The ocean hosts a fundamental component of Earth’s biosphere. Marine organisms play a pivotal role in the cycling of life’s building blocks such as nitrogen, carbon, oxygen, silica, and sulfur. About half of the global primary production—the process by which CO2 is taken up by plants and converted to new organic matter by photosynthesis—occurs in the ocean. Most of the primary producers in the ocean comprise microscopic plants and some bacteria; these photosynthetic organisms (phytoplankton) form the base of the ocean’s food web. Scientists are exploring how future climate change and sea surface warming might impact the overall abundance of phytoplankton. A long-term change in phytoplankton biomass would have major implications for the ocean’s ability to take up atmospheric CO2 and support current rates of fish production. Therefore, sustaining a global record of the abundance of phytoplankton and their contribution to global primary productivity is required to assess the overall health of the ocean, which is currently threatened by multiple stresses such as increased temperature and ocean acidification (both due to anthropogenic CO2 emissions), marine pollution, and overfishing.
Because the ocean covers roughly 70 percent of Earth’s surface, ships alone cannot collect observations rapidly enough to provide a global synoptic view of phytoplankton abundance. Only since the launch of the first ocean color satellite (the Coastal Zone Color Scanner [CZCS] in 1978) has it been possible to obtain a global view of the ocean’s phytoplankton biomass in the form of chlorophyll. These observations led to improved calculations of global ocean primary production, as well as better understanding of the processes affecting how biomass and productivity change within the ocean basins at daily to interannual time scales.
THE OCEAN COLOR TIME-SERIES IS AT RISK
Currently, the continuous ocean color data record collected by satellites since the launch of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS, in 1997) and the Moderate Resolution Imaging Spectroradiometer (MODIS, on Terra in 1999 and on Aqua in 2002) is at risk. The demise of SeaWiFS in December 2010 has accentuated this risk. MODIS on Aqua is currently the only U.S. sensor in orbit that meets all requirements (see below) for sustaining the climate-quality1 ocean color time-series and products. However, this sensor is also many years beyond its design life. Furthermore, it is no longer possible to rectify problems with the Aqua sensor degradation that were addressed through comparisons with SeaWiFS in the past few years. Therefore, it is uncertain how much longer data from U.S. sensors will be available to support climate research. Although the European Medium-Resolution Imaging Spectrometer (MERIS) meets all the requirements of a successful mission, it is also beyond its design life. Because of the many uncertainties surrounding the next U.S. satellite mission (more specifically the Visible Infrared Imager Radiometer Suite [VIIRS] sensor scheduled to launch fall 2011); data acquired through the VIIRS mission threaten to be of insufficient quality to continue the climate-quality time-series.
Even if fully successful, the VIIRS sensor’s capabilities are too limited to explore the full potential of ocean color remote sensing. Thus, the U.S. research community is looking to National Aeronautics and Space Administration (NASA) to provide ocean color sensors with advanced capabilities to support new applications and for significant improvements to current research products beyond what is possible with data from SeaWiFS and MODIS or will be possible from VIIRS. However, the Pre-Aerosol-Clouds-Ecosystem (PACE)—the first of NASA’s planned three missions that would advance the capabilities for basic ocean color research—is not scheduled to launch before 2019.
Without the ability to sustain high-quality ocean color measurements or to launch next generation sensors with new
1 Climate-quality observations are a time-series of measurements of sufficient length, consistency, and continuity to assess climate variability and change (following NRC, 2004b).
capabilities, many important research and operational uses are compromised, including the capability to detect impacts of climate change on primary productivity. Therefore, it is imperative to maintain and improve the capability of satellite ocean color missions at the accuracy level required to understand changes to ocean ecosystems that potentially affect living marine resources and the ocean carbon cycle, and to meet other operational and research needs. Given the importance of maintaining the data stream, the National Oceanic and Atmospheric Administration (NOAA), NASA, the National Science Foundation (NSF), and the Office of Naval Research (ONR) asked the National Research Council to convene an ad hoc study committee to review the minimum requirements to sustain global ocean color radiance measurements for research and operational applications and to identify options to minimize the risk of a data gap (see Box S.1 for the full statement of task). Because the ability to sustain current capabilities is at risk, the report focuses on minimum requirements to sustain ocean color observations of a quality equivalent to the data collected from SeaWiFS. Meeting these requirements will mitigate the risk of a gap in the ocean color climate data record but will be insufficient to explore the full potential of ocean color research and will fall short of meeting all the needs of the ocean color research and operational community. To meet all these needs, a constellation of multiple sensor types2 in polar and geostationary orbits will be required. Note that satellite requirements for research leading to the generation of novel products would vary depending on the question addressed and are difficult to generalize.
