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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) Assessment of Satellite Earth Observation Programs 1991 2 Earth Science from Space INTRODUCTION The two-volume report from the Committee on Earth Sciences, A Strategy for Earth Science from Space (SSB, 1982a, 1985—hereinafter referred to as the 1982 and 1985 SSB/CES reports), together with the Committee on Planetary Biology's Remote Sensing of the Biosphere (SSB, 1986—referred to as the SSB/CPB report), established a broad set of scientific objectives, formulated primarily for NASA, for the study of the Earth from space. It is important to emphasize at the outset, however, that although these reports have assisted NASA in developing its Earth observation flight programs and related research plans, the agency also receives advice in this area from other groups. Numerous other advisory reports-by several other NRC committees and internal NASA REPORT MENU advisory bodies-have collectively produced an interrelated advisory framework NOTICE covering all aspects of civil Earth observation programs and policies. MEMBERSHIP FOREWORD The various fields of earth and environmental sciences have progressed SUMMARY to the point where a unified approach—based on the view that the Earth's CHAPTER 1 physical, chemical, and biological processes constitute a coupled global CHAPTER 2 system—is required to understand the changes that are occurring. The CHAPTER 3 realization that the requisite scientific investigations must be conducted in an REFERENCES interdisciplinary context has led to the conceptual definition of an "earth system ABBREVIATIONS AND science" as the proper approach for such studies. Two key objectives of earth ACRONYMS system science are (1) to obtain a scientific understanding of the Earth as a APPENDIX system on a global scale, by describing how its component parts and their interactions have evolved, how they function, and how they may be expected to evolve on different time scales; and (2) to develop the capability to predict those changes that will occur in the next decade to a century, both naturally and in response to human activity. The ultimate goal is to gain a deeper understanding of the processes responsible for the evolution of the Earth as a coupled system. Achieving these objectives will require a comprehensive observational strategy and program consisting of space-based and in situ observations on a file:///C|/SSB_old_web/seo91ch2.htm (1 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) continuous basis over extended time periods. A major enhancement in the scientific computing and modeling capabilities also will be necessary to process the voluminous data sets and to develop effective methods for projecting environmental change. These activities are being pursued on the national level through the U.S. Global Change Research Program (USGCRP) and coordinated by the federal interagency Committee on Earth and Environmental Sciences (CEES). The USGCRP, in turn, is coordinated with a number of major international research initiatives, notably the International Geosphere-Biosphere Program (IGBP) of the International Council of Scientific Unions (ICSU), and the World Climate Research Program (WCRP), which is sponsored jointly by the World Meteorological Organization and ICSU. NASA's contribution to the USGCRP is the Mission to Planet Earth, which includes the Earth Observing System (EOS) program and its data and information system (EOSDIS), the Earth Probe line of small- to moderate-size missions, and a number of independent precursor research missions. These elements of NASA's Mission to Planet Earth are augmented significantly by the operational environmental spacecraft of the National Oceanic and Atmospheric Administration (NOAA) in polar and geostationary orbits, by the Landsat system operated on a commercial basis by the Earth Observation Satellite (EOSAT) Company, as well as by certain declassified data from operational and experimental satellites of the Department of Defense. Internationally, there are numerous experimental, operational, and commercial spacecraft already in orbit or under construction by the European Space Agency and its individual member states in western Europe, and by Canada, Japan, the Soviet Union, China, and India that can be expected to contribute to the global research and monitoring effort. This chapter provides an assessment of these flight programs in the context of the following scientific disciplines: (1) atmospheric sciences, (2) climate studies, (3) physical oceanography, (4) cryospheric research, (5) hydrology, (6) geology, (7) geodynamics, and (8) global biology, ecology, and biogeochemical cycles. The major scientific objectives for space-based observations, as outlined in the previous SSB reports (1982a, 1985, 1986), are presented for each discipline, and the status of their implementation, as well as suggestions for their improvement, are discussed briefly. In conducting this review, the committee found that although all three SSB reports still provide valid scientific guidance according to which NASA programs can be assessed, sufficient scientific and technological progress has been made to warrant their detailed reexamination and revision within the next few years. In particular, the previous discipline-specific advice should be reconsidered in an interdisciplinary context, consistent with the evolution of scientific research. ATMOSPHERIC SCIENCES The Troposphere file:///C|/SSB_old_web/seo91ch2.htm (2 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) Science Objectives The highest-priority objectives established by the 1985 SSB/CES report for the study of the troposphere from space are to accomplish each of the following: 1. To obtain global data sets for the internal and boundary forcing processes that maintain the atmospheric circulation. The required data sets are for (i) surface wind, atmospheric temperature and humidity, and stress over the oceans, and land and sea surface temperature; (ii) precipitation and closely related surface characteristics including soil moisture, snow and ice cover, and vegetative biomass; (iii) surface radiation and albedo, radiation at the top of the atmosphere; and (iv) cloud characteristics including type, amount, height, temperature, liquid water content, and radiative properties. 2. To obtain temporally continuous global data sets of sufficient spatial density and accuracy to determine the large-scale structure of the troposphere. The required data sets are for (i) wind, temperature, and moisture in the free atmosphere, and (ii) sea level pressure. Current Status The measurement of boundary forcing requires the simultaneous determination of wind stress, temperature, and humidity in the surface boundary layer; of surface boundary temperature and moisture variables; and of radiation fluxes at the surface and at the top of the atmosphere. The feasibility of satellite measurements of stress at the sea surface was clearly demonstrated with NASA's Seasat scatterometer. During the past decade, surface temperature measurements have been routinely made with the Advanced Very High Resolution Radiometer (AVHRR) on NOAA polar-orbiting meteorological spacecraft. Radiation fluxes at the top of the atmosphere have been assessed with the Earth Radiation Budget Experiment (ERBE) measurements on several NASA and NOAA satellites. In view of fundamental limitations on the achievable vertical resolution of temperature and moisture fields of passive infrared or microwave sounding, global distributions of temperature and humidity in the surface boundary layer can only be inferred from measurements made with relatively low vertical resolution that are