GeoCARB (Geostationary Carbon Cycle Observatory)
GeoCARB is a NASA Earth science mission with the goal to measure key greenhouse gases and vegetation health from space to advance our understanding of Earth’s natural exchanges of carbon between the land, atmosphere and ocean. The PI mission is led by Berrien Moore of the University of Oklahoma in Norman, Oklahoma. The primary objectives are to monitor plant health and vegetation stress throughout the Americas, and to probe, in unprecedented detail, the natural sources, sinks and exchange processes that control carbon dioxide, carbon monoxide and methane in the atmosphere. 1)
The investigator-led mission will launch on a commercial communications satellite to make observations over the Americas from an orbit of approximately 35,786 km above the equator. In December 2016, NASA selected the GeoCarb mission was competitively from 15 proposals submitted to the agency's second Earth Venture - Mission announcement of opportunity for small orbital investigations of the Earth system.
GeoCARB will measure daily the total concentration of carbon dioxide, methane and carbon monoxide in the atmosphere with a horizontal ground resolution of 5 to 10 km. GeoCARB also will measure solar-induced fluorescence, a signal related directly to changes in vegetation photosynthesis and plant stress.
GeoCARB is the second spaceborne investigation in the EVM-2 (Earth Venture - Mission) series of rapidly developed, cost-constrained projects for NASA's Earth Science Division. The CYGNSS (Cyclone Global Navigation Satellite System), selected in 2012, is the first mission in the series and launched from Florida on Dec. 15, 2016.
The University of Oklahoma-led GeoCARB team will build an advanced payload that will be launched on a commercial communications satellite, employing otherwise unused launch and spacecraft capacity to advance science and provide societal benefit. By demonstrating GeoCARB can be flown as a hosted payload on a commercial satellite, the mission will strengthen NASA’s partnerships with the commercial satellite industry and provide a model that can be adopted by NASA’s international partners to expand these observations to other parts of the world.
Mission partners include the Lockheed Martin Advanced Technology Center in Palo Alto, California; SES Government Solutions Company in Reston, Virginia; the Colorado State University in Fort Collins; and NASA/ARC (Ames Research Center) in Moffett Field, California, GSFC (Goddard Space Flight Center) in Greenbelt, Maryland, and JPL (Jet Propulsion Laboratory) in Pasadena, California.
The Earth Venture missions are part of NASA's ESSP (Earth System Science Pathfinder) program. The Venture Class small, targeted science investigations complement NASA's larger research missions. A National Academies 2007 report, 'Earth Science and Applications from Space Decadal Survey', recommended NASA undertake these regularly solicited, quick-turnaround projects.
In early December 2016, NASA HQ announced that the GeoCARB (Geostationary Carbon Cycle Observatory) was selected among 15 submitted proposals as the EVM-2 mission. The mission will monitor plant health and vegetation stress throughout the Americas, and probe the natural sources, sinks, and exchange processes that control carbon dioxide, carbon monoxide, and methane in the atmosphere.
GeoCARB will launch as a hosted payload on a commercial geosynchronous communications satellite. The intent is for GeoCARB to employ otherwise unused launch and spacecraft capacity to advance science and provide societal benefits. By demonstrating that it can be flown as a hosted payload on a commercial satellite, GeoCARB will strengthen NASA’s partnerships with the commercial satellite industry and provide a model that can be adopted by NASA’s international partners to expand these observations to other parts of the world.
Carbon is a building block of life on our planet. It is stored in reservoirs on Earth—in rocks, plants and soil—in the oceans, and in the atmosphere. And it cycles constantly between these reservoirs. Understanding the carbon cycle is crucially important for many reasons. It provides us with energy, stored as fossil fuel. Carbon gases in the atmosphere help regulate Earth’s temperature and are essential to the growth of plants. Carbon passing from the atmosphere to the ocean supports photosynthesis of marine phytoplankton and the development of reefs. These processes and myriad others are all interwoven with Earth’s climate, but the manner in which the processes respond to variability and change in climate is not well-quantified. 2)
The famous “Keeling curve,” which tracks CO2 concentrations in Earth’s atmosphere, is based on daily measurements at the Mauna Loa Observatory on Hawaii. It shows that global CO2 levels are rising over time, but also change seasonally due to biological processes. CO2 decreases during the Northern Hemisphere’s spring and summer months, as plants grow and take CO2 out of the air. It rises again in fall and winter when plants go relatively dormant and ecosystems “exhale” CO2.