THE REQUIREMENTS TO OBTAIN HIGH-QUALITY GLOBAL OCEAN COLOR DATA
Satellite ocean color sensors measure radiance at different wavelengths that originate from sunlight and are backscattered from the ocean and from the atmosphere. Deriving the small ocean component from the total radiance measured by satellite sensors is a complex, multi-step process. Each step is critical and needs to be optimized to arrive at accurate and stable measurements. Using a set of algorithms (starting with removal of the contribution from the atmosphere, which is most of the signal), radiance at the top of the atmosphere is converted to water-leaving radiance (Lw) and then to desired properties such as phytoplankton abundance and primary productivity. To detect long-term climactic trends from these properties, measurements need to meet stringent accuracy requirements. Achieving this high accuracy is a challenge, and based on a review of lessons learned from the SeaWiFS/MODIS era, requires the following steps to sustain current capabilities:
1. The sensor needs to be well characterized and calibrated prior to launch.
2. Sensor characteristics, such as band-set and signal-to-noise, need to be equivalent to the combined best attributes from SeaWiFS and MODIS.
3. Post-launch vicarious calibration3 using a Marine Optical Buoy (MOBY)-like approach with in situ measurements that meet stringent standards is required to set the gain factors of the sensor.
4. The sensor stability and the rate of degradation need to be monitored using monthly lunar looks.4
5. At least six months of sensor overlap are needed to transfer calibrations between space sensors and to produce continuous climate data records.
6. The mission needs to support on-going development and validation of atmospheric correction, bio-optical algorithms, and ocean color products.
7. Periodic data reprocessing is required during the mission.
8. A system needs to be in place that can archive, make freely available, and distribute rapidly and efficiently all raw,5 meta- and processed data products to the broad national and international user community.
9. Active research programs need to accompany the mission to improve algorithms and products.
10. Documentation of all mission-related aspects needs to be accessible to the user community.
Meeting these requirements would contribute to sustaining the climate-quality global ocean color record for the open ocean. However, further enhancements to sensors and missions, such as higher spectral and spatial resolution, will be required to meet the research and operational needs for imaging coastal waters and for obtaining information about the vertical distribution of biomass or particle load. High frequency sampling (e.g., imagery every 30 minutes for a fixed ocean area), such as can be obtained from geostationary orbit, are desirable enhancements for applications such as ecosystem and fisheries management, as well as naval applications.
2 Type 1: Polar orbiting sensors with relatively low spatial resolution (1 km) with 8 (or many more) wave bands.
Type 2: Polar orbiting sensors with medium spatial resolution (250-300 m) and more bands to provide a global synoptic view at the same time as allowing for better performance in coastal waters.
Type 3: Hyper-spectral sensors with high spatial resolution (~30m) in polar orbit.
Type 4: Hyper- or multi-spectral sensors with high spatial resolution in geostationary orbit.
3 Vicarious calibration refers to techniques that use natural or artificial sites on the surface of Earth and models for atmospheric radiative transfer to provide post-launch absolute calibration of sensors.
4 Monthly lunar looks refers to the spacecraft maneuver that looks at the surface of the moon once a month as a reference standard to determine how stable the sensor’s detectors are. The information from the lunar looks is then used for determining temporal changes in sensor calibration.
5 Raw data is defined as data in engineering units to which new calibration factors can be applied to generate radiance values at the top of the atmosphere.
Statement of Task
Continuity of satellite ocean color data and associated climate research products are presently at significant risk for the U.S. ocean color community. Temporal, radiometric, spectral, and geometric performance of future global ocean color observing systems must be considered in the context of the full range of research and operational/application user needs. This study aims to identify the ocean color data needs for a broad range of end users, develop a consensus for the minimum requirements, and outline options to meet these needs on a sustained basis.