extended with the aid of modeling. Similarly, surface radiation fluxes cannot be measured directly from space, but progress has been made in inferring these fluxes by combining satellite measurements, surface synoptic measurements, and models. In view of this dependence on modeling, there is a continuing need for in situ validation of surface energy and moisture flux parameters. Passive microwave measurements have proven useful for inferring precipitation from space, but the quantitative reliability of these determinations remains uncertain. This is partly a result of the scale mismatch between the satellite measurements and ground-based radar and rain gauge network measurements, and partly because there is a great amount of uncertainty in all methods used to estimate precipitation distributions, especially over the sea. The vertical distribution of latent heat release due to precipitation, which is of central file:///C|/SSB_old_web/seo91ch2.htm (3 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) dynamical importance, has not yet been assessed from space. The usefulness of satellite instruments such as AVHRR for assessing snow and ice cover and relative changes in biomass has been demonstrated, but much remains to be done to advance the capability for assessing these surface properties, as well as soil moisture, from space. Significant progress also has been made in the use of space observations to determine the properties of clouds from space. Microwave imagers such as the Scanning Multichannel Microwave Radiometer (SMMR), flown on Nimbus-7 and Seasat, and the Special Sensor Microwave Imager (SSM/I), flown on the Defense Meteorological Satellite Program (DMSP) series, have provided useful information on cloud liquid water and ice, as well as on precipitation. The ERBE program in conjunction with the International Satellite Cloud Climatology Project (ISCCP) has provided the research community with global data sets that are now being widely used for relating the properties of clouds to wind, temperature, and moisture fields, and to the radiation fluxes at the top of the atmosphere. In addition, space observations have been widely used in coordinated cloud process studies aimed at boundary layer clouds and cirrus clouds. The global spaceborne measurements of temperature, moisture, cloud fields, and cloud drift winds that are key elements of the weather forecasting system are also essential measurements of the tropospheric structure component of the climate system. Polar orbiting and geostationary satellites play important complementary roles. The NOAA Polar-Orbiting Operational Environmental Satellite (POES) series currently has two orbiting spacecraft with AVHRRs and atmospheric sounders (TIROS Operational Vertical Sounder, TOVS). The NOAA Geostationary Operational Environmental Satellite (GOES) spacecraft carry infrared and visible imagers (Visible and Infrared Spin-Scan Radiometer, VISSR) as well as sounders (VISSR Atmospheric Sounder, VAS), which provide data at 30- to 60-minute intervals throughout the day. Because of launch and satellite system failures, NOAA has been operating only one GOES instead of the two needed to complement the Japanese GMS, European METEOSAT, and Indian INSAT geostationary satellites for global coverage. Despite significant technical difficulties, cost overruns, and delays, development of the next scheduled GOES satellite is moving forward, and NOAA is developing backup plans in the event that further problems threaten a gap in operational coverage. Recommendations in the 1985 SSB/CES report identified surface pressure as an important atmospheric variable. Current spaceborne observational techniques do not provide direct measurements of surface pressure with meteorologically useful precision. For the foreseeable future, the global distribution of this parameter will have to be inferred from in situ measurements combined with indirect techniques, such as data assimilation and modeling techniques that utilize observations from space in a dynamically consistent way. The use of scatterometer measurements to infer surface wind distributions, and from these, surface pressure gradients, has been shown to be a particularly promising approach to this problem. file:///C|/SSB_old_web/seo91ch2.htm (4 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) Anticipated Improvements During the 1990s, implementation of the current plans of NASA, NOAA, and space agencies of other countries will make significant advances in several areas. In the near term, the European Space Agency's (ESA) Earth Remote- Sensing Satellite (ERS-1), scheduled for launch in the summer of 1991, and the ERS-2 and Japanese Advanced Earth Observing Satellite (ADEOS), both scheduled for launch in the mid-1990s, will carry scatterometers and will resume global measurements of stress at the sea surface. The Tropical Rainfall Measurement Mission (TRMM), a cooperative program between NASA and Japan that is currently planned for launch in 1996, will obtain radar measurements of precipitation at low and middle latitudes. NASA has also proposed that the TRMM carry an Earth radiation budget scanner, which would resume those critical radiation measurements that ended with the failure of the last ERBE scanner in 1990, as well as a visual infrared sounder and a lightning imaging sensor. In the longer term, NASA's Earth Observing System (EOS), which is being planned in cooperation with ESA, Japan, and Canada, is expected to significantly improve the capabilities for atmospheric research from space. (For additional assessments of the EOS program by the NRC, please see the "Space Studies Board Position on the NASA Earth Observing System" [SSB, 1991] and The U.S. Global Change Research Program: An Assessment of the FY 1991 Plans [NRC, 1990].) Both the EOS measurement strategy and the selected EOS instruments are generally well suited for advancing atmospheric research. The instruments selected for flight on EOS-A include the Stick Scatterometer (STIKSCAT), Multifrequency Imaging Microwave Radiometer (MIMR), Atmospheric Infrared Sounder (AIRS), Advanced Microwave Sounding Unit (AMSU-A and -B), Moderate-Resolution Imaging Spectrometer (MODIS), Earth Observing Scanning Polarimeter (EOSP), and Clouds and the Earth's Radiant Energy System (CERES). These instruments will provide substantial improvements in the measurement of surface stress over the sea, of land and sea surface temperature, of cloud properties, and of vertical profiles of temperature and water vapor. Further, the synergisms among this set of sensors, collocated on the same platform, will make especially useful contributions to the inference of the global distribution of surface boundary forcing (momentum, sensible and radiative heat, and water vapor fluxes) and should also contribute strongly to our understanding of the relationships among cloud properties, surface fluxes, and atmospheric circulation. With the CERES instrument included in the EOS-A series, continuing data on the energy fluxes at the top of the atmosphere and of the relationships between clouds and top-of-the-atmosphere radiative fluxes will be obtained. The MODIS and MIMR sensors will contribute to the determination of soil moisture, snow and ice properties, and vegetation characteristics. Several of the instruments under consideration for the EOS-B series could make important contributions to tropospheric science objectives. In particular, the Laser Atmospheric Wind Sounder (LAWS) could improve our understanding of surface-atmosphere exchange processes. The LAWS file:///C|/SSB_old_web/seo91ch2.htm (5 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) instrument was originally scheduled for launch on a Japanese satellite, but that does