This is the second of two Space Studies Board reports that address the complex issue of incorporating the needs of climate research into the National Polar-orbiting Operational Environmental Satellite System (NPOESS). NPOESS, which has been driven by the imperative of reliably providing short-term weather information, is itself a union of heretofore separate civilian and military programs. It is a marriage of convenience to eliminate needless duplication and reduce cost, one that appears to be working. The same considerations of expediency and economy motivate the present attempts to add to NPOESS the goals of climate research. The technical complexities of combining seemingly disparate requirements are accompanied by the programmatic complexities of forging further connections among three different agencies, with different mandates, cultures, and congressional appropriators. Yet the stakes are very high, and each agency gains significantly by finding ways to cooperate, as do the taxpayers. Beyond cost savings, benefits include the possibility that long-term climate observations will reveal new phenomena of interest to weather forecasters, as happened with the El Nino/Southern Oscillation. Conversely, climate researchers can often make good use of operational data. Necessity is the mother of invention, and the needs of all the parties involved in NPOESS should conspire to foster creative solutions to make this effort work. Although it has often been said that research and operational requirements are incommensurate, this report and the phase one report (Science and Design) accentuate the degree to which they are complementary and could be made compatible. The reports provide guidelines for achieving the desired integration to the mutual benefit of all parties. Although a significant level of commitment will be needed to surmount the very real technical and programmatic impediments, the public interest would be well served by a positive outcome.

ISSUES IN THE INTEGRATION of RESEARCH and OPERATIONAL SATELLITE SYSTEMS for CLIMATE RESEARCH

II. Implementation

NATIONAL ACADEMIES PRESS

Copyright © 2000 National Academy of Sciences
All right reserved.
ISBN: 978-0-309-06994-6

Contents

EXECUTIVE SUMMARY..........................................................................11 INTRODUCTION.............................................................................7Characteristics and Requirements of Research and Operational Missions,.....................7Key Implementation Issues,.................................................................8Climate Data Records,......................................................................9References,................................................................................92 CALIBRATION AND VALIDATION...............................................................11Introduction,..............................................................................11Instrument Characterization,...............................................................13Sensor Calibration,........................................................................14Calibration Verification,..................................................................16Data Quality Assessment,...................................................................17Data Product Validation,...................................................................17Conclusions and Recommendations,...........................................................18References,................................................................................193 DATA CONTINUITY..........................................................................20Key Issues and Lessons Learned,............................................................20NPOESS Replenishment Strategy,.............................................................24Recommendations,...........................................................................27References,................................................................................284 DATA SYSTEMS.............................................................................29Introduction,..............................................................................29Operational Versus Research Needs,.........................................................30Long-term Archiving of Raw Data Records,...................................................31Architecture for the NPOESS Climate Data System,...........................................32Evolution, Reprocessing, and Multiple Versions of Data Sets,...............................33Existing NASA and NOAA Data Centers,.......................................................34Conclusion,................................................................................34Recommendations,...........................................................................35References,................................................................................355 TECHNOLOGY INSERTION.....................................................................36Introduction,..............................................................................36Basic Considerations,......................................................................37Technical Issues,..........................................................................39Programmatic Issues,.......................................................................41A Continuing NPOESS System Augmentation Project,...........................................43NASA Strategies and Plans for Technology Development,......................................43Findings,..................................................................................46Recommendations,...........................................................................47References,................................................................................48APPENDIXESA Statement of Task,.......................................................................51B Workshop Discussion and Participants,....................................................53C Solar Reflection Region Measurements,....................................................76D Acronyms and Abbreviations,..............................................................80

Chapter One

Executive Summary

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).

DATA STABILITY

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 Nio/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.

Calibration and Validation

Findings

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.

Recommendations

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.

Data Continuity

Findings

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.

Recommendations

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.

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.

Data Systems

Findings

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.

Recommendations

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.

SYSTEM FLEXIBILITY

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.

Technology Insertion

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.

Findings

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.

(Continues...)

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