ISS Utilization: OCO-3 (Orbiting Carbon Observatory-3)
OCO-3 is a NASA/JPL space instrument designed to investigate important questions about the distribution of carbon dioxide on Earth as it relates to growing urban populations and changing patterns of fossil fuel combustion.
NASA plans to develop and assemble the instrument using spare materials from the successful development and launch of the OCO-2 (Orbiting Carbon Observatory-2) on July 2, 2014 and host the stand-alone instrument on the ISS (International Space Station) or another space-based platform. 1)
The OCO-3 instrument consists of three high resolution grating spectrometers which collect space-based measurements of atmospheric carbon dioxide (CO2) with the precision, resolution, and coverage needed to assess the spatial and temporal variability of CO2 over an annual cycle. After launch and docking with the International Space Station, the OCO-3 instrument will be installed on the ISS JEM-EF (Japanese Experiment Module- Exposed Facility) where it will be operating for the duration of the mission. 2)
Primary science objective: Collect the space-based measurements needed to quantify variations in the column averaged atmospheric carbon dioxide (CO2) dry air mole fraction, XCO2, with the precision, resolution, and coverage needed to improve our understanding of surface CO2 sources and sinks (fluxes) on regional scales (≥1000 km). Measurement precision and accuracy requirements same as OCO-2. Operation on ISS allows latitudinal coverage from 51ºS to 51ºN.
Legend to Figure 1: All geographic locations between 51.6º North and South latitude can be observed with NADIR pointing. The orbit provides coverage of 85% of the Earth’s surface and 95% of the world’s populated landmass every 1-3 days. 3)
Figure 2: OCO-3 high bay video (video credit: NASA)
Key messages of OCO-3
1) OCO-3 will measure and map carbon dioxide from space in great detail improving our understanding of the interaction between carbon and climate. Carbon dioxide is one of the most important and long-lived greenhouse gases. In fact, increases in atmospheric carbon dioxide — both manmade and natural — are responsible for about two-thirds of the total energy imbalance that is causing Earth's temperature to rise. OCO-3 will measure and map CO2 with such high spatial resolution, that, combined with the valuable 4.5 year dataset of its predecessor OCO-2, will paint the most detailed picture ever of human and plant influences on the carbon cycle and in turn, climate. Specifically, the measurements will help us to understand whether the land and oceans will continue to absorb roughly half of the CO2 that is emitted each year through human consumption, or whether that rate will decrease in the future as demonstrated by these findings.
2) OCO-3 will be the first instrument to measure SIF (Solar-Induced Fluorescence), an indicator of photosynthesis efficiency in high definition from dawn to dusk from space. OCO-3 will be mounted on the International Space Station whose orbit will enable it to measure plant fluorescence from dawn to dusk anywhere between 52º north and south latitudes — London to Patagonia — for a period of at least three years. How do we do this? Because of the space station's orbit, OCO-3 will pass over any given location a little earlier each day, spanning all sunlit hours of that location in a period of about 30 days. This will enable scientists to study how factors such as light, water, and temperature affect plant activity over the course of a day, weeks, months, and years. These insights will enable better management of water, forests, and food supplies.
3) OCO-3 will demonstrate a new "snapshot" mode capable of mapping local differences in CO2 from space for the first time. OCO-3 is equipped with an innovative targeting mechanism that will allow it to measure carbon dioxide emissions from almost any 50 mile by 50 mile region of interest. The instrument will sample emission sources and gradients, areas where plants and crops are being studied, volcanos, and other local carbon sources from space. These observations will provide data necessary to better understand how well we can determine emissions from space-based observations.
4) OCO-3 and two other NASA Earth-monitoring missions on the space station will take mutually beneficial measurements of global ecosystems. OCO-3 will share the space station with two other NASA instruments — ECOSTRESS and GEDI. Together, these instruments will tell us how plants respond to weather, heat stress, and climate from the warm tropics to the frozen tundra. This will enable improved understanding of the interaction of carbon and climate at different time scales. This combination of data will provide a more complete picture of the carbon cycle because, as the old adage goes, "the whole is greater than the sum of its parts."
