3
Data Continuity

KEY ISSUES AND LESSONS LEARNED

The most useful data for climate purposes are time series that are continuous and for which the characterization of errors, in terms of precision and bias, is known. Time series that are overwhelmed by data gaps or instabilities in the error properties are far less useful because the magnitude of such errors is often of the same order as the small climate signal being sought. Continuity, therefore, is concerned with more than the presence or absence of data: it includes the continuous and accurate characterization of the error properties of the time series. The following sections summarize the lessons that have emerged, lessons that, if learned from, would enhance the generation of climate data records.

Overlapping Observations Are Required (Data Gaps Should Not Occur)

From the perspective of climate research, temporal data gaps in satellite observations introduce uncertainties in time series construction that are usually greater than the climate signal under investigation and, therefore, are intolerable. Not only do gaps create noncontinuous time series, but they also prevent the performance of a key climate analysis function, that is, the calculation of relative instrument bias from data collected simultaneously when a new spacecraft replaces an old. In other words, overlapping observations give an opportunity for old and new spacecraft to be simultaneously calibrated against a common target (Earth). Without a proper knowledge of instrument bias, the continuity of the time series is jeopardized.

Instrument biases tend to develop during the long period between instrument characterization, spacecraft integration, launch (with its associated vibration and forces), and commissioning. By the time it is in space, the instrument has many opportunities to undergo a response change in the radiometer itself or in its calibration system. For example, laboratory tests for channel 2 of the Microwave Sounding Unit (MSU) on NOAA-12 indicated that the instrument view of cold space would produce about 700 digital counts. Once in space, however, the cold view reported about 1,800 cold counts, significantly altering the dynamic range and gain and requiring empirically based adjustments to the original laboratory calibration coefficients (Mo, 1995).

Although the intersatellite instrument biases may be minuscule from the operational standpoint (for temperature they are usually about 0.5 K), they are probably much larger than the magnitude of the climate signal scientists generally require (e.g., a decadal trend of only 0.1 K). Experience has shown that overlapping space observations



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 20
Issues in the Integration of Research and Operational Satellite Systems for Climate Research: II. Implementation 3 Data Continuity KEY ISSUES AND LESSONS LEARNED The most useful data for climate purposes are time series that are continuous and for which the characterization of errors, in terms of precision and bias, is known. Time series that are overwhelmed by data gaps or instabilities in the error properties are far less useful because the magnitude of such errors is often of the same order as the small climate signal being sought. Continuity, therefore, is concerned with more than the presence or absence of data: it includes the continuous and accurate characterization of the error properties of the time series. The following sections summarize the lessons that have emerged, lessons that, if learned from, would enhance the generation of climate data records. Overlapping Observations Are Required (Data Gaps Should Not Occur) From the perspective of climate research, temporal data gaps in satellite observations introduce uncertainties in time series construction that are usually greater than the climate signal under investigation and, therefore, are intolerable. Not only do gaps create noncontinuous time series, but they also prevent the performance of a key climate analysis function, that is, the calculation of relative instrument bias from data collected simultaneously when a new spacecraft replaces an old. In other words, overlapping observations give an opportunity for old and new spacecraft to be simultaneously calibrated against a common target (Earth). Without a proper knowledge of instrument bias, the continuity of the time series is jeopardized. Instrument biases tend to develop during the long period between instrument characterization, spacecraft integration, launch (with its associated vibration and forces), and commissioning. By the time it is in space, the instrument has many opportunities to undergo a response change in the radiometer itself or in its calibration system. For example, laboratory tests for channel 2 of the Microwave Sounding Unit (MSU) on NOAA-12 indicated that the instrument view of cold space would produce about 700 digital counts. Once in space, however, the cold view reported about 1,800 cold counts, significantly altering the dynamic range and gain and requiring empirically based adjustments to the original laboratory calibration coefficients (Mo, 1995). Although the intersatellite instrument biases may be minuscule from the operational standpoint (for temperature they are usually about 0.5 K), they are probably much larger than the magnitude of the climate signal scientists generally require (e.g., a decadal trend of only 0.1 K). Experience has shown that overlapping space observations

