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Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
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2

Suitability of the Large Platform for Earth System Explorers

In this chapter, the committee examines the suitability of the notional large platform for Targeted Observables that were prioritized in the ESAS 2017 decadal survey1 as candidates for implementation in the recommended new program line of Earth System Explorers.

GREENHOUSE GAS FLUXES

The ESAS 2017 decadal survey identified a candidate Earth System Explorer that would enable observation of CO2 and methane fluxes and trends at global and regional scales with quantification of point sources, and identification of sources and sinks. These measurements have also been identified by NASA ESD as a priority; indeed, in part as a response to the recent cancellation of the GeoCARB Venture-class mission, NASA stated that the ESAS 2017 objectives for GHG observations will be a priority for the first Earth System Explorer mission.2

Currently operating satellites with sensitivity to atmospheric boundary-layer CO2 and CH4 fluxes include global-to-regional scale flux mappers such as NASA’s OCO series, JAXA’s GOSAT (Greenhouse gases Observing Satellite) series, and ESA’s Sentinel-5P (TROPOMI, Tropospheric Monitoring Instrument) mission as well as point source imagers such as NASA’s EMIT instrument on the ISS and ASI’s PRISMA mission, to name a few.3 Instruments on these spacecraft are providing important new insights both for carbon cycle science and decision support.

However, the committee notes there are major gaps remaining that need to be addressed by future GHG observations from space including that anticipated to be selected in the first Earth System Explorer call. These gaps include (1) increased global spatial density of column-averaged dry air mole fractions of CO2 and CH4 (particularly CO2 soundings which remain relatively sparse globally), (2) higher spatial-temporal resolution and improved sensitivity for point source detection and quantification, (3) simultaneous observations of co-emitted tracers (e.g., CO and NO2) to support sectoral attribution (source apportionment) at regional scale, (4) routine high frequency observations across the daylight interval to address source variability not captured by traditional midday measurements, and (5) active sensing of nocturnal and high-latitude fluxes which are largely unobservable with passive sensors.

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1 NASEM, 2018, Thriving on Our Changing Planet.

2 J. Foust, 2022, “NASA Cancels GeoCarb Greenhouse Gas Monitoring Mission,” SpaceNews, November 29, https://spacenews.com/nasa-cancels-geocarb-greenhouse-gas-monitoring-mission.

3 A comprehensive review of how methane emissions may be monitored from space appears in D.J. Jacob, D.J. Varon, D.H. Cusworth, et al., 2022, “Quantifying Methane Emissions from the Global Scale Down to Point Sources Using Satellite Observations of Atmospheric Methane,” Atmospheric Chemistry and Physics 22(14):9617–9646, https://doi.org/10.5194/acp-22-9617-2022.

Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
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Suitability of the Large Platform

A single platform in an SSO orbit could be compatible with addressing one or two, but not all, of the above gaps with an Earth System Explorer (or future Earth Venture-Instrument) investigation. For example, investigations seeking to improve the global spatial density of CO2 and CH4 observations (such as a follow-on mission to the OCO and GOSAT programs) could be achieved with one or more wide-swath SWIR spectrometers on a single platform in an SSO orbit at greater than or equal to 700 km altitude with a midday crossing time optimized for peak solar illumination and well-mixed boundary layer conditions.

Studies of nocturnal and high-latitude carbon fluxes would not be possible with the same platform since active lidar instruments generally require altitudes below 500 km due to power constraints and because of the necessary dawn-dusk crossing times. Additionally, it is unlikely that the other observational gaps described above could be addressed with a single SSO platform including the need for higher spatial-temporal resolution and higher sensitivity point source observations (requiring multiple platforms in lower orbits) or characterizing variability in fluxes across the daylight interval (requiring either precessing LEO, constellations of satellites in SSOs and/or geostationary Earth orbit [GEO] platforms). On the other hand, if a large SSO platform became available with suitably low-cost accommodations, it might offer proposers flexibility to pair it with a SmallSat in a different orbit in order to achieve multiple objectives.

ICE ELEVATION

The ESAS 2017 decadal survey lists “Ice Elevation” as a candidate for selection in the Earth System Explorer line for “global ice characterization including elevation change of land ice to assess sea level contributions and freeboard height of sea ice to assess sea ice/ocean/atmosphere interaction.” The survey report states that, “Land ice and sea ice are both important components of the cryosphere that play different roles in Earth’s climate system; a fundamental parameter that should be monitored for both of them is surface elevation.”

Measuring land-ice surface elevation and sea-ice freeboard height by satellite or radar laser altimeter along repeated ground tracks provides an estimate of the volume change of land ice and sea ice over time. Space-based measurements of ice surface elevation would include a polar-orbiting satellite (to 88°) carrying a scanning laser or radar altimeter, as a follow-on to ICESat-2 and CryoSat-2. Over land ice, the spatial sampling should be at least 1 km over the central parts of the ice sheets, with 0.1 km sampling around the ice-sheet margins and should be accurate to 10–20 cm over areas with slopes greater than 1°. The repeat period should be weekly or better. Over sea ice, the spatial sampling should be at least 1 km with a precision of at least 3 cm. The repeat period should be weekly.