An ad hoc committee will assess lessons learned in global ocean color remote sensing from the SeaWiFS/MODIS era to guide planning for acquisition of future global ocean color radiance data to support U.S. research and operational needs. In particular, the committee will assess the sensor and system requirements necessary to produce high-quality global ocean color climate data records that are consistent with those from SeaWiFS/MODIS. The committee will also review the operational and research objectives, such as described in the Ocean Research Priorities Plan and Implementation Strategy, for the next generation of global ocean color satellite sensors and provide guidance on how to ensure both operational and research goals of the oceanographic community are met. In particular the study will address the following:
1. Identify research and operational needs, and the associated global ocean color sensor and system high-level requirements for a sustained, systematic capability to observe ocean color radiance (OCR) from space;
2. Review the capability, to the extent possible based on available information, of current and planned national and international sensors in meeting these requirements (including but not limited to: VIIRS on NPP and subsequent JPSS spacecrafts; MERIS on ENVISAT and subsequent sensors on ESA’s Sentinel-3; S-GLI on JAXA’s GCOM-C; OCM-2 on ISRO’s Oceansat-2; COCTS on SOA’s HY-1; and MERSI on CMA’s FY-3);
3. Identify and assess the observational gaps and options for filling these gaps between the current and planned sensor capabilities and timelines; define the minimum observational requirements for future ocean color sensors based on future oceanographic research and operational needs across a spectrum of scales from basin-scale synoptic to local process study, such as expected system launch dates, lifetimes, and data accessibility;
4. Identify and describe requirements for a sustained, rigorous on-board and vicarious calibration and data validation program, which incorporates a mix of measurement platforms (e.g., satellites, aircraft, and in situ platforms such as ships and buoys) using a layered approach through an assessment of needs for multiple data user communities; and
5. Identify minimum requirements for a sustained, long-term global ocean color program within the United States for the maintenance and improvement of associated ocean biological, ecological, and biogeochemical records, which ensures continuity and overlap among sensors, including plans for sustained rigorous on-orbit sensor inter-calibration and data validation; algorithm development and evaluation; data processing, re-processing, distribution, and archiving; as well as recommended funding levels for research and operational use of the data.
The review will also evaluate the minimum observational research requirements in the context of relevant missions outlined in previous NRC reports, such as the NRC “Decadal Survey” of Earth Science and Applications from Space. The committee will build on the Advance Plan developed by NASA’s Ocean Biology and Biogeochemistry program and comment on future ocean color remote sensing support of oceanographic research goals that have evolved since the publication of that report. Also included in the review will be an evaluation of ongoing national and international planning efforts related to ocean color measurements from geostationary platforms.
ASSESSMENT OF CURRENT AND FUTURE SENSORS IN MEETING THESE REQUIREMENTS
As Figure S.1 indicates, all current sensors except for Ocean Colour Monitor on-board Oceansat-2 (OCM-2) are beyond their design life. The recent demise of SeaWiFS is also putting into question the future of the MODIS sensors because their recent rapid degradation resulted in a reliance on SeaWiFS data to calibrate the MODIS data. Without this calibration, it is unclear how long MODIS data can be made available at the necessary accuracy. MERIS is a high-quality mission but also beyond its design life.
Therefore, the launch of VIIRS planned for fall 2011 comes at a very critical time. Unless there is a successful transition from European Space Agency’s (ESA) MERIS to ESA’s Ocean Land Colour Instrument (OLCI) sensor, and data from OCLI are available immediately, the success and the continuity of the global ocean color time-series will be dependent on the success of the VIIRS mission, because OCM-2 does not collect global data.
The research community has long questioned the ability of VIIRS to deliver high-quality data because of a manufacturing error in one of its optical components. Since this issue has been raised, the sensor has been mounted onto its launch vehicle and undergone additional testing and char-
FIGURE S.1 The launch sequence of past, current, and planned ocean color sensors in polar orbit are displayed. The sensors still operational are shown with a one-sided arrow; the hatched area indicates when a sensor is beyond its design life. The gray shaded background indicates a data gap in the past and a potential data gap if MODIS sensors and MERIS cease today. The question marks are used to indicate sensors that either do not yet meet the minimum requirements or are vulnerable to changes in funding allocation. Future sensors are shown having either a five- or seven-year lifetime, according to their individual specifications. CZCS: Coastal Zone Color Scanner; OCTS: Ocean Color and Temperature Scanner; SeaWiFS: Sea-viewing Wide Field-of-view Sensor; OCM/OCM-2: Ocean Colour Monitor; MODIS-Terra/MODIS-Aqua: Moderate Resolution Imaging Spectroradiometer on Terra/Aqua, respectively; MERIS: Medium Resolution Imaging Spectrometer; GLI: Global Imager; VIIRS: Visible Infrared Imager Radiometer Suite; OLCI: Ocean Land Colour Instrument onboard Sentinel-3; PACE: Pre-Aerosol-Clouds-Ecosystem; GCOM-C: Global Change Observation Mission for Climate Research; JPSS: Joint Polar Satellite System.