OCR for page 20
Issues in the Integration of Research and Operational Satellite Systems for Climate Research: II. Implementation of virtually every instrument are necessary to empirically determine the magnitude of these climatically large and unpredictable intersatellite biases. The overlapping observations should last at least 1 year as some biases depend on season. The present two-orbit configuration (nominal northbound equatorial crossing times of 1330 and 1930) is manageable for bias calculations for certain climate data sets. Overlapping observations from a 1330 spacecraft may be used to determine the bias of the 1930 instrument as long as their respective biases relative to the diurnal component are known. This is possible for quantities such as stratospheric temperature, which is fairly coherent in space and time. However, quantities with substantial diurnal variations, such as cloudiness or surface properties, will require overlapping observations between old and new spacecraft in the same orbit node. (The shift from 0730/1930 to 0530/1730 in the AM node, as has been proposed for the National Polar-orbiting Operational Environmental Satellite System (NPOESS), will probably cause some discontinuity. Additionally, some instruments will fly on only one node, again requiring overlapping observations of the old and new spacecraft in each node.) The following two examples demonstrate the critical value of overlapping observations. A time series of total solar irradiance (TSI) solar variations was pieced together by Willson (1997) using data from two consecutively flown Active Cavity Radiometer Irradiance Monitor instruments (ACRIM I and II). A gap of about 2 years in the ACRIM data was bridged by using mutually overlapping observations from two other TSI instruments, the Nimbus-7 Earth Radiation Budget (ERB) and the Earth Radiation Budget Satellite (ERBS). The results from the ERB and ERBS were less precise than those from ACRIM I and II and the absolute irradiance values differed by 0.5 percent (Figure 3.1). However, the extent of the overlap in the data made possible a sufficient reduction of noise in the bias calculations, allowing for the production of a time series with a trend of 0.032 (±0.0009) percent per decade. Christy et al. (1998, 2000) utilized the MSUs on National Oceanic and Atmospheric Administration (NOAA) polar orbiters in both 1330 and 1930 orbits, alternating between the two nodes, to merge data from nine satellites into a time series of daily global atmospheric temperatures. The duration of most overlapping periods was 1 to 3 years; however, two periods were less than 7 months long. Biases, which could be as large as 0.5 K, were determined to be globally accurate to a precision of 0.01 to 0.02 K (as estimated through a variety of subsampling experiments) only because data from overlapping observations were available. Flexible strategies for providing overlapping data should be investigated because data from the older spacecraft are not required in real time for climate purposes. Thus, information from instruments for which overlapping observations are necessary may be stored on board for downloading at more convenient times. Additionally, though an overlap period of 1 year is a requirement, the sampling during this year may be less than continuous, being whatever is sufficient to characterize the annual cycle of the bias. Most instruments will need overlapping observational periods for each orbit node (e.g., from an old 0530 to a new 0530 spacecraft). Spacecraft Orbit Should Be Stable The past generation of polar-orbiting satellites was injected into orbits that included slow longitudinal (east-west) drifting relative to local equatorial crossing time (LECT), to prevent the spacecraft from approaching local solar noon. The rate of the drift was up to 15 degrees per year in some cases, representing up to an hour drift per year in terms of LECT. Two consequences of this drift conspire to corrupt the observations from the standpoint of their stability. Because these spacecraft drifted to earlier or later LECTs, observations were influenced by the local diurnal cycle (e.g., afternoon temperatures are warmer than morning temperatures) in the quantity measured. Trend calculations are thus skewed by the local time at which observations are taken and may be misinterpreted as a trend in the absolute instrument bias. This is particularly important for most surface quantities, atmospheric temperature, and cloud observations, where diurnal signals and solar angle changes, which might affect the upwelling radiance, are substantial relative to decadal trends. One method of dealing with this problem was developed by Waliser and Zhou (1997) for outgoing longwave radiation (OLR) and highly reflective cloud (HRC) data sets. Because the spatial variation of the changes in OLR and HRC is substantial as a satellite drifts through the diurnal cycle, Waliser and Zhou based their corrections on

