OCO-2 – Orbiting Carbon Observatory 2

Image: NASA/JPL
Image: NASA/JPL
Photo: NASA Kennedy
Photo: NASA Kennedy

OCO-2 – the Orbiting Carbon Observatory 2 is a NASA mission studying carbon dioxide in Earth’s atmosphere on a global scale for a better understanding of the carbon cycle, and the natural processes and human activities that have an effect on the abundance and distribution of CO2, the most important greenhouse gas. Tracking and quantifying CO2 sources and sinks on a global scale over time and will allow scientists to improve forecasts of climatic changes.

The mission will uncover the processes that control the distribution of carbon dioxide in the atmosphere, quantifying the sources (natural and man-made) that emit CO2 into the atmosphere and also study the sinks of carbon dioxide. The CO2 sinks are of great importance as they absorb about half the carbon dioxide emitted by human activities, but it is not known how long this reduction of CO2 can continue which has significant implications for the rate of the CO2 build-up and its impact on Earth’s climate.

OCO-2 is the re-flight of the original OCO that was launched in February 2009 aboard a Taurus XL rocket that failed to reach orbit due to a payload fairing separation failure. After the failure, studies were made for flying the OCO instrument as an integrated payload aboard a different platform or secure funding for a re-flight of the original design of the mission. Funding was secured in 2010 and construction was underway in 2011 and 2012 before testing started in 2013 working towards launch in 2014 aboard a Delta II rocket from Vandenberg Air Force Base.

Spacecraft Overview

Photo: Orbital Sciences
Photo: Orbital Sciences

OCO-2 uses the LEOStar-2 spacecraft bus manufactured by Orbital Sciences to facilitate a range of payloads with masses up to 150 Kilograms. The OCO-2 satellite carries a single instrument consisting of three high-resolution spectrometers to detect atmospheric carbon dioxide.

Overall, the spacecraft has a mass of 455 Kilograms and consists of a hexagonal primary structure that is 0.94 meters wide and 2.12 meters long providing the mounting structure for all satellite systems.

Aluminum alloy struts and composite panels make up the internal and external satellite structure, providing installation locations for all satellite components.

OCO-2 uses two deployable solar arrays, each consisting of two panels to create a total array length of three meters. Each panel is 0.66 by 1.47 meters in size for a total surface area of 3.88 square meters. With the arrays deployed, the OCO-2 spacecraft has a span of 9 meters.

The solar array booms that interface with the spacecraft include motors for solar array rotation to track the sun based on sun sensor data. The solar arrays use triple-junction Gallium Arsenide solar cells to deliver an average power of 900 Watts to a 35 Amp-hour Nickel-Hydrogen Battery. A Power Conditioning and Distribution Unit controls the state of charge of the battery and provides the redundant 28-Volt power bus to the various satellite systems.

Image: NASA
Image: NASA
Star Trackers - Photo: Sodern
Star Trackers – Photo: Sodern
MIMU - Image: Honeywell
MIMU – Image: Honeywell
Reaction Wheel - Image: Ithaco Space Systems
Reaction Wheel – Image: Ithaco Space Systems

OCO-2 uses a combination of active and passive thermal control. Heaters are installed to maintain the internal components and avionics of the spacecraft within their operational range during the night portion of the orbit. Blankets and thermal paints used on the exterior of the spacecraft provide insulation when the satellite is illuminated by the sun. Excess heat is removed from the electronics using heat pipes connected to radiators that remove heat from the interior of the spacecraft. The Focal Plane Assemblies of the instruments use a dedicated cryocooler to maintain their operational temperature.

The Attitude Control Subsystem of the OCO-2 spacecraft consists of star trackers, a 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 per second. 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 30 degrees. The star stackers can continue tracking acquired stars up to a spacecraft motion of 10°/s. Each star tracker unit weighs 3.5kg and is 16 by 17 by 30 centimeters in size.

Three-axis angular measurement is provided by a Miniature Inertial Measurement Unit MIMU manufactured by Honeywell. The Inertial Measurement Unit has extensive flight heritage and features a robust design using the GG1320 Ring Laser Gyro that provides precise rotation measurements.

