Issues in the Integration of Research and Operational Satellite Systems for Climate Research: Part II. Implementation (2000)

A key objective of climate research and monitoring programs is to deliver scientifically valid knowledge that can be used by the public and by policymakers to make informed decisions about large-scale environmental issues. Because Earth’s climate involves a complex interplay among the atmosphere, oceans, cryosphere, and biosphere, meeting this objective will require a comprehensive strategy that includes observations, data analysis, technology development, modeling, and data archiving and distribution. Satellite observations are an essential part of this strategy as they can record global-scale phenomena and collect information on many critical physical, chemical, and biological processes. However, there are challenges in utilizing current satellite observation programs to support climate research and monitoring. The requirements of the climate research community are sometimes at odds with the capabilities of both the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA). Further, both agencies are likely to continue to operate in a highly constrained fiscal environment. For these reasons, this report and its phase one companion, Science and Design (NRC, 2000), focus on approaches to leverage existing and planned operational and research satellite assets to meet the needs of climate research.

Operational satellite missions are designed primarily to provide observations to support short-term environmental forecasts, while research satellite missions are often designed primarily to study specific processes of scientific interest or to test new observing technologies. Obtaining long-term, well-calibrated measurements from space often falls between these agency objectives. Yet the Committee on Earth Studies believes that, while challenging, the integration of operational and research missions to advance the objectives of climate research is possible and that a unique opportunity to demonstrate such integration is presented by the National Polar-orbiting Operational Environmental Satellite System (NPOESS) and the redesigned NASA/Earth Science Enterprise (ESE) missions.

NPOESS and the NPOESS Preparatory Project (NPP) offer significant improvements over the capabilities of the two existing separate operational polar-orbiting systems: NOAA’s Polar-orbiting Operational Environmental Satellites (POES) and the Department of Defense’s Defense Meteorological Satellite Program (DMSP). Moreover, the redesigned NASA/ESE missions focus on critical science questions in the area of climate research, and NASA’s new strategy of employing a larger number of smaller spacecraft provides a high level of flexibility.

NPOESS will collect critical data sets on variables that are not currently included in operational measurements (such as radiation budget, total ozone, wind speed and direction, ocean topography, and ocean color) and will offer improved quality for some variables now being measured (such as atmospheric moisture and temperature profiles, all-weather sea surface temperature, and vegetation indices). Moreover, the orbits of NPOESS

satellites will have stable equator-crossing times, which will significantly improve the utility of the data for climate research. The next set of NASA/ESE missions will not be based on copies of the first Earth Observing System (EOS) series. Instead, they will be divided into systematic missions (i.e., emphasizing measurements of processes dominated by long-term variability) and exploratory missions (i.e., focused on specific scientific questions that can be answered with a single mission). Because systematic measurements are an essential element of the NASA/ESE strategy, special attention is being given to NPOESS. In this context, NPP is important as a testbed for the incorporation of NASA/ESE science requirements into an operational mission.

The present report emphasizes two themes. First, data stability—enabled by long-term, consistent data sets—is a critical requirement for climate research. Second, system flexibility is necessary to enable pursuit of new science objectives as well as new technology and to respond to surprises that will emerge in the Earth system. Further discussion of both themes can be found in the “Pathways” report (NRC, 1998).

Because natural signals are often small, it is difficult to ascribe particular events or processes to climate change. This is especially true in the area of anthropogenic forcing, or global warming. Natural events such as the El Niño/Southern Oscillation represent enormous, global-scale perturbations in a variety of Earth system variables, such as ocean winds and sea surface temperature, precipitation, and atmospheric carbon dioxide. For this reason, long-term, high-quality measurements are needed to discern subtle shifts in Earth’s climate. Such measurements require an observing strategy emphasizing a strong commitment to maintaining data quality and minimizing gaps in coverage. Operational satellites represent a unique asset that could produce long time series with sufficient quality, although their primary mission is not climate research. NPOESS officials appear to be making significant progress toward facilitating such data records, particularly in their attempts to set stability requirements for some of the critical data sets. Currently, however, some NPOESS environmental data records do not have stability requirements, while others have incomplete or insufficient requirements. In addition, no strategy to test the stability requirements for NPOESS measurements has been defined or developed.

The committee considered data stability from three perspectives:

• Sensor calibration and data product validation,

• Requirements for and approaches to data continuity, and

• Data systems.

Long-term studies such as those needed for documenting and understanding global climate change require not only that a remote sensing instrument be accurately characterized and calibrated but also that its characteristics and calibration be stable over the life of the mission. Calibration and validation should be considered as a process that encompasses the entire system, from the sensor performance to the derivation of the data products. The process can be considered to consist of five steps. In the approximate order of performance they are (1) instrument characterization, (2) sensor calibration, (3) calibration verification, (4) data quality assessment, and (5) data product validation.

The committee makes the following recommendations with regard to calibration and validation:

• A continuous and effective on-board reference system is needed to verify the stability of the calibration and sensor characteristics from the launch through the life of the mission.