Implementation: The optimal orbit has an inclination of close to 90° to map the freeboard of the floating Arctic sea ice, as well as to map the land ice elevation over the entire Antarctic continent. The optimal altitude is ~500 km, reflecting the need to minimize the laser power while avoiding atmospheric drag effects that introduce uncertainties in knowledge of the orbit. The orbital accuracy needs to be better than 30 mm in the radial direction and 70 mm in the along/cross-track directions. Knowledge of the pointing of the laser needs to be better than 10 microradians, while pointing accuracy needs to be better than 100 microradians.

Suitability of the Large Platform

The notional large platform would not achieve the orbital and geodetic requirements of the decadal Targeted Observable—its altitude is too high for the lidar measurements and its assumed inclination of 98° would not allow complete coverage of the important Arctic sea ice. The platform’s orbit would also

Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
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not be suitable for a continuity mission with ICESat-2, which has an orbit with an inclination of 92° at an altitude of approximately 500 km. Although the nominal platform could fly in a lower orbit (at the cost of a larger gap around the poles), this would lead to more fuel for orbit maintenance and would lead to smaller swaths for optical instruments.

OCEAN SURFACE WINDS AND CURRENTS

The ESAS 2017 decadal survey identified the need for coincident high-accuracy measurements of ocean currents and vector winds to assess air-sea momentum exchange and to infer upwelling, upper ocean mixing, and sea-ice drift. The survey report notes that winds are critical elements in the coupling between ocean and atmosphere, strongly influencing the fluxes of heat and momentum transferred at the interface. Ocean surface winds are also a central driver of upper ocean currents, and thus the interaction between winds and currents provides a measure of momentum exchange between the atmosphere and ocean. Small-scale variations in sea-surface temperature modulate heat and momentum exchanges, which can vary on timescales of hours to days.

The proposed Winds and Currents Mission (WaCM) mission features a Ka-band vertically polarized pencil-beam Doppler scatterometer with a long (~5 m) skinny (~0.3 m) rotating antenna and would be able to map ocean winds and currents globally.4 One of the mission cost drivers is the radar radio frequency source, since power drives the size and complexity of the spacecraft. The proposed platform could be a benefit to this mission in that respect. The summary below is derived from data in the ESAS 2017 report, with additional information provided by the committee along with its assessment of the suitability of the large platform.

  • Spatial coverage: Global, including polar oceans. The proposed SSO inclination of 98° of the notional platform would be sufficient, provided Sun-synchronous signals, such as tides, can be removed reliably. Note that sea-ice drift can be inferred from the surface currents with an appropriate polar orbit.
  • Spatial and temporal sampling: One or two times per day, simultaneous sampling of winds and currents.
  • Spatial resolution: Approximately 5 km to avoid contamination of signal from rain and land with a large swath of approximately 1,800 km. Continuous spatial coverage without significant gaps between orbits. The proposed altitude of 700–800 km for the notional platform would provide for swaths in the 1,700–1,900 km range.

Suitability of the Large Platform

The notional platform could be augmented by other scatterometers (in different orbits) for improved coverage and faster refresh rates.5 Other sensors on the notional large platform could aid identification of rain and land contamination. The committee notes that in addition to inclination, equator crossing is relevant if this ESE is implemented on WaCM, particularly if the satellite is envisioned as part of a global

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4 E. Rodríguez, M. Bourassa, D. Chelton, et al., 2019, “The Winds and Currents Mission Concept.” Frontiers in Marine Science 6:438. https://doi.org/10.3389/fmars.2019.00438.

5 Winds and currents undergo diurnal variability pointing to another limitation of the proposed single platform. A constellation of satellites in different orbits and different measurement times would be needed to resolve the diurnal cycle and to evaluate the evolution of storm systems. For a review of the current status of satellite measurements of ocean surface winds and currents (see D. Hauser, S. Abdalla, F. Ardhuin, et al., 2023, “Satellite Remote Sensing of Surface Winds, Waves, and Currents: Where Are We Now?,” Surveys Geophysics, https://doi.org/10.1007/s10712-023-09771-2).

Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
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constellation. The committee is unable to comment on potential engineering challenges that might arise in accommodating the proposed 5-m-long rotating antenna of WaCM alongside other hosted instruments.6

OZONE AND TRACE GASES

The ESAS 2017 decadal survey identified a candidate Earth System Explorer that would address the need for vertical profiles, globally and with high spatial resolution, of ozone and trace gases including water vapor, CO, NO2, CH4, and N2O, chemical species that have important implications for human health, air quality, and climate. For example, characterizing the relationships between these different species and ozone in different regions of the stratosphere will provide important information for understanding the factors controlling ozone concentrations. The summary below is derived from data in the decadal survey report with additional information provided by the committee along with its assessment of the suitability of the large platform.

