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Ensuring the Climate Record from the NPOESS and GOES-R Spacecraft: Elements of a Strategy to Recover Measurement Capabilities Lost in Program Restructuring 4 Elements of a Long-Term Climate Strategy The dramatic reduction in NPOESS and GOES-R contributions to climate measurement following Nunn-McCurdy certification and program cost-reduction efforts necessitates a near-term strategy of the kind outlined in Chapter 3 for recovery of crucial climate capabilities. Yet without proactive consideration of the elements of a viable long-term climate observational measurement strategy, important lessons from the well-intentioned but poorly executed merger of the nation’s weather and climate observation systems (Box 4.1) will be of little benefit. This chapter outlines the key elements of a long-term climate strategy. OPERATIONAL VERSUS SUSTAINED CLIMATE OBSERVATIONS The NRC decadal survey Earth Science and Applications from Space describes the overlapping nature, similarities in, and differences between exploratory, operational, and sustained observations.1 Briefly, exploratory measurements are new observations designed to shed light on poorly understood processes and so advance scientific understanding; operational measurements serve day-to-day critical activities such as weather forecasting that require high reliability and near-real-time data availability; and sustained measurements support the development of long-term records of key variables that are required to uncover slowly evolving dynamics or long-term climate changes. The inclusion of sensors that make climate-relevant measurements in an operational observing system muddles the distinction to some extent. Whereas a sensor’s inclusion on an operational system in theory provides long-term access to space and clearly identified launch opportunities, it also implies significant overhead associated with requirements for high reliability and near-real-time data availability. For the climate sensors originally to be flown on NPOESS, being part of an operational system meant sharing both a satellite platform and a ground data system tailored to meet operational objectives rather than climate science needs. From a climate science perspective, this resulted in suboptimal spacecraft capabilities (e.g., the absence of a lunar calibration) and ground system design (e.g., the inability to archive or provide data for reanalysis) at increased cost compared to traditional narrowly focused research missions. Clearly, the desirability of conducting sustained climate observations as part of an operational program should be balanced with consideration of the pragmatics of combining the two and the overall cost-effectiveness of such an approach. 1 See Chapter 3 of Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond (NRC, 2007).
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Ensuring the Climate Record from the NPOESS and GOES-R Spacecraft: Elements of a Strategy to Recover Measurement Capabilities Lost in Program Restructuring BOX 4.1 NPOESS, EOS, and the Search for Sustained Environmental Measurements The NPOESS program was, at the outset, driven by a single imperative—convergence of weather measurements, which would eliminate duplication in observations in the early afternoon and still maintain the same temporal robustness that characterized the combination of the Polar-orbiting Operational Environmental Satellites and the Defense Meteorological Satellite Program. The cost savings from eliminating duplication could then be reallocated to improve weather observations and models. By the mid-1990s, it was clear that NASA would not sustain a long-term, broad observation and information-processing program (like the Earth Observation System); therefore, the community developed a new strategy for obtaining climate measurements from NPOESS. That led to a second NPOESS program imperative—operationalizing a climate observing system, which would enable sustained, long-term measurements for climate studies and other environmental issues. However, that was done after consideration of optical designs, orbits, and data systems needed for weather forecasts; additional requirements for climate were then added, invoking very different objectives and thus requirements for optical designs, orbits, and other mission and instrument characteristics. Attempting to satisfy the two imperatives simultaneously constituted a difficult challenge, both technically and programmatically. Part of the challenge arose from trying to balance the inherent mismatch of data requirements. Weather forecasts demand frequent observations and rapid data dissemination, whereas climate studies and research demand accurate and consistent long-term records. The