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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
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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
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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
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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.
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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
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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
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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
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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-
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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.
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