  • Measurement approaches: Ultraviolet/visible/infrared (UV/Vis/IR) microwave limb/nadir sounding and UV/Vis/IR solar/stellar occultation. Lidar. For example, differential absorption lidar (DIAL) operating on one of the ozone lines in UV and a nearby window.
  • Measurement objectives: Vertical profiles of ozone and speciation of trace gases. The vertical profile capability is crucial, as this gives the tropospheric and Planetary Boundary Layer (PBL) components.
  • Spatial coverage: Global coverage is preferable, with vertical profile information including the PBL troposphere.
  • Spatial and temporal sampling: Daily observations with a 1–2 h revisit preferable and spatial scale 1–10 km2. Fine vertical resolution of less than 500 m. Extreme events may need higher temporal resolution and spatial (horizontal and vertical) resolution.
  • Spatial resolution: Spatial resolution of 1–10 km2. Tropospheric ozone requires fine horizontal resolutions of 5 km2 or better. Tropospheric applications with high spatial variations (e.g., GHG monitoring) may need higher horizontal resolution.
  • Note: Geostationary satellites may be advantageous over polar orbiters to meet the required high temporal, vertical, and horizontal resolution requirements.

Suitability of the Large Platform

The notional large platform may not meet requirements for long-term monitoring. A HIRDLS7 type instrument could be a candidate for the platform (sacrificing horizontal coverage and resolution and temporal revisit times).

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6 RapidScat, a scatterometer that operated on the ISS from October 2014 through August 2016, illustrates other potential problems: The large size of the ISS meant that wind measurements were occasionally blocked, and disruptions associated with deliveries to ISS resulted in extended periods of data dropout.

7 HIRDLS (High Resolution Dynamics Limb Sounder) is an instrument launched on NASA’s Aura spacecraft in 2004. Its scanning infrared limb sounder was designed to observe the global distribution of temperature and concentrations of O3, H2O, CH4, N2O, NO2, HNO3, N2O5, CFC 11, CFC 12, and ClONO2. HIRDLS operations were limited by an 80 percent blockage in its optical path thought to be the result of a dislodged piece of thermal blanket material. See NASA, “The Aura Mission-HIRDLS,” https://aura.gsfc.nasa.gov/hirdls.html.

Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
×

SNOW DEPTH AND SNOW WATER EQUIVALENT

The ESAS 2017 decadal survey identified a candidate Earth System Explorer that would address needs for information on snow depth and snow water equivalent.8

Snow-covered area is monitored primarily from the vantage point of space.9 Snow water equivalent depth has been estimated since 1978 using passive microwave-based techniques,10 as the ground emissivity changes with snow cover (emissivity is affected by melt11). However, such microwave measurements are heavily constrained and only work in flat terrain with little vegetation and only when the snowpack is dry. Ground measurements are used to calibrate satellite data and constrain snow models and are also assimilated in numerical weather prediction (NWP) and reanalysis systems.

Empirical retrieval models are limited by the lack of ground observations over most areas of the world, and where these are available, they are sparse; therefore, uncertainty is very large. Passive microwave remote sensing of seasonal snow is characterized by coarse spatial resolution on the order of tens of kilometers is not sufficient to capture subgrid scale variability of seasonal snowpacks and landscape heterogeneity at scales of 50–250 m.12

A key geophysical variable for hydrology and water supply forecasting is snow water equivalent (SWE; how much water is contained in snow, equal to snowpack depth multiplied by average snowpack density). SWE is important for hydrological modeling and runoff prediction; snowfall as a fraction of total precipitation is important in hydrology models as it determines snowpack accumulation and snow cover extent. Changes in snowfall and snow accumulation patterns are indicative of hydroclimatic changes. Sublimation and melt processes and associated land-atmosphere coupling fluxes are tied to seasonal SWE, albedo, snowpack topography, and the weather near the ground with nonlinear feedbacks that impact regional air temperature and water availability.

The summary below is derived from data in the decadal survey report with additional information provided by the committee along with its assessment of the suitability of the large platform.

  • Observational approach: Snow depth may be inferred from lidar measurements (snow-on minus snow-off) and change in SWE from L-band Interferometric Synthetic Aperture Radar (InSAR)13

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8 This committee also notes the importance of measuring snow albedo to model the temporal evolution of the mass and energy balance of the seasonal. Albedo measurements require adequate sunlight and optimal measurements are close to solar noon, especially at high latitudes. Revisit interval of 1 to 5 days would capture changes in snowpack characteristics that affect albedo and thermal emissivity, sublimation, and synoptic-scale storm snowfall and accumulation events. An appropriate sensor would take measurements in the hyperspectral Vis-NIR.