added requirements for instrument stability and accuracy, driven by the more stringent climate requirements, placed additional challenges on the instruments. Moreover, the expanded mission’s requirements to address climate and other environmental issues established demands for additional observations such as ocean altimetry, which were not weather-related. That expanded the scope of the mission, increased its complexity, and added to the pressure for larger platforms. Finally, although the mission of one of the operational partners (the Department of Commerce’s NOAA) included climate and other broad environmental issues, the mission of the other (the Department of Defense’s Air Force) did not. That led to conflicting priorities between the two agencies, which by law were required to share program costs on a 50-50 basis. SOURCE: Reprinted from Box 3.1 in Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond (NRC, 2007), p. 63. The situation from the perspective of both research and operational agencies was characterized in a previous NRC report (NRC, 2000c, p. 8): Although the operational and the research approaches can appear to conflict, there are features of both that are essential for climate research and monitoring. However, the operational agencies are necessarily wary of assuming responsibility for new requirements that may be open ended in an environment that is cost constrained. The research agencies are similarly concerned about requirements for long-term, operational-style measuring systems that might inhibit their ability to pursue new technologies and new scientific directions. Despite their need for long-term commitments to measure many critical variables, they wonder about relying on operational programs that might decrease the level of scientific oversight as well as opportunities for innovation. In some cases there is clear overlap between sustained and operational measurement objectives, and a single measurement can provide for both needs. However, the specific measurement requirements for each application can vary, leading to less synergism than was initially intended. The ALT instrument on NPOESS, for example, is a well-designed and very capable instrument; however, climate science clearly requires an orbit different from
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Ensuring the Climate Record from the NPOESS and GOES-R Spacecraft: Elements of a Strategy to Recover Measurement Capabilities Lost in Program Restructuring what can be accommodated by the NPOESS architecture, rendering the hoped-for dual-purpose measurement useful only for operational applications. A long-term climate strategy should explicitly take into consideration both the similarities in and the differences between the sustained measurements required for climate research and the exploratory and/or operational measurement objectives of other agency programs so as to maximize synergies while avoiding incompatible implementations arising from inadequate understanding of each application’s specific measurement requirements. Calibration, Characterization, Stability, and Continuity Data used for characterizing climate trends must meet tighter requirements for calibration, characterization, and stability than are needed for non-climate trend-environmental data because the variations in signals of interest for climate studies tend to be small compared with the variations among sequential environmental measurements made during a single orbit (Box 4.2). Yet without exception, the space-based measurements of the climate system—including climate forcings, feedbacks, and responses—are made with instruments for which the degree of uncertainty in their absolute calibration exceeds the magnitude of the changes that they seek to measure, and whose stability is limited by drifts in optical, electrical, thermal, geometrical, orbital, and other in-flight parameters. The acquisition of reliable climate records therefore relies (until the measurements are adequately benchmarked against absolute standards) on the measurements having high repeatability (precision) over the years to decades-long time scales of climate change. Sensor calibrations can change between the ground and space, and they tend to change more rapidly during the beginning of a mission. Individual instruments are calibrated in-flight by using on-board sources or repeated reference observations of the Sun, stars, the Moon, or stable terrestrial features (e.g., deserts). However, since the design lifetimes of individual instruments and space missions are generally of insufficient duration for continuous observation on the longer time scales associated with climate change, most climate records are compiled from data gathered in overlapping missions with