SOURCE: Based on data from http://www.ioccg.org/sensors_ioccg.html.
acterization. The most recent tests have resulted in a more optimistic assessment about its performance, and a software solution to overcome part of the optical hardware issue has been proposed.
However, based on the committee’s assessment of the overall planning and budgeting, it is currently unlikely that this mission will provide data of sufficient quality to continue the ocean color climate data record. This conclusion reflects inadequacies in the current overall mission design and provisions to address all the key requirements of a successful ocean color mission (see above for 10 requirements). In particular, NOAA has not developed a capacity to process and reprocess the data such as is available at NASA.
Conclusion: VIIRS/NPP has the potential to continue the high-quality ocean color time-series only if NOAA takes ALL of the following actions:
1. implement spacecraft maneuvers as part of the mission, including monthly lunar looks using the Earth-viewing port to quantify sensor stability;
2. form a calibration team with the responsibility and authority to interact with those generating Level 16 products, as well as with the mission personnel responsible for
6 There are five different levels of processing of satellite data:
Level 0: Raw data as measured directly from the spacecraft in engineering units (e.g., volts or digital counts).
Level 1: Level 0 data converted to radiance at the top of the atmosphere using pre-launch sensor calibration and characterization information adjusted during the life of the mission by vicarious calibration and stability monitoring.
Level 2: Data generated from Level 1 data following atmospheric correction that are in the same satellite viewing coordinates as Level 1 data.
Level 3: Products that have been mapped to a known cartographic projection or placed on a two-dimensional grid at known spatial resolution.
Level 4: Results derived from a combination of satellite data and ancillary information, such as ecosystem model output.
the sensor, to provide the analyses needed to assess trends in sensor performance and to evaluate anomalies;
3. implement a vicarious calibration process and team using a MOBY-like approach;
4. implement a process to engage experts in the field of ocean color research to revisit standard algorithms and products, including those for atmospheric correction, to ensure consistency with those of heritage instruments and for implementing improvements;
5. form a data product team to work closely with the calibration team to implement vicarious and lunar calibrations, oversee validation efforts, and provide oversight of reprocessing; and
6. provide the capability to reprocess the mission data multiple times to incorporate improvements in calibration, correct for sensor drift, generate new and improved products, and for other essential reasons.
Conclusion: If these steps are not implemented, the United States will lose its capability to sustain the current time-series of high-quality ocean color measurements from U.S. operated sensors in the near future, because the only current viable U.S. sensor in space (MODIS-Aqua) is beyond its design life.
Regardless of how well VIIRS performs, it has only a very limited number of ocean color spectral bands and thus cannot provide the data required by the research community for advanced applications. Under ideal conditions of international cooperation, data from U.S. and non-U.S. sensors planned for the future could be made readily available to meet the many needs for research and operations, but ideal conditions are difficult to negotiate for many complicated reasons. The European MERIS mission is currently providing high-quality global data, albeit with somewhat less frequent global coverage owing to its narrower swath as compared to the U.S. missions. The European Space Agency (ESA) expects MERIS will continue to operate until its follow-on sensor (OLCI) is launched on ESA’s Sentinel-3 platforms in 2013. ESA, NASA and NOAA have ongoing discussions about full exchange of MERIS mission data, including raw satellite data and calibration data. The Indian space agency launched the OCM-2 sensor in 2009. OCM-2 has excellent technical specifications, but to date, data access is very limited. Furthermore, OCM-2 is not a global mission; its data collection priority focuses on the Indian Ocean. The Japanese space agency is planning an advanced ocean color sensor, Second-Generation Global Imager (S-GLI), for launch in 2014 that has high potential based on its technical specifications.