Watching the Planet Breathe: The Story of SIF (Solar Induced Fluorescence) 4)
What is SIF?: SIF is the measurement of infrared and red light that plants emit as a byproduct - much like oxygen - when they undergo photosynthesis. Since SIF emanates directly from photosynthetic processes, studying it can give us insight on plant health and productivity. However, the amount of infrared light they emit is so small, that previously it could only be measured on a leaf-by-leaf basis. Plants are highly reflective in the same region of the electromagnetic spectrum that fluorescence occurs in, reflecting up to 70% of the light in this region, compared to the small fraction (1-2%) of light emitted as fluorescence. So how can such a tiny signal be measured from space? 5)
Seeing SIF from Space: The ability to study SIF globally comes from NASA’s Orbiting Carbon Observatory-2 (OCO-2) satellite and was only discovered because of a similar satellite’s specific design. OCO-2 was built to measure carbon dioxide in the atmosphere, and to do so it needed a spectrometer (an instrument that splits light into separate colors) that was very detailed in the amount of spectral lines it covered. In these narrow spectral lines, there are some wavelengths of light that never leave the Sun’s photosphere (Fraunhofer lines). As such, Fraunhofer lines are narrow gaps (dark lines) in the electromagnetic spectrum of Earth’s atmosphere. OCO-2 was designed to use those dark lines as a reference point for carbon dioxide in the atmosphere, as they should have no light in them. However, when OCO-2 scientists looked in the Fraunhofer lines, the team found that there was light. But if the Sun doesn’t emit that wavelength of light, making it not present on Earth, where was the light coming from? It turns out that light was SIF! The glow of light in the Fraunhofer lines was in fact coming from plants, as fluorescence.
Figure 3: Fraunhofer lines (A-K) in the electromagnetic spectrum
SIF data improvements: Previous plant photosynthesis estimates were created by scaling up individual leaf data or measurements from canopy towers. With SIF data gathered from space using OCO-2, a global map of fluorescence is now possible, instead of inferences that came about from scaling up single measurements. A more accurate picture has emerged thanks to the improved spectral and spatial OCO-2 resolution, allowing scientists to see plant and ecosystem health globally, including in places where it might be hard to send in scientists to do the manual, leaf-by-leaf analyses.
Figure 4: Global SIF data map from August through October 2014. Photosynthesis is highest over the tropical forests of the Southern Hemisphere (where it is spring) but still occurs over much of the U.S. The northern forests have shut down for winter (image credit: NASA/JPL)
SIF’s Bright Future: Applications Studying SIF data can provide unprecedented data to help many different fields. Because SIF is a measurement of how much plants are photosynthesizing, it can indicate areas of drought long before plants show any outward signs of stress, such as discoloration. This information can help agricultural areas to prepare and try to offset the effects of a drought far earlier. SIF data can also help improve climate models; land uptake of carbon dioxide is the greatest uncertainty in current models, and now this data can be used to more accurately quantify exchanges of carbon dioxide between the atmosphere and land.
Figure 5: Scientists at JPL and Caltech are matching ground based measurements of photosynthesis with spectral measurements similar to those made on OCO-2 (image credit: NASA/JPL)
The Orbiting Carbon Observatory (OCO) was a NASA Earth System Science Pathfinder Project (ESSP) mission designed to make precise, time-dependent global measurements of atmospheric carbon dioxide (CO2) from an Earth orbiting satellite. Unfortunately, on February 24, 2009, due to a launch vehicle payload fairing anomaly, OCO failed to reach orbit.6)
However, in December 2009, the Congressional Conference committee directed NASA to allocate no less than $50M for the 2010 fiscal year (FY10) for the initial costs associated with an OCO replacement. Released on February 1st, 2010, the President's Budget provided adequate funding to support the launch of an OCO re-flight mission (now known as OCO-2). The OCO-2 mission underwent critical design review (CDR) in August 2010 and key design point-C (KDP-C) in September 2010. On October 2010, it began the implementation phase.
On July 16, 2012, NASA announced that it had awarded launch services contracts for three United Launch Alliance Delta 2 rockets. A little over 5 years after the OCO launch failure, OCO-2 launched from Vandenburg Air Force Base on Wednesday, July 2, 2014. Originally flown on a Taurus XL, OCO-2 flew on a Boeing Delta II 7320-10C. The Delta II is one of the most successful launch vehicles ever flown with well over 100 successful launches.