cross-calibrated instruments. A strategy for maintaining the long-term precision of successive measurements of climate parameters is to build in sufficient overlap of successive missions to enable reliable cross-calibration between instruments, in principle compensating for uncertainties in absolute calibration. In contrast, a gap between the measurements made by successive climate-monitoring instruments (even by instruments that are nominally “identical”) increases uncertainty in the time series of the derived climate parameter because two non-overlapping time series can be connected only via knowledge of their absolute uncertainties. An added degree of uncertainty that exceeds the magnitude of the geophysical change being measured effectively terminates the usefulness of that climate time series for trend analysis. An adequate period of overlap is therefore essential both to remove calibration biases among independent measurements and to achieve in-flight calibration of the new sensors by in-flight comparison with the more stable, in-flight-characterized existing sensor. The actual period of overlap for achieving continuity of climate time series such that the long-term repeatability is higher than the geophysical variations may depend on a number of factors, including the amplitude and time scales of variability inherent in the quantity being measured and the rate and nature of the sensors’ sensitivity to changes. The required overlap period has been evaluated in a number of ways and is typically reported to be about 1 year. The passive microwave record of sea ice (Figure 4.1), for example, is one of the longest consistent climate records obtained from satellite observations and has become a key indicator of climate change, signaling unexpectedly large and accelerating decreases in sea ice extent, especially in the recent past. Maintaining the consistency of this 30-year record, as well as ensuring that the recorded trends signify real climate change rather than instrument artifacts, requires sustained climate-quality observations. Overlapping coverage and inter-sensor calibration between successive instruments over at least one annual cycle are essential to maintaining the integrity of the time series for analysis of climate change. A gap in a critical time series for a variable such as sea ice would not simply represent a temporal lapse—it would in fact end a long-term record of climate impacts at a time when researchers have witnessed record-low sea ice extent for multiple years. Prior NRC reports have identified and discussed a variety of other climate records for which continuity of observations is essential, including solar irradiance (measured by TSIS), energy balance quantities (measured by CERES), ozone (measured by OMPS), and surface and atmospheric temperatures.
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Ensuring the Climate Record from the NPOESS and GOES-R Spacecraft: Elements of a Strategy to Recover Measurement Capabilities Lost in Program Restructuring BOX 4.2 Measurement Requirements to Detect Climate Trends Calibration refers to the process of comparing the output of a sensor to “truth” radiance to provide conversion factors (calibration coefficients) that allow in-flight sensor digital numbers to be converted to radiances. Generally limited to radiometry, calibration is an example of characterization, which has a broader meaning. Characterization is the assessment of the radiometric performance of a sensor not only in terms of “accuracy” but also in terms of other parameters, including, for example, sensitivity (signal-to-noise ratio), dynamic range, spatial resolution (e.g., modulation transfer function), spectral bandpass, and other parameters that must be known, but do not require calibration coefficients to convert sensor output to a geophysical parameter. Both calibration and characterization are essential to knowing how well a sensor measures the spectral radiance and/or spatial character of the scene within its field of view. Stability refers to how well a sensor maintains constancy in response over time to a constant input. Instruments are seldom inherently stable enough to guarantee accuracy over time using the initial calibration coefficients. For this reason, sensors such as MODIS have been developed with built-in calibration reference capability so that the radiometric calibration coefficients can be updated frequently enough to guarantee that the corrected data are accurate over time. Moreover, other characterization parameters can also be updated in orbit, including spectral bandpass, spectral response, and spectral band registration. FIGURE 4.1 Arctic sea ice extent from passive microwave sensors 1979-2007. SOURCE: Courtesy of the National Snow and Ice Data Center.
