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7
EOS Platforms
Depending on the outcome of the question of simultaneity, are the EOS
platoons, as currently configured, the optimal means for collecting
data, or are Here better allernanves ~ are more cost effective or
timely? These alcoves could include, for example, smatter mul-
fiple platforms flying in formation or additional near~erm precursor
missions that are capable of flying subsets or preliminary versions of
EOS instruments.
As discussed in Chapter 6, scientific arguments for simultaneous mea-
surements have been developed by NASA for two specific research areas:
the role of clouds in climate and the fluxes of the trace gases. With regard
to these two cases, we conclude that the number of instruments that must
fly together requires at least one large satellite. Dividing the proposed
instruments for these measurements among several smaller satellites and
flying them in close formation is technically feasible, but the smallest co-
herent set of instruments for one of the smaller satellites is still sufficiently
large to require a launch vehicle larger than the Delta rocket.
The scientific requirements for continuity in data sets has led the
community of researchers and NASA to plan for a long time-series of
measurements. EOS plans call for a 15-year record of observations using
series of identical satellites, each with a 5-year lifetime, for each set of
measurements. Measurements to carry out the USGCRP emphasis on the
role of clouds and the fluxes of trace gases, for example, are planned for
a series of large spacecraft called the EOS-A series. It seems likely that
61
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scientific understanding and technical capabilities will change during the
course of the EOS program. Accordingly, although continuity of specific
data sets will be an important consideration, it may also be desirable to
alter the instruments or the platforms, or both, at some time in the future.
NASA has incorporated the design feature of common interfaces into the
EOS program. This feature should make changing instruments easier than
on many past missions. There is, however, no current plan for incorporating
new understanding, concepts, or technology into EOS as they evolve during
the life of the EOS program.
Scientific arguments for simultaneity in terms of the research objectives
of the second proposed, large EOS-B satellite have not been developed, and
it appears that these objectives could be achieved with a number of smaller,
independent satellites. NASA s current assessment of comparative costs as
presented to us, suggests that flying the projected EOS-B instruments on
a large platform is the least expensive option, although the differences in
cost among some alternative configurations appear to be relatively small.
In principle, the science investigations proposed for EOS-B could be done
by a suite of smaller satellites. Since a number of the instruments do not
require extensive development, these could perhaps be launched sooner.
Significant opportunities exist for gathering key global change data
though a number of U.S. and foreign research and operational satellite
missions, including the proposed Earth Probes series, prior to the scheduled
first launch of EOS in 1998. Some of the EOS instruments are intended to
continue monitoring certain environmental parameters so that the precursor
missions that fly similar instruments will be prerequisites, not substitutes.
Interim missions, including WARS, TOPEX, and the currently proposed
missions in the Earth Probes series are intended to gather data that are
essential for the USGCRP. It is our view that if budget constraints arise,
it would be preferable to delay the the launch of EOS rather than to
forego or diminish the effectiveness of these near-term projects. (See the
section below on Precursors, Small Missions, Earth Probes, and Operational
Systems.)
ENGINEERING CONSIDERATIONS
Four alternative configurations of satellites to carry EOS instruments
have been analyzed to date by NASA: the baseline mission comprising two
large satellites, EOS-A and EOS-B, on identical platforms; a mix of one
large satellite and three satellites of intermediate size; six intermediate
satellites; and 12 small satellites. Each satellite would be designed to last
five years and would be replicated twice, for a net mission of 15 years.
The large satellites would be flown on the Titan-IV launch vehicle, the
intermediate ones would be sized to fly on the Atlas-IIAS, and the small
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payloads could be launched on Delta-II rockets. The NASA analyses do
not account for the potential that some sets of satellites would have to fly
in close formation to achieve the desired degree of simultaneity.
Launch Vehicles
Space missions are constrained by the availabilities and capabilities
of the launch vehicles available for the purpose. NASA, in its analysis,
considered a large number of factors, including cost, mass, power, data
rate, launch vehicle, the ability of the launch option to satisfy the mission
requirements, the number of spacecraft required, launch schedule, produc-
tion schedule, operational complexity, data processing requirements, ability
to fit the instruments on existing spacecraft, and direct data downlink and
broadcast requirements.
The NASA analysis is constrained in two ways. First, there is currently
no planned Atlas launch capability for the Western Test Range. As a
consequence, the costs of a launch pad and the ground support crews
would have to be provided, adding to the overall cost of any program
that planned to use such a capability. It is not clear whether the launch
rate would justify maintaining the crews at the Western Just Range or
transferring them from the Eastern Test Range on demand.
