The Earth Observer

--Mark R. Abbott (mark@oce.orst.edu), Chair

Introduction

NASA continues to search for ways to reduce the size, cost, and development time for its flight missions, to increase the rate of satellite launches, and to develop new technologies. These initiatives have helped to reshape NASA flight missions in all areas of science, including the post-2002 series of spacecraft in the Earth Observing System (EOS). These initiatives are interrelated in that smaller missions may be more cost-effective and provide more-frequent access to space. Reductions in the volume, mass, and power required by science payloads enable the use of smaller spacecraft and launch vehicles. The challenge is to achieve the needed reductions in spacecraft resources without jeopardizing the measurement capabilities required to achieve mission science objectives. The infusion of new technologies into the design and building of instruments and the spacecraft that carry them offers one means of meeting this challenge.

In response to these NASA initiatives, the Mission to Planet Earth (MTPE) Office at the Goddard Space Flight Center (GSFC) undertook a study to assess the feasibility of using new technologies and a new approach to implementing the EOS Series 1 Chemistry (CHEM-1) mission. The launch of CHEM-1 is baselined for December 2002, and its experiment teams are just starting detailed design and fabrication of the science instruments. Thus, there was time to consider whether an aggressive approach to infusing new technology could significantly reduce the resources required by the CHEM-1 instruments and so achieve substantial cost savings, possibly through multiple launches on smaller spacecraft. Investigating alternative launch scenarios was deemed particularly valuable because, at the start of the study in February 1996, the legal challenge to the contract award for procuring a common spacecraft for both the EOS PM-1 and CHEM-1 missions had not been adjudicated.

The GSFC CHEM-1 Study lead was Charles Vanek of GSFC, who was supported by the GSFC directorates and the EOS (CHEM-1) Project Staff. Science participation included: the Science Investigator Teams of the three U.S.-funded instruments (one funded jointly with the UK); Jim Gleason (CHEM-1 Project Scientist), Rich Zurek (Chair of the IWG Atmospheres Panel), P. K. Bhartia (TOMS Project Scientist), and Mark Schoeberl (EOS IDS PI). Preliminary results of the study were presented in late March to the GSFC management and in early April to the NASA MTPE (Code Y) Management. Mark Schoeberl also made a presentation to the Code Y Earth System Science and Applications Advisory Committee (ESSAAC) on April 19.

The GSFC CHEM-1 Study results were presented to the EOS Payload Panel on May 15, 1996, at a meeting appended to the Investigators Working Group Meeting, May 13-15, in Greenbelt, MD. Overviews of the study objectives, the impacts on science, and possible mission scenarios for deploying the CHEM-1 instruments were given by Al Diaz (GSFC Deputy Director), Rich Zurek (Atmosphere Panel Chair), and Peg Luce (EOS Chemistry and Special Flights Project), respectively. These were followed with presentations by the Principal Investigators of the instruments involved: John Gille and John Barnett, High-Resolution Dynamics Limb Sounder (HIRDLS, a US-UK collaboration); Reinhard Beer, Tropospheric Emission Spectrometer (TES), and Joe Waters, Microwave Limb Sounder (MLS). The Ozone Dynamics Ultraviolet Spectrometer (ODUS), provided by NASDA to NASA in exchange for the flight of the SeaWinds instrument on ADEOS-2, was not reviewed in this study, although it was necessarily included when considering various spacecraft options.

Study Guidelines and Methodology

The study guidelines were to aggressively pursue new technology options and to achieve substantial resource reductions, while maintaining the essential scientific measurement capabilities and a launch date no later, and possibly earlier, than the original December 2002 date.

Instrument teams were challenged to meet targets of 50% reductions in cost, weight, and power requirements. Teams were encouraged to embrace new technical approaches where feasible, to remove excess margins, and to streamline and shorten development schedules. When instrument costs were assessed, allowance for the cost of technology development was included. The instrument measurement capabilities were scrutinized for duplicate capabilities and were judged against the suite of specific observations required to meet the EOS science objectives for the CHEM-1 mission.