The system uses the basic principle that counter-propagating laser beams have different frequencies with the difference dependent on rotation rate which can be measured to calculate the rotation rate about the RLG’s sensitive axis. The MIMU weighs 4.5 Kilograms being 23 by 17 centimeters in size. It has an operational measurement range of +/-375°/sec at a low bias of under 0.005°/hour. The system can tolerate the radiation conditions in Low Earth Orbit and handles accelerations of up to 25G.

OCO-2 uses a GPS receiver for orbit determination and for location reference data added to the instrument data of the spacecraft.

Attitude control is primarily provided by a 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 RWA weighs about 24 kilograms and is 40 centimeters in diameter and 18 centimeters high. The wheels can deliver a torque of up to 2Nm and an angular momentum of 26.7Nms. To allow the reaction wheels to slow down, OCO uses magnetic torque rods that counter the momentum imparted when spinning down the wheels.

The OCO-2 spacecraft uses a Hydrazine monopropellant propulsion system for orbit-raising maneuvers at the start of its mission and orbit maintenance during its operational mission to keep flying in a precisely planned orbit. 45 Kilograms of hydrazine are stored in the vehicle’s tank and feeds four 0.5-Newton low-thrust engines operating in blowdown mode. The thrusters generate thrust by the catalytic decomposition of hydrazine propellant using heated catalyst beds.

The OCO-2 data system is in charge of command reception and execution, payload system operations, housekeeping operations and spacecraft control. The C&DH system uses the VxWorks operating system for realtime operations. OCO-2 uses a 1553 data bus and an analog RS-422 system for data transfer within the spacecraft. Housekeeping data and science data is stored in a solid-state recorder before downlink or is processed and downlinked via TDRSS in real time.

The central part of the Flight Control System of OCO-2 is a BAE RAD-6000 32-bit single board computer that offers radiation hardening to operate in the harsh space environment. The computer operates at a maximum clock rate of 33MHz and a processing speed of 35 MIPS (Million Instructions per Second). The CPU consists of 1.1 million transistors. It has an L1 cache memory of 8KB and controls up to 128MB of SRAM memory. The spacecraft control unit also includes 1GB of Random Access Memory and 3MB of non-volatile memory storing the flight software and command sequences without a loss of data in case of a power outage.

Images: Lockheed Martin/BAE
Images: Lockheed Martin/BAE

 

S-Band Transceiver - Image: Thales Alenia
S-Band Transceiver – Image: Thales Alenia

Science data is stored in a 96Gbit solid state recorder that interfaces with the communications system to be able to downlink the acquired science data. OCO-2 uses an S- and X-Band communications system for the uplink of commands, downlink of telemetry and housekeeping data and the downlink of science data.

Housekeeping and science data can be downlinked to the ground via a high-speed X-Band terminal that includes a transmitter and patch antenna that is installed on the exterior of the satellite body, requiring pointing of the entire spacecraft to track a ground station. The X-Band system reaches data rates of 150Mbit/s.

Science data and telemetry can also be downlinked to the ground via an S-Band terminal that is also capable of sending data to a Tracking & Data Relay Satellite in Geosynchronous Orbit which allows real time science data downlink. Two omni-directional S-Band antennas are installed on the spacecraft to receive uplinked commands from ground stations. OCO-2 uses a different S-Band system than the original OCO switching from analog to digital technology enabling data rates of up to 2Mbit/s.

Instrument Overview

Images: NASA/JPL
Images: NASA/JPL

The OCO-2 instrument is almost an exact copy of the OCO instrument with minor changes due to the unavailability of parts no longer in production and the mitigation of known performance issues known from OCO calibrations and testing which allows OCO-2 to surpass the accuracy of the original spacecraft in certain fields. The instrument, consisting of three high-resolution imaging spectrometers, was designed and developed by Hamilton Sundstand Sensor Systems.

The OCO-2 instrument has a mass of around 131 Kilograms measuring 1.6 by 0.4 by 0.6 meters and requires 125 Watts of power. Overall, the instrument covers a 1.29 by 2.25-Kilometer footprint at nadir and is capable of acquiring eight cross-track footprints creating a swath width of 10.3 Kilometers.