Conclusion: Under the following conditions non-U.S. sensors can be viable options in replacing or augmenting data:
1. A U.S. program is established to coordinate access to data from non-U.S. sensors, including full access to pre-launch characterization information and timely access to post-launch Level 1 or Level 0 data, and direct downlink for real-time access; and
2. This program includes sufficient personnel and financing to collect independent calibration and validation data, assess algorithms and develop new algorithms as required, produce and distribute data products required by U.S. users, support interactions among U.S. research and operational users in government, academia and the private sector, and has the capability to reprocess data from U.S. missions (e.g., MODIS, SeaWiFS) as well as the non-U.S. sensors to establish a continuous time-series of calibrated data.
The committee finds that non-U.S. sensors can be viewed as a source of data to complement and enhance U.S. missions. For example, merging calibrated data from multiple sensors, particularly if the sensors have different equatorial crossing times, can provide much more complete global coverage than is possible from a single sensor. Mean coverage from a single sensor averages about 15 percent of the global ocean per day, owing to cloud cover and limitations imposed by swath width and orbit characteristics. Daily coverage can be increased by merging data from multiple sensors, if they are in complementary orbits. Furthermore, sensors such as MERIS, OLCI, and OCM-2 have much better capabilities—including higher spatial and spectral resolution—for imaging coastal waters than current U.S. sensors or VIIRS. Routine access to the data from these non-U.S. sensors, particularly MERIS and OLCI, is essential to advance the research and operational uses of ocean color data for U.S. coastal applications. OCM-2 has potential but is not currently operated for global observations.
Finally, non-U.S. space agencies are taking some of the development risk for new approaches to ocean color data collection. For example, South Korea in 2010 became the first country to put an ocean color imager into geostationary orbit (viewing the East China Sea), and thus will help the international user community understand the potential of this approach, including the capability to view the same ocean area about every 30 minutes during daylight hours.
MINIMIZING THE RISK OF A DATA GAP
The risk of a data gap in the U.S. ocean color time-series is very real and imminent because MODIS is not likely to deliver high-quality data for much longer. Many issues remain unresolved regarding the VIIRS missions, and the next U.S. ocean color mission, NASA’s Pre-Aerosol-Clouds-Ecosystem (PACE) mission, will not launch before 2019. To minimize this risk, the principal recommendation of the committee is:
Recommendation: NOAA should take all the actions outlined above to resolve remaining issues with the VIIRS/NPP. In addition, NOAA needs to fix the hardware problems on the subsequent VIIRS sensors and ensure all the above actions are incorporated into the mission planning for the subsequent VIIRS launches on JPSS-1 and JPSS-2. Taking these steps is necessary to generate a high-quality dataset, because VIIRS is the only opportunity for a U.S. ocean color mission until the launch of NASA’s PACE mission, currently scheduled for launch no earlier than 2019. In addition, if MERIS ceases operation before Sentinel-3A is launched in 2013, VIIRS/NPP would be the only global ocean color sensor in polar orbit.
To develop quality ocean color products requires highly specialized skill and expertise. Currently, the NASA Ocean Biology Processing Group (OBPG) at Goddard Space Flight Center (GSFC) is internationally recognized as a leader in producing well-calibrated, high-quality ocean color data products from multiple satellite sensors. NOAA currently lacks the demonstrated capacity to readily produce high-quality ocean color products and provide the comprehensive services currently available from the OBPG, although NOAA is in the process of building its capacity. For example, although NOAA’s National Climate Data Center (NCDC) plans to archive a climate-level7 radiance data record, it is unclear how NOAA can generate the products or make them easily accessible to U.S. and foreign scientists.
Both NASA and NOAA support ocean color applications, with NASA focused primarily on research and development and NOAA focused on operational uses. Because both agencies have a strong interest in climate and climate impacts, they share a common interest in climate data records.If NOAA builds its own data processing/reprocessing group, two independent federal groups will then be developing ocean color products and climate data records. While this can be justified given the distinct missions of NOAA and NASA, it can also raise problems when discrepancies appear in the data records. Moreover, the committee anticipates major challenges to generating high-quality products from the VIIRS/NPP data, which call for involving the expertise currently only available at NASA’s OBPG. For these reasons, the committee concludes the following:
Conclusion: NOAA would greatly benefit from initiating and pursuing discussions with NASA for an ocean color partnership that would build on lessons learned from SeaWiFS and MODIS, in particular.8
Recommendation: To move toward a partnership, NASA and NOAA should form a working group to determine the most effective way to satisfy the requirements of each agency for ocean color products from VIIRS and to consider how to produce, archive, and distribute products of shared interest, such as climate data records, that are based on data from all ocean color missions. This group should comprise representatives from both agencies and include a broad range of stakeholders from the ocean color research and applications community.
