designed to facilitate the coordination of multiple teams around a daily data uplink and downlink (Figure 34).
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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 (email@example.com).