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Review oj~ Scientific Aspects of the NASA Triana Mission INTRODUCTION In a letter of October 14,1999,1 the National ResearchCouncil (NRC) was asked to evaluate the scientific goals of Triana, as specified in House Report 106-379! Accordingly, the l-.IRCestablishedthe Task Group on the Review of Scientific Aspects of the NASA Triana Mission] (referred to here as the task group) under the auspicesof the Space Studies Bo(lfd (SSB), the Board on Earth Sciencesand Resources(BESR), and the Board on Atmosp.heric Sciencesand Climate (BASC). The charge to the task group was to review (1) the e:xtentto which the mission's goals and objectives are consonant with published science strategies and priorities, (2) the likelihood that the planned measurementscarl contribute to achieving the stated goals and objectives, and (3) the extent to which the mission can enhanceor complement other missions now in operation or in development. The task group met on January 12 and 13,2000, at the National Academies' Georgetown officl~s in Washington, D.C. Prior to this meeting, it held two teleconferences to discuss the charge to the task group and plans for the meeting, and it also reviewed all relevant NRC reports, relevant government reports, and background materials.4 On the fIrst day of the meeting, the task group received presentations from NASA 's Triana s<:ience team, among others.5These presentationsdiscussedthe technical aspectsof the mission, including the science goals and objectives, data products, and instrument specifications and included a variety of opinions regarding the mission. One presenter made a number of recommendationsto improve the sciencereturn from the mission, includin~~significant redesign of the mission, as well as changesin the science team and data analysis efforts. For example, he proposed postponing the mission "to allow the science analysis efforts to catch up and to possibly reverse some of the downgrades to as~.ure successful scientific Triana mission that achieves its stated a scientific objectives." The task group discussedthese recommendations and concluded that several of them were beyond its statementof task; others are adequately covered in this report.6 1 See Appendix I. 2 This conference repDrt accompanied the V A-HUD-Independent Agencies appropriations bill for FY 2000, P.L. 106-379, Title III, p. 158, enacted October 13,1999. 3 See Appendix 2 for the task group roster. 4 Valero, Francisco P.J., Jay Herman, Patrick Minnis, William D. Collins, Robert Sadourny, Warren Wiscombe, Dan Lubin, and Keith Ogilvie, Triana-a Deep Space Earth and Solar Observatory, NASA background report, December 1999. Available at posted as pdffile. s See Appendix 3 for the agenda. 6 This report has been reviewed by individuals chosen for their diverse perspectives and technical expertise in accordance with procedures approved by the NRC's Report Review Committee. See Appendix 4. 1

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GENERAL MISSION DESCRIPTION Previous aJldexisting solar and magnetosphericmissions demonstratethe suitability of Lagr:mgian point 1 (L 1)7 as a unique and opportune deep-spacelocation for solar and spaceobservation.8Triana was proposed as an exploratory mission to investigate the sci(~ntific and technical advantagesof L 1 for Earth observations. It will have a continuous and simultaneous view of the sunlit face of Earth that is not possible to achieve with low I~arth orbit (LEO)9 or geostationary Earth orbit (GEO)!O satellites. Triana is intended to provide a global synoptic view of Earth. It is designed to make measureme11lts a range of spectral channelsto observe spatial and temporal in variations in Earth's geophysical parameters,such as ozone, aerosols, clouds, and surface ultraviolet (UV) fluxes. Triana is designedto measureozone and cloud distributions to enhancestudies of their effects on climate and the amount of UV radiation that reaches the ground. The v(~getationcanopy structure is also intended to be observed in order to contribute to monitoring the status of Earth's vegetation. The global aerosol optical thickness!! will be measuredto increaseknowledge of how pollution generatedby humans and as a r(~sultof natural processesaffects Earth. Simultaneously, instruments on Triana are designedto determine Earth's planetary albedo in three regions of the spectrum-broadband long wave, near-infrared (IR), and UV/visible-to better characterize Earth's radiation budget. These measurementswolJld provide the fIrst direct determination of the radiant power emitted by the full sunlit disk of Earth in the direction of the Sun (i.e., Earth' s radiance from which planetary albedo is determined by ratioing to solar irradiance), and therefore increaseresearchers' understanding of how much of the Sun's energy is absorbed in the atmosphere. In addition to Earth-viewing instruments, Triana includes an instrument package designed to meaS1JIe solar wind and the interplanetary magnetic field at L 1. Based on these observations Triana, during its limited lifetime, could provide early warning (about 1 hour) to commutrication satellites and ground-basedsystemsthat are susceptible to solar-related disturbances during spaceweather events. Triana imagery and science data would also be ma OCR for page 4
Instrumentation To accomplish its science goals, Triana has three instruments: the Scripps-Earth Polychromatic Imaging Camera (EPIC), the Scripps-National Institute of Standardsand Technology (NIST) Advanced Radiometer (NISTAR), and the Goddard SpaceFlight Center (GSFC) Plasma-Magnetometer Solar Weather Package(Plasma-Mag). EPIC The EPIC i][lStrumentis designedto provide ozone, aerosol, and cloud reflectivity data for the full surllit disk of Earth. EPIC is a framing camera with a charge-coupled detector (CCD) foc:al plane array that will image the whole Earth disk from the L1 vantage point. The size of the array, 2048 by 2048 pixels, coupled with the Cassegrain telescope of 30.5-cm aperture and 282-cm focal length (f9.38),8rovides a nominal spatial resolution of about 8 bl8 km for pixels viewed at nadir, yielding a ground- projected pixel area of 64 km .When observationsapproach the edge of the Earth disk, the effective pixel :;ize grows and the pixel changesshapeas Earth's surface tilts away from the instrument. At 70° view zenith angle, the nominal pixel area is 187 km2; at 800, the nominal pixel size is 369 km2. The changing size and shapeof the pixels at the edge of the disk will de~;radthe effective spatial resolution of the measurements.The effective spatial resolution is somewhat coarser due to the point-spread function of the optics, which is expected 10be about 10 by 10 km (nadir). Earth's illuminated disk is expected to occupy about 60 percent of the array. The Epic c~unera' CCD array, operatedat -40° C, has a high quantum efficiency s beginning at about 250 nanometers(nm), thus permitting imaging in wavelengths from the UV to the near..IR. Through the use of a filter wheel fitted with filters whose surfaces are hardened by ion-assisted deposition, the camerarecords images of Earth in 10 spectral bands (Table 1). Shutter speedsare programmable to adjust for the wavelength- dependent sensitivi.ty of the camera's detectors and for in-band scenebrightness. The digital intensity conversion provides 12 bits of precision (0 to 4095) in the output signal. The signal-to-nois(: ratio of the array's detectors is designed to equa1200:1 at median signal intensities. fI,1easured, calibrated radianceswill be observed hourly for bands 1 to 5 and 9 to 10, and every 15 minutes for bands 6 to 8. These radiances will be Earth-located by attaching a latitllde and longitude tag to each pixel. They will be archived in Earth Observing System -Hierarchical Data Format (EOS-HDF). The Triana science team intends to calibrate this instrument before it is launched and to track its calibration in flight when the cameraviews the far side of the Moon as it comes between L 1 and Earth. This event occurs about once per month and permits the monitoring of detel:tor and filter degradation for the life of the mission. The technique assumesthat the M[oon's surface has a highly stable brightness and can thus be used as a reflectance standard. )2 At the nadir view, the instrument looks directly "down" at the surface from directly above the surface- that is, at an angle perJ>endicularto the surface. 3