• Radiometric characterization of the Moon should be continued and possibly expanded to include measurements made at multiple institutions in order to verify the NASA results. If the new reflectance calibration paradigm is adopted (see Appendix C), then the objective of the lunar characterization program should be to measure changes in the relative reflectance as a function of the phase and position of Earth, the Sun, and the Moon rather than absolute spectral radiance.

• The establishment of traceability by national measurement institutions in addition to the National Institute of Standards and Technology should be considered to determine if improved accuracy, reduced uncertainty in the measurement chain, and/or better documentation might be achieved, perhaps even at a lower cost.

• The results of sensitivity studies on the parameters in the data product algorithms should be summarized in a requirements document that specifies the characterization measurements for each channel in the sensor. Blanket specifications covering all channels should be avoided unless justified by the sensitivity studies.

• Quality assessment should be an intrinsic part of operational data production and should be provided in the form of metadata with the data product.

• Validation, an essential part of the information system, should be undertaken for each data product or data record to provide a quantitative estimate of the accuracy of the product over the range of environmental conditions for which the product is provided.

• Wavelengths and bandwidths of channels in the solar spectral region should be selected to avoid absorption features of the atmosphere, if possible.

• Calibration of thermal sensing instruments such as CERES (Clouds and the Earth’s Radiation Energy System) and the thermal bands of MODIS (Moderate-resolution Imaging Spectroradiometer) should continue to be traceable to the SI unit of temperature via the Planckian radiator, blackbody technology.

Continuity is concerned with more than the presence or absence of data. It includes the continuous and accurate characterization of the properties that affect the construction of the time series. The most useful data for climate research purposes are time series that are continuous and for which the characterization of error, in terms of precision and bias, is known. Such errors should be minimized as much as possible in order to detect the often small, climate-related signal.

The committee recommends taking the following steps to ensure data continuity:

• A policy that ensures overlapping observations of at least 1 year (more for solar instruments) should be adopted. The IPO should examine the relation between this requirement and the launch-on-failure strategy and should include a clear definition of spacecraft or instrument failure and an assessment of still-functioning instruments.

• Competitive selection of instrument science teams should be adopted to follow the progress of the instrument from design and fabrication through integration, launch, operation, and finally, data archiving, thereby promoting more thorough instrument characterization.

• As instruments are developed for future missions, the IPO should make a determination of threats to the continuity of currently monitored radiances in the design requirements.

• Out-year funding should be provided to maximize the investment made in climate and operational observing instruments.¹

• Free-flier status should be evaluated for key climate parameters such as solar radiance and sea-level altimetry whose measurement appears to be endangered by the NPOESS single-platform configuration.

• Proven active microwave sensors should be considered for ocean vector winds, another key climate (and operational) parameter.

The development of an NPOESS climate data system (NCDS) represents a significant challenge. Care will be needed to ensure that the design and specifications for the data system are given a broad review prior to their implementation. In addition, special attention will be needed in areas including calibration and validation, data product continuity, data archiving, archive access, reprocessing, and cost. The NPP will serve for the early testing of instruments and data systems. It will be a joint activity between NASA and the Integrated Program Office (IPO) and as such will provide an opportunity for NPOESS to benefit from the progress NASA is making in data system development.

The development of an NCDS can clearly benefit from adopting the best elements of the current NASA and NOAA data systems. However, it will not be enough to simply expand existing facilities. A successful NCDS will also require a new vision in which innovation and competition play a central role. Observations of Earth will increase by an order of magnitude when NPOESS begins operation, which could lead to an enormous increase in our understanding of Earth. To realize this potential, the huge volumes of raw data must be converted to usable products and information. The responsibility for doing this should be given to those groups and organizations that demonstrate the vision, innovation, and expertise needed to meet the NPOESS challenge.

The committee recommends meeting the following basic data-systems requirements in addition to what is needed for operational processing:

• A long-term archiving system is needed that provides easy and affordable access for a large number of scientists in many different fields.

• Data should be supported by metadata that carefully document sensor performance history and data processing algorithms.

• The system should have the ability to reprocess large data sets as understanding of sensor performance, algorithms, and Earth science improves. Examples of sources of new information that would warrant data reprocessing include the discovery of processing errors, the detection of sensor calibration drift, the availability of better ancillary data sets, and better geophysical models.

• Science teams responsible for algorithm development, data set continuity, and calibration and validation should be selected via an open, peer-reviewed process (in contrast to the approach taken with the operational integrated data processing system (IDPS) and algorithms, which are being developed by sensor contractors for NPOESS).

• The research community and government agencies should take the initiative and begin planning for a research-oriented NCDS and the associated science participation.

Because the forcing and response of Earth’s climate to natural and anthropogenic variability is a complex, nonlinear process, it can be anticipated that unforeseen properties will emerge. These are the “surprises” discussed in the “Pathways” report (NRC, 1998). Scientific advances will require new observing tools. Moreover, technological advances may reduce costs or improve system performance. A rigid plan of flying exact copies of sensors

will not accommodate such changes. Therefore, a way will have to be found to infuse new technology into the system while maintaining data continuity and without driving up costs. Technology insertion is defined as introduction of any new and/or improved capability (either through hardware or software innovations) into an established operational system. NASA/ESE will play an especially important role in this regard, given its experience in technology development. The committee considered the issue of system flexibility primarily from this vantage point.