not now appear possible. The potential LAWS contribution of direct wind measurements would be of significant value for deducing tropospheric structure, particularly if errors as low as 1-2 m/s can be achieved. This is especially true for the tropics, where determination of global wind distribution by indirect means is notoriously difficult. The proposed EOS SAR, which would fly as a separate mission, could make important contributions to the determination of the properties of sea ice and snow, soil moisture and surface water distribution on land, and vegetative structure, all of which interact in significant ways with the troposphere. Additional Needs The AVHRR instrument on the NOAA polar orbiters, and the VAS and VISSR instruments on the NOAA GOES are providing important historical data sets for future climate change studies, in particular, for the early versions of the EOSDIS. The temperature and moisture sounders on the NOAA POES are collecting the critical background information for the future EOS atmospheric profiling systems. While the measurements from geostationary and other polar orbiter spacecraft will provide a somewhat lower resolution than some research instruments selected for EOS, they will continue the long time series started in the 1970s that have provided the heritage of observations that led to the design of the EOS program and instruments. The geosynchronous spacecraft also will continue to obtain some of the essential information throughout the daily cycle. Because global change and climate research studies all require long series of measurements, it will be important to interrelate the future space observations with earlier NOAA satellite data in order to produce long records of important variables. During the EOS time frame, NASA's and NOAA's existing plans would lead to a number of observational deficiencies. For studies in the atmospheric sciences, these deficiencies include (1) the inherent limitations of satellite instruments for comprehensively measuring such quantities as surface fluxes of heat, water vapor, and solar and thermal radiation, as well as distributions of surface pressure and key surface properties such as soil moisture and snow depth; (2) the absence of measurements of the three-dimensional distribution of precipitation beyond TRMM; (3) the lack of observations on a frequent basis, particularly coverage of the daily cycle with high-resolution imaging, microwave imaging, and scatterometer measurements comparable to those of EOS; and (4) the potential absence of global wind field measurements. The first issue can be addressed by coordinated satellite, field, and modeling programs that cover those processes and their electromagnetic signatures. Important studies using currently available data have been carried out, and others are under way or in the planning stage. The International Satellite Land Surface Climatology Project is an example of a successful research program already in progress. There will be a continuing requirement for file:///C|/SSB_old_web/seo91ch2.htm (6 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) coordinated satellite, in situ, and modeling programs as new instruments with different spatial, spectral, and signal-to-noise characteristics are flown. Although EOS from its polar orbit will obtain some precipitation measurements with the MIMR and MODIS instruments, there are currently no plans to obtain radar measurements or low-inclination orbital sampling of precipitation beyond TRMM. The possibility of a gap in precipitation measurements from space is critical because latent heat release due to precipitation is the single most important internal driver of the atmospheric circulation. Therefore, there will be a need to measure precipitation on a global basis throughout the EOS time frame, and the committee considers it important for NASA to develop plans to continue such measurements in cooperation with our foreign partners in Mission to Planet Earth. The limited coverage of the daily cycle by EOS is particularly serious for the assessment of boundary fluxes and cloud and precipitation processes, since they vary greatly on diurnal or shorter time scales. To address this issue, it is necessary for NASA and NOAA to continue to improve observations from geosynchronous satellites and to obtain data from complementary spacecraft flown by other countries. Full development and coordination of the NOAA polar and geosynchronous satellite programs with the EOS program are particularly important in this context, as well as for assuring long-term continuity of observations. Current plans at NOAA call for a suite of instruments on future geosynchronous satellites that would provide an excellent complement to EOS, but it is not yet clear whether future funding levels will allow these plans to be achieved on time. Although coordination between NOAA and the outside scientific community, and between NASA and NOAA, have improved, an even stronger integration of efforts is needed. Finally, it is important for NASA to continue the development of an instrument that could obtain the important global wind field measurements. The Stratosphere and Mesosphere Science Objectives According to the 1985 SSB/CES report, the highest-priority objectives for the study of the stratosphere and mesosphere from space are as follows: 1. To measure continuously total ozone and its vertical profile over the globe with sufficient accuracy to test theoretical predictions. 2. To measure simultaneously the vertical profiles of atoms and radicals involved in ozone chemistry and the source and sink species of these atoms and radicals, as a function of latitude and time of year. Current Status file:///C|/SSB_old_web/seo91ch2.htm (7 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) One of the remarkable scientific achievements of the past decade has been the discovery of the seasonal ozone hole in the Antarctic and the rapid scientific response, including aircraft, balloon, and surface-based campaigns coordinated with space-based measurements. These observations verified that industrial chlorofluorocarbon emissions, activated by the unique meteorological conditions in the Antarctic (and, to a lesser extent, the Arctic) stratosphere, were the principal cause of the phenomenon. NASA, NOAA, NSF, and the agencies of other nations played key roles in this cooperative scientific effort. The rapid incorporation of the results of these programs into the formulation of policy responses can be attributed in part to the timeliness and quality of this research. Following the important contributions of the Nimbus-7 Limb Infrared Monitor of the Stratosphere (LIMS) and Pressure Modulator Radiometer (PMR) to our knowledge of the dynamical and chemical structure of the stratosphere and mesosphere, data on distributions of ozone and oxides of nitrogen were extended by measurements from the Solar Mesosphere Explorer (SME) spacecraft. Aerosol and ozone measurements were also obtained by the Stratospheric Aerosol and Gas Experiment (SAGE) mission. Currently, space-based observations of this region are limited primarily to the measurement of total ozone and vertical distribution of ozone with the Total Ozone Mapping Spectrometer (TOMS) on Nimbus-7, and the Solar Backscatter Ultraviolet (SBUV) instruments on Nimbus-7 and on the NOAA POES. These measurements, when combined with surface-based total ozone measurements, have led to new assessments for the trends in ozone levels, which have indicated that chlorofluorocarbons may be contributing to worldwide depletion, not just polar depletion, of stratospheric ozone. A follow-on TOMS is to be launched on a USSR Meteor-3 satellite during the summer of 1991 to provide continuity of the