OCR for page 20
Issues in the Integration of Research and Operational Satellite Systems for Climate Research: II. Implementation FIGURE 3.1 Total solar irradiance measurements (daily means and uncertainties) obtained during solar cycles 21 through 23 from space-based sensors. See discussion in Willson (1997). Courtesy of Richard C. Willson. empirical orthogonal functional analyses of the patterns of change whose time series correlated with LECT variations. However, there remains some uncertainty owing to the possible convolution of real interannual changes in OLR or HRC during a drifting period and changes caused by the diurnal cycle alone. As mentioned earlier, instruments on board Sun-synchronous polar-orbiting spacecraft undergo substantial variations in incident solar radiation. If station-keeping is rigorous and the spacecraft positioning is maintained at a consistent LECT, these effects are systematic on a latitudinal basis. As the spacecraft drifts relative to a fixed LECT, there are changes in the shadowing effects on the various instruments, and this in turn induces considerable changes in instrument body temperatures. Precise laboratory characterization of the instrument responses apparently has not provided enough information to determine the effects due to variational solar heating. The magnitude of the error introduced by these effects, at least in the case of the MSU temperatures, is as large as the signal of climate change. One way to compensate for this effect of spurious instrument response to varying instrument body temperatures relies on simultaneous observations of Earth from two orbiting instruments. Comparing the Earth observations from each with the instrument body temperatures of each allows for estimating the error. In this case, the intersatellite differences in Earth-viewed temperatures might be highly correlated with the individual instrument body temperatures, so that adjustments might be applied (Christy et al., 2000). This effect could be better dealt with by requiring a more thorough characterization of the instrument response in the calibration chamber tests. For

OCR for page 20
Issues in the Integration of Research and Operational Satellite Systems for Climate Research: II. Implementation example, simulated solar radiation could be made to strike the instrument from all angles in a simulated orbit cycle to determine the effect on instrument response. A further source of orbit variability relates to changes in the altitude of the Sun-synchronous satellites (~840 km). This orbit is occupied by a rarified atmosphere that nevertheless is dense enough to exert a drag on the orbiting spacecraft. This drag is accentuated during solar maxima as the upper atmosphere expands from below, causing an increase in the atmospheric density at orbit altitude. The increased drag results in a loss of altitude for the spacecraft, which in the case of the TIROS-N family of polar orbiters accumulated to about 20 km. Measurements that require a fixed altitude are thus affected, and corrections are required (Wentz and Schabel, 1998). The current intention of the Integrated Program Office (IPO) to require a fixed orbit node to within ±10 minutes (±2.5 degrees longitude at the equator) has been greeted with particular satisfaction by the climate community. Thorough Instrument Characterization Is a Requirement Once in space, the instrument’s behavior and response may be quite different from its laboratory behavior and response. In thermal/vacuum testing, the instruments are tested (characterized) individually as isolated pieces of equipment, without the impact of changing solar illumination and often without all their components (e.g., mirrors removed where small emissivity adjustments may not be properly accounted for). Later, each instrument is integrated into the spacecraft along with other instruments, all of which compete for space and power. The instrument configurations generate complex shadowing effects, and after enduring the forces of a launch into orbit, the instruments can experience changes in the very components that were designed for measurement and calibration. Ideally, to anticipate the impact of these on-orbit effects, the instrument should be tested after all the spacecraft components have been fully integrated to document potential anomalies arising from variations in sunlight and thermal loads. However, the environmental chambers used in characterization are not large enough to accommodate a complete spacecraft. An alternative approach would be to enhance the on-board monitoring devices, which would allow for a higher degree of on-board characterization (see Chapter 2). Data Sets Should Be Uniformly Archived and Continuously Available A significant impediment to the construction of continuous, space-based climate data sets is the lack of attention given to the archiving methodology. Often, investigators must piece together observational data sets held at various locations, written in various formats, and residing on various media before work can begin. For example, in the case of the MSU archive (part of TIROS-N operational vertical sounders (TOVS)), pre-1985 data were stored on terabit memory (TBM) reels (5-cm-wide videotape weighing 7 kg each) at the National Center for Atmospheric Research (NCAR), which housed the last operating TBM reader in the world. Post-1984 data were available on IBM cartridges from the National Environmental Satellite Data and Information Service in Washington, D.C. Knowing the TBM reader was to be decommissioned, NCAR personnel copied the data to a newer medium in 1989. The TOVS orbit files were complex with many duplicate orbits, so various checks were employed to ensure that only the proper data were extracted and copied. One such check was the satellite identifier number, which if not recognized as one of the operational spacecraft would cause those orbit files to be skipped. Unknown to NCAR personnel, NOAA-6’s identifier number was changed after its third commissioning in space, when NOAA-8 failed for the second time. The identifier change was not discovered until after the copying had been completed and the TBM reader scrapped. Several months of operational data from NOAA-6 were lost as a result. The lesson here is that as new devices become available for data storage, all previously archived data should be copied to the new media and archived in one place. Thus, the archive itself will have uniformity and continuity. The delivery of these data sets conveniently to researchers is the first part of this overall issue. The second is that agencies must be prepared to provide continued funding to researchers to allow them to study the data and get the most out of the substantial investment. Budget cuts tend to fall disproportionately on the end-researcher, preventing the mission from achieving its full results. This is especially the case for climate issues, as they are