Qualifying technological innovations span a wide range of implied changes and, thus, impose a wide range of risk levels on the operational performance of the system. For example, replacing a computer with a faster model that preserves the form, fit, and function of the earlier model is quite different from changing the computer’s operating system or data processing algorithm. There is risk in any change to the design, but some changes may ripple throughout the system, forcing additional changes to accommodate the first. Additional risk is anathema for an operational system, for which reliability and continuity are the prime considerations. Any potential change must be examined carefully and conservatively, no matter how well justified the augmented capabilities may be from a scientific point of view.

The committee’s findings are as follows:

• Operational agencies exhibit a natural tendency to resist change; any candidate technology enhancement to increase the science content of data products must satisfy rigorous prequalification before being accepted into an operational payload.

• The challenge for an operational system such as NPOESS is to accommodate technological change in a timely manner, while ensuring that the modified system will sustain operational functionality.

• In general, the means of technology insertion into operational missions is not well determined. Indeed, there appears to be a gap between the development of instruments in the science stream and their adoption in the operational stream.

• If the NPOESS program is to be used to support the science community as well as the operational weather agency, then a careful assessment of the pertinent science requirements must be made in the early phases of the program.

• Technology insertion always will be subject to limitations. Any downstream change in the on-board technology must fit within the spacecraft resources (mass, power, data bandwidth, data volume, etc.) that may remain over and above the requirements of the baseline system.

• It is likely that the development and qualification of any new measurement capability that might be required for scientific purposes would have to be funded from non-IPO sources, unless that instrument were deemed to be critical to the NPOESS operational mission. Clearly, vision and well-coordinated interagency planning are needed to sustain the development of suitable instruments in synchronization with NPOESS flight opportunities.

• Unlike the relatively short design lifetimes of their predecessors, the NPOESS satellites are meant to have a 7-year lifetime. Although a 7-year design life is laudable for an ongoing operational facility, it adds further roadblocks to the process of technology insertion.

• Under current policy, whether an instrument provides data that are important to a climate science record has no bearing on the criteria for launch of an NPOESS replacement spacecraft. Partial failure, even of a mission-critical instrument, may have such a small impact for operational weather purposes that it does not trigger a replacement launch. However, the same fault could induce degradations that would be far more significant for scientific purposes.

• An opportunity to prove in practice the value of a candidate instrument is often a pivotal step in the effort

to transform a scientific measurement into an operational tool. A satellite program such as NPP could provide such opportunities.

• It is noteworthy that NASA’s ESE Technology Development Plan does not provide for the transitioning of technology from scientific status to operational status. This fact is central to the question of technology insertion into NPOESS in support of climate or other scientific objectives. Even if a new technological innovation is proven to offer unique scientific value and is shown to be technically feasible, there are no firm plans to guide its transition onto NPOESS.

The IPO and NASA should strive to accommodate technological change in a timely manner, while ensuring that the modified system will sustain operational functionality. The committee’s recommendations with regard to technology insertion are as follows:

• The IPO should identify a person or group to review the system requirements and the design to ensure that both the Integrated Operational Requirements Document (IORD; IPO, 1996) and the contractor approaches will support flexibility and change.

• NASA should provide a list of science requirements (ostensibly from the Science Plan) and climate requirements that are candidates for implementation on NPOESS.

• The IPO should plan for the insertion of new or enhanced measurement capabilities into NPOESS that would likely have to be funded from non-IPO sources.

• NASA ESE should implement its Technology Development Plan with firm plans linked to missions and ensure that any necessary NPOESS enabling technologies are covered in the plan.

• NASA and the IPO should devise an approach to support announcing and accepting additional experiments on NPOESS.

• It is essential that the process of incorporating research requirements into NPOESS be started now and be allowed to influence the program development and risk reduction phase that is in progress, without disrupting the primary NPOESS mission. Opportunities for change after the launch are limited by the longer satellite life and longer time between launches.

Integrated Program Office (IPO), National Polar-orbiting Operational Environmental Satellite System (NPOESS). 1996. Integrated Operational Requirements Document (First Version) (IORD-1) 1996. Issued by Office of Primary Responsibility: Joint Agency Requirements Group (JARG) Administrators, March 28. The updated IORD and other documents related to NPOESS are available online at <http://npoesslib.ipo.noaa.gov/ElectLib.htm.>

National Research Council (NRC). 1998. Overview, Global Environmental Change: Research Pathways for the Next Decade. National Academy Press. Washington, D.C.

National Research Council (NRC), Space Studies Board. 2000. Issues in the Integration of Research and Operational Satellite Systems for Climate Research: I. Science and Design. National Academy Press, Washington, D.C.