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Ensuring the Climate Record from the NPOESS and GOES-R Spacecraft: Elements of a Strategy to Recover Measurement Capabilities Lost in Program Restructuring TABLE 4.1 Data Requirements for Operational and Climate Applications Data System Attribute Operational Applications Climate Applications Archiving Focus is on rapid assimilation into Numerical Weather Prediction models; “old” data have limited operational utility Archiving is critical, enabling both multiple reprocessing cycles and library-like access by the climate community to climate data records Reprocessing Typically out of scope; no resources allocated Mandatory, with the throughput capacity requirements growing at approximately “1X” (1X permits the processing of 24 hours worth of data in 1 day) per year—for every year of measurements Latency The ideal latency is “zero” (real time); new innovations such as SafetyNet for NPOESS support <30-minute data collection Latency is much less important than data completeness, calibration continuity and stability, robust on-orbit instrument characterization, and rigorously validated and improved algorithms (both code and theoretical basis) Reliability Assured “up times” >99% are mandatory, as the production supports mission critical (e.g., life-saving) applications Determined through best-value model that balances the total life-cycle cost of the system against throughput efficiency Configuration management Must facilitate frequent and immediate “reactive” (event-driven) and “science-based” upgrades Scope includes algorithms, primary and ancillary input data, output products and metadata, and calibration coefficients; usually frozen for an entire reprocessing cycle Appropriate Data System Design Considerable attention should be paid to the development of data system architecture that best serves the multitude of stakeholders needing access to climate-relevant data.Data systems used for exploratory research (e.g., NASA DAACs)provide demonstrated long-term archival capabilities and reprocessing capacity but lack intuitive interfaces for applications end users.Operational data systems,such as the NPOESS data system,are designed for low data latency and high reliability rather than the active archival and evolutionary reprocessing required for establishment and maintenance of climate data records. An appropriate and cost-effective data system for climate measurements should seek to leverage the best attributes of both systems, while avoiding cost-driving capabilities that are not associated with climate needs. While the need for sustained climate measurements is clear, the need for “operational” climate data streams in near-real time is less obvious. The requirements for climate versus other operational systems typically differ in several significant dimensions, specifically including divergent needs for archiving, reprocessing, latency, reliability, and configuration management (Table 4.1). Given the extent of these disparities, the agencies should consider the level of ground system integration that is both desirable and cost-effective in future observation systems. A flexible data system architecture is required to accommodate the multitude of climate data sources (e.g., interagency, international, space-based, ground-based) while allowing for upgradability as technology improves. Clear standards and well-defined interfaces are needed to enable more streamlined user access to existing and planned climate-relevant data sets, without having to negotiate the myriad specialized and uncoordinated systems currently in use. Numerous NRC reports provide guidance on the attributes of an effective climate data system (NRC, 1999a,c, 2000a,b, 2004a). CLEAR NATIONAL POLICY FOR PROVISION OF LONG-TERM CLIMATE MEASUREMENTS Much of climate science depends on long-term, sustained measurement records. Yet, as noted in many previous NRC and agency reports, the nation lacks a clear policy to address these known national and international needs. For example, an ad hoc NRC task group (NRC, 1999b, p. 4) stated as follows: No federal entity is currently the “agent” for climate or longer-term observations and analyses, nor has the “virtual agency” envisioned in the [U.S. Global Change Research Program] succeeded in this function. The task group
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Ensuring the Climate Record from the NPOESS and GOES-R Spacecraft: Elements of a Strategy to Recover Measurement Capabilities Lost in Program Restructuring endorses NASA’s call for a high-level process to develop a national policy to ensure that the long-term continuity and quality of key data sets required for global change research are not compromised in the process of merging research and operational data sets.2 A coherent, integrated, and viable long-term climate observation strategy should explicitly seek to balance the myriad science and applications objectives basic to serving the variety of climate data stakeholders. The program should, for example, consider the appropriate balance between (1) new sensors for technological innovation, (2) new observations for emerging science needs, (3) long-term sustainable science-grade environmental observations, and (4) measurements that improve support for decision making to enable more effective climate mitigation and adaptation regulations (NRC, 2006). The various agencies have differing levels of expertise associated with each of these programmatic elements, and a long-term strategy should seek to capitalize on inherent organizational strengths where appropriate. Elements of this needed national policy are discussed in the following sections, which draw heavily on previous NRC reports and careful consideration of the needs of sustained versus operational observations as discussed above. Clear Agency Roles and Responsibilities The issues noted above were recognized explicitly in the NRC decadal survey Earth Science and Applications from Space, whose authors stated, “The committee is concerned that the nation’s