CHEM-1 Science Objectives

The EOS CHEM-1 science objectives are derived from requirements addressing five areas of the EOS 24 Measurements: (i) Tropospheric Chemistry, (ii) Stratospheric Chemistry (emphasizing ozone and its precursors), (iii) Atmospheric Temperature, (iv) Atmospheric Humidity, and (v) Aerosol Properties (stratospheric). These five areas map directly and indirectly into four themes for the EOS CHEM-1 mission: (i) Tropospheric Chemistry, (ii) Stratospheric Chemical Cycles, (iii) Transport and Dynamics, and (iv) Long-Term Trends. These themes place special emphasis on beginning a global survey of key tropospheric chemical species, on acquiring global measurements of meteorological fields, and on obtaining distributions of trace gases and aerosols in the upper troposphere and lower stratosphere.

Launch of the CHEM-1 instruments in or before 2002 is critical to establishing long-term trends in key chemical species, by shortening the gap with present measurements, such as those of the continuing, but aging, Upper Atmosphere Research Satellite (UARS). Furthermore, the peak in stratospheric chlorine concentrations is projected to occur at the turn of the century, and observations are needed to capture the possibly nonlinear response of the ozone layer to these high chlorine concentrations. The many policy issues involving tropospheric chemistry will benefit from an early global survey of key tropospheric trace gases, and understanding the potential links of stratospheric ozone and upper tropospheric water vapor to climate change requires observations with greater precision and spatial resolution than are currently available. Thus, the National Academy of Sciences (NAS) Board on Sustainable Development recommended that the CHEM-1 stratospheric and tropospheric measurements not be delayed [Ref: "A Review of the U. S. Global Change Research Program and NASA's Mission to Planet Earth/Earth Observing System, " NRC, 1995].

General Findings of the CHEM-1 Study

Major reductions in the cost of the CHEM-1 mission could be achieved only if at least one instrument were deleted from the CHEM-1 payload(s). However, this would result in an unacceptable loss of science. Given past scrutiny of the CHEM-1 payload, e.g., see the report of the November 1995 Payload Panel Meeting, it was not surprising that the present study confirmed that deletion of any instrument or any descope that significantly reduced any instrument's observational capability would result in an unacceptable loss of science. Each instrument contributes in a major way to at least one, and usually more than one, of the EOS CHEM-1 science themes. Furthermore, in those cases where more than one instrument measures a given field, e.g., temperature (T) or ozone (O3), they do so with different spatial resolutions, coverage, instrument biases and sensitivities to clouds and aerosols, or they require nearly simultaneous and co-located measurements, e.g., T or water vapor, to retrieve other, instrument-unique fields.

For the Microwave Limb Sounder (MLS), rapid advances in the development of Microwave Monolithic Integrated Circuits (MMICs) at ever higher frequencies presents a real opportunity for reductions in the mass (380 kg 255 kg) and power of one of the largest single EOS instruments. The use of these devices could permit a small array of detectors that would provide multi-azimuth limb scanning for some of the fields measured by MLS; microwave frequencies have the advantage that they are less sensitive to the ice clouds and aerosol layers in the upper troposphere and lower stratosphere.

No such breakthrough is available for HIRDLS or TES, which observe infrared radiation. These instruments are incorporating the use of improved infrared detectors, coolers, and composite materials into their instrument designs. Some mass and cost savings were identified; these are discussed below in detail. The major reductions in the weight of the MLS and pro-posed smaller reductions in HIRDLS (161 kg 110 kg) and TES (290 kg 255 kg) do enable launch on smaller spacecraft. The use of multiple launch vehicles provides opportunities for earlier launch of some of the EOS CHEM-1 instruments. [Note: The original instrument masses quoted here already take into account reductions identified in preliminary design work.]

The proposed modifications to MLS, HIRDLS, and TES increase technical risk to the extent that new technologies are being considered for MLS, and all three instruments have less margin in weight and power. The study concluded that overall these risks could be managed appropriately. Furthermore, the benefits of technology infusion, e.g., enabling of cross-track limb scanning for MLS and the use of MMIC technology that will undoubtedly be used in later missions, and of potential deployment on small spacecraft, e.g., enabling earlier launches and protecting against loss of the entire payload due to a single launch failure, were considerable.