Based on its review of previous ocean color missions, the committee concludes that a long-term national program to support ocean color remote sensing involves multiple agencies—NOAA and NASA in particular, with input from the academic research community, and continuous funding that goes beyond the lifetime of any particular satellite mission. Such a mechanism is required to ensure that:
1. continuity is achieved and maintained between U.S. and non-U.S. satellite missions;
2. lessons learned from previous missions are incorporated into the planning for future missions;
3. mission planning and implementation are timed appropriately to ensure continuity between satellite missions;
4. capability for data processing and reprocessing of U.S. and non-U.S. missions is maintained; and
5. planning for transition from research to operation occurs early for each mission and is implemented seamlessly via cooperation and interaction between government, academic, and private-sector scientists.
Recommendation: To sustain current capabilities, NOAA and NASA should identify long-term mechanisms that can:
• provide stable funding for a MOBY-like approach for vicarious calibration;
• maintain the unique ocean color expertise currently available at NASA’s OBPG over the long term and make it available to all ocean color missions;
• nurture relations between NASA and NOAA scientists so that both agencies meet their needs for ocean color data in the most cost-effective manner and without needless duplication;
• establish and maintain validation programs, and maintain and distribute the data over the long term;
• provide the planning and build the will for continuity in the satellite missions over the long term; and
• sustain the viability of the scientific base by supporting research and training.
The committee envisions that such a mechanism could be a U.S. working group modeled after the International Ocean Colour Coordinating Group (IOCCG). The establishment of a working group with representation from all the
7 Climate-level means repackaged data to look like a MODIS granule and all metadata repackaged accordingly to ease the reprocessing of the Level 0 data.
8 Consistent with the conclusions and recommendations of “Assessment of Impediments to Interagency Collaboration on Space and Earth Science Missions” (NRC, 2010).
interested federal agencies, from U.S. academic institutions and the private sector could provide the necessary long-range planning to meet the needs of U.S. users, provide external advice to the individual missions, interact with foreign partners, and develop consensus views on data needs and sensor requirements.
The diverse applications of, and future enhancements to, ocean color observations will require a mix of ocean color satellites in polar and geostationary orbit with advanced capabilities. Although the three missions described in NASA’s Decadal Survey (Aerosol-Cloud-Ecosystem/PreAerosol-Cloud-Ecosystem, Geostationary Coastal and Air Pollution Events [GEOCAPE], and Hyperspectral Infrared Imager [HyspIRI]) will potentially provide many advanced capabilities, meeting all user needs within the next decade will likely surpass the capability of a single space agency or nation.
Conclusion: U.S. scientists and operational users of satellite ocean color data will need to rely on multiple sources, including sensors operated by non-U.S. space agencies, because the United States does not have approved missions to sustain optimal ocean color radiance data for all applications.
Recommendation: NOAA’s National Environmental Satellite, Data, and Information Service and NASA’s Science Mission Directorate should increase efforts to quickly establish lasting, long-term data exchange policies, because U.S. users are increasingly dependent on ocean color data from non-U.S. sensors.
The IOCCG presents an effective body through which NASA and NOAA can engage with foreign space agencies and develop a long-term vision for meeting the research and operational needs for ocean color products. Through the IOCCG, space agencies can identify options for collaborations and approaches mutually beneficial to all interested parties. The group has been active in communicating user needs and is working with the Committee on Earth Observation Satellites (CEOS) to develop plans for the Ocean Colour Radiometry Virtual Constellation9 (OCR-VC). In the long term, international partnerships will be needed to sustain the climate-quality global ocean color time-series, and at the same time, to advance ocean color capabilities and research.
9 A virtual constellation is a set of space and ground segment capabilities operating together in a coordinated manner; in effect, a virtual system that overlaps in coverage in order to meet a combined and common set of Earth Observation requirements. The individual satellites and ground segments can belong to a single or multiple owners.