civil space institutions (including NASA, NOAA, and USGS) are not adequately prepared to meet society’s rapidly evolving Earth information needs. These institutions have responsibilities that are in many cases mismatched with their authorities and resources: institutional mandates are inconsistent with agency charters, budgets are not well matched to emerging needs, and shared responsibilities are supported inconsistently by mechanisms for cooperation. These are issues whose solutions will require action at high levels of the federal government” (NRC, 2007, p. 13). In turn, this prompted one of the report’s most important recommendations: “The Office of Science and Technology Policy, in collaboration with the relevant agencies and in consultation with the scientific community, should develop and implement a plan for achieving and sustaining global Earth observations. This plan should recognize the complexity of differing agency roles, responsibilities, and capabilities as well as the lessons from implementation of the Landsat, EOS, and NPOESS programs” (p. 14). The present committee fully endorses the need for clarified agency roles and responsibilities, consistent with inherent agency strengths, and reiterates this important recommendation of the decadal survey. International Coordination The committee recognizes the importance of international cooperation in obtaining climate quality measurements from space. As noted in the June 2007 workshop report (NRC, 2008, p. 38): With limited financial and human resources, a response to GCOS [Global Climate Observing System] requirements can be achieved only through enhanced international cooperation. Such cooperation should involve global planning with international contributions, in such a way that implementation problems encountered by an individual agency do not dramatically affect the global system. It was recognized that a number of missions planned in Europe will be of great value for climate analysis and that there is an acute need for better international collaboration and awareness spanning the full spectrum of activities from high-level data access agreements to pragmatic documentation exchange. The committee agrees with a workshop participant’s assessment that “there has not been a concerted strategy for sustained climate observations from space [and that] … the climate community has relied on suboptimal sen- 2 A similar view was expressed in Adequacy of Climate Observing Systems, which stated, “There has been a lack of progress by the federal agencies responsible for climate observing systems, individually and collectively, toward developing and maintaining a credible integrated climate observing system” (NRC, 1999a, p. 5).
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Ensuring the Climate Record from the NPOESS and GOES-R Spacecraft: Elements of a Strategy to Recover Measurement Capabilities Lost in Program Restructuring sors to create a climate record, resulting in significant challenges in terms of handling bias differences, orbit drift, data gaps, and spectral differences between follow-on instruments when reprocessing multi-satellite data—often at considerable cost” (NRC, 2008, p. 37). That there remains no internationally agreed upon and ratified strategy for climate observations from space remains an area of grave concern. While the committee has no doubt that organizations such as CEOS, CGMS, WMO, and the evolving GEO will provide important insight and support for developing such a strategy, it believes the issue is of such paramount importance that it must be addressed, and eventually implemented on an intergovernmental level where binding agreements can be reached. The committee also finds merit in recent international plans to establish so-called virtual constellations consisting of a set of existing (or planned) ground or space-based assets from different partners that are mobilized in a coordinated manner for greater efficiency or improved data products. Constellations may involve formation flying or coordinated operation scheduling and data distribution, thus potentially including both real and virtual constellation elements. Virtual constellations can be designed to provide better coverage, in temporal, spatial, and/or spectral information, and improved data management and dissemination. This can both improve information products and reduce net costs for operating agencies. With respect to climate missions, the creation and use of constellations provide significant benefits through: Coordinating use of existing systems providing global data; Generating the potential for advanced, integrated products; Coordinating the analysis of gaps in current operations and future mission deployment plans; Providing standards for interoperability and facilitating data uptake into models; and Provision of routine global coverage for sustained observations and increased redundancy. Recommendation: To obtain benefits of the kind that virtual constellations of assets could provide, it is strongly recommended that the research and operational agencies coordinate their development, operations, standards, and products with international partners. In addition to the obvious coordination of observations and ground systems, national agencies should incorporate standard calibration and validation processes including the use of ground-based and lunar virtual calibrations, and establish “best practices” recommendations for measurements, calibration, and use of standards. COMMUNITY INVOLVEMENT IN THE DEVELOPMENT OF CLIMATE DATA RECORDS The National Research Council has produced a number of reports on the subject of climate data records (CDRs), many having been motivated by concerns over the future availability of satellite-based climate-quality data records. Some of these reports offer opinions about what is needed to ensure CDRs from satellites and recommend guiding principles for their development (e.g., NRC, 1999a,c, 2000a-c, 2003, 2004a,b). The implied demise of climate-focused satellite observations from NPOESS, a