Second, the Delta option could not accommodate some of the larger
instruments, such as HIRIS, ITIR, MLS, GLRS, and HIMSS, without
changes in the instruments or the spacecraft. Based on the NASA analysis,
the all-Delta scenario seems to us to be technically unrealistic.
Thus from consideration of the capabilities of the respective launch
vehicles, there are several discrete "levels" of capability rather that a
continuum. If, as discussed above, the scientific investigations or the
measurement capabilities to support them require the simultaneous flight
of sets of instruments, the designers are led at this time toward the large
spacecraft as the "optimum" configuration. Nonetheless, we believe it
would be prudent at this time to continue to consider a mixed launch vehicle
scenario so that the scientific return of instruments currently designated
for EOS-B can be increased or achieved sooner. The option should not be
eliminated solely on the basis of consideration of the launch vehicles.
The Platform Systems
There is no inherent risk in a structure of the size of the EOS-A
satellite. The major development risk lies in the subsystems and complex
interactions inherent in integrating and flying many instruments together.
Systems interactions in cooling, viewing angles, data management, me-
chanical and electronic noise, and other factors have complicated multiple
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instrument space missions in the past. These are engineering challenges
that can usually be identified and solved. Nonetheless, there is always a
risk that some interaction will not be anticipated and mitigated in advance.
This is not an issue that we could deal with in our deliberations, and it is
in any case never conclusively solved at this stage in the development of a
space mission.
The data system will require tape recorders that are a significant
extrapolation from current technology. While we accept the assertion that
the extrapolation is straightforward, past experience suggests that extra tape
drives should be supplied where they will be critical to the operation of the
mission. The experience in the NOAA TIROS series has been that half of
the tape drives fail on orbit before the end of the mission and usually early
in the mission. Providing extra tape decks, which is NASA:s current plan,
will require extra space on the spacecraft, but it can be supplied with the
larger platform.
Similar arguments can be made for most of the subsystems. In general,
the larger platform has greater capability to provide backup or redundancy.
Extra weight or volume capacity on the part of the launch system further
allows for the use of cheaper or less exotic materials and simpler designs,
and makes possible arrangements for passive cooling of several instruments
and unobstructed views of Earth by several instruments in relatively simple
ways.
The large platform also allows NASA to consider a direct downlink for
data as another service for users and as a backup for telecommunication via
the TDRSS, which is the baseline configuration. The user community has
derived much benefit from the use of low-rate, real-time direct broadcast
from the current meteorological satellites. The capability would be advan-
tageous for EOS instruments, and it is currently planned for the EOS-A
and EOS-B satellites.
CONIINUI1Y AND RELIABILITY
The USGCRP gives high priority to the establishment of an integrated,
comprehensive, long-term program of documenting the Earth system on a
global scale. The EOS will be a key component of that overall program,
which will include surface-based measurements as well. It is essential that
EOS be both comprehensive in its coverage of parameters in space and
time and that it provide for continuity of calibrated data for the most
critical of them.
Data continuity is particularly important when the required information
is used both to establish a baseline about global change and to monitor
change as it occurs. If the critical instrument is not in place when major
events occur, such as an E1 Nino or a major volcanic eruption, then
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opportunities to document and learn about the Earth system will have
been lost. Furthermore, identifying trends in highly varying data is always
compromised when a portion of the time series is missing. If data on
clouds or the radiative balance were missing for either extreme of the
solar cycle, for example, understanding of the related phenomena would
be severely degraded. Loss of continuity also severely compromises the
warning capability of the observing system. Consequently, the reliability
of EOS data is important to the USGCRP. Considering the potential for
long-term drift in instruments, the issue of calibration warrants particular
attention.
In our view, NASA has established reasonable criteria for the needed
reliability for the scientific mission. For example, the large spacecraft plat-
form is being designed to have a 75 percent probability of full capabilities
for its expected operating life of 5 years, as well as a 75 percent probability
of having 80 percent of its design power capability at the end of 7.5 years.
NASA's current requirement for the instruments is a probability of at least
85 percent that they will be working satisfactorily at the end of 5 years,
while for certain measurements of the facility instruments the requirement
is at least 90 percent. NASA's preliminary analyses indicate that these
probabilities are within reach and can be met.