global stratospheric ozone observations. Anticipated Improvements The next major step forward will be the launch of the Upper Atmosphere Research Satellite (UARS) in the fall of 1991. The LIARS mission will provide global measurements of a suite of trace gases and free radicals together with ozone and temperature measurements, and for the first time in the stratosphere and mesosphere, direct measurements of winds. These measurements should go far toward meeting the scientific objectives for the stratosphere and mesosphere, which are aimed largely at understanding the natural and anthropogenic mechanisms for seasonal and interannual variability and long-term trends in ozone, associated trace gases, and large-scale circulation. Beginning with the launch of EOS-A, the High-Resolution Infrared Dynamical Limb Sounder (HIRDLS) instrument will take a further step toward meeting the objectives of the 1985 SSB/CES report by obtaining global measurements of ozone, temperature, and key trace gases with high horizontal and vertical resolution from the upper troposphere (in cloud-free regions) into the mesosphere. Additional relevant instruments currently proposed for flight on EOS- file:///C|/SSB_old_web/seo91ch2.htm (8 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) B include the Spectroscopy of the Atmosphere Using Far Infrared Emission (SAFIRE) instrument, which would make high-precision measurements of key trace species, including free radicals; the Solar Stellar Irradiance Comparison Experiment (SOLSTICE), which would monitor the critically important solar ultraviolet flux; SAGE-III, which would provide precise data on aerosols, as well as on the distributions of ozone and other trace gases; and the Stratospheric Wind Infrared Limb Sounder (SWIRLS), which would directly measure stratospheric winds. The latter is important since geostrophic winds derivable from the temperature distribution are not accurate enough for calculation of the transport of trace species on a day-to-day basis. This suite of instruments would provide an excellent basis for long-term monitoring of ozone and the factors that control its variations. Additional Needs The UARS has a nominal mission life of 18 months, and although it is likely to provide measurements for a longer time, there is a need for continued monitoring of the ozone distribution in the period between the termination of UARS and the launch of EOS-A. In the meantime, measurements of total column ozone with the TOMS instrument will be continued on several spacecraft, and the SBUV instruments will be flown on the NOAA POES series. The TOMS should make it possible to continue tracking trends in total ozone, and the SBUV instruments will provide very useful information on the three-dimensional ozone distribution in the stratosphere. However, recent studies have shown that calibration drifts of the SBUV instruments can confuse the detection of trends using SBUV alone and that the previously flown SAGE instrument would provide a more reliable trend assessment. For this reason, the committee considers it important to complement the post-UARS ozone measurement program with SAGE measurements. CLIMATE STUDIES Science Objectives The highest-priority objectives established by the 1985 SSB/CES report for the study of long-term climatic changes from space are the following: 1. To measure the long-term global and regional trends in external and internal climate forcings: the variables that must be measured are the solar flux, the radiative fluxes at the top of the atmosphere, radiatively important trace gases and aerosols, and certain land-surface properties (vegetative cover, soil moisture, albedo, and emissivity). 2. To measure the long-term global and regional changes in climate: the variables that must be measured are surface and tropospheric temperatures, precipitation, water vapor, and cloud, snow, and ice cover. file:///C|/SSB_old_web/seo91ch2.htm (9 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) Current Status Most of the measurements that must be made to characterize long-term global and regional trends are a subset of those required for the atmospheric science objectives discussed above. The global data sets of wind, temperature, and moisture required for studies of the troposphere, stratosphere, and mesosphere are essential for the future development of climate models, particularly for refining the parameterization of small-scale, chemical and microphysical processes required by these models. There are, nevertheless, some special considerations for climate monitoring. These include long-term, precise, and stable measurements of atmospheric variables, such as temperature and ozone, to monitor climatic trends. Satellite passive microwave measurements have recently been shown to provide very stable temperature trend information averaged over the troposphere and lower stratosphere, while, as mentioned above, SAGE measurements, which use solar (and possible lunar) occultation techniques, can produce information on trends in stratospheric ozone and aerosols. A critical component of climate system monitoring is the measurement of radiation balance quantities at the top of the atmosphere, including incident solar, reflected solar, and emitted thermal radiation. The contributions of the ERBE instruments to our knowledge of these variables were mentioned above. The data from ERBE, together with increased availability and usefulness of cloud information developed through the ISCCP studies, have augmented our understanding of the interactions between clouds and the top-of-the-atmosphere radiation budget parameters. Anticipated Improvements Precise monitoring of the solar input radiation will be extended by UARS. The CERES instrument on UARS will help to extend the observational record of the top-of-the-atmosphere radiation budget, and continued processing of historical data sets on cloud properties under the ISCCP will add to our ability to monitor the radiation budget and cloud interactions. The French-Soviet Scanner for Radiative Budget (SCARAB) mission, expected to be launched in 1992, will provide some additional data on the Earth's radiation. Other new measurements discussed in the section on the troposphere will contribute to the climate objectives as well. Additional Needs As mentioned above, there has been a gap in the precise measurement file:///C|/SSB_old_web/seo91ch2.htm (10 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) of reflected solar and emitted thermal radiation since the failure of the ERBE scanner. It is essential that the long-term recording of these observations be resumed as early as possible. The CERES instrument planned for TRMM would go far toward solving this problem, but would still leave a gap at high latitudes. Moreover, diurnal variations are an essential factor in measuring the radiation flux at the top of the atmosphere. For both of these reasons, it would be desirable to place additional Earth radiation budget sensors in orbit before and during the EOS time frame, perhaps in cooperation with the space agencies of other countries. One way of meeting these needs would be with long-term, well- calibrated measurements that could complement TRMM and EOS measurements to provide complete diurnal and latitudinal coverage of top-of-the-atmosphere radiation balance, as well as stratospheric aerosols and ozone measurements suitable for precise determination of trends in vertical profiles of ozone. The potential seriousness of the problem of global climate change and the importance of avoiding gaps in key long-term data sets together underscore the need to ensure their continuation on a cooperative international basis. PHYSICAL OCEANOGRAPHY Science Objectives As stated in the 1982 