consequence of the Nunn-McCurdy certification, adds to the ongoing concern about the lack of organized commitment to the development of CDRs. It has been stressed in many NRC and other reports that CDRs are far more than time series of EDRs and that their production requires considerable scientific insight. The careful sensor calibration, sensor characterization, and algorithm refinements necessary for high-quality CDRs require access to uncalibrated measurements (see Box 4.3). CDRs require periodic reanalysis and reprocessing as data sources improve, error characteristics become clearer, and retrieval algorithms and assimilation methods advance. In addition, the requirements for generating CDRs vary greatly from one record to another. Thus development of CDRs requires a broader engagement of the community than is required for the development of EDRs, and independent teams, funded to calculate CDRs from uncalibrated satellite measurements, are essential if the requirements for quality expected of CDRs are to be met.3 3 This recommendation can be found in NRC reports dating back to at least 2000. See, for example, the following: “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)” (NRC, 2000c, p. 4).
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Ensuring the Climate Record from the NPOESS and GOES-R Spacecraft: Elements of a Strategy to Recover Measurement Capabilities Lost in Program Restructuring The NRC report Satellite Observations of the Earth’s Environment: Accelerating the Transition of Research to Operations identifies key state variables—temperature, precipitation, humidity, pressure, clouds, sea-ice and snow cover, SST, carbon fluxes, and soil moisture—for establishment of CDRs (NRC, 2003). To ensure that data records comprise more than a series of isolated variables, these data records must be produced in such a way as to provide a means to monitor processes of the climate system through the construction and monitoring of joint distributions of parameters. Although the need to institutionalize the management and oversight of CDR data stewardship is obvious, the CDR implementation process and its stewardship have to be distributed across the community where relevant expertise and competencies exist, typically outside any one agency. An important lesson from the NPOESS process is that putting together an advisory council or analogous product teams does not necessarily ensure the preservation of the community’s interest throughout the process. Rather, there is a need to establish “new community relationships by engaging a broader academic community, other government agencies, and the private sector in the development and continuing stewardship of satellite climate data records” (NRC, 2004a, p. 99) by integrating the external community into the CDR implementation processes, taking it well beyond using this community in a superficial advisory role. The committee believes that the research community has to be involved at every level of CDR development, from identification of the variable to be measured over time as a CDR, to the determination of the level of maturity of products, to their maintenance and the eventual improvement via reprocessing. The way to develop this partnership is to solicit the community’s involvement from the outset, giving it ownership in the process and funding its involvement. As noted in footnote 3, these issues are recognized in recommendations from a 2000 NRC report. Thus, while management of CDRs may be delegated to a single agency, their development, maintenance, and archiving do not necessarily need to remain exclusively within that agency, and in fact are more appropriately distributed to relevant communities who are already engaged in the production of CDRs. Plans for implementing a CDR program have to explain how the necessary new community relationships are to be developed, how and when an advisory group that represents the external community might be formed to review CDR maturity, and how external CDR science teams responsible for both implementation and stewardship are to be formed, managed, and supported through a peer review process. Historically, insufficient allocation of resources to support this wider engagement has often been perceived as the principal impediment to progress. The key elements of successful CDR generation are outlined in Box 4.4. CONCLUSION: A WAY FORWARD In this report, the committee provides a prioritized, short-term strategy for recovery of crucial climate research capabilities lost in the NPOESS and GOES-R program descopes. However, mitigation of these recent losses is only the first step in establishing a viable long-term climate strategy that builds on the lessons learned from the well-intentioned but poorly executed merger of the nation’s weather and climate observation systems. Specifically, a coherent, integrated, and viable long-term climate observation strategy is needed that explicitly seeks to balance the myriad science and applications objectives of climate data stakeholders, while properly considering both the similarities in and the differences among the exploratory, sustained, and operational measurements needed for climate science. The long-term strategy must provide for the requisite characterization, calibration, stability, and continuity of sensors and data, as well as for data systems designed to enable a range of climate applications. The development of a coherent national strategy for Earth observations (NRC, 2007) would support these efforts by establishing clear agency roles and responsibilities for the provision of climate-quality data and ensuring community involvement in the development of climate data records. REFERENCES NRC (National Research Council). 1999a. Adequacy of Climate Observing Systems. National Academy Press, Washington, D.C. NRC. 1999b. “Assessment of NASA’s Plans for Post-2002 Earth Observing Missions,” letter report. National Academy Press, Washington, D.C., April 8. NRC. 1999c. Making Climate Forecasts Matter. National Academy Press, Washington, D.C.