Continuity failures are more difficult to quantify. Three types of
failures can occur, threatening the continuity of measurement: first, the
launch vehicle can fail. NASA:s plan for this contingency is to provide a
spare platform and set of instruments that would be available for launch
as soon as possible after the failure. In the interim, the lifetime of a
platform already operating in orbit could be extended until such time as
the replacement is launched. The longer the lifetime of an operational
platform is extended, the greater will be the likelihood that the continuity
of some data streams will be lost. If a failure occurs during the launch of
the first platform in either of its proposed series, NASA projects that the
delay for the beginning of that series would be 2.5 years. An interruption in
the time series of this magnitude would seriously disrupt USGCRP research
objectives.
The second type of failure is of the platform itself, once it is in orbit.
Platform systems, such as the attitude control system, data management
system, power supply, and telecommunications system, are essential to the
operation of all instruments. The least reliable element of the platform
infrastructure is widely regarded to be the tape recorders, six of which are
currently planned to assure that at least two are always operational. The
spare platform and set of instruments is intended to provide backup in the
event of a platform failure, but it is clear that continuity in some of the
data records would be lost. An interval would be necessary to determine
the cause of the failure, take corrective actions as required, and launch a
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replacement platform, even if the platform, instruments, and launch vehicle
were available at the time of failure.
The third type of failure is of an instrument. Most of the instru-
ments have a technical heritage in devices that have already been flown.
Nonetheless, several proposed instruments require significant advances in
technology. NASA currently estimates that development of the most com-
plex instruments on EOS-A will take between 42 and 60 months, with some
technological risk entailed.
One way to help assure the continuity of critical measurements in
the event of an instrument failure is to place redundant sensors on the
platform. The approach would, however, inevitably result in a reduction of
the size and scientific scope of the complement of sensors on the platform.
Another approach, applicable to either partial or catastrophic failures,
would be to provide so-called "hot spares" for critical instruments, spare
instruments, and launch vehicles ready to go with minimum delay. This
approach also has disadvantages: it is costly, and program managers are
likely to want to determine the cause of failure and take corrective actions
before launching a spare, increasing the interval of discontinuity of the
data. NASA estimates the cost of providing for a typical hot spare to
be in the $250M to $350M range. Further, depending on the instrument
and the acceptable degree of simultaneity with other measurements, close
formation flying may be required. If the lost instrument were to be in a
set requiring congruent measurements, the hot spare could not restore the
relationship.
A third approach, which might also apply in the event that an in-
strument is not ready at the scheduled time for launching, would be to
use existing instruments on the platform to provide backup. In a working
document entitled EOS Instnument Standard Data Products, NASA is as-
sembling backup strategies for each data product in the event of loss of
an instrument or channel in an instrument. The products being examined
include such parameters as biomass characteristics, broken ice distribu-
tion, carbon monoxide profiles, temperature profiles, ice sheet height, and
many others. Backup products would be degraded from the originals but
would be constructed from channels of other instruments to give crude but
workable data for gap-filling purposes. This approach and other aspects of
contingency planning are still under development by NASA
Finally, given the extensive remote sensing capabilities and plans of
other nations, full coordination of EOS with the foreign programs could
contribute to maintaining critical redundancies. NASA should place high
priority on such coordination.
Any approach to providing for the contingency of instrument failure
will entail balancing costs, technical capabilities, and other considerations
with the scientific goals of the program. The question of the reliability
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and continuity of data is central to the mission, so that careful contingency
planning Is unportant. We conclude that an important first action is to
identify the mission-critical sensors for which contingency plans have highest
priority. NASA considers the MODIS-N and -T as the "mission critical"
instruments for the first platform series, so much so that the agency is
prepared to delay the mission until these instruments are ready. We agree
with this assignment of priority, particularly since the MODIS instruments
are part of a suite that will provide significant benefits from simultaneous
measurement and which serve the highest priority scientific objectives. A
careful analysis needs to be done to determine whether there are others.
A related question is the need for companion instruments with any such
mission-critical instruments.
After the mission-critical sensors have been identified, the next step
is to identify ways in which an EOS instrument failure could be covered,
and with what degradation to the science, with other instruments that are
already flying. For example, the following types of questions need answers
for each standard data product. ~ what extent can a data gap in MODIS
be met with data from AVHRR, Landsat, and SPOT? How well can a gap
in observations by AIRS be met with data from HIRS and AMSU? How
well will ALT coverage be continued with ERS-2, U.S. Navy altimeters,
and others? What will be the alternative SAR coverage with the Canadian,
European, and Japanese missions?