OCO-2 was built based on the original Orbiting Carbon Observatory mission to minimize cost, schedule and performance impacts. OCO-2 is designed to have a nominal mission time frame of at least two years, but the spacecraft could continue to fly well beyond its prime mission. OCO-2's primary science objective is still to substantially increase our understanding of how carbon dioxide sources and sinks are geographically distributed on regional scales and how their efficiency changes over time.
As of December 22, 2015, OCO-3 was given the green light to move forward as the next installment in the OCO legacy that has and continues to build on using innovative technologies to continue NASA's space borne study of carbon dioxide. Initially, the OCO-3 Project was not included in the President’s Proposed Budget for FY2018 when it was released in February 2017. However, funding for the project was restored in March 2018 with the Enacted Budget for FY2018. At his first NASA town hall, NASA Administrator Jim Bridenstine mentioned OCO-3 and said, “... It’s not been cut. In fact, it’s going to be on orbit very, very soon.”
OCO-3 is a NASA-directed mission on the International Space Station (ISS). The primary mission objective is to collect the space-based measurements needed to quantify variations in the column averaged atmospheric carbon dioxide (CO2) dry air mole fraction, XCO2, with the precision, resolution, and coverage needed to improve our understanding of surface CO2 sources and sinks (fluxes) on regional scales (≥1000 km). The precision requirement is identical to that of OCO-2. Operations on ISS allows latitude coverage from 51º N to 51º S.
Figure 6: NASA’s OCO-3 mission is ready for launch to the International Space Station. This follow-on to OCO-2 brings new techniques and new technologies to carbon dioxide observations of Earth from space (video credit: NASA/JPL, Published on Apr 2, 2019)
The PI of the OCO-3 mission is Annemarie Eldering, Ph.D. of NASA/JPL, Pasadena, CA. OCO-3 continues the global carbon dioxide record started by OCO-2, but adds complementary information with sampling at all sunlit hours, a unique feature of sampling from the International Space Station (ISS). In addition to global sampling, OCO-3 capabilities allow for targeted local mapping of emissions hotspots. Megacities with massive carbon emissions are a potential target for measurements. Regional measurements in areas of high rates of carbon exchange that could be useful for process studies include snapshot maps over agricultural regions and forests, and mangroves.
Figure 7: Overview of Earth science instruments on the ISS (installed or planned) in the second decade of the 21st century (image credit: NASA) 7)
Launch: The SpaceX CRS-17 (Commercial Resupply Service-17) with a Dragon spacecraft on a Falcon 9 Block 5 rocket was launched on 04 May 2019 (02:48 EST, or 06:48 UTC) from the Space Launch Complex 40 at Cape Canaveral Air Force Station in Florida. Major payloads on this flight were: 8)
• OCO-3 (Orbiting Carbon Observatory-3) of NASA
• STP-H6-XCOM (Space Test Program-Houston 6-X-ray Communication)
• PBR (Photobioreactor)
• Hermes Facility
• Organs on Chips
Orbit: Near-circular ISS orbit of ~400 km, inclination = 51.6º.
The spacecraft will take two days to reach the space station before installation on May 6. When it arrives, astronaut David Saint-Jacques of the Canadian Space Agency will grapple Dragon, with NASA astronaut Nick Hague serving as backup. NASA astronaut Christina Koch will assist by monitoring telemetry during Dragon’s approach. After Dragon capture, mission control in Houston will send commands to the station’s arm to rotate and install the spacecraft on the bottom of the station’s Harmony module.
Once the payload reaches ISS, OCO-3 will be taken off the Dragon spacecraft and robotically installed on the exterior of the station's Japanese Experiment Module (JEM) Exposed Facility Unit.
Figure 8: OCO-3 ISS installation animation (video credit: OCO2/NISAR, Uploaded on Nov 12, 2018)
The Dragon spacecraft will spend about four weeks attached to the space station, returning to Earth with more than 1900 kg of research, hardware and crew supplies.