SSB/CES report, the primary science objectives for the study of ocean dynamics from space, in order of priority, are: 1. a. To measure the time-variable sea-surface elevation; b. To measure the time-independent sea-surface elevation relative to the geoid; 2. To determine wind stress and its distribution over the ocean; 3. To measure directly the near-surface circulation; 4. To measure subsurface ocean properties; 5. To measure sea-surface temperatures. The science objectives for biological oceanography are discussed later in this chapter. Current Status Starting with the NASA GEOS-3 and Seasat radar altimeters, and continuing more recently with the U.S. Navy's 3-year Geosat mission, the ocean science community has gathered experience in the application of satellite altimeter measurements to the study of ocean circulation variations. In many cases, the spatial coverage of the altimeter data has proved to be uniquely suited to mapping well-known but seldom observed phenomena such as oceanic Rossby waves. The lack of an accurate reference geoid (or mean gravity field on scales of a few hundred kilometers) has restricted many of these studies to file:///C|/SSB_old_web/seo91ch2.htm (11 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) present-day vertical uplift due to postglacial rebound? Furthermore, estimates of vertical displacements along coastlines with millimeter-level accuracies could help remove tectonic effects from tide gauge data to better constrain the global rise in sea level. Also, precise observations of local surface subsidence could help indicate the degree of water depletion in underground reservoirs. Finally, accurate measurements of the time-varying, long-wavelength components of the Earth's gravity field, and the associated variations in the Earth's rotation are important in understanding the interactions among the atmosphere, ocean, and solid earth. Space-geodetic techniques, including satellite laser ranging (SLR), very- long-baseline interferometry (VLBI), and radio interferometry using the GPS satellites, are beginning to answer some of these questions. Land-based techniques are competitive over distances of approximately a few tens of kilometers and especially when continual monitoring of a particular baseline is desired. Over longer baselines, however, space-based techniques are far superior. The space-based positioning techniques (particularly VLBI, SLR, and lunar laser ranging) can also be used to infer time-dependent variations in the mantle's rotation rate and in the position of the rotation axis. Variability can be caused by the exchange of angular momentum between the mantle and the fluid regions of the Earth: the atmosphere, the oceans, and the core. These observations can be used to help constrain certain properties of the core and lower mantle, including the mantle's electrical conductivity and the shape of the core-mantle boundary. They also provide data for an independent assessment of certain atmospheric and oceanic variables, and insight into how angular momentum is exchanged between the solid earth and the atmosphere, oceans, and hydrosphere. Measurements of the time variations of the Earth's gravity field are possible by SLR to special satellites such as Lageos. During the past decade, the NASA program has been responsive to a number of the recommendations in the 1982 and 1985 SSB/CES reports. In particular, the development of space-based techniques with the capability for measuring global tectonic motion has led to the confirmation of plate tectonics theory. The development and demonstration of the SLR and VLBI techniques for measuring plate motion and for measuring variations in the Earth's rotation rate have been major accomplishments by NASA during the 1980s. The SLR and VLBI techniques provide the capability for defining and maintaining an accurate terrestrial reference frame, which is necessary for monitoring long-term changes in global plate motions and for monitoring secular trends in ice sheet topography and mean ocean surface. These latter signals will be important in the study of global warming and will play a major role in the long-term global change research initiatives. Anticipated Improvements file:///C|/SSB_old_web/seo91ch2.htm (23 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) The plans during the 1990s include placing in orbit Lageos-2 and Lageos3 as targets for further satellite laser ranging studies. These targets, along with improved ranging precision in the SLR systems, will provide high temporal resolution for a better discrimination of the time variations in both the Earth's rotation and the tectonic plate motion. In addition, the specific configuration identified for Lageos-3, which is driven by a requirement for attempting to measure the Lense-Thirring frame rotation predicted by general relativity, will be an especially important satellite configuration for measuring the tidal variation and time-dependent gravity signals, and as a means of providing a better tie to an inertial reference frame. The full deployment of the Defense Department's GPS will provide the capability for performing low-cost, regionally dense measurements of relative position at accuracies that are consistent with the accuracy achieved by the SLR and VLBI techniques. The GPS has the potential for fulfilling the dense grid measurement requirements inherent in measuring time-dependent deformation in the major worldwide seismic zones. The deployment of the 21-satellite GPS by the mid-1990s also will provide the potential for achieving a large number of precise measurements to support the long baseline (continental and intercontinental length) measurements by VLBI and SLR systems. If the research community can get unrestricted access to the GPS signals, the data will provide a significant improvement in the ability to rapidly produce a large number of precise regional measurements. Some gravity data over the oceans are expected to be obtained by the ALT instrument on EOS-B. Flying the ALT in a polar orbit as currently planned, however, would not be optimal for this measurement. Although the EOS-B ALT will be a TOPEX/Poseidon-class nadir-pointing instrument, its flight in a polar orbit would not provide either a more accurate oceanic geoid or more precise measurements of ocean circulation than earlier altimeters; however, it would provide estimates along a different ground track. Improved point positioning measurements for augmenting the GPS data are expected to be provided by the GLRS on EOS-B. The GLRS is the only instrument proposed for EOS-B that would be used primarily for geodynamic purposes, although it also would have applications for snow, ice, and cloud studies. The instrument also would complement existing VLBI, SLR, and GPS space techniques in determining plate motions and deformation. In addition, the GLRS would provide information on Earth's rotation, although it is not yet clear whether the results would be comparable in quality to the results obtained with other techniques. The GLRS would be particularly effective for monitoring areas near fault zones and volcanos, where relatively rapid deformation can be expected. Those areas would be covered with a dense array of retroreflectors, which would be sampled repeatedly by the onboard laser to detect sudden motion. Finally, the proposed EOS Geomagnetic Observing System (GOS) instrument would be able to provide valuable information on the time dependence file:///C|/SSB_old_web/seo91ch2.htm (24 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) of the magnetic field originating in the core. The magnetometer that would be carried on the proposed ARISTOTELES mission also would be useful for determining how the magnetic field has