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TABLE 1 EPIC's Filters ~pe~i~~~tions- Center Bandwidth Previous Space Band Waveleng;th (nm) Flight Heritage Frequency Purpose (nm) - I 317.5 I TOMS I hour Ozone 2 325 I TOMS I hour Ozone, SO2 3 340 3 TOMS I hour Aerosols 4 388 3 TOMS I hour Aerosols, clouds 5 393.5 I (New) I hour Cloud height 6 443 (bluc:) 10 MODIS 15 minutes Aerosols 7 551 (green) 10 MODIS 15 minutes Aerosols, ozone 8 645 (red) 10 MODIS* 15 minutes Aerosols, vegetation 9 870 15 MODIS I hour Clouds, vegetation 10 905 3~ --MODIS I hour Precipitable water *The MODIS band has a 50-nm bandwidth. NISTAR The balance between incoming radiation from the Sun (in the near-UV , visible, and near-IR regions of the spectrum) that Earth reflects and absorbs,and radiation outgoing from Earth to space (in the thermal infrared spectrum) determines the budget of energy available for climate processes.By providing the fIrst determination of the radiation reflected and emitted by the full sunlit disk of Earth in the direction of the Sun, the NISTAR instn:lll1entat Ll can contribute to researchers'knowledge of this radiation balance. NIST AR is a suite of four radiation detectors mounted together with a filter wheel, shutter wheel, front-end baffles, and rear-end control and detection electronics, and boresight aligrled with EPIC. Three of the four detectors are absolute devices, called electrical substitution active cavity radiometers,13 which measurethe integrated power from a single sourc:eof radiation (i.e., irradiance), in this caseEarth as a planet. The NIST AR instrument is designed so that during a typical observation of Earth's radiation flux, two filters in the filter wheel placed over two of the three active cavities permit the measurementoft\\'o bands of radiation (from 0.2 to 4 J.tmand from 0.7 to 4 J.1In) while an open position in th.efilter wheel admits the entire radiation spectrum at all wavelengths. Becausethe time rt~sponse active caVity radiometers is on the order of 3 minutes, a of fourth channel ofl'~IST AR contains a photodiode that has a much faster time response but inferior accura(~y and stability. In addition to providing higher time resolution, the photodiode channel permits in-flight measurementsof the transmittances of the filters (which can be positioned over the cavities or the photodiode). NISTAR is designed to use the in-flight filter transmittance measurementsand periodic use of redundant filters to track the stability of the radiation flux measurementsthroughout the mission. Preliminaf)' laboratory operations indicate that the goal of 0.1 percent accuracy and noise levels of 10 nW are attainable. Stabilities are unknown, but NIST reported that it has made efforts in the design ofNISTAR to minimize drift and to monitor in-flight the I3 An active cavity radiometer makes accurate measurementsof optical power by comparing it with equivalent electrical power at constant temperature when a shutter successively exposes and blocks the source of radiation. The active cavities respond to the electromagnetic spectrum from 0.2 to 100 ~, and thus to solar radiation Ihat Earth reflects and to longer wavelength radiation that Earth emits. 4

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radiometric sensitivity .Extensive preflight testing, calibration, and characterization are also planned using;the laboratory standardsat NIST . The combination ofNISTAR's full-disk measurementsof Earth's radiance with EPIC's spatially rt~solvedradiance measurements potentially offers a capability for future radiation budget monitoring with improved in-flight calibration and stability. The technology in NIST AR is basedon well-established laboratory practices,14 its use in but spacewill be new. P/asma-Mag The Plasm:3.-Mag instruments are designedto measurethe velocity distributions of solar wind electrons and ions (protons and alpha particles), and the interplanetary magnetic field at tJ~e 1 location. These are standardmeasurementsthat have been made L previously and are currently being made on the Advanced Composition Explorer (ACE)IS and the Solar Wiruj Observatory (WIND), except that a ~30-fold improvement in the time resolution of the solar wind ion measurementscan be accomplished on a 3-axis stabilized spacecraft such as Triana using existing designs. The magnetic field vector is determined with a sensitivity level of less than 0.1 nanoTesla (nT) and a dynamic range of 108using standardtechnolo~';yoptimized for small size and low power. Both the solar wind and magnetic field are sampled once every second. The Plasm.i-Mag instrument package consists of four parts: (1) a Faraday cup to measurethe velocity distribution of solar wind protons and helium nuclei (typically about 1 kiloelectron volt per atomic mass unit [keV/amu]), (2) a "top-hat" type electrostatic deflection analyzejrthat is operated in the range of 3 electron volts (eV) to 2 keV and has a sufficiently broacj field of view to allow inference of the 3-dimensional solar wind electron velocity s:pectra,(3) a triaxal flux-gate magnetometer,and (4) a data handling unit for processing:the signals from the three instruments. The magnetometerand electron analyzers are mounted on a 3-meter boom to minimize the effects of spacecraft potential and the magnetic field. All three instrument designs have been used extensively in spaceapplications,16 and algorithms for deriving the physical parameters(e.g., solar wind density, bulk speed and temperature, magnetic field strength and direction) from the raw data are well established and tested, but have only been used with instruments on spinning spacecraft.17 14RiceI.P ., S.R. Lorel1ltz,and T.M. lung, "The Next Generation of Active Cavity Radiometers for Space- based Remote SensinE:,"American Meteorological Society conference proceedings: lOth Conference on Atmospheric Radiation: A Symposium with Tributes to the Works of Vemer E. Suomi, pp. 85-88, 1999. ISfor more information about the NASA missions and instruments referred to in this report, see . 16Ogilvie, K. W., D.I. Chomay, R.I. fritzenreiter, f. Hunsaker, I. Keller, I. Lobell, G. Miller, I.D. Scudder, E.C. Sittler, Ir., R.B. Torbert, D. Bodet, G. Needell, A.I. Lazarus, I. T., Tappan, A. Mavretic, and E. Gergin, "SWE, A Comprehen!:ive Plasma Instrument for the Wind Spacecraft," Space Science Reviews 71(1/4):55- 77, february 1995. 17Scudder I., f. Hunsacker, G. Miller, I. Lobell, T. Zawistowski, K. Ogilvie, I. Keller, D. Chomay, f. Herrero, R. fitzenreiteT, D. faiI'field, I. Needell, D. Bodet, I. Googins, C. Kletzing, R. Torbert, I. Vandiver, R. Bentley, W. fillius, C. McIlwain, E. Whipple, and A. Korth, "Hydra -A 3.Dimensional Electron and Ion Hot Plasma Instrument for the Polar Spacecraft of the GGS Mission," Space Science Reviews 71(1/4):459-495, february 1995. 5

