Minimize ISS: OCO-3

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.


Figure 1: ISS as a platform for Earth Science (image credit: NASA)

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)


Some background:

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: A launch of the OCO-3 instrument to the ISS is scheduled for no earlier than 25 April 2019 on a Northrop Grumman Cargo Dragon capsule on the SpaceX CRS-17 flight with a Falcon-9 Block5 vehicle. 8) 9)

Orbit: Near-circular ISS orbit of ~400 km, inclination = 51.6º.

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)



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.


OCO-2 mission

OCO-3 on ISS

Land Sampling

Every day (using glint and nadir measurements)

Every day

Glint/Ocean Sampling

16 days on/16 days off

Every day

Latitudinal coverage


±51.6º (on ISS)

Local time of day sampling and repeat

~13:30 hours with 16 day routine and repeated measurements

Ranges across all sunlit hours with variable revisit (0 to multiple per day)

Expected XCO2 single sounding precision



Expected XCO2 precision for collection of 100 cloud-free soundings

≤ 0.3% (1 ppm)

≤ 0.3% (1 ppm)

Target mode capability

Yes, with spacecraft pointing

Yes, expanded with pointing mirror assembly

Polarization approach

Keep instrument slit in principal plane

Include optical element to depolarize incoming radiation

Table 1: Comparison of OCO-2 and OCO-3 missions

Instrument mass, power

500 kg, 600 W

Instrument volume

1.85 m x 1.0 m x 0.8 m

Data rate

3 Mbit/s

Thermal subsystem

Fluid cooling loop

Table 2: OCO-3 payload interface parameters


Figure 9: OCO-3 payload exterior (image credit: NASA/JPL) 10)


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). 11) 12)

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 [LINK].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 10: 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.


Measurement concept

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. 13)

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 11: How diffraction grating works (image credit: NASA/JPL, Caltech)


Figure 12: This illustration shows NASA's OCO-3 mounted on the underside of the International Space Station (image credit: NASA) 14)

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 13, using a COTS C-mount lens.


Figure 13: OCO-3 Internal Context Camera (image credit: NASA/JPL)


Figure 14: 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)


1) "Orbiting Carbon Observatory 3," NASA, URL:

2) "OCO-3," NASA Science, 22 May 2016, URL:

3) Rod Jones, "ISS Orbit and Ground Track Applicability for Earth Observation, Astrophysics and Heliophysics," Presentations of the ISS Research & Development Conference, Chicago, Illinois, USA, June 17-19, 2014, URL:

4) Kalina Velev, "Watching the Planet Breathe: The Story of SIF," NASA/JPL, 14 April 2017, URL:

5) Esprit Smith, Carol Rasmussen, "More Than a Carbon Copy: OCO-3 on the Space Station," NASA/JPL, 2 April 2019, URL:

6) "OCO-3 History," NASA/JPL, URL:

7) Julie A. Robinson, William L. Stefanov,"Earth Science Research on the International Space Station," Committee on Earth Science and Applications from Space (CESAS) Space Studies Board National Academies of Science, Engineering, Medicine, 29 March 2016, URL:

8) Stephen Clark, "Launch Schedule," Spaceflight Now, 13 March 2019, URL:


10) Annmarie Eldering and the OCO-3 team, including Chris O'Dell, Tommy Taylor, Ryan Pavlick, Thomas Kurosu, Greg Osterman, and Brendan Fisher & OCO-3 Cal team, Rob Rosenberg, Richard Lee, Peter Lawson, Lars Chapsky, Shanshan Yu, & Matt Bennett, Muthu Jengenathan, Gary Spiers, Ralph Basilio, "The OCO-3 Mission: Science Objectives and Instrument Performance," NASA/JPL, Caltech, 9 May 2018, URL:

11) "OCO-3, Pointing Mirror Assembly (PMA) and Modes," NASA/JPL, URL:

12) Annmarie Eldering, Tommy E. Taylor, Chris W. O'Dell, and Ryan Pavlick, "The OCO-3 mission; measurement objectives and expected performance based on one year of simulated data," Atmospheric Measurement Techniques Discussions, Discussion started: 5 November 2018,, EGU, URL:

13) OCO-3 instrument," NASA/JPL, URL:

14) "More Than a Carbon Copy: OCO-3 on the Space Station," NASA/JPL, 2 April 2019, URL:

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 (