Figure 1: Recorded starting in 1958 by the late geochemist Charles David Keeling, the Keeling curve measures atmospheric carbon dioxide concentrations (image credit: Scripps Institution of Oceanography)
A closer look shows that every year’s cycle is slightly different. In some years the biosphere takes more CO2 out of the atmosphere; in others it releases more to the atmosphere. We want to know more about what causes the year-to-year differences because that contains clues on how the carbon cycle works.
For example, during the El Niño of 1997-1998, a sharp rise in CO2 was largely driven by fires in Indonesia. The most recent El Niño in 2015-2016 also led to a rise in CO2, but the cause was probably a complex mixture of effects across the tropics – including reduced photosynthesis in Amazonia, temperature-driven soil release of CO2 in Africa and fires in tropical Asia.
These two examples of year-to-year variability in the carbon cycle, both globally and regionally, reflect what we now believe – namely, that variability is largely driven by terrestrial ecosystems. The ability to probe the climate-carbon interaction will require a much more quantitative understanding of the causes of this variability at the process level of various ecosystems.
GeoCarb will be launched into geostationary orbit at roughly 85 degrees west longitude, where it will rotate in tandem with the Earth. From this vantage point, the major urban and industrial regions in the Americas from Saskatoon (the largest city in the Canadian province of Saskatchewan) to Punta Arenas (in the very south of Chile) will be in view, as will the large agricultural areas and the expansive South American tropical forests and wetlands. Measurements of carbon dioxide, methane and carbon monoxide once or twice daily over much of the terrestrial Americas will help resolve flux variability for CO2 and CH4.
GeoCarb also will measure SIF (Solar Induced Fluorescence) – plants emitting light that they cannot use back out into space. This “flashing” by the biosphere is strongly tied to the rate of photosynthesis, and so provides a measure of how much CO2 plants take in.
NASA pioneered the technology that GeoCarb will carry on an earlier mission, the OCO-2 (Orbiting Carbon Observatory-2). OCO-2 launched into a low Earth orbit in 2014 and has been measuring CO2 from space ever since, passing from pole to pole several times per day as the Earth turns beneath it.
Figure 2: This map, pieced together with data from NASA's OCO-2, shows average global atmospheric carbon dioxide concentrations from Oct. 1 through Nov. 11,2014 (image credit: NASA/JPL)
Though the instruments are similar, the difference in orbit is crucial. OCO-2 samples a narrow 10 km track over much of the globe on a 16 day repeat cycle, while GeoCarb will look at the terrestrial Western Hemisphere continuously from a fixed position, scanning most of this land mass at least once per day.
Where OCO-2 may miss observing the Amazon for a season due to regular cloud cover, GeoCarb will target the cloud-free regions every day with flexible scanning patterns. Daily revisits will show the biosphere changing in near-realtime alongside weather satellites such as GOES 16, which is located at 105 degrees west, helping to connect the dots between the components of Earth’s system.
Nuances of the carbon cycle: Many processes affect levels of CO2 in the atmosphere, including plant growth and decay, fossil fuel combustion and land use changes, such as clearing forests for farming or development. Attributing atmospheric CO2 changes to different processes is difficult using CO2 measurements alone, because the atmosphere mixes CO2 from all of the different sources together.
In addition to CO2 and CH4, GeoCarb will measure CO. Burning fossil fuel releases both CO and CO2. This means that when we see high concentrations of both gases together, we have evidence that they are being released by human activities. - Making this distinction is key so we do not assume that human-induced CO2 emissions come from a decrease in plant activity or a natural release of CO2 from soil. If we can distinguish between man-made and natural emissions, we can draw more robust conclusions about the carbon cycle. Knowing what fraction of these changes is caused by human activities is important for understanding our impact on the planet, and observing and measuring it is essential to any conversation about strategies for reducing CO2 emissions.
GeoCarb’s measurement of methane will be a crucial element in understanding the global carbon-climate system. Methane is produced by natural systems, such as wetlands, and by human activities such as natural gas production. We do not understand the methane portion of the carbon cycle as well as CO2. But just as with CO2, methane observations tell us a lot about the functioning of natural systems. Marshes release methane as part of the natural decay in the system. The rate of release is tied to how wet/dry and warm/cool the system is.
It is uncertain how much natural gas production contributes to methane emissions. One reason to quantify these emissions more accurately is that they represent lost revenue for energy producers. The EPA (Environmental Protection Agency) estimates a U.S. leakage rate of around 2 percent, which could add up to billions of dollars annually.