• January 30, 2020: NASA's Orbiting Carbon Observatory-3 (OCO-3) is getting a bright start to the new year. Since December, the agency's newest carbon dioxide (CO2) monitoring mission has collected more than 20 million measurements of sunlight reflected off Earth's surface, or radiance. This light data lies at the heart of the mission's ability to measure atmospheric CO2, a major contributor to climate change. The public release of the data began on January 30, 2020. 9)
- Here's why it matters.
- OCO-3's primary instrument is a highly accurate, three-channel spectrometer. When sunlight moves through the atmosphere to Earth's surface and is reflected back to the spacecraft, the instrument splits the incoming light into a spectrum of colors — much like a light shone through a prism creates a rainbow.
- Each of those colors corresponds to a different frequency and wavelength of light. Because different gasses in the atmosphere absorb only specific colors (or frequencies), and do so in their own way, each has a unique absorption "fingerprint." OCO-3's spectrometer detects and count these fingerprints for both CO2 and oxygen.
- "Measuring the number of oxygen molecules lets us determine the ratio of CO2 molecules to air molecules," said Annmarie Eldering, OCO-3 project scientist. "When we talk about a CO2 concentration like 400 parts per million, it is that ratio."
- They then run the radiance (light) data against a fine-tuned model of the atmosphere that takes into account variables like water vapor and the sun's location relative to the ground at a given time which can influence the results.
- "We filter out the poor-quality data, where it might just be too cloudy or where the Sun may have been too low on the horizon, too dim for us to learn anything useful," Eldering said. "If the data and the radiance model don't match, we make adjustments to find the very best explanation for our observed light."
- Thus, the radiance data is the foundation on which OCO-3's CO2 measurements are based. The mission team produced several maps to demonstrate what the data looks like, including images over Buenos Aires and San Jose, California (see images above/or wherever we're putting them). The data from darker, less reflective areas is shown in dark purple. The brightest areas are shown in white.
- OCO-3, which launched to the International Space Station in May 2019, is tasked with continuing the CO2 record of its still-operational predecessor OCO-2; however, there are some distinct differences between the two missions. The OCO-2 spacecraft was launched into a near-polar orbit, which means that every time is passes over a given point on Earth's surface, it does so at the same time of day. The space station, on the other hand, makes about 16 orbits of Earth per day, each shifting slightly to the west on its longitudinal axis. This orbit, combined with the fact that Earth itself is also rotating, allows OCO-3 to measure CO2 over the same areas at different times of day.
- OCO-3 also has a new pointing mechanism that can capture "snapshot maps" — detailed mini-maps of CO2 over specific areas of interest like cities and volcanoes. The mechanism can map areas of 50 by 50 miles (80 by 80 km) in just two minutes. In order for the mechanism to work, though, it has to know what time and over what location it is flying. While the space station's variable orbit makes this a challenge as well, it's not one too great for the OCO-3 mission team.
- The teamwork, the complicated process and collaborations required, all of that has worked really well and has really been a success," Eldering said.
- OCO-3 is on track to produce its much-anticipated CO2 maps, including hotspot maps over various cities, this April.
- To access the newly released radiance data, visit: https://disc.gsfc.nasa.gov/datasets?keywords=OCO3
• July 12, 2019: NASA's OCO-3 (Orbiting Carbon Observatory-3), the agency's newest carbon dioxide-measuring mission to launch into space, has seen the light. From its perch on the International Space Station, OCO-3 captured its first glimpses of sunlight reflected by Earth's surface on 25 June 2019. Just weeks later, the OCO-3 team was able to make its first determinations of carbon dioxide and solar-induced fluorescence - the "glow" that plants emit from photosynthesis, a process that includes the capture of carbon from the atmosphere. 10)
- OCO-3 was also able to make its first measurements of solar-induced fluorescence. The image of Figure 10 shows solar-induced fluorescence in western Asia. Areas with lower plant glow - indicating lower photosynthesis activity - are shown in light green; areas with higher photosynthesis activity are shown in dark green. As expected, there is significant contrast in plant activity from areas of low vegetation near the Caspian Sea to the forests and farms north and east of the Mingachevir Reservoir (near the center of the image).