9 A.J. Dietz, C. Kuenzer, U. Gessner, and S. Dech, 2012, “Remote Sensing of Snow—A Review of Available Methods,” International Journal of Remote Sensing 33(13):4094–4134.

10 Passive microwave-based SWE measurements date back to 1978 by the Nimbus-7 satellite using the Scanning Multi-channel Microwave Radiometer (SMMR) sensor, followed by the inter-calibrated sensors of the special sensor microwave/imager (SSM/I) onboard the Defense Meteorological Satellite Program (DMSP), the advanced microwave scanning radiometer for Earth Observing System (AMSR-E) on the Aqua spacecraft of NASA’s EOS, AMSR2 on JAXA’s Global Change Observation Mission 1st-Water (GCOM-W1), and the Chinese FengYung series. (See M. Taheri and A. Mohammadian, 2022, “An Overview of Snow Water Equivalent: Methods, Challenges, and Future Outlook,” Sustainability 14:11395, https://doi.org/10.3390/su141811395.)

11 Radar measurements will not work for wet snow, but lidar integrated with modeled snowpack density can provide adequate estimates of changes in SWE throughout the melt season. Snow albedo from about 0.4 to 2.2 microns is needed to estimate net shortwave fluxes. Snow surface temperature from multi- or hyperspectral thermal sensors (8–14 microns) is needed to characterize snowpack condition and identify melt onset. For temporal resolution, the optimal would be about 1–5 days (to capture changes in snowpack characteristics that affect albedo and thermal emissivity, sublimation, and synoptic-scale snow accumulation events).

12 J.S. Deems, S.R. Fassnacht, and K.J. Elder, 2006, “Fractal Distribution of Snow Depth from Lidar Data,” Journal of Hydrometeorology 7:285–297.

13 J. Tarricone, R.W. Webb, H.P. Marshall, A.W. Nolin, and F.J. Meyer, 2022, “Estimating Snow Accumulation and Ablation with L-band InSAR,” The Cryosphere Discuss, [preprint], https://doi.org/10.5194/tc-2022-224.

Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
×

    (or possibly C-, X-, and Ku-band SAR instruments that can do InSAR).14 SWE can be directly estimated from C-, X-, and Ku-band SAR instruments15 at high spatial resolution on the order of tens of meters; P-band SAR may also be useful as its longer wavelength can potentially penetrate forests to provide sub-canopy SWE, conditional on explicitly modeling scattering processes within the canopy and in the forest floor. Signals of Opportunity (e.g., SNOOPI16) measurements at P band are constrained by the transmitter orbits and revisit times and latitudes at 50° and below.17

  • Model integration with data from the SAR on ESA’s Sentinel-1 mission has been effective in very steep watersheds (without vegetation).18
  • Any L-band reflectometry should also be able to map changes in SWE as, for instance, from Global Navigation Satellite System Reflectometry (GNSS-R).19
  • Measurement objectives: SWE, surface temperature, wetness condition.
  • Viewing geometry: For SWE: 30°–50° in complex terrain. For measurement of snow depth with lidar: nadir viewing.
  • Spatial and temporal sampling: For SWE: pre-dawn (observations are needed before surficial melt). SWE estimates from radar require dry or refrozen snow; revisit interval of 3–10 days would capture synoptic-scale snow accumulation and ablation event.
  • Spatial resolution: In mountain watersheds where snowmelt runoff is critical, measurements of snow depth and changes in snow water equivalent at high spatial scales may be required to capture the spatial variability in forested landscapes, including forest gap sizes (most frequent, 10–100 m2)20 and complex topography. Scaling studies21 show that 100–250 m is the range of scales at which variance is minimum both in lidar and SAR snowpack measurements in

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14 For background information on synthetic aperture radar, with details on wavelength and frequency, polarization, scattering mechanisms, and interferometry, see Earthdata, 2020, “What Is Synthetic Aperture Radar?,” April 10, https://www.earthdata.nasa.gov/learn/backgrounders/what-is-sar. For background on InSAR, see R. Burgmann, P. Rosen, and E. Fielding, 2000, “Synthetic Aperture Radar Interferometry to Measure Earth’s Surface Topography and Its Deformation,” Annual Review of Earth and Planetary Sciences 28:169–209, https://doi.org/10.1146/annurev.earth.28.1.169.

15 L. Tsang, M. Durand, C. Derksen, et al., 2022, “Review Article: Global Monitoring of Snow Water Equivalent Using High-Frequency Radar Remote Sensing,” Cryosphere 16(9):3531–3573, https://doi.org/10.5194/tc-16-3531-2022.

16 SNOOPI (Signals of Opportunity P-band Investigation) is a 6U CubeSat mission intended to demonstrate and validate the in-space use of P-band signals of opportunity to measure root zone soil moisture and snow water equivalent. See NASA, 2023, “SNOOPI-NASA Earth Science and Technology Office,” https://esto.nasa.gov/invest/snoopi.