Instrument Changes Recommended

MLS would measure the same fields in the same spectral intervals as previously proposed. The higher frequency channels at 240 and 640 GHz would continue to use the planar diodes originally proposed, but the 112 and 190 GHz channels would use MMIC-based devices. The small size and weight of these devices permit, with little mass and power penalty, sparsely populated MMIC arrays to be used to provide five or more azimuths, greatly increasing the horizontal resolution of the MLS measured fields of T, P, H2O, ClO, O3, SO2, N2O (a long-lived trace gas), and HNO3.

One of two technical approaches to implementing the MLS 2.5 THz channel which measures OH, involves a high-tech high Tc superconducting hot electron bolometer in conjunction with a solid state laser and photomixer local oscillator. These approaches were already being evaluated by the EOS project prior to this CHEM-1 study.

HIRDLS proposed to reduce its mass by removing the outer housing of the instrument, and downsizing the instrument by reducing its optical aperture (telescope and optical bench). The resulting lower signal-to-noise was originally thought to affect measurements mainly in the mesosphere and upper stratosphere, requiring more spatial averaging of measurements of these still important, but lower-priority, regions. The lower signal-to-noise would also degrade the vertical resolution of measurements to be made in the upper troposphere and lower stratosphere, though HIRDLS would still achieve better resolution than that achieved by present instruments, e.g., on UARS.

However, more-recent and detailed algorithm simulations indicate significant degradation in the precision of the retrieved temperature and some species concentration fields in the critical upper troposphere and lower stratosphere. Roughly 60% of the mass savings for HIRDLS are achieved by eliminating the outer shell; the factor of 2 in increased noise only saves about 20 kg in mass. Investigations reconfirmed that potential new material and detector technologies were not mature and would raise costs by requiring substantial development efforts.

This proposed downsizing of HIRDLS retains the innovative horizontal limb-scanning capability of the instrument and the complement of trace gases and aerosols that can be measured. A more-radical descope of the instrument would likely jeopardize the participation of the UK in this international collaboration. However, because of its loss of capability in the critical tropopause region, this option was rejected.

TES proposed to implement changes to its electronics assembly and to replace its dual coolers with a single cooler; these options had been identified in studies conducted in the previous year, as part of the assessment of the TES tropospheric capability [Ref: "Tropospheric Chemistry: Measurement and Modeling Strategy," Proceedings of the GISS Workshop, November 7-9, 1995]. As had been reported previously, more-severe modifications of the TES instrument, e.g., replacement of the Connes interferometer with a Michelson device, would jeopardize the central TES measurements of tropospheric ozone and nitrogen species. Thus, this option was rejected.

TES also proposed transferring some of its science budget to augment instrument development. This proposal raised the issue of whether the TES data production algorithms would be ready at launch, particularly if the instrument were launched as early as December 2001.

Recommendations by the Payload Panel: Instruments

• MLS should proceed with implementation of the MMIC technology in its lower frequency channels, with normal and full evaluation by the EOS Chemistry and Special Flights (C&SF) Project of technical risk, including potential impacts on cost profiles and schedule.

• MLS should incorporate the proposed MMIC detector/spectrometer array (with 5 or more azimuths) to achieve multi-azimuth limb scanning for the lower frequency channels, subject to normal evaluation by the EOS C&SF Project. Because of the relative insensitivity of the microwave measurements to the presence of ice clouds and of (volcanic) aerosols, this high horizontal resolution scanning provides a new and desirable capability in pursuit of EOS science goals.

• HIRDLS should not implement the proposed aperture reduction as it will jeopardize key measurements in the upper troposphere and lower stratosphere, a region of high scientific priority. Mass savings due to removal of the instrument's outer shell appear to have minimal impact on science and are therefore acceptable.

• TES and the EOS C&SF Project should continue evaluation of the use of single or dual coolers, including impacts on instrument optical alignment and thus on the schedule for instrument delivery. The schedule and budget for delivery of TES data products should be reviewed by the EOS Project Science Office, and a strategy developed for accommodating potential early launch dates.

• The Panel agrees that there is unlikely to be significant cost savings associated with these instrument changes, given the redesign and technology development activities necessitated by the changes. The Panel concurs that deletion of an instrument or a major descoping beyond the changes recommended here would have a major impact on the CHEM-1 mission science and would result in an unacceptable loss to EOS science.