- "The team is so excited to see how well OCO-3 is performing," said Project Scientist Annmarie Eldering, who is based at NASA's Jet Propulsion Laboratory in Pasadena, California. "These preliminary carbon dioxide and solar-induced fluorescence retrievals look fantastic and will only improve as calibration improves."
- OCO-3 launched to the space station on May 4. Although one of its main objectives is to continue the five-year data record started by OCO-2, it has two unique capabilities. First, OCO-3 is equipped with a new pointing mirror assembly that will allow scientists to map local variations in carbon dioxide from space more completely than can be achieved by OCO-2.
- Second, the space station's orbit will allow OCO-3 to see the same location on Earth at different times of day, which will allow scientists to study how carbon dioxide fluctuates throughout the day. OCO-2, not mounted on the space station, is in a near polar orbit that only allows it to see the same location at the same time of day.
- OCO-3's data will complement data from two other Earth-observing missions aboard the space station - ECOSTRESS, which measures temperature stress and water use by plants, and GEDI, which assesses the amount of above-ground organic plant material present particularly in forests. The combined data from all of these instruments will give scientists both an unprecedented level of detail about how plants around the globe are responding to changes in climate and a more complete understanding of the carbon cycle.
Figure 9: This image shows CO2 over the United States during OCO-3's first few days of science data collection. These initial measurements are consistent with measurements taken by OCO-3's older sibling, OCO-2, over the same area — meaning that even though OCO-3's instrument calibration is not yet complete, it is right on track to continue its (currently still operational) predecessor's data record (image credit: NASA/JPL-Caltech)
Figure 10: This image shows OCO-3's first preliminary SIF (Solar-Induced Fluorescence) measurements over western Asia. SIF is the glow plants emit from photosynthesis — the process of plant growth that includes the capture of carbon from the atmosphere. Areas with lower photosynthesis activity are shown in light green; areas with higher photosynthesis activity are shown in dark green. As expected, there is significant contrast in plant activity from areas of low vegetation near the Caspian Sea to areas of more dense vegetation like the forests and farms north and east of the Mingachevir Reservoir (near the center of the image), image credit: NASA/JPL-Caltech
- The mission team expects to complete OCO-3's in-orbit checkout phase — the period where they ensure all instruments and components are working and calibrated correctly — in August 2019. They are scheduled to release official CO2 and solar-induced fluorescence data to the science community a year later; however, this data will likely be available sooner given the quality of the measurements that OCO-3 is already making.
• May 10, 2019: NASA's OCO-3 was removed from the Dragon spacecraft and robotically installed on the exterior of the space station's Japanese Experiment Module-Exposed Facility as of approximately 9 p.m. PDT on May 9 (12 a.m. EDT on May 10). Over the next two days, a functional checkout will be performed and the OCO-3's Pointing Mirror Assembly (PMA) will be deployed. The PMA and context cameras will then perform an initial survey of OCO-3's surroundings to make sure nothing unexpected is interfering with its view of Earth. 11)
OCO-3 (Orbiting Carbon Observatory-3) Instrument
OCO-3 is a complete stand-alone payload built using the spare OCO-2 flight instrument, with additional elements added to accommodate installation and operation on the ISS.
The OCO-3 instrument consists of three high resolution grating spectrometers which collect space-based measurements of atmospheric carbon dioxide (CO2) with the precision, resolution, and coverage needed to assess the spatial and temporal variability of CO2 over an annual cycle. Two of OCO-3's spectrometers record two sets of wavelengths where carbon dioxide absorption is strong; the third records wavelengths with strong absorption of oxygen, which researchers need in order to calculate the total number of molecules in the part of the atmosphere where the measurement was made. Combining the data from the three spectrometers allows researchers to obtain a measurement of CO2 so accurate that it records the difference between, for example, 405 and 406 molecules of the gas in every 1 million molecules of air.
Table 1: Comparison of OCO-2 and OCO-3 missions
Table 2: OCO-3 payload interface parameters
Mirrors, Motors and Mapping Mode: OCO-3's new capabilities depend heavily on an innovative swiveling mirror assembly, which Bennett described as a "very agile pointing mechanism." 12)
"When OCO-2 points toward an observation target, the entire spacecraft has to rotate," Bennett said. "Since OCO-3 is a 'passenger' on the space station, we had to add the pointing mirror assembly to point independently of the station."