changed since the 1979-1980 Magsat mission. Additional Needs The question arises as to whether some SLR and VLBI systems should be replaced by GPS systems, and if so, when. Further validation and collocation studies are an essential prerequisite to understanding the accuracy of the GPS and to evaluating the possible problems encountered as a result of restricted access to GPS data. At present, the SLR is the only technique with the demonstrated ability to ensure that the origin of the terrestrial reference system is coincident with the Earth's center of mass. Further, VLBI is the only technique that ties the terrestrial reference system to the quasar-based inertial reference system. Consequently, the committee views the SLR and VLBI techniques as being crucial to the task of maintaining the reference frame required for monitoring long-term global change, and they should continue to have a significant role in the future complement of space-based geodetic techniques. This issue is discussed in greater detail in the NRC report International Network of Global Fiducial Stations (BESR, 1991), which is fully consistent with the CES views. The committee notes further that there is a continuing strong requirement for a dedicated gravity satellite, such as the Gravitational Research Mission proposed under NASA's Earth Probes program, or ESA's ARISTOTELES mission. (See also the Physical Oceanography section above.) GLOBAL BIOLOGY, ECOLOGY, AND BIOGEOCHEMICAL CYCLES Land-Surface Vegetation Science Objectives The scientific objectives identified in the 1986 SSB/CPB report for studying land-surface vegetation on a global basis are as follows: Measure total area covered and geographic distribution of major biomes. Measure the rate of change of distribution of major biomes. Measure biomass density for each biome. Vegetation production (annual): 1. Use leaf area index as key variable relating vegetation reflectance file:///C|/SSB_old_web/seo91ch2.htm (25 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) to biomass and biological production. 2. Test active microwave techniques to measure biomass, canopy moisture, and soil moisture. Current Status The understanding of the interaction between vegetation and the land surface is progressing, but at a slow pace. The land surface is very complex in its chemical, biological, and geological composition; thus, the slow rate of progress is understandable. An additional issue is the definition of the satellite capability to address these questions. The spectral and spatial resolution necessary to map globally the vegetal communities or biomes has not been addressed adequately. The NOAA AVHRR data have been used extensively to map vegetation over the globe, but the transformation of those data into physiological or biological information has not yet been accomplished. Remote sensing technology has been used to map forest cover, areas under cultivation, and areas of deforestation. It has also been used to study causes of land degradation and eventual desertification. The technology has helped determine which areas may be brought into agricultural production and which ones are more fragile and should be kept in natural vegetation cover. However, the scientific synergism of combining remote sensing measurements from space with ground-based data has not been adequately proven globally. Physical variables such as slope, soil type, erosion, and rainfall distribution, combined with the major biomes observed from space, can more efficiently assist in monitoring land use. The United Nations Environment Program estimates that 6 million hectares of land are becoming desert each year owing to soil degradation. Multitemporal remote sensing data combined with ground-based data can be used to establish baselines of land-use patterns and to monitor seasonal changes in desert interfaces, soil moisture, vegetation, and human settlements. The rate of change on the land surface varies. If the change is induced by human activity, such as the clearing of trees or the planting of new crops, the change can be rapid. If the change is climate-induced, the change can be slow and subtle. The spectral, temporal, and spatial resolutions required to detect vegetal changes vary with the type of disturbance or change being addressed. Tropical deforestation has been successfully studied using AVHRR and Landsat. Vegetation changes induced by climate will be detected first in the areas between ecological units, and the spectral and spatial resolution requirements could be quite different. Anticipated Improvements During the EOS time frame, the MODIS will provide greatly improved spectral resolution of the land surface. Studies have been and are being conducted to develop sampling strategies on selected biomes. An excellent file:///C|/SSB_old_web/seo91ch2.htm (26 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) model was provided by the work done through the International Satellite Land Surface Climatology Project in 1987 and 1989, and similar studies are being planned. Because of the investment in data collection for such large-scale field studies, it will be important to continue to support the data analysis before moving to new biomes. New sensors with high spectral and spatial resolutions, such as HIRIS, would be useful for identifying soils and vegetation communities. Measurements from space in the thermal infrared (e.g., ASTER, MODIS) and microwave (e.g., EOS SAR) regions would also better define the surface condition and global changes of Earth. In agricultural crops, the spectral reflectance is highly correlated to biomass and production because the plants have developed to efficiently capture light energy with their leaves. Thus there is a high correlation between leaf area and biomass over much of the growing season. However, in many other vegetation communities, such as native grasslands and forests, leaf area and biomass are not strongly correlated. Light intercepted by the vegetal canopy appears to have the potential to be directly quantified by remote sensing observations from HIRIS and MODIS. Biophysical models are then necessary to simulate net primary production. The EOS SAR has the potential to measure global biomass and to assess surface soil moisture, but not the soil water content throughout the root zone of most plants. Surface soil moisture is useful in assessing runoff and, therefore, important in constructing the water balance. In many ecological units, soil moisture is a primary limitation to net primary production. The greatest potential for using radar backscatter information is through the use of models and their inversion to obtain canopy characteristics. Additional Needs Information about net primary production, biogeochemical cycling, carbon pools, and vegetal condition (e.g., physiological stress) is essential to fully assess global change. For some areas, such as wetlands, the spectral features of vegetation have a high correlation to biomass; in others, such as native grassland, the correlation is much weaker. This inconsistency among biomes is due to the complex interaction between the radiation field and the vegetal elements, as well as the background features such as soil or organic residues. It has become evident that higher spectral resolution than that provided by the Landsat MSS or TM is necessary to reduce the background noise. In addition, the temporal resolution requirements have not been well identified for the assessment of major biomes. Transitions between vegetation communities, or "ecotones," can be used to study climatic changes because the plants there are at the frontier of their physiological limits and are therefore most responsive to environmental change. Ecotones are quite varied in