• The Panel notes that the major risk to the CHEM-1 science may not be technical, but programmatic, in that prolonged delay of the start of instrument fabrication and of algorithm development poses significant schedule risk. The EOS C&SF Project needs to ensure that cost profiles, as well as total cost and instrument delivery margins, are adequate. Furthermore, a decision is needed very soon on which spacecraft will be used to deploy the various CHEM-1 instruments (see below).

• The Panel strongly recommends that the EOS C&SF Project not perturb the risk-mitigation strategies now being pursued by the various instrument activities. This includes development of engineering models, test and delivery of processing software, pre-launch sensor characterization, etc. Attempts to save money in these critical areas should not be pursued at the expense of increased risk of sensor failure or poor performance on orbit.

Potential Mission Scenarios

The GSFC CHEM-1 Study identified several mission options, with different combinations of MLS, TES, HIRDLS, and ODUS deployed on various sets of the PM-1/CHEM-1 Common bus and/or smaller spacecraft. Additional options are still being considered and costed; one of these involves a smaller version of the Common spacecraft, discussed with TRW after the dismissal of the challenge to the procurement of the Common spacecraft. These options (still preliminary) are shown in Table 1. Cost data for the various mission scenarios are still being acquired and evaluated.

Table 1. Potential Mission Scenarios

MISSION OPTION	INSTRUMENT PAYLOAD	SPACECRAFT & ELV CONFIGURATION	LAUNCH DATES
Baseline	HIRDLS, MLS, ODUS, TES	Common S/C Delta 7920	12/02
1	HIRDLS, AMLS*, ODUS, TES [+400 kg, 250 W]	Common S/C Delta 7920	6/02
2	HIRDLS, AMLS, ODUS [+700 kg, 600 W], TES	Common S/C Delta 7920 Small/Taurus	6/02 9/01
3	TES, HIRDLS, AMLS, ODUS	Small/Delta Small/Delta	9/01 6/02
4	HIRDLS, TES, ODUS, AMLS	Common-Lite** Delta 7320 Small/Taurus	12/01 6/02
5	TES, HIRDLS, ODUS, AMLS	Small/Taurus Small/Taurus Small Taurus	9/01 3/02 12/02

* AMLS=MMIC-array version of MLS
** Common-Lite = smaller version of Common S/C bus (deck modules removed)

One consequence of the downsizing of the CHEM-1 instruments, through the infusion of new technology into MLS and through more-conventional descopes of HIRDLS and TES, is that the deployment of the CHEM-1 payload on the Common spacecraft bus leaves considerable mass and power resources for the flight of other instruments. These resources could be used to accommodate EOS instruments now slated for flights-of-opportunity or other instruments flown as part of other programs, including the Earth System Science Pathfinders (ESSP) or New Millennium Program (NMP).

Inclusion of such instruments is not free, due to integration costs. The best use should be made of these "extra resources," which may be as great as 700 kg & 600 W additional capability; see Table 1. However, the EOS measurement capabilities must not be jeopardized, whether by driving spacecraft requirements, by imprudently severe reduction of margins, or otherwise diverting EOS resources.

Science Implications of Splitting The CHEM-1 Payload

In the past, the Payload Panel has opposed moving the CHEM-1 payload onto multiple spacecraft. Concerns typically focused on two issues: (i) the feared loss of measurement capability if one or more instruments did not fly in the same time frame, and (ii) the loss of nearly simultaneous coverage. It is important to note that the instruments would not have co-located fields-of-view, even if they were on the same platform. However, the CHEM-1 instruments would sample overlapping regions within minutes of one another's observations and would observe at similar local times, if deployed on a single platform.

The synergy between HIRDLS and TES is that the stratosphere is a source for tropospheric ozone and some nitrogen species. With its high-spatial-resolution observations, HIRDLS can capture much of the temporal variation of this influx to the troposphere, while TES observes the variation in the lower troposphere itself. The addition of ODUS, with its column measurements of ozone, together with the stratospheric ozone profiles provided by HIRDLS or MLS, also complements the TES measurements of tropospheric ozone. For HIRDLS and MLS, the synergism is that they measure different parts of the chemical cycles involved in ozone destruction.