The pointing assembly uses two pairs of mirrors to rotate in two complementary directions - one parallel to Earth's surface, the other perpendicular. This setup allows OCO-3 to point to just about anywhere within view of the space station but also allows it to capture "snapshot maps" - detailed mini-maps of carbon dioxide - over areas of interest.
This snapshot mapping mode can measure emissions from sources ranging from relatively small areas surrounding power plants to large urban areas up 1,000 square miles (2,590 km2) in just two minutes. That means OCO-3 can measure the entire Los Angeles Basin in just a single pass - a task that would take OCO-2 several days.
Measuring large urban areas is particularly important to scientists since about 70% of total fossil-fuel emissions come from large cities.
"These targeted measurements will help us disentangle which sources of carbon dioxide are in nature and which are anthropogenic, or human-caused," Bennett said.
While measuring carbon dioxide, OCO-3 can simultaneously measure how well plants are performing photosynthesis by measuring how much their chlorophyll "fluoresces" - or emits a specific wavelength of light - while illuminated by the Sun. This will help carbon-cycle scientists observe how well vegetation is absorbing carbon dioxide on the ground and how the surrounding atmosphere is responding.
Figure 11: OCO-3 payload exterior (image credit: NASA/JPL) 13)
OCO-3 Observation Modes
OCO-3 will be using the same instrument as OCO-2, but it has been adapted to work on the ISS. The instrument functions in three modes in flight: Nadir viewing (straight down), glint (reflected) and a pointing mode for target sites. Unlike OCO-2, which performs complex maneuvers of the entire satellite bus to observe ground targets, the OCO-3 instrument will be fitted with an agile 2-D pointing mechanism, i.e., a Pointing Mirror Assembly (PMA) that will allow for rapid transitions between nadir and glint mode (less than 1 minute). 14) 15)
The PMA will also allow for target mode observations, similar to those taken by OCO-2, typically at Total Column Carbon Observation Network (TCCON) ground sites for use in validation.The PMA will provide the ability to scan large contiguous areas (order 80 km by 80 km), such as cities and forests, on a single overpass. This mode will be known as "snapshot" mode and will allow for fine scale spatial sampling of CO2 and SIF variations unlike what can be done with any current satellite system.
Unlike OCO-2, which flies in a polar orbit around the Earth, OCO-3 on board ISS will follow a precessing orbit. This means that overpasses will progress earlier and earlier in local time of day for a given point on the earth over periods of days. In about 30 days, at a given location, measurements progress from late in the day to early in the day. For some locations at the higher latitudes, there are periods where measurements are taken both in the morning and in the afternoon of the same day. This variable time of say sampling has implications with respect to the diurnal cycle of both clouds and aerosols (known contaminates when observing XCO2), and studies of the carbon cycle, which itself has a strong diurnal variation. The precession in time-of-day sampling will be especially informative for the SIF observations with respect to studying the biosphere response (both natural and anthropogenic) to changes in sunlight. When OCO-2 and OCO-3 operate concurrently, they will collect overlapping data and continue the important baseline begun by OCO-2, although the primary method of comparing OCO-2 and OCO-3 will be through the TCCON measurements.
Figure 12: Illustration of the OCO-3 PMA (Pointing Mirror Assembly), image credit: NASA/JPL, Caltech
The PMA is required to allow non-nadir (straight down) observations from the fixed position on the ISS. Two important design requirements of the PMA were 1) to allow quick movement through a large range of angles, and 2) that the movement not affect the measurements through angular dependent polarization or radiance changes. To meet these objectives a variation of the pointing system designed for the Glory Aerosol Polarimetry Sensor (APS) was selected.
This system relies on a single pair of matched mirrors in an orthogonal configuration that impart less than 0.05% change to the polarization . For the OCO-3 PMA, the concept was extended to a 2-axis pointing system - one controlling the azimuthal (cross-track) angle, and the other controlling the elevation (along-track) angle. Although the PMA itself does not change the polarization of the light more them 0.1%, there are polarization implications because the slit image is rotated as a function of the change in the PMA, driven primarily by the elevation (along-track) angle. It is worth noting that reflected sunlight is naturally polarized by its interaction with the earth’s surface and atmosphere, especially over water.