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The plasma and magnetometer instruments are nearly identical to c:orrespondingsensors flown successfully on the WIND and Polar spacecraft.18 Triana's Orbit and Earth-Viewing Geometry The Ll point provides a unique view of Earth for the EPIC camera and NISTAR radiometers and also allows observations of the solar wind upstream from Earth with the Plasma-Mag instnllnent. The Ll point is located on the direct line between Earth and the Sun, about one-hundredth of the distance from Earth to the Sun. The mission is designed so that the spacecraft will not actually be located directly at the L 1 point. If it were, radio communication would be too noisy, since earthbound antennasfocused on the spacecraft would also seethe Sun, a strong source of radio noise directly behind the spacecraft. Instead, Triana is designed to orbit around the Earth-Sun axis in a ]tlear-circular ellipse centered on the Ll point. This small orbit (Lissajous orbit) require:sabout 6 months for a complete revolution and provides a view of Earth that diverges from the Earth-Sun axis by 4°. The orbit also changesshapeon a 4-year cycle such that the initial 4° divergence of view point expclndsto 15° through the cycle. Thus, Triana's EPIC and NISTAR instruments will view Earth from a direction that diverges from th~ direction of the Sun's illumination by an angle of 4 to 15°.19 The near-coincidence of view and illumination direction hcLS important implications for thlealgorithms that transform EPIC radiancesand NISTAR irradiances into geophysical data products. For example, the scattering angle of aerosol and cloud phase functions will be 165 to 177°, indicating scattering in nearly the backward scattering directio.tl.2oSince some scattering functions show rapid I:hangewith angle in this angular region, Triana data reduction algorithms are designedto accommodatethe effects of the change in viewing geometry that will be experienced. over the life of the mission. Over water, Sun glint can brighten surface reflectance when the Sun is near the overhead position. As a result, some ocean retrievals will be limited to morning and afternoon observa"tions when glint is not a problem. For land observations, the view is very near to the "hot-spc.t" (perfect backscatter) direction, at which surface bidirectional reflectance in reflective wavelengths is known to reach a peak. The hot-spot effect is produced by shadow hiding, il], which structures or projections that cast shadows (e.g., plant canopies,individual plant leaves) also hide their own shadows when viewed from the sameposition as their illumirlation. While these directional effects may need to be "corrected" in some algorithms (e.g., to deduce albedos from NISTAR and EPIC observations), they can be a source ofin1:Ormation for other algorithms (e.g., yielding potential Triana geophysical data produc:tsdescribing surface 18Lepping R.P., M.H" Acufta, L.F. Burlaga, W.M. Farrell, J.A. Slavin, K.H. Schatten, F. Mariani, N.F. Ness, F.M. Neubauer, Y.C. Whang, J.B. Bymes, R.S. Kennon, P.V. Panetta, J. 'Scheifele, and E.M. Worley, "The Wind Magnetic Field Investigation," Space Science Reviews 71(1/4): 207..229, February 1995. Acufia, M.H., K.W. Ogilvie, D.N. Baker, S.A. Curtis, D.H. Fairfield, and W.H. Mish, "The Global Geospace Science Program and Its Investigations," Space Science Reviews 71(1/4):5-21, February 1995. Harten, Ronald, and .~enn Clark, "The Design Features of the GGS Wind and Polar Spacecraft," Space Science Reviews 71(1/4): 23-40, February 1995. 19For clarity, this simple description assumesa static Earth-Sun axis, whereas the axis is actually in constant motion as &Irth revolves around the Sun. 20Radiation that is sc;ittered in the backward scattering direction is exactly reversed in direction and so proceeds directly on a line toward its source. 6

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vegetation structure). Becauseof the unique viewing point, observations from L 1 may also help to fill in ltheangular observation domains of LEO and GEO imagers, which acquire hot-spot data only under very limited conditions. A continuous view of Earth from the Ll point shows the changesin Earth's disk with the seasons.During the northern hemispheresummer, the arctic regions will be tilted toward the Sun and thus continuously visible, while antarctic regions will be continuously visible during the southern hemisphere summer. Polar visibility also dependson the position of Triana on its Lissajous orbit, which in turn dependson its launch date. If Tri~mais "above" the plane of the ecliptic during the northern hemisphere summer, its view of the arctic region will be better. The Triana scienceteam prefers this scenario, as it will improve the quality and area of continuous measurementof ozone in the arctic. Data Processing and Distribution Triana's primary data products, as reported by the Triana scienceteam, are shown in Table 2. Some of the data products will require both Triana data and ancillary data from other sources,such as ground-basedinstruments or other satellites. As envisioned, the Triana data system will provide multiple streamsto accommodatedifferent user needs.The Triana data would be received on Earth at five to seven ground stations and from there would be transmitted to the Mission Operations Center (MOC) at the Goddard SpaceFlight Center. At a ground station, the data would be parsed into three streams-spacecraft status,time-critical science and image data, and data that are not time-critical. Time-critical data, which would be forwarded immediately to the MOC, include EPIC visible channels (443-,551-, and 645-nm bands) observed every 15 minutes, aerosol and ozone channels observed every hour, and the entire Plasma-Mag data ~itream.The remaining data would be forwarded within 8 hours. Becauseof their potential urgency, the Plasma-Mag data are proposed to be transmitted directly to the National Oceanic and Atmospheric Administration (NOM) for use in spaceweather forecasts and advisories. Geophysical and image processing of data would occur at the Triana, Science and Operations Center (TSOC) at Scripps Institution of Oceanography, UIJdversityof California, San Diego. EPIC visible channels will be calibrated, geolocatted,georegistered,and posted on the Triana Web site within 30 to 45 minutes after acquisition. The NIST AR data will be received as a continuous stream, processed,and stoJred the TSOC. NIST will then confmn that the data were collected at properly and did not arrive during filter movement, spacecraft slew, or during an instrument calibra1:ionperiod. The TSOC will store all raw and processedscience and image data for the life of the mission (2 to 5 years) plus 3 years. The EPIC and NIST AR data will be managed at the Langley Distri1butedActive Archive Center. The task group did not review the data archiving or manaJgement plans. 7

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Data Product Relevant NRC and Accuracy Government Reports* EPIC Total column ozone 8-16 kIn 2, 3,4, 5, 9, 12 :i: 3% Aerosol index 8-16 kIn 3, 10 :i: 3% Aerosol optical depth 8-16 kIn 2,3,4,5,9, 10, 12 :i:10% Cloud height 16 kIn 4,5,9,10,11 :i: 30 mb UV radiance 8-16 kIn 3,4, 5 :1:10% Precipitable water 8-16 kIn 3,4,5,9, 10, II, 12 :I: 10% Volcanic S02 8-16 kIn 4,5.. :I: 10% Cloud reflectivity 8-16 kIn 2,4,5, 10, II, 12 :i: 5% NISTAR Broad band radiance's 2, 12 Sunlit full 0.2 to >loo 10, II :!:0.1% disk of microns Earth 0.2 to 4 microns 4, 10, 12 :t:0.1% 0.7 to 4 microns 10 :1:0.1% Planetary albedo 10,11 :t: 0.003% Sunlit full Measurements disk of absolute Earth Plasma-Mag Solar wind proton minute 1,4,6,7.8, 9 1.5 :!:2% second density Solar wind velocity :t 10% minute 1.5 4,6,7,8,9 second Solar wind proton minute 1.5 .t10% .4, 6, 7, 8, 9 thermal speed second NA Solar wind electron 1,4,6,7,8, 9 1.5 :f: ]0% thermal speed second Magnetometer Vector 1 minute 20 milli- 1,4,6,7,8,9 :t: 1% measurements seconds each of the component interplanetary magnetic field Note: Except for that in the right-hand column, the infonnation in Table 2 was provided by the Triana science team aJldrepresentsNASA's program plans and objectives. *Compiled by the task group, this column lists previously published NRC and government reports that describe the value of1thesekinds of data for advancing understanding. Seethe key below for corresponding full references. One of the ways the task group addressedthe issue of whether the Triana mission and goals are consonant with published science strategies was to compare Triana's primary data products as defined by the science team ~,ith priorities in relevant NRC and government reports. **This report indicat(:s the need to understand the contribution of volcanoes to the sulfur budget, radiation balance, and impact an stratospheric chemistry and physics. 8