Experience with aircraft data and with UARS observations has relaxed some concerns that understanding atmospheric chemical processes requires essentially simultaneous measurements of the key chemical species. Often, understanding the chemical processing of air requires knowledge of the environments encountered by the air parcel being observed, in addition to its present trace-gas composition. The ability to use available wind fields, derived from conventional meteorological data, to assess the effects of transport, together with the constraints provided by observing long-lived tracers, permits the chemical modeler to synthesize observations taken at different times and places.

This at least holds true for the lower stratosphere at mid to high latitudes, where characteristic spatial scales and variability can be captured by fields mapped globally once or twice a day by the satellite. In the upper troposphere, changes may be more rapid and involve smaller spatial scales, but the horizontal and vertical resolutions provided by HIRDLS and the array-MLS are much improved over past and present capabilities, e.g., UARS. Finally, the measurement by both HIRDLS and an array-MLS of long-lived tracers, e.g., N2O, with good horizontal resolution provides a standard of comparison for their uniquely measured species concentrations, e.g., NOx for HIRDLS and ClO by MLS.

Deployment of the CHEM-1 instruments on multiple spacecraft has the advantage that earlier launches of some instruments are possible, if budget profiles allow, and different instruments could be deployed at different times of day. The TES, for example, would prefer a late morning, as opposed to afternoon, orbit to increase the number of cloud-free fields-of-view as it measures tropospheric trace gases in its nadir mode. Furthermore, formation flying of small satellites could provide adequate near-simultaneity of several minutes in orbits at the same local time.

Recommendations of The Payload Panel: Mission Implementation

The Payload Panel recommends:

• That the EOS C&SF Project evaluate various mission implementation options first on the basis of cost savings, provided that the implementation does not delay launch of any instrument beyond the 2002 baseline, and that all of the CHEM-1 instruments will operate simultaneously from orbit for at least a four-year period.

• That the possibility of formation flying of multiple satellites carrying CHEM-1 instruments be evaluated as part of the various mission implementation studies. This evaluation should consider all costs and risks, including launch vehicle performance, platform pointing stability, etc.

• That TES be launched early, if feasible, to begin the high-priority global survey of key tropospheric chemical species. This assumes that the processing algorithm activity proceeds in step with the instrument development.

• That, in declining order of science preference, the mission options are: (i) all instruments on the same platform or in formation; (ii) TES, HIRDLS and ODUS on the same platform or in formation; or (iii) TES and HIRDLS together, whether on the same platform or on platforms flying in formation. These preferences should be weighed against potential cost savings and the desire for an earlier launch of TES (see above).

• That, should there be separate launches of HIRDLS and the array-MLS, there is preference for moving HIRDLS first, in part to provide some schedule margin for the development of the MMIC-based devices or terahertz technologies required by MLS and also to achieve an earlier pairing of TES and HIRDLS on orbit.

• That "divertible resources" not be created at the expense of jeopardizing, e.g., through reduced margins or instrument descopes, the approved EOS science measurements.

• That, if "divertible resources" are available, their use be based on the overall advantage to the EOS program, and first consideration be given to EOS instruments needing flights of opportunity, e.g., SOLSTICE, and second to the advantage of the MTPE program. This assessment must include the broader Earth science community, together with the Mission to Planet Earth Office at GSFC and NASA Headquarters.

Final Comments

The Payload Panel appreciates the efforts of the GSFC CHEM-1 Study team and, in particular, the contributions by the EOS Chemistry and Special Flights Project personnel and EOS investigators. The need to respond to NASA initiatives attempting to preserve and enhance timely access to space and to provide the scientific observations vital to Mission to Planet Earth in an era of declining federal research budgets is understood. The present study has identified an infusion of new technology and instrument changes which enable launch on the Common (or derivative) bus with significant spacecraft margins or on multiple spacecraft, including small spacecraft, while maintaining and possibly advancing the launch date(s) and the overall science.

Scrutiny of the technical risks associated with the recommended changes must continue to be evaluated by the Mission to Planet Earth Office. Close interaction of the science community and the GSFC engineering assessment teams will continue to be essential to the success of EOS. The Payload Panel further notes that a decision on mission implementation is needed soon to avoid further delays that may increase costs and risk, and possibly impact launch dates.

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