OCR for page 20
Issues in the Integration of Research and Operational Satellite Systems for Climate Research: II. Implementation often not apparent until years after the spacecraft and instruments have been decommissioned and (unfortunately) long after agency interest has peaked. The NASA and NOAA Enhanced Data Set Project states as its central theme “producing new and/or enhanced climate or global change data sets for analysis, applications, assessments, or climate impact studies.” Satellite data are included in this project. However, this is a fairly small program in which investigators must compete against those who study nonsatellite data sets. Compared with the excitement of instrument design and fabrication and of spacecraft integration, launch, and operation, the issues of accessible archives and out-year science funding are rather unglamorous. Yet for any mission to be considered a success, these two issues must be considered as equally important requirements when the mission is defined. This argues for assigning funds for these tasks when the mission is planned, either from the agency in charge of the instrument or from a program whose priority is climate data and research. Because climate date records cross multiple satellite missions, this particular funding is crucial to the generation of long, continuous data records. Backward Compatibility Is Highly Recommended Many space-based measurements go back at least 20 years. As new technology is developed, there is always a chance that continuity will be lost in the time series of radiances (the fundamental observation). Most requirements for space-based measurements are stated in terms of geophysical parameters (e.g., atmospheric temperature at height corresponding to pressure of 500 hPa). These measurements may be delivered using new technology and instrumentation and, therefore, with different fundamental radiance observations (e.g., frequency or bandwidth). Many climate data sets are produced from reanalyzed radiance-based data, so new technology may become a threat to the continuity of these level 1 time series. In cases where operational quality is not compromised, it is recommended that the backward compatibility of proposed radiances with currently archived radiances be considered as a climate requirement. NPOESS REPLENISHMENT STRATEGY The first lesson listed above (overlapping observations) is fundamentally related to the NPOESS replenishment strategy. Launch-on-Failure Versus Launch-on-Schedule The decision to launch a replacement spacecraft depends on many factors, not the least of which is budgetary constraint. Accordingly, for NPOESS missions, a launch-on-failure strategy has apparently been adopted. By this strategy, the decision to launch a replacement spacecraft is based on the failure or anticipated failure of one of the critical instruments (which are indicated in Table 3.1). The key driver is the data stream that supports the operational mission. Depending on the readiness of the next spacecraft, a launch-on-failure strategy might take up to 12 months to execute from the moment an unanticipated failure occurs. The launch-on-failure strategy virtually guarantees discontinuities in the radiance time series and thus will create considerable difficulties for climate research. By this strategy, overlapping periods of observations will not be possible in cases of instrument failure. In the case of incremental failure of an instrument, such as increasing noise, the launch decision is more difficult. The guidelines are unknown and perhaps impossible to set because a multitude of minor problems might occur. As a problem develops, assessments would presumably be made as to when the accumulated effect of the increased noise would prevent accurate weather monitoring. With the launch-on-failure strategy, measurements that have little value for operations but great value for research will be allowed to expire so that the associated time series would become discontinuous. Data sets for solar radiance and altimetry, for example, would become discontinuous if either instrument failed and all other operationally critical instruments maintained nominal performance. This raises the question of the use of still-working assets on board decommissioned spacecraft.