The spectrometers measure the sunlight reflected by the surface of Earth meaning that the sunlight entering the instrument has passed though the atmosphere twice. Carbon dioxide and molecular oxygen absorb radiation at distinct energy levels or wavelengths. A spectrum of radiation that passed through the atmosphere will show a reduced amount of energy at those characteristic wavelengths when compared with pure sunlight. OCO-2 acquires spectra in three wavelength ranges with high spectral resolutions using detectors that are cooled by a cryocooler to achieve the required accuracy of the measurements.

OCO-2 Optical Layout - Image: Liebe et al. 2009
OCO-2 Optical Layout – Image: Liebe et al. 2009
Image: NASA/JPL
Image: NASA/JPL

The instrument consists of three long-slit imaging grating spectrometers sharing a common bore-sight and covering the following spectral bands: the Oxygen A-Band at 0.764 µm and two carbon dioxide bands at 1.61 and 2.06 µm. The three spectrometers feature a similar optical design and share a mounting structure to improve rigidity and thermal stability. All spectrometers share a single telescope.

The telescope uses a Cassegrain design with a primary concave mirror and a secondary convex mirror aligned about the optical axis to focus the light onto the spectrometer entrance slit. Using an 11-centimeter aperture, the telescope is equipped with a series of folding mirrors and dichroic beam splitters to be able to split the light entering the telescope and directing it to the three spectrometers. The relay optics also use band isolation filters and re-imaging mirrors.

Each of the spectrometers consists of an entrance slit, a two-lens collimator, a diffractive grating and a two-lens camera with detectors. The spectrometers are basically identical – the only changes are associated with coatings, lenses and gratings to optimize each spectrometer for the spectral band it is covering.

Before entering the spectrometer, the light passes through a narrow-band pre-disperser filter that transmits light within +/-1% of the desired central wavelength of each to the O2/CO2 bands that are of interest for OCO-2. The rest of the light is rejected and does not enter the spectrometer. A reverse Newtonian telescope (concave primary mirror and flat diagonal secondary mirror) re-focuses the light onto the entrance slit of the spectrometer. Just before the entrance slit is a polarizer that rejects light that is not polarized parallel to the slit since the diffractive gratings can only disperse light polarized parallel to the slit. Eliminating the unwanted polarization before entering the spectrometer prevents light scatter issues inside.

The entrance slits used on OCO-2 are 3mm long and 25 µm wide creating a field of view of about 0.0001 radians in width and 0.0146 radians in length. After passing through the slit, the light reaches a two-lens collimator that directs collimated light (focused at infinity) to a diffraction grating.

The planar holographic grating uses a gold-coating and grooves that are finely tuned to spread the light spectrum into a large number of very narrow wavelength bands. A two-element camera lens focuses the dispersed light onto the Focal Plane Assembly after passing a narrowband filter that rejects thermal emissions from the instrument.

The spectral range and resolving power of each channel includes the complete molecular absorption band plus some of the nearby continuum that is used to deduce constraints of the properties of the surface and aerosols and absorbing gases which can be used to put measurements into contact and provide additional data for other studies such as atmospheric pressure and water concentration.

The spectrometer creates a 2D image of a spectrum on the 1024 by 1024-pixel Focal Plane Assembly that uses 18 µm pixels. The 1024-pixel spectrum is dispersed in the direction perpendicular to the long slit axis creating the spatial field of view of 190 pixels in the dimension orthogonal to the dispersion direction. The active science pixels create a 160 by 1,016-pixel strip, the rest of the detector is basically unused.

Images: NASA/JPL
Images: NASA/JPL

 

Image: NASA/JPL
Image: NASA/JPL

The Oxygen A-Band channel covers a spectral range of 0.758 to 0.772 µm with a resolving power of 17,500, the two carbon dioxide channels cover spectral regions of 1.594 to 1.619 µm and 2.042 to 2.082 µm at a resolving power of 20,000.

For science operations, the Focal Plane is operated at a read-out frequency of 3Hz. To decrease the generated data volume and to increase the signal-to-noise ratio, 20 adjacent pixels in the spatial direction (the detector dimension parallel to the slit) are summed up onboard to provide eight spatially-averaged spectra along the slit.