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Key 1. Space Studies Board, National ResearchCouncil, An Assessmentof the Solar and Space Physics Aspects ofNASA 's Space Science Enterprise Strategic Plan, National Academy Press, Washington, D.C.,1997. 2. Space Studies Board, National ResearchCouncil, Issues in the Integration of Research and Operational Satellite Systems for Climate Research: I. Science and Design, National Academy Press, Washington, D.C., in preparation, February 2000. 3. National Researc]!lCouncil, A Review of the U.S. Global Change Research Program and NASA 's Mission to Planel' EarthlEarth Observing System,National Academy Press, Washington, D.C., 1995. 4. National Research Council, Global Environmental Change: Research Pathways for the Next Decade, National Academy Press, Washington, D.C., 1998. 5. National Research Council, The Atmospheric SciencesEntering the Twenty-First Century, National Academy Press, 'Washington, D.C., 1998. 6. Space Studies Board, National Research Council, A Science Strategy for Space Physics, Committee on Solar and Space Physics, National Academy Press, Washington, D.C., 1995. 7. Space Studies Board, National Research Council, Space Weather: A Research Briefing, Committee on Solar and Space I~esearchand Board on Atmospheric Sciencesand Climate Committee on Solar- Ten-estrial ReseaJ'ch,National Academy Press, Washington, D.C., 1997. Available only as an electronic docum,~ntat . 8. National Research Council, Toward a New National Weather Service-Continuity ofNOAA Satellites, National Academy Press, Washington, D.C., 1997. 9. National Research Council, A Visionfor the National Weather Service: Road Map for the Future. National Academy Press, Washington, D.C., 1999. 10. Office of Science and Technology Policy, Our Changing Planet: A US. Strategy for Global Change Research. Committee on Earth Sciences, Washington, D.C., 1989. 11. National Research Council, Research Strategies of the US. Global Change Research Program, Committee on G1obal Change, National Academy Press, Washington, D.C., 1990. 12. Space Studies Board, National Research Council, Issues in the Integration of Research and Operational Sate/lite Systems for Climate Research: II. Implementation, National Academy Press, Washington, D.C" in preparation, 2000. TECHNI CAL ASSESSMENT I. Ar.~ Triana's goals and objectives consonant with published science strategies and priorities? The goals and objectives of the Triana mission fall within two general categories: (1) to launch a modest exploratory mission to demonstratethe value of remote sensing observations from Ll for Earth scienceand (2) to gather global climate data and fill operational needs related to global change and solar weather. In general, the task group found that the scilentific goals and objectives are consistent with the strategies and priorities for coU.ectionof climate data sets, and the need for development of new technologies, as articulated in relevant reports published by the National Research Council and other similar organizations. The task gJ"OUPcould not find within any of the recently published reports of the NRC a specific re(;ommendation to use L 1 as the point from which to gather Earth science information. Nevertheless, the task group found that observation from Ll has the potential to provide data that can addressseveral high-priority and conceptual issuesthat the reports highlight. For example, the proposed Triana mission is consistent with some recommendations made in the recent NRC report ResearchPathwaysfor the Next 9

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Decade,21such as the need to elucidate "the connections among radiation, dynamics, chemistry and climate" and the need for "a scientific understanding of the entire Earth System on a global scale" (p. 5), with the caveat that although Triana views the full sunlit disk of Earth it cannot determine the thermal budget of the planet as a whole. The Pathways report stressesthree objectives: (1) the nee'dfor well-calibrated observations,which Triana is designed to accomplish by using both the Moon and absolute radiometry; (2) the need to adopt multiple observational approaches,which Trilana is designed to provide in conjunction with LEO and GEO missions; and (3) the need for te'chnical innovation, which the use ofboth Ll for Earth observations and the NIST AR instrument exemplifies. The Pathways report also recommendsthe use of "smaller and more focused missions along the lines of the new Earth System Science Pathfinders" (p. xi). Triana is a relatively 5;mallmission comparable to an Earth System Science Pathfinder, but its focus is on exploring 1the technique of using L 1 for Earth observations, rather than addressing a specific scientific problem. PerhapsTriana's most important contribution to Earth science observations is the potential for using Ll observations of Earth to integrate data from multiple spaceborneas well as surface and airborne observation platforms into a self-consistent global databasefor studying the planet and documenting the extent of regional and global change. The Ll view allows the continuous acquisition of data from the entire full sunlit disk of Earth. These data overlap in both spaceand time the da1:a gatheredby essentially all other networks. The caveat here, however, is that Triana observations have a particular scattering geometry (close to backscatter), and the integration will theretDre require additional processing of the data sets.Data from L 1 may be useful for cross-calibrating independent observations and hence for assembling improved, self- consistent global data1bases from the diverse set of existing observations. Moreover, becauseof its large spatial cover~lgeand temporal continuity , the data from Triana at L 1 can be used to fill in data gaps left by otJler networks and spaceborneplatforms. Triana at the 1.1 view also has the potential to provide atmospheric observations (particularly of ozone) at a finer temporal and spatial resolution for a larger portion of the globe than can currently be t)btained from either LEO or GEO. For example, it is well known that both the planetary-scale circulation and small-scale mixing are equally important to the transport of chemical substancesi:n the stratosphere.Few LEO and GEO measurementsof trace species encompassthese wide~lyseparatedscalessimultaneously. The hemispheric, high-resolution (8 kIn) ozone and aerosol data to be sampled by EPIC on Triana will be a unique set of observations for elucidating transport processesat both large and small scales.Such data should be valuable in furthering understandilngof the chemistry of the stratosphere(e.g., ozone layer) and its response to anthropogenic and natural perturbations. The observations proposed by the Triana scienceteam also have the potential to addressa number of more specific scientific issuesrelated to climate and spaceweather. As Table 2 indicates, most of the principal data products anticipated from Triana are identified as priorities in relevant NRC repoJ1s. These reports were produced over a number of years and using a variety of methodologies. Thl~task group concluded that it would be difficult, if not impossible, to establish more refmecl estimatesof priorities among these reports. Therefore, for the primary data products listed in Table 2, the task group has noted which earlier reports have indicated that the data were desirable, but it has not attempted to establish relative priorities. The observations from EPIC and NISTAR are designedto addressthe connections between radiation dynamics, chemistry, and climate, a theme that is highlighted in many recent NRC 21National Research Council, Global Environmental Change: Research Pathways for the Next Decade, National Academy Press, Washington, D.C., 1998. 10