We expect, based on simulations, that GeoCarb will produce maps that highlight the largest leaks with only a few days of observations. Finding leaks will reduce costs for energy producers and reduce the carbon footprint of natural gas. Currently, energy companies find leaks by sending personnel with detection equipment to suspected leak sites. Newer airborne sensors could make the process cheaper, but they are still deployed on a limited basis and in an ad hoc manner. GeoCarb’s regular observations will provide leakage information to producers in a timely manner to help them limit their losses.
Watching the planet breathe: With daily scans of landmasses in the Western Hemisphere, GeoCarb will provide an unprecedented number of high-quality measurements of CO2, CH4 and CO in the atmosphere. These observations, along with direct measurements of photosynthetic activity from SIF observations, will raise our understanding of the carbon cycle to a new level.
For the first time we will be able to watch as the Western Hemisphere breathes in and out every day, and to see the seasons change through the eyes of the biosphere. Equipped with these observations, we will begin to disentangle natural and human contributions to the carbon balance. These insights will help scientists make robust predictions about Earth’s future.
SES-Government Solutions has been selected as the GeoCarb host spacecraft provider as it delivers the best overall combination of flight experience, ability to provide an effective and suitable S/C host, experience and willingness to work with the PI and NASA, available launch windows and longitudes, and cost. 3)
The GeoCarb instrument will be hosted and launched on a commercial geostationary communications platform. This approach leverages the large size of these vehicles, their frequent launch to orbits over large population areas, their wide communications downlink bandwidth, and the availability of competitive pricing rates for hosting.
GeoCarb accommodation: The instrument will be accommodated on the nadir deck of the SES-GS selected communications S/C. The GeoCarb team worked with SES-GS to validate the GeoCarb assumptions for accommodation by performing a study where GeoCarb is accommodated on a representative communications spacecraft.
The instrument is designed to be flexible and will need minor changes to interfaces for easy accommodation on the spacecraft. The instrument configuration ensures the visible and thermal Field of View meet the requirements of the instrument.
Due to the 10-to-15-year life requirement for a communications S/C and the end-of-life power requirements for the solar arrays and power subsystem, there is an excess of power available during the three years required for the GeoCarb project that far exceed worst-case estimates for the payload. No special pointing of the S/C is required due to the flexible pointing capabilities of the instrument.
An SES Government Solutions commercial satellite has provided economical access to geostationary orbit above 85° West longitude. GeoCarb benefits from existing infrastructure for command and control, and mission data delivery. The communications satellite can easily accommodate the mass, data downlink rate and power requirements of the GeoCarb instrument payload.
Launch: A launch of the hosted GeoCarb instrument is targeted for 2022 on a commercial SES-Government Solutions communications satellite flying in geostationary orbit, at about 85º W longitude.
From this vantage point, the GeoCarb instrument will produce maps of the column averaged dry air mixing ratios of CO2 (XCO2), CH4, (XCH4), CO (XCO) and SIF (Solar Induced chlorophyll Fluorescence) at a spatial resolution of 5-10 km multiple times each day. 4) 5)
The GeoCarb instrument views reflected light from Earth through a narrow slit. When the slit is projected onto Earth's surface, it sees an area measuring about 2,700 km from north to south and about 5.2 km from east to west. In comparison, OCO-2's swath is about 10 km wide. GeoCarb stares at that area for about 4-1/2 seconds, then the slit is moved half a slit width — 3 km — to the west, allowing for double sampling. With this technique, GeoCarb can scan the entire continental United States in about 2-1/4 hours, and from Brazil to South America's West Coast in about 2-3/4 hours. It is not designed to observe the oceans, as reflectivity over the oceans is too low to provide useful data. 8) 9) 10)
Figure 3: Artist's illustration of a concept for NASA's GeoCarb mission, which will map concentrations of key carbon gases above the Americas from geostationary orbit (image credit: NASA/Lockheed Martin/University of Oklahoma)
The GeoCarb instrument consist of the aperture assembly, telescope, spectrometer, and electronics boxes. It is a 4-Infrared channel, single-slit imaging spectrograph optimized to measure carbon dioxide, carbon monoxide and methane, and Solar-Induced Fluorescence (SIF) from geostationary orbit. The fourth channel, measures oxygen dioxide column concentration and, SIF, providing valuable information on aerosol and cloud contamination.