This generates an along-slit angular field of view for each of these averaged super pixels of 0.1 degrees corresponding to 1.3 Kilometers on the ground. The angular FOV of the narrow dimension of the slit would be 0.14mrad but is widened by intentional blurring by the telescope that increases this value to about 0.6mrad and simplifies the boresight alignment of the three spectrometers.

In addition to the 8-binned averaged 1024-element spectra, 4 to 20 spectral samples are collected and recorded without spatial binning to provide the full along-slit spatial resolution.

These samples cover a 220-pixel region of the detector including the full 190 pixels of the slit length and pixels beyond the end of the slit. The samples are used to characterize the thermal emission and scattered light inside the instrument, but also allow the detection of spatial variability within the super pixels.

Photo: NASA/JPL
Photo: NASA/JPL
Photo: NASA/JPL
Photo: NASA/JPL

OCO-2 uses substrate-removed HgCdTe (mercury cadmium telluride)detectors that are sensitive in all the desired spectral bands. These detectors have high dark-current characteristics and require cooling to around –120°C to achieve the required accuracy.

The original OCO employed a pulse tube cryocooler that was the flight spare of the TES (Tropospheric Emission Spectrometer) project flying on the Aurora spacecraft. This cryocooler is no longer in production and OCO used the last available unit, requiring the re-flight mission to switch to a different cooler.

A cryocooler with the specifications needed by OCO-2 was found in the GOES-R (Geostationary Operational Environmental Satellite Series R) program of NOAA. A GOES-R spare unit was transferred to the OCO program for use aboard the spacecraft while a new unit for GOES-R was purchased and started construction.

Although the two-stage, split tube cryocooler assembly meets the OCO-2 requirements, it is not fully identical to the previously used system on OCO. The new cooler is smaller in size and operates on different electronics requiring a number of accommodations to be implemented. A new bracket was built to facilitate the smaller cooler in the correct position and some changes were necessary on the conductive heat pipes that thermally connect the cooler to an external radiator.

Facilitating the different electronics was a bigger challenge since the original cooler used and RS-422 data connection while the new unit uses low-voltage differential signaling (LVDS) instead. Also, the format of command and telemetry data is different between the two coolers. To overcome these differences, there were two options – changing the onboard software and implementing LVDS which would have required a complete re-certification of the systems or to design a converter to be installed ahead of the cryocooler.

It was decided to implement a converter that is capable of converting between the RS-422 and LVDS protocols and also provides conversion in between commands – recognizing the incoming command using the TES protocol and re-formatting or substituting it with the proper command for the ABI cooler. This approach allowed the flight software and data system to use the same designs qualified for the original OCO mission.

Observation Modes & Calibration

Images: NASA/JPL
Images: NASA/JPL
Image: NASA
Image: NASA

The overall objective of the OCO-2 mission is to provide a global measurement of CO2 distribution including carbon dioxide sources and sinks. To accomplish that, OCO-2 does not directly measure sources and sinks. Instead, the mission detects the column averaged carbon dioxide mole fraction that builds the foundation of computer-based data assimilation that will infer the sources and sinks that are present. This requires OCO-2 to make measurements of the complete atmospheric column in the absence of clouds and aerosols. To achieve that, the spacecraft acquires densely-spaced samples with each frame covering about 3 square Kilometers (1.29 by 2.25km). With samples of this size and density, a number of adequate high-quality soundings can be acquired even in locations with topographic variations, clouds and aerosols.

The weak CO2 band wavelength at 1.61 µm is used to determine the CO2 density at the surface since other atmospheric species do not absorb at this wavelength making it an unambiguous indicator of the CO2 mole fraction.

The oxygen A-Band is used as a comparative absorption measurement of the oxygen concentration which can be used as a reference since the oxygen distribution is very uniform throughout the entire atmosphere and its concentration is constant and well-understood. The O2 spectra will be used to assess the presence of clouds and optically thick aerosols and select the data frames that are discarded. Furthermore, the 0.764 µm oxygen channel is used for for the measurement of the total atmospheric pressure and the light path length through the atmosphere that depends of the geometry of the spacecraft boresight and the position of the sun.