Figure 13: OCO-3 PMA (Pointing Mirror Assembly ) animation (video credit: NASA/JPL, Caltech)
Figure 14: OCO-3 sits on the large vibration table (known as the "shaker") in the Environmental Test Lab at the Jet Propulsion Laboratory. The exposed wires lead to sensors used during dynamics and thermal-vacuum testing. Thermal blankets will be added to the instrument at Kennedy Space Center, where a Space-X Dragon capsule carrying OCO-3 will launch in on a Falcon 9 rocket to the space station on May 1, 2019 (image credit: NASA/JPL-Caltech) 16)
OCO-3 will not be measuring CO2 directly; but actually, the intensity of the sunlight reflected from the presence of CO2 in a column of air. This measurement is unique like a fingerprint, and can be used for identification. The OCO-3 instrument (like the current OCO-2 instrument) will use a diffraction grating (like the back of a compact disk) to separate the incoming sunlight into a spectrum of multiple component colors. 17)
The instrument measures the intensity of three relatively small wavelength bands (Weak CO2, Strong CO2 and Oxygen O2) from the spectrum, each specific to one of the three spectrometers. The absorption levels will indicate the presence of the different gases. By simultaneously measuring the gases over the same location and over time, OCO-2 will be able to track the changes over the surface over time.
The OCO-3 spectrometers will measure sunlight reflected off the Earth's surface. The sunlight rays entering the spectrometers will pass through the atmosphere twice - once as they travel from the Sun to the Earth, and then again as they bounce off from the Earth's surface to the OCO-3 instrument. Carbon dioxide and molecular oxygen molecules in the atmosphere absorb light energy at very specific colors or wavelengths.
The OCO-3 instrument uses diffraction grating to separate the inbound light energy into a spectrum of multiple component colors. The reflection gratings used in the OCO-3 spectrometers will consist of a very regularly-spaced series of grooves that lie on a very flat surface.
The characteristic spectral pattern for CO2 can alternate from transparent to opaque over very small variations in wavelength. The OCO-3 instrument must be able to detect these dramatic changes, and specify the wavelengths where these variations take place. So, 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. In fact, the OCO-3 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.
OCO-3 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-3 instrument design will include a cryocooler, which is a refrigeration device. The cryocooler keeps the detector temperature at or near -120° C (-184° F).
Figure 15: How diffraction grating works (image credit: NASA/JPL, Caltech)
Figure 16: This illustration shows NASA's OCO-3 mounted on the underside of the International Space Station (image credit: NASA) 18)
OCO-3 Internal Context Camera
The OCO-3 instrument is the first mission to use the flexible camera architecture, producing two Context Cameras that aid in the instrument’s calibration campaign. Each Context Camera consists of an identical electronics chassis with either a medium- or narrow-angle lens. The unique ruggedized COTS lenses are accommodated by the common electronics chassis, highlighting the modularity of the design. The OCO-3 Internal Context Camera is shown in Figure 17, using a COTS C-mount lens.
Figure 18: Right: Internal context camera (red image) specifically for geolocation. Gold mirrors will alter the color balance of the image. Left: External context camera (left) will collect a large image in false color (image credit: NASA/JPL)
Figure 19: Artist interpretation of OCO-3 measurements (image credit: NASA/JPL, Caltech)
NASA's OCO-3 Measures How Plants Grow - and Glow
When plants take in too much energy, they don't get fat - they lighten up. They absorb more sunlight than they need to power photosynthesis, and they get rid of the excess solar energy by emitting it as a very faint glow. The light is far too dim for us to notice under normal circumstances, but it can be measured with a spectrometer. Called solar-induced fluorescence (SIF), it's the most accurate signal of photosynthesis that can be observed from space. 19)
Figure 20: This honeysuckle is glowing in response to a high-energy ultraviolet light rather than to the Sun, but its shine is similar to the solar-induced fluorescence that OCO-3 will measure (image credit: ©Craig P. Burrows)
That's important because, as Earth's climate changes, growing seasons worldwide are also changing in both timing and length. These changes may affect world food production and the pace of greenhouse warming. It's not possible to measure photosynthesis globally from ground level, and lab experiments can't easily replicate all of the environmental factors affecting plant growth, such as water availability, wildfires and competition from other plants - factors that also are changing with the climate.