There are two grating spectrometers, four focal plane assemblies cooled to 125 K (degrees Kelvin) and Cold Optical Bench cooled to ~190 K . Cooling by Northrup Grumman thermal-mechanical unit. 11)
Table 1: Key parameters of the GeoCarb instrument
Figure 4: Left: Illustration of the GeoCarb on the spacecraft; Right: schematic view of the GeoCarb instrument (image credit: GeoCarb Team)
Figure 5: On orbit calibration (image credit: GeoCarb Team)
Instrument Design and Trace Gas Retrievals
The GeoCarb instrument will be hosted on a SES Government Solutions satellite in GEO orbit at 85º (±15º) West longitude, and it will be launch in 2022. The ~85º W slot allows observations of major urban and industrial regions, large agricultural areas, and the expansive South American tropical forests and wetlands, which will help resolve climate-critical flux variability for CO2 and CH4. GeoCarb deploys, as noted, a 4-channel slit imaging spectrometer that measures reflected near-IR sunlight at wavelengths 1.61 and 2.06 µm for column integrated CO2 dry air mixing ratio (XCO2), and 2.32 µm for XCH4 and XCO. The fourth channel, 0.76µm, measures total column O2, which allows determination of mixing ratios. The 0.76 µm channel also allows measurement of SIF and provides valuable information on aerosol and cloud contamination. It is worth noting that the O2 and CO2 channels are similar to those used for the OCO-2 mission and that the O2 spectral band is identical to that of OCO-2. 12)
Figure 6: An SNR-optimized scanning strategy for GeoCarb (image credit: GeoCarb Team, (Ref. 10)
The retrieval of SIF (Solar-Induced Fluorescence), XCO2, XCH4, and XCO is accomplished through the use of an optimal estimation technique that draws upon the heritage of the Atmospheric Carbon Observing System (ACOS) algorithm developed initially for OCO and GOSAT, and with numerous refinements updates for OCO-2. In brief, a first guess of each gas profile as well as other atmospheric parameters are propagated through a radiative transfer model to produce a simulated spectra, which is compared against the measured spectra in each band. The difference between the simulated and measured spectra is propagated back into updated gas concentrations and atmospheric parameters, and the process is repeated until the algorithm converges. SIF is produced separately by a different algorithm, called the Iterative Maximum a Posteriori Differential Optical Absorption Spectroscopy algorithm, which is also used to screen clouds. GeoCarb will employ the OCO-2 algorithm, modified to include the longest wavelength band, in which OCO-2 does not take spectra.
The instrument scan slit design allows for a large North-South (N-S) extent with high spatial resolution along the scan. The on-board N-S extent of the scan is fixed at a 4.4º view angle, which corresponds to 25º in latitude or 2,800 km at nadir on the Earth’s surface. Each scan is composed of 1016 N-S samples spaced 2.7 km apart on center and collected with 3 km East-West (E-W) double sampling. Scans involve a 4.08 s integration time, followed by a 0.3825 s E-W step. This instrument configuration allows GeoCarb to scan the conterminous United States (CONUS) in <2.5 h. The scan patterns are flexible; scan blocks can be changed, and the scan strategy can be updated to observe areas of greater interest or uncertainty, for calibration and validation, or for transient events in a campaign mode. Figure 7 shows a sample coverage map for the Mexico City area for both OCO-2 and GeoCarb. By sweeping the slit from East to West, GeoCarb provides continental-scale “mapping-like” coverage, producing daily maps of XCO2, XCH4, XCO, and SIF over regions of interest, which enables CO2 and CH4 flux estimation and attribution at unprecedented temporal and spatial scales.
Figure 7: Sample field of view for the area surrounding Mexico City. The green parallelograms represent individual OCO-2 soundings, and each track is traversed in a few minutes, but with a 16 day latency between revisits. By comparison, the red rectangles represent individual GeoCarb soundings, and the shaded region depicts a single observing slit projection. Every 4.4625s, the shaded red area will shift a half width to the left, allowing the area to be scanned in full in a few minutes. The map of North America in the upper left shows the full N/S extent of the slit projection. The green triangles represent power plants that produce 3–10 TgC (Teragram of carbon) per year in emissions. GeoCarb would make it possible to verify the reported emissions rates with top down emissions estimates (image credit: GeoCarb Team)
The ~85º W slot enables, as mentioned, observations of most major urban and industrial regions in the Americas, large agricultural areas, and the expansive South American tropical forests and wetlands. Each of these regions plays a key role in the global carbon cycle, and thus GeoCarb observations will help to provide a climate-critical insight into the Carbon-Climate connection as well as monitoring large “point” sources (e.g., cities) of all three gases, and helping to disaggregate anthropogenic and biogenic emissions using all three gases in tandem.
Several Observing Systems Simulation Experiments (OSSEs) were performed in order (a) to determine useful measurement requirements that are technically feasible and (b) to examine the potential for significant scientific advances that will be made possible with observations from GeoCarb. These OSSEs were designed with a set of hypotheses in mind, though there are numerous other important questions that could be addressed.
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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).