As a second, completely independent measurement of the carbon dioxide concentration, OCO-2 uses the 2.06 µm channel. This spectral band is very sensitive to the presence of aerosols and to variations in atmospheric pressure and humidity.

Flying in a polar sun-synchronous orbit, OCO-2 provides global coverage at a repeat cycle of 16 days.

The spacecraft crosses the equator at 1:35 p.m. local time – a setup that was carefully selected since it maximizes the signal-to-noise ratio because of the high sun angle at that time of day. Additionally, studies have shown that the column averaged dry air CO2 mole fraction is near its average value at that time.

OCO-2 can collect science data in Nadir, Glint and Target Mode.

Image: NASA/JPL
Image: NASA/JPL
Image: NASA/JPL
Image: NASA/JPL

In Nadir Mode, the instrument is pointed local nadir – looking straight down to Earth. Science data is collected along the ground track just below the satellite providing the highest possible spatial resolution of the surface and returns more usable soundings in regions with abundant cloud cover or locations with significant topography. Nadir Mode will be selected for all latitudes were the solar zenith angle is less than 85 degrees. Problematic for Nadir Observations are dark ocean surfaces that may not provide the required signal to noise ratio.

In Glint Mode, OCO-2 points the instrument to the bright glint spot on Earth where solar radiation is directly reflected off the Earth’s surface. For high latitudes over the ocean, Glint Observations will return up to 100 times as much signal as measurements in Nadir Mode. Therefore, Glint Mode will be selected for passes over dark ocean.

Glint soundings will also be collected for all latitudes where the local solar zenith angle is below 75°. OCO-2 alternates between the primary use of Nadir and Glint Modes over sequential 16-day global coverage so that the entire planet is mapped in each of the two modes on a monthly cadence. Data from both modes will be compared to correct biases that may be introduced by the viewing geometry.

For the target mode, OCO-2 will observe a selected target for the entire length of a ground pass, using its attitude control system to keep pointing at the location. This observation mode is utilized when OCO-2 passes over ground calibration sites where solar-looking Fourier Transform Spectrometers are deployed to provide CO2 concentration data that can be compared in order to identify and correct systematic and random errors in the data provided by OCO-2.

A typical ground pass lasts for about nine minutes during which OCO-2 can acquire 12,960 samples at local zenith angles from 0° to 85°. Target mode will also be coupled with airborne sensors. Additionally, OCO-2 can include a slight oscillation in the target mode to observe a ground area of 15 by 30 Kilometers which can be useful for observation of carbon dioxide point sources of natural cause or due to human activity in cities. OCO-2 will pass over a ground validation site once every day.

OCO Coverage Map & CO2 Surface Measurement Stations - Image: NASA/JPL
OCO Coverage Map & CO2 Surface Measurement Stations – Image: NASA/JPL

 

For all observation modes, OCO-2 uses the same Focal Plane Mode – being able to collect up to eight soundings over the 0.8° wide swath every 0.33 seconds. In Nadir Mode, this read-out rate allows the spacecraft to acquire frames with sufficient overlap in the along-track direction. The cross-track dimension changes due to different orientations of the entrance slit with respect to the orbital path that changes as the spacecraft travels from south to north. In the polar regions, the slit is oriented perpendicular to the orbital track leading to a swath width of 10.3 Kilometers. At sub-solar latitudes, the slit is nearly parallel to the orbital track and the cross-track dimension of the swath is reduced.

For nominal operations, the read-out from the Focal Plane Assembly is processed onboard (spatial-binning) and goes through analog to digital conversion before being stored in the solid state mass memory of the spacecraft to await downlink to the ground. The raw data that is gathered represents Data Numbers DN without physical units. Each pixel (being illuminated by a specific wavelength after diffraction) provides a DN representing the radiance – these numbers stretch from 0 to 2^16. During data processing on the ground, these numbers are converted to wavelength-dependent measurements expressed in units for spectral radiance (photons per square meter per steradian per second). The conversion uses radiometric calibration that is performed before launch and checked in flight.