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reports!2 The Plasma-Mag instrument is designed to provide data on the small-scale structure of the solar wind with a htigh time resolution, objectives consistent with the recommendations of NRC reports.23 The Triana mission is also consistent with more general recommendations to adopt multiple observational approaches.24 It is also possible that the Triana Earth observations will secure useful near-real-time information on the occurrence and evolution of potentially harmful environmenta:l events (e.g., forest fires, volcanoes, UV irradiance peaks), thereby demonstrating the utility of L 1 imaging for future operational products of societal relevance. Without doubt, tile Triana mission will have valuable space weather operational applications, the impoJ1ance of which both NRC reports and the National Space Weather Program25 confirm. In conjunction with the present ACE mission (also at Ll but in a different orbit), Triana's Plasma-Mag enhances the ability ofNOAA's Space Environment Center to carry out its mission to provide warning of imminent solar storm events, especially those whose terrestrial impact is less certain. Because the environment at L 1 is very benign, it is expected that the ACE spacecraft and its instruments will remain healthy and thus will be able to provide space weather data to NOAP~ 's Space Environment Center for at least 4 years beyond the end of ACE's prime mission in 2002 (providingNASA funds the mission's extension). However, if the ACE spacecraft is lost or its plasma or magnetometer instrument fails, then Triana as the only upstream monitor of solar wind and interplanetary magnetic fields could be critical to the Space Environment Center's mission. As an exploratory mission Triana has experimental and innovative aspects that carry higher than usual risks but have the potential to make unique scientific contributions. The use of L 1 for making Earth observations is itself experimental, since it will test the algorithms used to reduce remotely sense.:ldata from a new combination of solar zenith angle and 22SpaceStudiesBoard,National Research Council,Readiness the UpcomingSolar Maximum,National for AcademyPress, Washington, D.C., 1998.Space StudiesBoard,National Research Council,Earth Observations .from Space:History, Promise,and Reality,NationalAcademyPress, Washington, D.C., 1995.Space StudiesBoard, National Research Council,An Assessment the Solar andSpace of Physics AspectsofNASA's Space Science EnterpriseStrategicPlan, NationalAcademyPress, Washington, D.C., 1997.SpaceStudiesBoard,National Research Council, Letter Report:"Assessment ofNASA 's Plansfor Post-2002 EarthObservingMission," National AcademyPress,Washington, D.C., 1999.Space StudiesBoard,National Research Council,Issuesin the Integration of Research and Operatio1:lal SatelliteSystems ClimateResearch: Science Design,NationalAcademy for I. and Press,Washington, D.C., iJ1preparation, February2000.Space Studies Board,NationalResearch Council, TheRole of Small Satellitesin NASAand NOAA Earth Observation Programs,NationalAcademyPress, Washington, D.C., in press,February2000.National Research Council,A Reviewof the US. Global Change Research Program and NASA's Mission to Planet £arthlEarth Observing System, NationalAcademyPress,Washington, D.C., 1995. National Research Council, Global Environmental Change:Research Pathways the Next Decade,National for AcademyPress, Washington, D.C., 1998.NationalResearch Council, TheAtmospheric SciencesEntering the Twenty-FirstCentury,NatiionalAcademyPress, Washington, D.C., 1998. National Research Council,Adequacyof Climate ObservingSystem.\", National AcademyPress, Washington, D.C., 1999.Space StudiesBoard,National Research Council,A Scien,~e Strategy Space for Physics,NationalAcademyPress, Washington, D.C., 1995.Space StudiesBoard,National Rc:search Council, SpaceWeather: Research A Briefing, National AcademyPress, Washington, D.C., 1997.,A,vailable only asan electronicdocument online at . Office of Science TeclmologyPolicy, Our Changing and Planet:A US. Strategy Global Change for Research, Committeeon Earth Scien.:es, Washington, D.C., 1989.NationalResearch Council,Research Strategiesforthe US. Global ChangeResearch }>rogram, National AcademyPress, Washington, D.C., 1990. 23National Research Council,AdequacyofClimate Observing Systems, National AcademyPress, Washington, D.C., 1999.SpaceStudiesBoard~ National Research Council,Earth Observations.from Space:History, Promise,and Reality,National AcademyPress,Washington, D.C., 1995.SpaceStudiesBoard,NationalResearch Council,An Assessment the Solar and SpacePhysicsAspects of ofNASA's Space ScienceEnterpriseStrategicPlan, National AcademyPress,Washington, D.C., 1997. 2~ational Research Coun<:il, Global EnvironmentalChange:Research Pathways the NextDecade,National for AcademyPress, Washington, D.C., 1998. 2S National SpaceWeatherProgram,The Implementation The Plan,FCM-P31-1997, Washington,D.C. 11

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viewinglbackscatterin,g angles. The NIST AR instrument is basedon an established laboratory technology, but one that has never before been used on a space-based platfonn; it is a completely new technological application of both hardware and algorithms. If the instrument perfonns properly and suitable c1lgorithmsare developed to provide sufficiently accurate data, NIST AR may provide unique observations of Earth's radiation parameters.Similarly, the proposal to use hot-spot data from EP[C to infer forest canopy structure is experimental but has the potential to make a significant contribution to the area of terrestrial ecology. 2. Can Triana's goals and objectives be achieved with the planned measurements? The task group conducted neither a technical review of the Triana instrumentation or satellite nor a risk analysis. Such activities were beyond its scope and were precluded by the time and budgetary constraints placed on the preparation of this report. Nevertheless, the task group agreed on a number oj'general issuesrelated to the likely scientific successof the mission based on a review of relevant documents, reports, and briefings by the Triana science team. The task group emphasizesthat the following discussion of the ability of Triana to achieve its goals and objectives is predicated on the assumption that the instruments and satellite have been and will continue to be subject to all necessaryand appropriate exploratory-mission technical and quality control reviews. Under no circumstances should this report or the statementscontained in it be used as a replacementfor these technical evaluations. Spacemission:5,by their very nature, are risky, and exploratory missions such as Triana typically carry additional risk. It appearsthat Triana has been subjectedto an unusually tight schedule and constrairled budget. It is not unreasonable,in the task group's view, to expect that missions implemented[on a short time line and with a constrained budget might carry more risk, although no specific evidence suggeststhat this is the casefor Triana. Suffice it to say that the short time line and tight budget for Triana should not be allowed to preclude the rigorous technical evaluations Imd quality controls normally carried out by NASA for exploratory missions. This applies especially to the NIST portion of the mission and NIST AR, since NIST has no experience in tJ~e construction, quality control, and implementation of space instrumentation and NlST AR has no prior flight heritage. Some aspectsof the mission led the task group to be optimistic. Becausethe radiation environment at Ll is more benign than for LEO and GEO, once the platform reachesLl, tJhe chancesof instrument damageor degradation from radiation will be significantly less tJhan for more typical space-based missions focusing on Earth. Becauseit is never eclipsed, the Triana spacecraft will experi(~nce less thermal stressthan most LEO and GEO missions. AnotJher encouraging sign is tJhe fact that all tJhree Triana' s instruments have been built and are now in of tJhetesting phase. Hov{ever, a critical part of this phase-the thermal and vibration testing-has yet to be conducted. Successful completion of these milestones will enhancethe prognosis for Triana' s success. EPIC The EPIC camera relies on largely proven technology, and its fabrication is not apparently a significaJlt technological challenge. According to the Triana science team, EPIC's basic CCD array techIlology has been applied in other spacebomeimagers (namely the Michelson Doppler Imager on the Solar and Heliospheric Observatory [SOHO] and the 12