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Ensuring the Climate Record from the NPOESS and GOES-R Spacecraft: Elements of a Strategy to Recover Measurement Capabilities Lost in Program Restructuring BOX 4.3 EDRs Versus CDRs: An Example The Total Irradiance Monitor (TIM) on SORCE measures total solar irradiance.1 Fundamentally, this involves measurement of radiant power and area (Figure 4.3.1). For TIM, area is determined by a precision aperture, and radiant power is determined by the electrical heating required to maintain constant temperature of an absorptive radiometer as sunlight is modulated. FIGURE 4.3.1 Irradiance (i.e., 1361 W/m2) requires two measurements: power and area. SOURCE: Courtesy of G. Kopp, Laboratory for Atmospheric and Space Physics, University of Colorado. Total solar irradiance EDRs are released to the scientific community in near-real time for short-term process-oriented studies. To generate the EDR, the fundamental instrument measurements are converted to TSI considering: Aperture area [watts/m2], Thermal background corrections (from on-orbit dark measurements), Distance and radial velocity between the radiometer and the Sun, and Change in cavity absorptance (from on-orbit degradation tracking). Total solar irradiance CDRs are needed by the scientific community for studies of climate change and short- and long-term solar and climate processes. Accuracy and consistency are crucial to such studies, which generally incorporate data from multiple instrument generations. Many more steps are involved to ensure that the best-known characterization, calibration, and stability information is fully incorporated for each instrument. For example, they might include: NRC. 2000a. Ensuring the Climate Record from the NPP and NPOESS Meteorological Satellites. National Academy Press, Washington, D.C. NRC. 2000b. Issues in the Integration of Research and Operational Satellite Systems for Climate Research: Part I. Science and Design. National Academy Press, Washington, D.C. NRC. 2000c. Issues in the Integration of Research and Operational Satellite Systems for Climate Research: Part II. Implementation. National Academy Press, Washington, D.C. NRC. 2003. Satellite Observations of the Earth’s Environment: Accelerating the Transition of Research to Operations. The National Academies Press, Washington, D.C. NRC. 2004a. Climate Data Records from Environmental Satellites: Interim Report. The National Academies Press, Washington, D.C. NRC. 2004b. Utilization of Operational Environmental Satellite Data: Ensuring Readiness for 2010 and Beyond. The National Academies Press, Washington, D.C.