OCO-2 Calibration - Image: NASA/JPL
OCO-2 Calibration – Image: NASA/JPL
Image: NASA/JPL
Image: NASA/JPL

An onboard calibrator is used for a series of calibration operations that are completed regularly while the spacecraft passes over the night side of Earth where no science observations can take place. The zero-point of the detector output (no incident radiation present) will be calibrated at two points on the night side of Earth. Dark calibration either takes place over the dark ocean or uses a cover that is placed over the telescope. One dark calibration takes place at the same position relative to the terminator to specify the absolute zero point on the radiance scale. The second dark calibration takes place at different points on the night side to be able to track the effect of the instrument temperature on the zero-point.

Another calibration mode is the direct observation of the sun that requires the Onboard Calibrator to move an attenuator into the telescope aperture. Directly observing the sun allows the instrument to collect the solar spectrum which is well known and thus serves as a calibrator. A solar calibration is followed up with a limb calibration in which the instrument continues to point to the sun, but allows the Earth’s limb to pass into the field of view as the spacecraft continues in its orbit. In that case, absorption lines that are characteristic of the atmosphere’s chemical content appear in the spectrum for assessment as part of calibration.

OCO-2 also uses calibration lamps. Three lamps with different spectral characteristics are part of the system. One lamp is used at a time and illuminates a reflector that diffuses the light and creates a uniform field that enters the telescope. Since the spatial and spectral distribution from the lamps is precisely known, the measurements of the lamps by the OCO detectors allow an accurate assessment of the detector’s radiometric response.

Further calibration methods include the Vicarious Calibration that collects in-situ measurements to estimate the solar radiation field at the top of the atmosphere for comparison with OCO data. Also, flat-fielding calibrations and doppler measurements are made.

Using calibration and data validation through ground-based sensors, OCO-2 can achieve an accuracy of its carbon dioxide measurements of better than 1ppm (0.3%), allowing the instrument to identify sinks and sources of atmospheric CO2 that usually show variations on the order to 2%.

Orbit

A-Train Spacecraft - Image: NASA
A-Train Spacecraft – Image: NASA

OCO-2 targets an operational sun-synchronous orbit at an altitude of 705 Kilometers and an inclination of 98.2 degrees, creating an orbital period of 98.8 minutes. The sun-synchronous orbit has a local time of ascending node of 13:15.

The spacecraft is launched into an orbit at 635 Kilometers and uses the onboard propulsion system to raise its altitude and phase into the planned sun-synchronous orbit and the proper slot within the A-Train of satellites. The A-Train is a series of Earth/atmospheric observation spacecraft in the afternoon sun-synchronous orbit flying in identical orbits to pass over any given location in close succession so that data from a variety of sensors can be correlated to create advanced scientific data products.

OCO-2 will be the leader of the A-train flying about 15 minutes ahead of the Aqua spacecraft that is the last of the six A-Train spacecraft. The A-Train and its science data is coordinated by the Afternoon Constellation Mission Operations Working Group that includes members of all participating agencies and missions.

Ground Segment & Data Products

Image: NASA/JPL
Image: NASA/JPL
Image: NASA/JPL
Image: NASA/JPL

The Ground Segment for the OCO mission includes a high-degree of automation due to the volume of data provided by the spacecraft. For nominal mission operations, OCO requires one command uplink per day that includes all planned observation sequences, calibration operations and target observations. Science data is downlinked to ground stations several times per day and the Tracking and Data Relay Satellite System can also be called up when needed.

The Ground Segment includes the Mission Operations Center at Orbital Sciences and NASA’s Goddard Spaceflight Center. The Science Operations Center is located at the Jet Propulsion Laboratory and is in charge of accepting all instrument data and processing it to the mission data products at Levels 1 through 4. Data is then distributed and stored at the Goddard Earth Science Data & Information Services Center.

Level 0 data is the raw data downlinked by the spacecraft with any and all communication artifacts removed. The next Level is 1A in which the unprocessed instrument data is provided at full resolution with time-stamps and radiometric, geometric and georeferencing parameters amounting to about 20Gbits per day. Level 1B data is 1A data processed to senor units, essentially a list of every sounding of OCO-2 with three spectra for each sounding (one for each spectrometer) which can amount to 72,000 soundings per orbit.