OCR for page 20
Issues in the Integration of Research and Operational Satellite Systems for Climate Research: II. Implementation TABLE 3.1 NPOESS Notional Payloads to Satisfy IORD-1 NPOESS Instruments     METOP NPP   0530 1330 0930 1030 IPO Developed: Visible/Infrared Imager Radiometer Suite (VIIRS)* x x x (AVHRR) x Cross-track IR Sounder (CrIS)*   x x (IASI/HIRS) x Conical Scanning Microwave Imager/Sounder (CMIS)* x x     Ozone Mapping and Profiler Suite (OMPS)   x x (GOME)   GPS Occultation Sensor (GPSOS) x x x (GRAS)   Space Environmental Sensor Suite (SESS) x x x (SEM)   Leveraged: Advanced Technology MW Sounder (ATMS)*   x x (AMSU/MHS) x Data Collection System (DCS) x x x   Search and Rescue (SARSAT) x   x   Earth Radiation Budget Sensor (ERBS)       x Total Solar Irradiance Sensor (TSIS) x       Radar altimeter (ALT) x       Advanced Scatterometer (ASCAT)     x   NOTE: “IORD-1” is the shorthand term for the Integrated Operational Requirements Document issued in 1996 by the Integrated Program Office and since updated. *Critical instrument—failure constitutes need to replace satellite. SOURCE: NPOESS IPO. From the perspective of climate research, a launch-on-schedule strategy is preferred. Launching a new spacecraft while a fully functional one is operating allows for the necessary period of overlap. This aids the operational mission, which also needs some overlapping observations for intersatellite calibration. Too, if the new spacecraft develops serious problems, the old, still-functional spacecraft could continue to fulfill its operational mission until the next spacecraft is launched. NOAA-6, for example, was recommissioned twice (May 1985 and November 1985) when problems developed with its replacement (NOAA-8), and it performed the operational mission until NOAA-10 was launched. Specific Concerns Related to Continuity Many space-based data sets are in some sense threatened by loss of continuity. For example, measurements of oceanic chlorophyll and dissolved organic material from the Moderate-resolution Imaging Spectrometer have no continuing commitment after the Earth Observing System (EOS) mission. Four other measurements viewed as critical to climate research and global change that at this time appear threatened with loss of continuity are discussed below. At present, policymakers are demanding to know which are the required long-term, precise time series of geophysical parameters and, of these, which are plagued by uncertainties (NRC, 1999). Fifty years and more from now, policymakers will still be asking scientists to provide societally relevant information, some of which will be based on observations discussed here. From the perspective of future generations, the points addressed below become necessary considerations because having useful long-term time series in the future is dependent on obtaining good data from monitoring that is accomplished now. The points discussed in the following four sections speak directly to the requirements for climate quality records stated in NRC (1999).

