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
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.