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Transition Region and Coronal Explorer [TRACE]). However, the array utilizes two new features-back side thinning and back side illwnination.26 Back side thinned and illuminated arrays are currently used in many earthbound astronomical instruments. Although there is a flight heritage for back side..illuminated arrays, Triana would be the first spacebomeapplication of a back side thinned and illwninated array. NASA has assuredthe task group that the filters, which are fabricated with ion-assisted surface deposition, have been built and tested, and closely meet the nominal specifications. NISTAR The NIST AR radiometers are absolute detectorsthat measurepower directly, thereby precluding the need for complex transfer algorithms and inversions to obtain geophysical data products from detector signals, other than the transmission functions of the filters that isolate the solar and thermal signals. The approach relected in NIST AR ' s inclusion on Triana for monitoring Earth's irr,adiancehas not been utilized in the past becauseof the lack of absolute radiometric devices with sufficient sensitivity when operating near room temperature. Similar types of devices have for two decadesmeasuredthe power from the Sun,27 and independent NASA instruments of this type will provide the measurementsof incident solar energy neededto derive planetary albedo during the Triana mission. NIST AR will implement analogous, simultaneous measurementsof integrated power from the full sunlit disk of Earth itself, permitting measureme:nts Earth's planetary albedo as a function of time, including visible and of near-IR bands separatl~ly. The NIST AR measurementsshould be possible becauseof significant radiometric advancesthat NIST h~1S pioneered in the construction of radiometers. These new radiometers are designed to achieve adequatesignals relative to noise at room temperature and are based on laboratory cryogenic radiometers used extensively as national power standards.28 The filters that separateradiant fluxe!; into visible and near-IR spectral regions have ion-assisted deposition on their surfaces and are multiply redundant, features that help, respectively, to minimize and permit tracking of their in-fliJght stability drifts. Dual carriage filter and shutter wheels are designed for adequatethermal isolation of the receiver cavities from the surrounding environment. The NIST AR hardware has been constructed and is currently undergoing laboratory testing at NIST. The tiSk group notes that algorithms remain to be developed to derive the planetary parameters j]-om the NISTAR radiation measurements.Since the sunlit disk albedo measurementsplanned by NIST AR are new observables,and the derived geophysical parameters from NISTAR and EF'IC are new data products, all of which lack algorithm heritage, it is not possible yet to assess effort required to deducereliable geophysical data from these .the observations. Howeve:r, experience with similar data sets (e.g., ERBE) suggeststhat a significant time investment will be required. 26The back side thinning process removes excess silicon to enhance sensitivity in the ultraviolet region. Back side illumination, in which the array is illuminated from the side from which the signal is read out, improves sensitivity and makes the array less sl~sceptibleto on-orbit radiation degradation. 27The Sun's signal of -100 milliwatt per square centimeter at LEO is five orders of magnitude higher than the I microwatt per square centimeter signal from Earth at LI. 28Examples oftechnologi,:al advances used in the design of these new radiometers include (I) positive temperature coefficient thennistors that achieve order-of-magnitude sensitivity detennination of cavity temperature compared with the nonnally used platinum resistance wire; (2) AC bridge circuitry that minimizes noise by isolating the frequency of the measuredlsignals; (3) stable phosphorous nickel surface coatings that maximize optical electrical equivalence; and (4) diam,~ndturned apertures and precision optical aperture area detenninations. 13

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p lasma- Mag The goals and scientific aims of the Plasma-Mag investigation are to (1) study plasma turbulence and structures in the solar wind using the fast (~1 sec) time resolution capabilities of Plasma-Mag, (2) stud~/the large-scale solar wind structures using multipoint and correlative observations from complementary spaceenvironment missions (ACE, WIND, Solar Terrestrial Relations Observatory [STEREO], and SOHO), and (3) provide real-time solar wind parameters required for spacewe~ltherforecasting. The task group concluded that the likelihood that these objectives and goals ~rill be achieved is high, becausemeasurementsof the type planned by Plasma-Mag are widely used in spaceenvironment missions. The interplanetary environment is a highly turbulent medium that supports a great variety of wave motions. Shoc:;ks, discontinuities, and small-scale structures such as "magnetic holes" (in which the magnetic fic~ldnearly vanishes) are often present. A fast sampling (~ l-sec time resolution) such as wo'uld be provided by the Plasma-Mag solar wind instruments on Triana could be useful for fwther progress on these problems:9 For example, high-time-resolution measurementsmay help researchersbetter understandthe wave damping and heating of particles expected to take place near the proton cyclotron frequency. Such measurementsare likely to be useful to properly chaJracterize discontinuities and shocks in the solar wind. High-time-resolution plasma data will enable studies of the smaller magnetic hole structures that have frequently been observed at lower tim(~resolutions.3o Solar wind and magnetic field observations from Plasma-Mag will be valuable in studies examining a variety oJ[large-scale structures such as the shocks, current sheets,and magnetic clouds often associate.d with coronal mass ejections. With a constellation of four spacecraft (Triana, ACE, WIND, and STEREO) separatedby large distances (on the order of200 Earth radii), the geometry o1: relatively stable structures in the near-Earth spaceenvironment can be determined. This configuration of four spacecraftwill also enable determination of, for example, the size and configuration of larger magnetic holes and would allow multi-spacecraft studies of the geometric configurations and structures of coronal transient related disturbances. The Plasma-Mag on Triana at Ll is designedto provide in near real time and on a continuous basis the primary set of measurementsrequired by the SpaceEnvironment Center of NOAA to monitor and forecast Earth's solar environment and provide accurate,reliable, and useful warnings of solar-terrestrial interactions. The required primary measurementsare the solar wind plasma ion densilty velocity , and temperature, and the magnetic field vector in standard , coordinates. The requjlredtime resolution is 1 minute or faster. The Plasma-Mag instrument package is intended tCItake the measurementsand compute on-board averagesof solar wind and magnetic field parame:ters real time once per minute. The launch of Triana in 2001 or later will in provide overlap with J\CE for many years, allowing for cross-calibration. The availability of real-time solar wind data from Ll spacecraftat two separatepoints in spacewould enhancethe reliability of detecting;the geoeffectivenessof disturbancesnot directly on the Sun-Earth line by providing additional iJrlformationabout the irregularities in the solar wind. 29Valero, Francisco P.J., Jay Herman, Patrick Minnis, William D. Collins, Robert Sadourny, Warren Wiscombe, Dan Lubin, and Keith Ogilvie, Triana- a Deep Space Earth and Solar Observatory, NASA background report, December 1999. A vailablt: at posted as pdf file. 30Burlaga. L.F ., "Micro-Scale Structures in the Interplanetary Medium," Solar Physics 4:67, 1968. Burgla, L.F ., and N.F. Ness, "Macro- and Microstructure of the Interplanetary Magnetic Field," Can. .I: Physics 46:S962, 1968. Fitzenreiter, R.J., and L.F. Burlaga. "Structure ofCurrent Sheets in Magnetic Holes at IAU," J. Geophysics Res. 83:5579, 1978. Turner, J.~rf.,L.F. Burlaga. N.F. Ness, and J.F. Lamarie, "Magnetic Holes in the Solar Wind, " .I: Geophysics Res. 82:1921-1924,1977. 14