In summary, NASA's current science strategy does not fully address
the issue of continuity of key data sets throughout the 15-year lifetime
of the mission. We conclude that NASA needs a contingency plan for
instrument failures. Changing scientific priorities may lead to different
designations of "mission-critical" instruments. The plan could distinguish
among instruments, but the scientific requirement for simultaneous, con-
gruent measurements, which provide the rationale for a large platform for
the highest priority science objectives, should be the guiding principle of
the plan. The panel's suggestion that the instruments for EOS-B could be
flown on a number of smaller satellites could also be used to help address
continuity and backup issues. The data continuity issue is so important that
it deserves continuing careful financial analysis and consideration.
NASA s analysis indicates that the large platform approach gives the
better overall reliability. Because this analysis is based necessarily at this
time on design criteria rather than actual designs, we find the prelimi-
nary analysis to be less than conclusive. Nevertheless, the NASA analysis
adds another argument to those already mustered in support of the large
spacecraft for the two highest research priorities.
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COSTS
At this time, NASAs cost estimates are not sufficiently precise to
support a conclusion about whether a configuration of one large platform
with several smaller complementary spacecraft is more or less costly than
the proposed two large platforms. The NASA cost estimate for an observing
system with a comparable payload that consists only of small spacecraft,
however, indicates that such a configuration would be roughly 50 percent
more expensive than either of the other two options.
The relative costs for four mission configurations as analyzed by NASA
are given in Figure 4. The analyses are normalized to the baseline concept
for two platform series. For each case, both an optimistic and a pessimistic
estimate are given. The analyses are based on NASA cost models and
the assumption that the same total set of instruments would be flown in
each case. The assumption is probably not valid. The costs associated
with contingency plans, which may differ among the configurations, are not
included either.
Aside from the totals, several features of these cost analyses are
striking. First, the vehicle costs in each case are a small fraction of the
total system costs.
At some number of satellites in orbit, which happens to just about
coincide with the number in a mixed fleet, an additional TDRSS is needed.
This is driven by the need for committed access to a high data rate channel
for several of the satellites. With the Atlas and mixed ELV scenarios,
additional costs will have to be met to sustain a dedicated launch capability
at the Western Test Range.
Obviously, the costs for integration and testing increase with the num-
ber of spacecraft. While the relationship is not linear, since the smaller
spacecraft are simpler, there is real increase just from the numbers. This is
especially true in the case of the Delta launch scenario where each space-
craft may have to be individually designed for each instrument to fit within
the shroud.
The costs of operations will also grow as the number of satellites
and the number of orbits are increased. Further, in the area of science,
atmospheric corrections or co-location of images will require increasingly
complex algorithms as simultaneity is lost and resulting costs increase.
Because the precision of the NASA estimates is not high, there is
little to choose in terms of cost between the baseline case, the mixed
case, and the lesser assessment of the all-Altas configuration. Questions
about the costs of establishing a polar orbit launch capability for Atlas-IIAS
rockets and providing the requisite ground support personnel suggest that
the differences in costs among the respective configurations may be larger
than the analysis indicates. Nonetheless, since the differences between the
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Optimistic Conservative Optimistic Conservative Optimism Conservative
BASELINE MIXED ATLAS DELTA
Note: Direct downlink communication aysteme not induded in the mixed, Atlas, or Delta options
FIGURE 4 NASA estimates of relative costs of alternative mission configurations with
identical instrument payloads. SOURCE: C. J. Scolese, NASA Goddard Space Flight Center,
April, 1990.
mixed scenario and the baseline are not large, we recommend that NASA
continue studies now under way to optimize the scientific return from the
instruments carried in NASA's strawman payload for EOS-B.
PRECURSORS, SMALL MISSIONS, EARTH PROBES,
AND OPERATIONAL SYSTEMS
The first of the EOS platforms is scheduled to be launched in 1998. In
the interim, significant opportunities exist for gathering key global change
data in precursor missions in the Earth Probes series. Also, the agency plans
to develop the EOSDIS as early as possible in order to make available to
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the research community other historical data sets and data streams from on-
going operational satellites that can be used to experiment with prototypes
of the data and information system in evaluating information management
concepts.
The scientific justification for several precursor missions, including
Earth Probes, and the use of data from existing operational systems (e.g.