17 J.L. Garrison, J. Piepmeier, R. Shah, et al., 2019, “SNOOPI: A Technology Validation Mission for P-band Reflectometry Using Signals of Opportunity,” pp. 5082–5085 in 2019 IEEE International Geoscience and Remote Sensing Symposium, https://doi.org/10.1109/IGARSS.2019.8900351.

18 Y. Cao and A.P. Barros, 2023, “Topographic Controls on Active Microwave Behavior of Mountain Snowpacks, Remote Sensing of Environment,” Remote Sensing of Environment 284:113373, https://doi.org/10.1016/j.rse.2022.113373.

19 See Global Geodetic Observing System, 2022, “GNSS Reflectometry.” Global Geodetic Observing System (GGOS), April 29, https://ggos.org/item/gnss-reflectometry.

20 T.R.H. Goodbody, P. Tompalski, N.C. Coops, J.C. White, M.A. Wulder, and M. Sanelli, 2020, “Uncovering Spatial and Ecological Variability in Gap Size Frequency Distributions in the Canadian Boreal Forest,” Scientific Reports 10:6069, https://doi.org/10.1038/s41598-020-62878-z.

21 G. Blöschl, 1999, “Scaling Issues in Snow Hydrology,” Hydrological Processes 13(14–15):2149–2175.

Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
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    heterogeneous landscapes and complex terrain22,23 and are suitable to capture seasonal snow patterns globally. In forest gaps, snow depth estimates can be obtained from lidar and snow water equivalent can be estimated from SAR volume scattering measurements.24

Suitability of the Large Platform

In general, meeting the need to map SWE at watershed scales in mountain regions, including where forests are present, and mapping and SWE across the highly heterogeneous high-latitude landscapes, including the boreal forests and tundra, will require a constellation of radar,25 or radar and lidar. The notional platform could contribute to that constellation; however, its crossing time would need to be dawn or very early morning, which conflicts with the desired crossing times of measurements associated with other Earth System Explorers. Its altitude would also be problematic for lidar measurements and might be higher than desired for radar measurements.

TERRESTRIAL ECOSYSTEM STRUCTURE

The ESAS 2017 decadal survey identified a candidate Earth System Explorer, “Terrestrial Ecosystem Structure,” to measure the three-dimensional (3D) structure of terrestrial ecosystems, including forest canopy and aboveground biomass and changes in aboveground carbon stock from processes such as wildfire, deforestation, and forest degradation.

Characterization of the 3D structure of land-based vegetation, particularly for forested ecosystems, is needed for multiple areas of research, resource management, and conservation. Canopy and understory structure reflects the species and functional composition of the ecosystem as well as competition for light, water, and nutrients across the landscape. Measurements of ecosystem structure inform rates of primary production, greenhouse gas fluxes, ecosystem disturbances, ecological functioning, carbon balances, and changing land use. Vertical canopy structure defines aspects of wildlife habitat, and surface vegetation structure is critical for quantifying grazing capacities.

The summary below is derived from data in the decadal survey report with additional information provided by the committee along with its assessment of the suitability of the notional large platform.

  • Observational approach: Satellite-based lidar, especially for seasonal phenology (springtime leaf growth and autumn senescence). Biomass separation of perennial forest versus deciduous understory illustrate the needed complexity of canopy measurements. Functionally, it is essential to separate live and dead vegetation for decomposition and full carbon balance algorithms. Observations in Arctic and boreal latitudes are critical for biosphere change detection and require high-inclination orbital tracks for polar coverage.

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22 P.A. Mendoza, T.E. Shaw, J. McPhee, K.N. Musselman, J. Revuelto, and S. MacDonell, 2020, “Spatial Distribution and Scaling Properties of Lidar-Derived Snow Depth in the Extratropical Andes,” Water Resources Research 56(12):e2020WR028480. https://doi.org/10.1029/2020WR028480.

23 S. Manickam and A.P. Barros, 2020, “Parsing Synthetic Aperture Radar Measurements of Snow in Complex Terrain: Scaling Behavior and Sensitivity to Snow Wetness and Landcover,” Remote Sensing 12(3):483, https://doi.org/10.3390/rs12030483.

24 Although the spatial resolution of SAR is on the order of meters, processing is necessary to eliminate noise including spatial averaging, and thus useful resolution is 30 m and above.

25 While radar saturation is a problem with wet snow, it may be used to detect melt onset and snow wetness state, which would be valuable, especially if used with snowpack evolution models since the improved estimates of SWE at the end of the accumulation season determine water availability in the warm season with implications for water resources, food production, enhancing or inhibiting extreme events from persistent floods to droughts and widespread wildfires at high latitudes.

Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
×
  • Measurement objective: Plant canopy density, vertical structure, leaf area index, separation of overstory/understory components.
  • Spatial resolution: 1–10 m to resolve plant canopies.
  • Revisit interval: 1–3 days, especially to resolve spring and fall ecosystem seasonality, and disturbance dynamics.

Suitability of the Large Platform

Extrapolating from GEDI, the lidars baselined for this mission would need to operate at a much lower altitude than that assumed for the notional platform.

ATMOSPHERIC WINDS

The ESAS 2017 decadal survey identified Atmospheric Winds26 as both a candidate for selection as an Earth System Explorer and as a candidate for support via the recommended new “Incubation” line, with the expectation that Incubation investment for lidar technology could achieve sufficient risk reduction to achieve readiness for competition within the Earth System Explorer program element. NASA’s ESD subsequently decided that the lidar and other enabling technologies were sufficiently advanced that they did not need further maturation within the Incubation line.

Implementation of this Targeted Observable would provide 3D wind profiles in the troposphere/Planetary Boundary Layer, a very high priority identified in ESAS 2017,27 as well as the inaugural ESAS decadal survey published in 2007. In recommending the Winds mission, ESAS 2017 noted its potential to improve prediction of high-impact natural hazards such as severe air pollution outbreaks and tropical and winter storms. The mission also is important for renewable wind energy applications and understanding the transport and distribution of global water and carbon in hydrological and energy cycles of the Earth system.

A precursor to Atmospheric Winds is the ESA wind mission, Aeolus, which was launched in April 2018 and carries the first space-based Doppler wind lidar worldwide. Its primary mission objective is to demonstrate the Doppler wind lidar technique for measuring wind profiles from space, intended for assimilation in NWP models in near real time (within 3 hours of sensing). Other applications include the advancement of atmospheric dynamics research and evaluation of climate models. Mission spinoff products are profiles of cloud and aerosol optical properties.

As stated in ESAS 2017, measurement of atmospheric winds is not only important to weather and air quality forecasts but also is fundamental to other components of the Earth system. Wind is a central driver for ocean currents and essential for determining air-sea-land-ice surface fluxes. The summary below is derived from data in ESAS 2017, with additional information provided by the committee along with its assessment of the suitability of the notional large platform.

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26 The ESAS 2017 decadal survey’s “Atmospheric Winds,” also referred to as 3D Winds (three-dimensional winds), involves space-based measurement of vertical profiles of horizontal wind vectors and vertical velocity in convective precipitation.

27 “One of the most pressing science and application priorities in the coming decade is to better observe the properties in the PBL and lower troposphere and improve prediction of high-impact natural hazards such as severe air pollution outbreaks and tropical and winter storms, renewable wind energy applications, transport and distribution of global water, and carbon in hydrological and energy cycles of the Earth system. Observing 3D winds is key to addressing these priorities to meet societal needs.” (From ESAS, 2017, p. 151.)

Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
×
  • Atmospheric 3D winds are an essential expression of the circulation of the atmosphere and the coupling between clouds and the general circulation is central to address cloud and climate grand challenges.28 Large-scale winds also transport energy and water through the atmosphere and, together with vertical motions of convection, are a principal input in quantifying transports of trace gases and other constituents around the globe.
  • Transports by winds are critical inputs to methodologies that invert concentration of trace gases into ecosystem fluxes. Winds are also fundamental to understanding the hydrological cycle and related water resource applications. For example, the narrow ribbons of water-laden tropospheric winds of the subtropics act like rivers of moisture bringing heavy rains and snows to the southwestern United States.
  • Observations of winds in the PBL are critical for better understanding and forecasting of extreme high winds in winter storms, tornadoes, hurricanes, and wind-induced storm surge.

The ESAS 2017 decadal survey report also noted that trade studies might be needed to design the most cost-effective strategy for wind measurements.

  • Measurement objectives: 3D winds in troposphere/PBL for transport of pollutants/carbon/aerosol and water vapor, wind energy, cloud dynamics and convection, and large-scale circulation. A variety of synergistic measurement approaches will likely be required to meet the objectives for Winds as defined in the ESAS 2017.
  • Measurement approaches: Active sensing (lidar, radar, scatterometer); passive imagery or radiometry-based atmospheric motion vectors (AMVs) tracking; or lidar.29
  • Spatial coverage, temporal sampling, and spatial resolution: The specifications for these parameters will depend on the science application/technology demonstration.
  • Horizontal and vertical resolution: For most of the applications listed above, a 3–20 km horizontal resolution with a 0.2–1 km vertical resolution is desirable. The measurement approaches described below are complementary, rather than exclusive.

Suitability of the Large Platform

Below are heritage sensors that inform the potential use of the notional large platform in making the measurements relevant to 3D winds and PBL missions.