structure and in their sensitivity to environmental file:///C|/SSB_old_web/seo91ch2.htm (27 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) change. Theory suggests that the morphology of ecotones should be useful in predicting the effects of alteration in the environment. Unfortunately, the important studies of vegetation using remotely sensed imagery have universally concentrated on general features using crude scalar analyses. Exploratory research needs to be performed that will determine the feasibility of analyzing satellite images of vegetation to locate transitions and boundaries that are especially sensitive for detecting changes in the Earth's environment. These structures are the "hot spots" at which climate- or pollution-induced change may first occur. For the study of ecotones, AVHRR and even Landsat MSS data will not have adequate spatial resolution. Spatial resolutions of 30 m or better, particularly those combined with the high spectral resolution that would be provided by an instrument such as HIRIS, will be required for this type of research as well. Fresh Water, Wetlands, and Estuaries Science Objectives The 1986 SSB/CPB report established the following scientific objectives for the study of fresh water, wetlands, and estuaries from space: For the 20 largest rivers, determine annual rate of transport of carbon, nitrogen, sulfur, and phosphorus from land to oceans. Determine area covered and geographic distribution of coastal wetlands. Determine production of greenhouse gases from wetlands (methane and carbon dioxide). Current Status During the past decade, NASA has supported a series of small research projects focused on riverine systems, wetlands, and estuaries, and in 1989, NOAA initiated a broad-based Coastal Ocean Program. Both agencies have begun several research programs that are making important contributions to a better understanding of wetlands and estuaries, and their significance to global ecology. For instance, since 1984, NASA has supported a highly focused program using satellites to measure wetland biomass production and to relate that to the emission of greenhouse gases such as methane. As a result, researchers have demonstrated that both Landsat and SPOT data can be used to provide accurately the geographic distribution of coastal wetlands. Major plant species in coastal wetlands can now be mapped with Landsat and SPOT with accuracies ranging from 80 percent to 95 percent. Biomass of Spartina marshes can be determined with TM and SPOT within 10 percent of ground-measured values. Remotely sensed above-ground biomass is being related to below-ground file:///C|/SSB_old_web/seo91ch2.htm (28 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) biomass production. The Landsat MSS and TM sensors, and the SPOT sensors have good spatial resolution, but their temporal coverage is very poor and their spectral bands are not ideal for measuring concentrations of suspended or dissolved substances in the water column. Nevertheless, these sensors are suitable for mapping several suspended-sediment and flow patterns. The NOAA AVHRR sensors provide daily coverage and are quite effective for tracking the dynamics of certain phytoplankton blooms, turbidity maxima, and Gulf Stream rings, among other parameters. The AVHRR has a spatial resolution of only 1.1 km, however, and its spectral bands are not ideal for mapping sediment or chlorophyll concentration. The Nimbus-7 Coastal Zone Color Scanner (CZCS) provided global maps of open-ocean chlorophyll between 1978 and 1986. Despite its name, however, the CZCS was not designed for near-shore coastal and estuarine remote sensing. Its sensors could not handle the wide dynamic range of radiances backscattered from turbid waters, and it had insufficient spatial resolution (0.8 km) for estuarine studies. The National Aeronautics and Space Administration has begun a Fresh Water Initiative (FWI), a multiyear interagency program devoted to acquiring a predictive understanding of freshwater systems in the context of global change. Study areas will include freshwater ecology, hydrology, and resources. The FWI is designed to coordinate ongoing and future programs aimed at acquiring a predictive understanding of freshwater ecosystems and resources. This can be used to improve detection, assessment, and prediction of environmental effects, and develop management and mitigation alternatives for potential global change scenarios. NASA is leading the planning process, but is cooperating with 14 bureaus and services representing eight federal agencies: the departments of Energy, the Interior, Defense, and Agriculture; NOAA; NSF; the Environmental Protection Agency; and the Tennessee Valley Authority. The National Aeronautics and Space Administration has already made substantial progress in developing satellite techniques for wetlands studies in support of the USGCRP. Similarly, NOAA's sponsorship through the Small Business Innovation Research program for small companies to develop a remote sensing instrument package for estuarine observations from small, single-engine aircraft is providing researchers with an inexpensive option for wetland and estuarine studies. Anticipated Improvements The committee expects that the spectral resolution of the future HIRIS instrument on EOS will enable researchers to further improve biomass and stress measurements in temperate wetlands, and to extend them to freshwater and tropical wetlands. Riverine and wetland features are frequently narrow and have complex spectral signatures. Thus high spectral and spatial resolution is required file:///C|/SSB_old_web/seo91ch2.htm (29 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) not only for studies of wetlands, but also for studies of rivers and estuaries. Estuarine waters typically contain high concentrations of dissolved and suspended materials that arise from or have an impact on biological activity in the water. These substances may also, in some cases, be used as tracers to study circulation patterns in the estuary. Many of these materials are optically active and influence the spectral and angular distribution of the light in the water column. As a result, measurements of the detailed spectral characteristics of the light within the water column and above the surface may be used to determine the concentrations of various materials in the water. Better spectral resolution, such as that expected with HIRIS, is required to discriminate between chlorophyll, dissolved organics, suspended sediments, and other dissolved or suspended substances and to correct for atmospheric effects. Satellite sensors alone, however, cannot provide the temporal and spatial resolutions required by watershed, hydrodynamic water quality, and living resource models of estuaries. Based on requirements for spatial and temporal resolution, it is obvious that no single spacecraft can provide the necessary tidal, daily, and weekly coverage required for studies of coastal and estuarine test sites. Aircraft can provide frequent overflights at good spatial resolution, but the large four-engine aircraft used in the past are too expensive to be flown repeatedly. A new sensor package is being developed with NOAA and NASA support, which will be small enough to fit on single- or twin-engine aircraft with a tenfold reduction in operating costs. The sensor package will include a small multispectral video camera for color measurements of the water from which chlorophyll, suspended sediment, and concentrations of dissolved organic solids can be estimated. A thermal infrared radiometer will be used to measure surface temperature, and a microwave radiometer, which is at present being reduced in size, will be used to measure water salinity. Deployed in conjunction with high- resolution satellite sensors such as HIRIS, these airborne sensors should be able to observe tidal, seasonal, and annual variations and spatial distributions of phytoplankton blooms, sediment plumes, estuarine fronts, circulation patterns, and other estuarine phenomena. Biogeochemical Cycles Science Objectives The highest-priority objectives established in the 1985 SSB/CES report and the 1986 SSB/CPB report for the study of global biogeochemical cycles from space are as follows: 1. Develop computer simulation models of the biospheric cycles of carbon, nitrogen, sulfur, and phosphorus as a function of the state of the biota, climate dynamics, and interactions among these cycles. 