Landsat and the meteorological satellites) was discussed earlier in Chapter
5.
Because Earth Probes are planned as smaller satellites with shorter
development times, they have considerable potential for providing high-
priority precursor measurements to EOS. They also can advance the time
in which some of the measurements critical to understanding global change
could be made (see the SSB/CES report, Strategy for Earth Explorers u'
Global Earth Sciences, 1988~. An important concern in the near term is
the discontinuity of key measurements such as global stratospheric ozone
levels, the Earth's radiation budget, ocean topography and winds; and the
biological productivity of the oceans, made by satellite missions launched
in the 1980s. Certain missions proposed for the Earth Probes line could
provide opportunities for extending those measurements until acquisition
of schemata sets is assumed by the EOS spacecraft. However, there is no
Earth Probe mission proposed to fill the gap in Earth radiation budget
measurements, as mentioned above. Certain other Earth Probes missions
could also provide information on parameters not measured before on a
global scale, e.g., the Earth's gravitational field and tropical rainfall.
There are other opportunities for early flight and for providing conti-
nuity of currently important measurements that do not seem to be as well
exploited as they might be. These are flights of opportunity on already
planned missions. Satellite series like the NOAA polar orbiters and the
satellites of other nations can provide opportunities for modest instruments.
The flight of the SBUV instrument on the NOAA polar orbiting series and
the planned flight of the TOMS instrument on a Soviet meteorological
spacecraft are instructive examples.
Several important existing measurement capabilities are candidates for
such "piggy-back" flight. They include flying another ERBE on the NOAA
series, an active cavity radiometer on almost any satellite series, and an
ocean color instrument. The contributions of these measurements to the
objectives of the USGCRP are described in Chapter 5.
In addition to possibilities for precursor-or interim flight of selected
instruments, some measurements could be better made, or must be made
from other orbits. For instance, biological processes and radiation studies
related to cloud motion require sampling at various times of day, which
cannot be made solely from sun synchronous polar-orbiting spacecraft such
as EOS. ~ the extent that these measurements are critical to achieving
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the objectives of global change research, the Earth Probe line can pro-
vide a flexible mechanism for such observations from more appropriate
orbits. During the EOS era, Earth Probe missions will also be essential in
complementing the EOS measurements by flying instruments that may be
incompatible with the design and orbit of EOS platforms. It is expected
that the Earth Probes will be augmented by foreign spacecraft, some of
which could provide flight opportunities for U.S. instruments. For instance,
discussions are currently under way to position the Japanese spacecraft
contribution to EOS in a lower inclination orbit. A plan is needed to
determine how to coordinate such possibilities with the USGCRP.
Of the several instruments that can be better flown in a different orbit,
we believe that the Synthetic Aperture Radar is of prime importance. If
its technological development is successful it will supply two quantitive
measurements unavailable on a global synoptic scale in any other manner.
These are soil moisture and an estimate of biomass in a standing plant
canopy. Both of these measurements are important for the two top priority
scientific areas, i.e., the hydrologic cycle and fluxes of atmospheric gases.
The SAR should be considered for inclusion in the EOS program as a free
flier as soon as possible.
Existing U.S. operational systems supply both interim data and a con-
tinuing contribution to global change research. They include the NOAA
polar orbiting and geostationary meteorological satellites, the Defense Me-
teorological Satellite Program (DMSP), and the Landsat system. Many
scientific studies proposed for EOS assume the continuation of these mea-
surements. For instance, the top priority study area the role of clouds in
the hydrologic cycle-assumes the continued measurement and mapping of
clouds by the meteorological spacecraft. The second priority area, fluxes
of atmospheric gases, assumes the continued global mapping capability of
a Landsat system intermediate in capability between MODIS and HIRIS.
NOAA and NASA are jointly planning for flight of atmospheric
sounders and for a common interface for a number of the instruments,
an approach that we endorse. Further, the European partners in EOS
are planning to fly a polar orbiting platform with a morning crossing of
the equator to cover the requirement for operational weather data, again
with a common interface. This continuing use of operational data and
integration of the hardware approaches will benefit scientific investigations
of the future.
These research and operational missions are complementary to, not
replacements for, the main EOS missions. They will improve the scientific
return of the space-based Earth observing program, and they can help to
ensure continuity of key observations for the next decade or longer. Some,
such as SeaWiFS, are important for the success of planned international
field programs that will improve understanding of global change.
Representative terms from entire chapter:
launch vehicles