  • Wind lidar: The example for this is the demonstration Aeolus Aladin wind lidar mission which launched in 2018. Aeolus exploits a dawn-dusk orbit at 0600, and the platform is actively stabilized over three axes. The notional mission life was projected to be relatively short (~3 years) due to the laser components. Aeolus flies in a 320 km orbit altitude with continuous sampling that yields an effective 87 km horizontal resolution. The 16 orbits per day give a repeat cycle of 7 days with a vertical resolution of 250 m in PBL to 2 km at 20 km. 16 orbits per day, weekly repeat cycle.

    An instrument with Aeolus’ power requirements (840 W) could likely be accommodated on the notional platform, but its planned altitude of 600–800 km would make operation problematic. Aeolus measures the Doppler shift of the collected return signal, backscattered at different levels in the atmosphere. Operation at a very low altitude is required for detectors to have sufficient signal to noise, but also limits the lifetime of the host spacecraft due to atmospheric drag and

___________________

28 S. Bony, B. Stevens, D. Frierson, et al., 2015, “Clouds, Circulation and Climate Sensitivity,” Nature Geoscience 8:261–268, https://doi.org/10.1038/ngeo2398.

29 The ESAS 2017 decadal survey report noted the potential to develop a multi-function lidar that would be designed to address two or more of the Targeted Observables.

Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
×

    limited onboard fuel for boosting. It should also be noted that the volume and mass of Aeolus exceed the assumptions regarding instrument characteristics given to CESAS by NASA.30

  • Passive imagery or radiometry-based atmospheric motion vectors (AMVs): Feature-tracked winds at various levels. Note that tracking of features is well established, but lacks required vertical resolution. These are generally provided by satellites in geostationary orbits, or in polar orbits with complementary viewing angles. As such, they are not a candidate for the notional platform. One exception to this has been demonstrated by the Terra MISR winds which makes use of the multiple viewing angles to derive AMVs at multiple vertical levels using cloud or water vapor tracers (daylight only due to wavelengths used).

There are also heritage sensors for making two-dimensional wind measurements of potential interest:

  • Radar (Ku-band) scatterometer for ocean surface wind vectors: The candidate instrument and desired orbit is informed by the NASA QuikScat mission. QuikScat was placed into a Sun-synchronous polar orbit at an altitude of 803 km; it measured winds in swaths 1,800 km wide on Earth’s surface. QuikScat’s orbit would generally result in twice per day coverage over a given geographic region. The proposed platform’s higher altitude orbit and high inclination orbit could similarly allow for global coverage once per day with high spatial resolution.
  • Passive microwave radiometry for ocean surface vector winds: The COWVR (Compact Ocean Wind Vector Radiometer) follows the Coriolis/WindSat heritage. It is a technological demonstration of a compact multi-polarimetric microwave radiometer and is currently flying on the ISS.31 A COWVR follow-on mission could be a good candidate for the notional platform given COWVR’s notional 3-year life span on the ISS. The proposed platform’s higher altitude orbit and high inclination orbit would be similar to Coriolis/WindSat.

Tables 2-1a and 2-1b provide a summary of the information in this chapter with a focus on the suitability of the SSO notional platform as a host for instruments that would meet the objectives of the survey-recommended Earth System Explorers. Table 2-1a summarizes limitations of the notional platform as a platform to implement the ESEs, or an incubation mission (for Surface Topography and Vegetation); Table 2-1b identifies the ESEs that could be hosted on the notional platform assuming compatibility with the crossing time. Table 2-1b also assumes the platform itself could meet instrument requirements (e.g., pointing accuracy).

___________________

30 The Aeolus payload is 266 kg; its size is 1.74 m × 1.9 m × 2.0 m in launch configuration, limited by the payload envelope. CESAS was asked to assume instruments of 50–200 kg and volume under 1 m3.

31 Designed and built at NASA’s Jet Propulsion Laboratory, and planned as a technology demonstration, COWVR was launched on December 21, 2021, to the ISS as part of a SpaceX commercial resupply mission. The instruments were deployed to the JEM-EF module of the ISS to commence a planned 3-year operation.

Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
×

TABLE 2-1 Implementing Earth System Explorers on the Notional Platform

(a)