OCR for page 20
Issues in the Integration of Research and Operational Satellite Systems for Climate Research: II. Implementation Solar Irradiance Within the climate research community, there is substantial support to make solar irradiance missions independent of the operational constraints of NPOESS. Few issues in the area of climate and global change research have sparked more uncertainty and controversy than the characterization of solar irradiance and its influence on climate, yet variations in solar irradiance are not considered a critical factor in supporting the operational task of NPOESS. A particular concern centers on the consequences of a gap in solar irradiance measurements between the planned NASA SORCE (Solar Radiation and Climate Experiment) mission in mid-2002 and follow-ons in the NPP/NPOESS era. Designers plan to operate and obtain data from the SORCE spacecraft for a period of 5 years (with a goal of 6 years), which would nominally extend solar irradiance data sets through mid 2007. However, the first NPOESS satellite might not be launched until 2009 or later. Since 1978, a total solar irradiance (TSI) data set has been continued through a strategy of overlapping satellite-based measurements and multiple redundant instruments in orbit at one time. The committee considers solar irradiance to be a key climate forcing parameter that should be monitored precisely during the NPP/NPOESS time frame for purposes of climate change detection and attribution. The committee believes free-flying satellites should be evaluated as an additional platform option for planned solar irradiance measurements. Sensors to monitor solar irradiance are relatively small and lightweight, making them particularly suited for small satellites (notional payloads of only approximately 40 kg would be necessary). In this way: The orbit selected gives the highest-quality observations; The NPOESS operational spacecraft would benefit from a reduction in the mechanical components required for Sun-pointing; and A discontinuity in measurements would become less likely because replenishment of a “noncritical” sensor would not be dependent on launching a new NPOESS multisensor spacecraft. However, the committee also notes the concern among some in the scientific community that executing the solar irradiance mission on a free-flier would make the program more vulnerable to the vagaries of the budgetary process versus execution of the mission as a secondary payload on an operational weather satellite. Ocean Altimetry The success of the present observations of sea-level height from the TOPEX satellite is due in large part to its non-Sun-synchronous orbit (~1,300 km). Such an inclined orbit is required to determine and eliminate the unknown spatial variations in random tidal components, which at present are poorly known. Placing the altimetry instrument on a Sun-synchronous spacecraft (~800 km) would degrade the operational and climate effectiveness of this data set. Improvements in seasonal forecasts of El Niño/Southern Oscillation have been demonstrated as a result of the more precise characterization of the ocean height (and thus temperature and circulation) brought about by TOPEX. The potential for understanding more of the causes of interannual, decadal, and climate-change time-scale fluctuations will be limited unless this data set continues with its present (or an enhanced) level of precision. The altimeter is not considered a critical measurement for NPOESS launch decisions should failure occur (see Table 3.1). Thus, as in the case of solar irradiance, this measurement would be in jeopardy of complete discontinuity should problems develop. Jason-1 is intended to extend the TOPEX-type measurements until about 2005, but there is no assurance of continued high-precision surveillance in the NPOESS Preparatory Project and NPOESS periods. Present attempts to produce high-precision ocean height measurements from other sources (e.g., Earth Resources Satellite (ERS-1 and -2)) are successful only because TOPEX data serve as a standard. A further requirement put forth by Mitchum et al. (forthcoming) points to the need for very long overlapping observations from the old and new instruments and for the spacecraft to be in very similar orbits. Precise ocean altimetry after Jason-1 is considered a critical issue that must be resolved.