2. To measure the concentration of chlorophyll-a in the world's oceans. 3. To measure the magnitudes of the terrestrial and oceanic sources and sinks file:///C|/SSB_old_web/seo91ch2.htm (30 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) for radiatively and chemically important tropospheric trace gases, in particular CO2, CO, CH4 and other hydrocarbons, N2O, NH3, (CH3)2S, H2S, OCS, and SO2. Current Status Work supported by NASA's Ecosystem Dynamics and Biogeochemical Cycles Branch and Biogeochemistry and Geophysics Branch, both within the Earth Science and Applications Division, specifically addresses this recommendation. These newly formed branches are focused on the following: Cycling of carbon and key nutrients within ecosystems, and between ecosystems and their abiotic environment; Identification of sources of radiatively and chemically active trace gases; and Quantification of major exchanges of these gases between the Earth's biosphere and its atmosphere. Progress over the previous five, years toward understanding the role of the ocean biota in global biogeochemical cycles of carbon, nitrogen, sulfur, and phosphorus has focused on efforts to quantify the effects of phytoplankton. In particular, the goal has been to study the spatial and temporal variability of phytoplankton biomass (as chlorophyll or "pigment" concentrations) and rates of primary production (carbon fixation). The principal satellite sensor for measuring phytoplankton was the CZCS on Nimbus-7. This sensor measured upwelling radiance in six narrow spectral bands in the visible to near-infrared range. These measurements were used to derive phytoplankton concentrations expressed as chlorophyll concentration. The CZCS differed from the land-oriented multispectral imagers, such as those on Landsat or SPOT, in that it had several narrow bands in the blue-green spectral region, much coarser spatial resolution, and gains set to accommodate the relatively low radiance levels reflected from the ocean. Because the CZCS was an experimental system, algorithms for interpreting the data evolved throughout the lifetime of the sensor. Since the end of the sensor's operational life in 1986, scientists at NASA's Goddard Space Flight Center, together with colleagues at the University of Miami, have reprocessed the entire CZCS archive to produce a self-consistent data set to be used by the oceanographic research community. These data have been distributed on optical disks to several NASA-supported facilities, which have been equipped with video disk systems for browsing the CZCS archive. The National Aeronautics and Space Administration has also supported work aimed at understanding the production and fate of calcite (calcium file:///C|/SSB_old_web/seo91ch2.htm (31 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) carbonate) particles produced by a group of phytoplankton species known as coccolithophores. Extensive blooms of these species have been observed as bright, highly reflective patches in CZCS and AVHRR visible-channel data throughout the North Atlantic. These species may play a significant role in atmospheric sulfur cycles as they produce dimethyl sulfide, which acts as a source of sulfate aerosols in the atmosphere. It has also been hypothesized that these organisms influence cloud formation over the ocean, and hence have a potential role in the global radiation budget. Anticipated Improvements The ability to quantify phytoplankton pigment concentrations in surface waters does not automatically translate into carbon fixation rates. A number of investigations have been conducted to derive algorithms for estimating primary productivity or photosynthetic carbon fixation rates from remote sensing data. These algorithms will be tested in the near term using data from an ocean color instrument called the Sea Wide Field Sensor (SeaWIFS) and later from the MODIS sensors on EOS. The SeaWIFS will fly onboard the commercial Sea Star mission, scheduled for a mid-1993 launch, whereby the contractor will sell the satellite data to commercial customers. NASA has agreed in advance to purchase data for distribution to the research community. Sensor Development for Remote Sensing of the Biosphere The 1986 SSB/CPB report had several recommendations concerning instrumentation: 1. Develop calibrated sensors capable of high spectral resolution measurement in the 0.4-µm to 2.5-µm region. 2. Develop calibrated, active microwave sensors at wavelengths from millimeters to 1 m. 3. Develop sensors to detect emissive infrared wavelengths in the 2-µm to 5.5- µm and 10-µm to 12-µm region. The flights of the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) over the past few years have met the objective for limited areal coverage in the 0.4-µm to 2.5-µm region. The HIRIS instrument, expected to eventually fly in the EOS program, would enable critical regional process studies. As discussed above, efforts are under way to build the SeaWIFS, which will have eight spectral bands ranging from about 400 nm to 890 nm. The additional bands will accommodate the need to differentiate substances other than phytoplankton that affect water color in near-shore regions. This addition is expected to improve our ability to study phytoplankton distributions in regions affected by terrigenous input. file:///C|/SSB_old_web/seo91ch2.htm (32 of 34) [6/18/2004 1:37:55 PM]

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Assessment of Satellite Earth Observation Programs 1991 (Chapter 2) There is currently no operational active microwave sensor operating at frequencies higher than about 5 GHz that is generally available to the scientific community. The successful implementation and calibration of the Jet Propulsion Laboratory's Airborne Imaging Radar provide limited areal coverage at the P- band (440 MHz, 70 cm), L-band (1.2 GHz, 25 cm), and C-band (5.3 GHz, 6 cm). The ERS-1, JERS-1, and Radarsat missions will provide valuable large-scale coverage, but they will have limited value for quantitative measurement because of their single-frequency limitation. The SIR-C, scheduled for Shuttle flights in 1993, 1994, arid 1996, will provide increased polarimetric coverage at regional scales for C-band and L-band, and X-band polarized data. However, there remain concerns related to the delay in the development of the EOS SAR for global-scale coverage. At this time there is no high-resolution infrared sensor operating in the 2- µm to 5.5-µm region that is available to the science community. Data in this region of the spectrum are expected to become available with flight of the ASTER instrument on the EOS-A satellite. Although the Thermal Infrared Multispectral Scanner (TIMS) acquires data in six channels between 8 µm and 12 µm, and ASTER will acquire data in several channels in this region, there is significant interest in higher spectral resolution measurements from both airborne and spaceborne platforms, and at the present time these are not planned. file:///C|/SSB_old_web/seo91ch2.htm (33 of 34) [6/18/2004 1:37:55 PM]