The Orbiting Carbon Observatory 3 (OCO-3), set to launch to the International Space Station later this month, will join its older sibling, OCO-2, in measuring SIF along with its primary target of carbon dioxide concentrations around the globe. The two satellites will be in different orbits: OCO-2 circles Earth from pole to pole, whereas OCO-3 will be mounted on the exterior of the space station, which circles between 52º north and 52º south latitude.
The view from the space station will enable OCO-3 to collect a denser data set than OCO-2 does over the parts of Earth where the most carbon is emitted and stored. The space station orbit will also bring the instrument over any given Earth location at a different time on each orbit, permitting the first dawn-to-dusk observations of how SIF varies over the course of a day.
Nicholas Parazoo of NASA's Jet Propulsion Laboratory in Pasadena, California, is the lead SIF scientist for OCO-3, and he's looking forward to the combined data set to gain insight into remote regions that are relatively little studied. "The two high-carbon, highly uncertain regions on Earth are the Arctic, where there's a lot of carbon in the ground, and the tropics, where there's a lot of carbon in the plants," Parazoo said. "With OCO-2 and OCO-3 combined, we're going to observe those regions in unprecedented detail."
Parazoo and his colleagues will use previously developed algorithms to extract the SIF signal from the full set of data collected by OCO-3. The instrument consists of three spectrometers, each observing different bands of wavelengths in the electromagnetic spectrum. Every kind of gas molecule in the atmosphere - oxygen, carbon dioxide and the others - absorbs sunlight in a unique set of wavelengths. A spectrometer looking at the right wavelengths will see this absorption as a distinctive series of dark lines, like the spectral bar code of a particular gas.
OCO-3's three spectrometers are tuned to two wavelength bands covering different parts of carbon dioxide's bar code and one band with an oxygen bar code. As it happens, the oxygen spectrometer records not only wavelengths absorbed by oxygen, but also nearby wavelengths where SIF shines particularly strongly. "So the SIF measurement wasn't by design but an extremely fortunate bonus," Parazoo said.
Since NASA scientist Joanna Joiner and colleagues produced the first spaceborne SIF measurements in 2010 - before OCO-2 was launched - SIF data has been generated from earlier European and Japanese satellites. However, OCO-2 has a much finer-scale field of view, or footprint, than any preceding satellite, with each image covering an area of about a square mile (< 3 km2).
OCO-3 will add to that advantage something OCO-2 cannot do: As OCO-3 orbits, it will turn its sensor quickly to point at instrumented towers on the ground below the spacecraft. These towers measure SIF and photosynthesis concurrently, with similar resolution to OCO-3. Validating the data this way provides critical information on OCO-3's performance and can increase scientific insight into the underlying SIF mechanics.
Data averaged over a large area suggest that there's a straightforward relationship between solar energy coming in and photosynthesis taking place. With OCO-2's fine-scale data, Parazoo said, "We're finding that the relationship between SIF, absorbed solar energy and photosynthesis is more complicated than we thought. We're trying to understand that." He hopes OCO-3 will be able to shed some light on the causes of this complexity.
Cities are another area where the SIF measurement is of interest. They are hotter than surrounding natural regions because of their many heat sources and heat-absorbing surfaces, like pavement. Comparing how the same species of plants grow and thrive in both a city and its natural surroundings gives a sort of sneak preview of how the these plants will respond to a warmer climate.
OCO-2 collects a single, narrow slice of data cutting through a few cities on each orbit, but OCO-3 will target and record SIF at almost every major midlatitude and tropical city. The measurements may prove helpful to urban planners in using their water resources wisely, as well as to biologists in understanding the effects of heat stress on plants.
With so many promising avenues of study arising from SIF, OCO-3's plant light measurements will illuminate new findings for years to come.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).