OCR for page 20
Issues in the Integration of Research and Operational Satellite Systems for Climate Research: II. Implementation Ocean Vector Winds The recent success of QuikSCAT, developed and launched following the failure of the Japanese Advanced Earth Observing Satellite (ADEOS), which contained the NASA scatterometer, NSCAT, has demonstrated the utility and limitations of a single broad-swath, Ku-band, active scatterometer system in the retrieval of surface vector winds (speed and direction) over the ice-free global ocean. Based on experience with ERS-1, ERS-2, NSCAT, and QuikSCAT, the case for tandem scatterometer missions in support of climate research and predictions has been made (Milliff et al., forthcoming). A roadmap for international cooperation to achieve the climate science requirements for surface vector winds awaits multiagency approval and support (see Milliff et al., forthcoming). The surface vector wind is the critical climate measurement, as opposed to surface wind speed alone. Further research and development of passive microwave methods for vector wind retrieval are encouraged. However, this technology is unproven in space, and satisfactory performance in a wide variety of environmental conditions (e.g., under cloudy conditions) has yet to be demonstrated. It is premature to regard the NPOESS/ CMIS instrument as sufficient to meet the needs of the climate community for surface vector winds. CERES The infrared measurements provided by the Clouds and the Earth’s Radiation Energy System (CERES) are especially sensitive to the diurnal cycle, the magnitude of which is easily comparable to the small changes sought in the detection of climate change. The placement of CERES on Tropical Rainfall Measuring Mission (TRMM) (inclined), EOS-AM (1030), and EOS-PM (1330) provides the opportunity to study problems requiring relatively high Sun angles, low cloud amounts (AM), and high cloud amounts (PM). However, because many useful quantities are surface or surface-related, a morning orbit is preferred because cloudiness is less prevalent. At present, NPP will provide a bridge for atmospheric correction and moderate resolution in the 1030 measurements (VIIRS, Cross-track IR Sounder (CrIS)), but these will end with the anticipated European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) Operational Polar Orbiter (METOP) missions designed to fill the 1030 (at 0930) slot with a less useful Advanced Very High Resolution Radiometer (AVHRR)-class instrument. Thus, the continuity of the climate-change quantities measured to high precision (e.g., broadband, top-of-the-atmosphere Earth radiation budget) at a specific point in the diurnal cycle will be compromised. High-resolution visible and infrared measurements in the mid-morning orbit will become discontinuous under the present plan. RECOMMENDATIONS The committee’s recommendations on data continuity are as follows: 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. It 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 1   For climate studies, there is a need for continuing investment in sensor studies and tests—programs for operational instruments typically do not fund such activities beyond initial checkout.

OCR for page 20
Issues in the Integration of Research and Operational Satellite Systems for Climate Research: II. Implementation 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. REFERENCES Christy, J.R., R.W. Spencer, and E. Lobl. 1998. “Analysis of the merging procedure for the MSU daily temperature time series,” J. Climate 5:2016-2041. Christy, J.R., R.W. Spencer, and W.D. Braswell. 2000. “MSU tropospheric temperatures: Dataset construction and radiosonde comparisons.” J. Atmos. Oceanic Techol. (in press). Milliff, R.F., M.H. Freilich, W.T. Liu, R. Atlas, and W.G. Large. “Global ocean surface vector wind observations from space,” in OCEANOBS-99, Proceedings of the International Conference on the Ocean Observing System for Climate, Centre National d’Etudes Spatiales (CNES), Saint-Raphael, France, forthcoming. Mitchum, G.T., R. Cheney, L-L. Fu, C.L. Provost, Y. Menard, and P. Woodward. “The future of sea surface height observations,” in OCEANOBS-99, Proceedings of the International Conference on the Ocean Observing System for Climate, Centre National d’Etudes Spatiales (CNES), Saint-Raphael, France, forthcoming. Mo, T. 1995. “A study of the Microwave Sounding Unit on the NOAA-12 satellite,” IEEE Trans. Geosci. Remote Sensing 33:1141-1152. National Research Council (NRC). 1999. Adequacy of Climate Observing Systems. National Academy Press, Washington, D.C. Waliser, D.E., and W. Zhou. 1997. “Removing satellite equatorial crossing time biases from the OLR and HRC datasets,” J. Climate 10:2125-2146. Wentz, F.J., and M. Schabel. 1998. “Effects of satellite orbital decay on MSU lower tropospheric temperature trends,” Nature 394:661-664. Willson, R.C. 1997. “Total solar irradiance trend during solar cycles 21 and 22,” Science 277:1963-1965.