Targeted Observable Notional Large Platform in Sun-Synchronous Orbit (600800 km; 98°): Limitations
Greenhouse Gases Detection and quantification of point sources requires daily/sub-daily temporal resolution, necessitating numerous platforms with lower (≤400 km) orbits. Characterization of diurnal variability in CO2 and CH4 fluxes requires either precessing LEO or GEO orbit.
Ice Elevation The optimal orbit inclination is closer to 90° with an altitude of approximately 500 km to enable the lidar measurements. The notional platform would also not follow the orbit of ICESat-2, a consideration for continuity. The platform would need to meet stringent requirements for orbital accuracy and precise laser pointing.
Ocean Surface Winds and Currents Other high-inclination non-SSOs between 82° and 98° may have better diurnal and tidal sampling (to remove the effect of tides).
Ozone and Trace Gases The large platform may not meet requirements for long-term monitoring.
Snow Depth and Snow Water Equivalent Orbits lower in altitude than the assumed 600–800 km orbits are needed for active sensors like lidar and radar. Early morning (crossing time ~6:00 am), near polar SSOs are optimal for measurements of snow depth and SWE. Pointing precision and fixed viewing geometry are critical for SAR revisits as time series is essential to detect change.
Terrestrial Ecosystem Structure Orbital crossing times need to be optimized for cloud-free observations to avoid significant data loss. Narrow lidar swath requires high repeat cycle, especially for seasonal phenology. Lidar on the notional platform problematical given the assumed orbit at 600–800 km altitude.
Atmospheric Winds The assumed altitude provides inadequate signal-to-noise ratio for lidar detectors. Passive imagery requires geostationary sampling or multiple polar orbiting platforms with complementary viewing angles that are not provided by the notional large platform.
Planetary Boundary Layer To meet the objectives, a combination of space-based and in situ measurement approaches will be required. Constrained to a particular orbit, instruments hosted on a single notional platform will be limited in their capability to meet temporal or vertical resolution requirements.
Surface Topography and Vegetation The requirements for spatial resolution, vertical accuracy, revisit time, and event latency are heterogeneous, with three types of instruments (e.g., lidar, radar, and stereogrammetry) contributing. The required host platforms range from drones to low- and high-flying aircraft to spacecraft.

(b)

Targeted Observable Notional Large Platform in Sun-Synchronous Orbit (600800 km; 98°): Potential Uses
Greenhouse Gases Quantifying and trending CO2 and CH4 fluxes at global to regional scale could be achieved with wide-swath, passive infrared spectrometers on a single SSO platform with midday crossing time, 14-day revisit.
Ice Elevation Possible use in the development and space testing of a multibeam lidar (>100 beams).
Ocean Surface Winds and Currents Possible use as a platform for technology demonstration.
Ozone and Trace Gases May be suitable for a HIRDLS-type instrument, provided the optical path is not blocked by the structure or other sensors.
Snow Depth and Snow Water Equivalent Could contribute to meeting this Targeted Observable as part of a constellation.
Terrestrial Ecosystem Structure Sensors drawing on the heritage of MODIS visible/near infrared would be complementary with leaf area index measurements and could provide continuity.
Atmospheric Winds Notional platform could augment polar orbiting platforms with complementary viewing.
Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
×
Targeted Observable Notional Large Platform in Sun-Synchronous Orbit (600800 km; 98°): Potential Uses
Planetary Boundary Layer Possible use for technology incubation/demonstration projects relevant to address the planetary boundary layer Targeted Observable.
Surface Topography and Vegetation There are perhaps several areas where the notional platform could contribute. These would most likely be related to global stereogrammetry at moderate spatial resolution (1–10 m) and multibeam lidar development. Data volumes from these instruments are much larger than most downlink capabilities, and a notional platform with onboard processing capability would provide needed capabilities. It could also be used to explore onboard processing and data fusion methods to reduce downlink requirements (e.g., see onboard processing for the NASA SWOT mission).

NOTE: GEO, geostationary Earth orbit; HIRDLS, High Resolution Dynamics Limb Sounder; ICESat-2, Ice, Cloud and land Elevation Satellite; LEO, low Earth orbit; MODIS, Moderate Resolution Imaging Spectroradiometer; SAR, Synthetic Aperture Radar; SSO, Sun-synchronous orbit; SWE, snow water equivalent; SWOT, Surface Water and Ocean Topography.

Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
×
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Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
×
Page 17
Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
×
Page 18
Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
×
Page 19
Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
×
Page 20
Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
×
Page 21
Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
×
Page 22
Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
×
Page 23
Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
×
Page 24
Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
×
Page 25
Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
×
Page 26
Suggested Citation:"2 Suitability of the Large Platform for Earth System Explorers." National Academies of Sciences, Engineering, and Medicine. 2023. Assessment of Commercial Space Platforms for Earth Science Instruments: Report Series—Committee on Earth Science and Applications from Space. Washington, DC: The National Academies Press. doi: 10.17226/27019.
×
Page 27
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Space-based Earth observations enable global observations of the land surface, biosphere, solid Earth, atmosphere, cryosphere, and oceans. Earth observations from space, combined with data acquired from in situ and ground-based instruments, help scientists understand the components of the Earth system and their interactions and enable wide-ranging applications, including forecasts of weather and air quality, projections of future climate, management of natural resources, ecological forecasting, disaster management, drought and wildfire prediction, and the mapping and prediction of vector borne/animal diseases.

At the request of NASA Earth Science Division, this report assesses the potential use of a proposed multi-user, robot-tended, uncrewed commercial space platform as a potential host for a large number of Earth remote sensing instruments. Assessment of Commercial Space Platforms for Earth Science Instruments evaluates the utility and practicality of a platform in a Sun-synchronous orbit, capable of hosting 20 or more instruments.

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