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Data Products The main advaJ1tage Triana is that it will view the full sunlit disk of Earth, of continuously and syno:ptically. The technique employed by EPIC (of combining a telescope with a CCD camera) allows particularly high spatial resolution considering the L1 vantage point. For stratospheric and uppeir-atmosphere studies basedon total ozone column data, EPIC's 8-km spatial resolution at nadir (and up to 14 km at the highest usable viewing angle) is far superior to that available for UV channels on the Total Ozone Mapping Spectrometer(TOMS) (~ 80 km)(which uses a scanning spectrometer and photomultiplier in LEO). Coupled with the monitoring of diurnal "ariations obtained from L1, EPIC's 8-km spatial resolution will permit preliminary studies of stratosphericprocessesat time and spacescalesnot resolved thus far with LEO satellites. The 8-};:mspatial resolution is also sufficient for lower-atmosphere studies of aerosol optical depth, precipitable water, and clouds (with some caveatsas discussedbelow) but is much less optimal for surface process investigations. For surface processesthe main advantage is the observation geornetry, the so-called hot-spot view, which is rarely realized from other spacecraft and thus offers new opportunities, in particular for studying canopy properties. Table 2 lists tht~data products that the Triana mission is intended to deliver in quasi-real time. Most algorithms neededto produce the EPIC and Plasma-Mag data have notable heritage, with some algorithms more mature than others. For instance, in the caseof total column ozone measurements,the TOMS heritage is significant,31and if the EPIC instrument functions according to specification, total column ozone should be a relatively straightforward data product for the Triana team to deliver from the start of the mission. On the other hand, in the case of aerosol optical depth estimation basedon a ratio ofUV radiances, the algorithm is less mature and has limited documentation. Some adjustments are likely to be necessaryafter launch, particularly over bright surfaces or at high viewing angles ( e.g., arctic). So, while it can be expected that the gene]~ation most data products will be achieved to a scientifically useful of accuracy, the accuracy of some data products is expectedto be higher than that of others. NIST AR in contrast is a new instrument, so that significant algorithm development, testing, and validation are needed10enable processing of its raw data into useful information. The relationship between tlle accuracy of the derived data products and the accuracy of the raw data is unclear. To take full ad'vantage the new opportunities offered by Triana requires special of attention to the accural~y and stability of the NISTAR and EPIC instruments. The accuracy will be obtained through on-board calibration of the instruments. For instance, NISTAR is a self- calibrating instrument by virtue of multiple redundant filters, an unfiltered absolute radiometric channel, and inter-calibration of EPIC with other spacebomeinstruments, while EPIC stability will be assessed thrOU!~ monthly monitoring of the back side of the Moon. The level of accuracy to be achieved from inter-calibration is difficult to assesssince most of the other instruments with which EPIC will be inter-calibrated are themselvespoorly calibrated (e.g., the Advanced Very High Resolution Radiometer [A VHRR], the Visible Infrared Spin-Scan Radiometer (VISSR) on the U.S. CreostationaryMeteorological Satellite [GOES]). The innovative and particularly attractive approach of using the Moon for perfonning instrument in-flight stability assessmentappearsto have been very well thought out, but operational experience may lead to refinements in the teclmiques with time. 31Valero, Francisco P.J., J;ly Herman, Patrick Minnis, William D. Collins, Robert Sadourny, Warren Wiscombe, Dan Lubin, and Keith Ogilvie, Triana- a Deep Space Earth and Solar Observatory, NASA background report, December 1999. Available at posted as pdffile. 15

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With regard to the generation of atmospheric data products besidesozone (aerosol optical depth, total precipitable water), one issue of concern is the determination of cloud data in pixels that are only partially cloudy acrosstheir areas.The accuracy of the retrieved parameterswill depend on the quality of the scenedetermination in cloud-free pixels. Cloudy pixel determination will be achieved usin~~ commonly applied radiance threshold method. With relatively small the pixels (~ 1 kIn), such ,asthose from A VHRR or the Moderate Resolution Imaging Spectroradiometer (MODIS) for instance, the cloud/clear distinction is relatively straightforward. However, it becomes'more difficult as the spatial resolution decreases(i.e., the size of the pixels increases).Due to the typical cloud size, an 8-kIn pixel is more likely than a l-kIn pixel to be partially cloudy. A lo,~er threshold value will ensurethat no clouds (or at least few clouds) are present and is likely to produce more accuratedata, but it will limit the number of pixels usable in retrievals of geoph~rsical data. A higher threshold will allow more partly cloudy pixels to be included, but will induce a reduction in the accuracy of the parameter retrieved. Also, since the Triana observations aJ.e made at high scattering angles (between 140 and 160°), the computed threshold value will h;a.ve account for this scattering angle. This meansthat thresholding to algorithms developed for other instruments such as A VHRR and MODIS will need to be adjusted to the EPIC s:patialresolution and observation geometry, and their performance evaluated. Some guidcmcecould be obtained from the work done with the Polarization and Directionality ofEartll'S Reflectances(POLDER) instrument,32which has a similar resolution. This, however, sugge~its that the heritage from other sensorsfor cloud detection will not be directly applicable and that a significant amount of work will have to be done both before and after launch to adjust :for the EPIC instrument and Triana viewing characteristics. Another issue of concern is estimation from NISTAR and EPIC of Earth's albedo. Becausealbedo concerns solar radiation reflected in all directions from the whole Earth disk, it cannot be measured directly from a look in a single direction. Extrapolating the data from one direction to others requires coming up with an angular distribution model that essentially transforms measureml~nts from one direction into another. Such a model varies with surface type. EPIC data will be usel.ito assign each pixel of the Earth disk to a particular surface type (cloud, water, vegetation, and so on). Given the surface type and the imaging geometry, a weight representing the angutar distribution model will be assignedthat accounts for the directional effects, and the weights will be aggregatedto provide whole-Earth albedo. Angular distribution models can be built from the observations of other instruments (e.g., Clouds and Earth's Radiant Energy System [CERES]). The procedure employed is rather complex-since it uses a combination of meastlrementsfrom NIST AR, EPIC, and CERES, for instance, to build the albedo of the sunlit side of the planet-and will likely need some testing and adjustment. For the concerns raised here, it is not the possibility of producing excellent data sets that is in question, but ratller the level of effort that will be required to do so. Indeed, for the Triana mission to produce useful geophysical parameterswill require that great care be taken in the development, testing, and validation of the operational algorithms. The expected resources neededfor these func,~ions inconsistent with the current, very limited, level of effort to are support development of these algorithms. In view of the extremely short time frame of the mission and the necessaryalgorithm adjustments alluded to above, substantial work on the data reduction algorithms :;hould start immediately. Operational algorithms can take a long time to implement and fully test. The scientific successof the Triana mission will be judged, in large part, on the quality of the initial data delivered to the scientific community .The task group 32The instrument will observe from space the polarization, and the directional and spectral characteristics, of solar light reflected by the Eartll-atmosphere system. 16