Derived geophysical variables are included in Level 2 products while Level 3 includes variables mapped on uniform space-time grid scales with high consistency. Level 2 data products only include retrievals of spectra acquired in favorable cloud and aerosol conditions (~9,000). These data products also include vertical atmospheric profiles of temperature, CO2 and water vapor as well as aerosol content, scalar measures of albedo, surface pressure and the column averaged dry air CO2 mole fraction (Xco2).

The Level 3 products will include global maps of carbon dioxide mole fraction with 1ppm accuracy over the Earth surface in bins with dimensions of 1° in latitude and 1° in longitude. Finally, Level 4 products will map the sources and sinks of carbon dioxide over Earth’s surface and also display the fluxes of CO2 abundance over Earth’s surface in bins dimensions of 4° in latitude and 5° in longitude.

Science Overview

Image: NASA/JPL
Image: NASA/JPL
Image: NASA/JPL
Image: NASA/JPL

Carbon Dioxide is a key greenhouse gas in Earth’s atmosphere playing a major role in Earth’s climate. Greenhouse gases trap thermal radiation in the atmosphere that would otherwise radiate into space and are capable of heating up the atmosphere. CO2 is also important to life on the planet as it is an integral part of the atmosphere, protecting life on Earth. However, variations of the carbon dioxide concentration can affect the atmospheric balance and lead to adverse effects in Earth’s climate on a global scale. Variations of atmospheric carbon dioxide can be caused by human activity, but there are also natural CO2 sources and sinks as the Earth system maintains a balance on CO2 through the carbon cycle.

In the 10,000 years ahead of the industrial revolution, atmospheric carbon dioxide abundance has risen by less than one percent. Since then until 2003, human activity has added 466 billion tons of carbon in the form of CO2 to the atmosphere. In 1970, the overall concentration of carbon dioxide was about 330ppm and in the 40 years to follow, man-made emissions have increased the concentration by 1 to 2 ppm per year reaching 400ppm in 2013.

Of all the carbon dioxide added to the atmosphere, 40% has remained in the atmosphere while the remaining 60% was absorbed by the ocean and continents. Inventories of the ocean – a known CO2 sink – account for half of the absorbed carbon. The other half must have been absorbed by sinks on land which are poorly understood and need further study to be identified and quantified which is exactly what OCO-2 will attempt to accomplish.

OCO-2 provides valuable data to be used by the atmospheric and carbon cycle science community to improve global models of the carbon cycle, reduce uncertainties in forecasts of total carbon dioxide abundance in the atmosphere and make more accurate predictions of global climate change in which CO2 is a key driver. By studying the location, nature and processes of natural carbon dioxide sinks, a better prediction of the rate of build-up of CO2 in the atmosphere and its impact on the climate becomes possible. It is unknown whether the CO2 sinks will continue to operate at their current efficiency or if their uptake will decrease over time which could lead to a significant increase in atmospheric Carbon Dioxide which would project a very different future for life on Earth.

Measurements by OCO-2 will allow scientists to monitor the geographic distribution of carbon dioxide sources and quantify their variability in order to map the natural and man-made processes that regulate the exchange of CO2 between the Earth’s surface and the atmosphere on both regional to continental scales.

The data provided by OCO-2 may also be of interest to policy makers and business leaders to make better decisions to ensure climate stability over the long-term.

OCO-2 will also contribute to a number of other scientific areas related to the global carbon cycle:

  • the dynamics of ocean carbon exchange
  • the seasonal dynamics of northern hemisphere terrestrial ecosystems in Eurasia and North America
  • the exchange of carbon between the atmosphere and tropical ecosystems due to plant growth, respiration, and fires
  • the movement of fossil fuel plumes across North America, Europe, and Asia
  • the effect of weather fronts, storms, and hurricanes on the exchange of CO2 between different geographic and ecological regions
  • the mixing of atmospheric gases across hemispheres

In addition to carbon cycle research, OCO-2 will support other operational applications that include precise measurements of surface pressure on a global scale, water column abundance, cloud and aerosol measurement, and solar irradiation data. This data can be used for climate research and meteorological research as well as operational applications such as weather forecasting.