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Ensuring the Climate Record from the NPOESS and GOES-R Spacecraft: Elements of a Strategy to Recover Measurement Capabilities Lost in Program Restructuring Using the most recent understanding from each instrument team of degradation, gain, and dark measurement trends, as available. Reprocessing entire instrument data sets throughout the mission(s) to incorporate the latest available trending data. Cross-calibration of current and successive TSI instruments using periods of measurement overlap to maintain continuity. Intercomparison of independent irradiance measurements to estimate differences in absolute accuracies and instrument stabilities with time. Estimation of the effects of orbit limitations on measurements used to construct daily means. Construction of composite TSI record by adjusting for instrument offsets and temporal drifts, using cross-comparisons from ground-based irradiance reference if available, including time-dependent uncertainties in the composite. Seeking consistency among measurements and models to identify areas needing improved understanding. As summarized below, the more extensive requirements for CDRs drive TIM instrument pre-flight and in-flight calibration and characterization needs: TIM pre-flight calibrations and characterizations supporting CDR development: Entrance aperture geometric area and diffraction/scatter effects Cavity absorptance (efficiency) across solar spectral region Electrical power applied to cavities from reference voltages and resistances Electrical linearity calibrations across dynamic range Radiometer non-equivalence (mismatch between electrical and radiant heating) Thermistor calibrations Shutter response Scattered light and field-of-view angular dependences End-to-end comparison with ground-based irradiance reference links current and future TSI instruments TIM on-orbit calibrations and calibration tracking supporting CDR development: Dark background measurements of deep space correct for instrument thermal contributions Degradation tracking due to solar exposure since pre-flight cavity absorptance calibration via regular infrequent simultaneous observations with lesser-used cavities Servo system gain calibrations Pointing sensitivity 1G. Kopp and G. Lawrence, “The Total Irradiance Monitor (TIM): Instrument Design,” Solar Physics 230(1):91-109, 2005. NRC. 2006. “A Review of NASA’s 2006 Draft Science Plan: Letter Report.” The National Academies Press, Washington, D.C. NRC. 2007. Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond. The National Academies Press, Washington, D.C. NRC. 2008. Options to Ensure the Climate Record from the NPOESS and GOES-R Spacecraft: A Workshop Report. The National Academies Press, Washington, D.C.
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Ensuring the Climate Record from the NPOESS and GOES-R Spacecraft: Elements of a Strategy to Recover Measurement Capabilities Lost in Program Restructuring BOX 4.4 Key Elements of Successful Climate Data Record Generation On the basis of its review of previous NRC studies (especially NRC, 2004a and 2000c), and its members’ own experience, the committee identified the following as particularly important elements of a sustained long-term program for deriving credible climate data records. Organizational Elements A high-level leadership council1—including members from different agencies, academia, and industry to oversee the process of defining and creating climate data records (CDRs) from satellite data. Ad hoc advisory committees can be established as needed. Science teams—formed for each CDR, consisting of a specifically appointed team of experts responsible for sensor calibration, algorithm design, validation, and research using the CDR. The science teams should have membership broadly distributed across different agencies, academia, and industry. Multiple subgroups within the science team should be funded to provide independent CDRs from raw (uncalibrated) satellite measurements. CDR Generation Elements Sensors must be thoroughly characterized before and after launch, and their performance should be continuously monitored throughout their lifetime. Sensors should be thoroughly calibrated, including nominal calibration of sensors in orbit, vicarious calibration with in situ data, and satellite-to-satellite cross-calibration. Algorithm design must consider differences in measurements due to changes in the sensor design or satellite navigation. Well-defined levels of uncertainty are required. An ongoing program that includes correlative in situ measurements and model analyses is required to validate CDRs. Multiple teams should be funded to independently create CDRs. The multiple teams are key to providing the best possible end product. These teams will be responsible for calibration of satellite measurements, developing CDR algorithms, validation of the CDR, and refinement of CDR processing. Researchers must be involved who, through their research, will provide feedback on the quality of the CDRs. Sustaining CDR Elements A long-term commitment of resources should be made to the generation and archiving of CDRs and associated documentation, data, and metadata. Access to reprocessing resources is required when better sensor calibrations and improved algorithms become available. 1See, in NRC (2004a), pp. 3-4, especially Supporting Recommendation 1: “NOAA should utilize an organizational structure where a high-level leadership council within NOAA receives advice from an advisory council that provides input to the process on behalf of the climate research community and other stakeholders. The advisory council should be supported by instrument and science teams responsible for overseeing the generation of climate data records.”