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therefore recommendsthat NASA seriously consider increasing as soon as possible the level of effort invested in development and testing of data reduction algorithm~;for the core Earth data products. The more re:)earch-orienteddata products can and will take more time to produce and test, and that is entirel:y acceptable.The Plasma-Mag algorithms have a long heritage and have been well proven; it is just a matter of transferring them to operational algorithms. Although this effort should not be ne:glected,it should require much less investment 1:han that needed for the EPIC or NIST AR algorithms, data reduction, and analysis effort. 3. I)oes Triana Enhance or Complement Other Missions Now in Operation or in Development? The Triana scil~nceteam assertsthat, in addition to providing unique capabilities for remote sensing observation of Earth, Triana will enhanceand complement other missions becauseof its Ll vantclgepoint for continuous imaging of the full sunlit disk of Earth. The task group generally supports this view, although the nature and extent of enhancementwill likely vary among the instrwnents. Many of the details of the complementar)' nature of Triana are discussed in the preceding sections. Interactions with Earth-viewing missions at LEO and GEO will extend in time and coverage, and in accur'acythrough cross-calibrations, the data quality and value of all of the missions. For example::(1) EPIC will significantly extend TOMS, whil~h samples data once a day at local noon at a nadiJrresolution of 80 km, to a near-continuous sampling at a nadir resolution of 8 km; (2) EPIC will also enhancethe temporal coverage of MODIS" which, unlike Triana, covers the entire Earthl'Ssurface but does so every 1 to 2 days; (3) EPIC and the Multi-angle Imaging Spectroradiometer (MISR), POLDER, and the Along Track Scanning Radiometer (ATSR-2) fill in angular spacefor each other; (4) NISTAR augmentsCERES with continuous planetary albedo near 180° backscatter in similar spectral bands; and (:;) Triana complements GEO satellites with high-Iatitude observations, although the utility of the data near the fringe of the disk is somewhat questionable. Triana's synoptic view of Earth will help to put localized, ground-based,and airborne field observations into a glDbal context. For example, measurementso:ftropical cirrus cloud microphysics and radi,ation during the Cirrus Regional Study of Tropic:al Anvils and Cirrus Layers (CRYSTAL) c;ampaigns, planned for 2002 and 2004, can be correlated with concurrent observations by Triarul at L 1. Work at Department of Energy -Atmos:pheric Radiation Measurement (DOE-A.RM) sites also, for example, will benefit from slllch correlative observation.33 Triana will alS;Daugment existing Sun-viewing satellites at Ll. Plasma-Mag will enhance the time resolution and spatial coverage of solar wind data from WINI) and ACE. It will complement, and may succeed,ACE in operational utility. In turn, Triana will benefit from the presenceof other satellites. Data from ins~ents with higher spatial resolution such as MODIS and the Sea- Viewing Wide Field Sensor34 (SeaWiFS) will improve EPIC data, especially aerosols, and add new imormation about cloud 33Valero, Francisco P.J., Jay Herman, Patrick Minnis, William D. Collins, Robert Sadourny, Warren Wiscombe, Dan Lubin, and Keith OgiJlvie, Triana- a Deep Space Earth and Solar Observatory, NASA background report, December 1999. Available: at posted as a pdffilc~. 34It provides global estimates of oceanic chlorophyll-a and other bio-optical quantities to the international research community . 17

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properties. Triana's in-flight validation should benefit from the calibration heritage of TOMS and MODIS. Radiation fields observed by CERES can be directly compared with NISTAR data. SUMMARY The task group's assessment Triana's scientific objectives and goals is basedon its of review of the relevant literature and presentationsregarding the proposed scientific mission. The task group found that (1) the scientific goals and objectives of the Triana mission are consonant with published science strategies and priorities for collection of climate data sets and the need for development of new technologies; (2) if successfully implemented, the planned measuremelllts will likely contribute to Triana's stated goals and objectives; and (3) the Triana missi(ln will complement and enhance data from other missions now in operation or in devellopment because of the unique character of the measurements obtainable at the Ll point in space, which allows continuous imaging of the full sunlit disk of Earth and monitoring of the space environment upstream from Earth. Nevertheless, the task group recommends that NASA seriously consider increasing the level of effort invested in development and testing of data .reduction algorithms for the core Earth data products as soon as possible and ensure that all the appropriate technical and management reviews are performed. In addition, if Triana lasts longer than its nominal 2 years, it will be important for NASA to support the data processing activities for the mission's useful duration. More specificlllly, the task group found that the scientific objectives and deliverable data products of the Triana mission as described by NASA's Triana science team are consonant with SCieIllCe strategies and priorities proposed by various NRC and government reports, as summarized in Table 2 of this report. The task group notes that Triana's primary focus is technique and technology development at Ll, as the Pathways report recommended for future Earth Science ~;ystemPathfinder missions, rather than anyone specific scientific problem. The task group concluded that the mission, if successfully implemented, is likely to achieve the stated goals and objectives, although as in most exploratory missions there can be no assurance of success.A detailed analysis of instrumentation, data collection and reduction, systems operations, management,cost, and risk was beyond the scope of the charge to this task group. However, it WiiSimpressed by the detailed efforts of the Triana science team and their extensive use ofheritige technology and data reduction algorithms where they were available. The task group found that the Triana mission will complement and enhance other missions because of 1the unique character of measurements made from the Ll point, which allow continuous imaging of the full sunlight disk of Earth and monitoring of solar wind properties relevant to spaceweather. Furthermore, such observations from Ll should provide a unique perspective to develop new databases and validate and augment existing and planned global and local intef))lanetary databases. Triana is an e}cploratorymission that may open up the use of deep-spaceobservation points such as L 1 for Earth science. The task group believes that the potential impact is sufficiently valuable to Earth sciencethat such a mission might well have been viewed as an earlier NASA priority had adequatetechnology been available at reasonablecost. The task group lacked the proper expertise, resources,and time to conduct a credible cost or cost-benefit analysis (such an effort might take many months and much detailed analysis) or an analysis of the mission goals and objectives within the context of a limited NASA budget or relative to other Earth.Science Enterprise missions. However, basedon the available information, the task group found that (1) the cost of Triana is not out of line for a relatively small 18

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mission that explore~:a new Earth observing perspective and provides unique data; (2) since a significant fr~Lction of the Triana funds (according to NASA and the Triana principal investigatolr, 50 percent of total funding and 90 percent of instrument development money) have already been expended, weighing cost issues would lead to only limited opportunities to save or transfer funds to other projects. 19