1. Moderate spatial resolution (30–45 m ground sample distance, hyperspectral resolution (10 nm; 400–2,500 nm), high fidelity (signal-to-noise ratio = 400:1 VNIR/250:1) imaging spectrometer is needed for characterizing land, inland aquatic, coastal zone, and shallow coral reef ecosystems.
2. 30–60 m TIR observations in the 10.5–11.5 µm and 11.5–12.5 µm spectral region are needed with a 2–4 day revisit frequency.

• TIR: 665 km SSO, 13:30 local time (supports measurement of evapotranspiration), 3–5 day revisit time.
• VSWIR: 632 km SSO, 10:45 local time, revisit time 16–22 days.

For details, see SBG working group Science and Applications Traceability Matrix.^d

• Option 1: Quantifying and trending CO₂ and CH₄ fluxes at global to regional scale could be achieved with wide-swath, passive IR spectrometers on a single SSO platform with midday crossing time, 14-day revisit, higher (≥700 km) orbit; or
• Option 2: Characterization of nocturnal and high-latitude CO₂ and CH₄ fluxes with narrow swath lidar sensors in lower (≤500 km) orbit and early morning/evening crossing times.

GHG objectives not achievable with this approach: (a) detection and quantification of point sources at high spatial resolution and daily/subdaily temporal resolution over large regions (requires many platforms with lower (≤400 km) orbits), (b) characterization of diurnal variability in CO₂ and CH₄ fluxes (requires either precessing LEO or GEO orbit).

Global coverage, including the polar ocean, requires high-inclination orbits. Most scatterometer missions are Sun synchronous (orbit inclination ~98°), which is sufficient to meet WaCM goals, provided Sun-synchronous signals, such as tides, can be removed reliably. While the elevation changes due to tides are well known in the deep ocean,^f their surface velocity expression has been less validated. Other

high-inclination non-SSOs in the range between 82° and 98°, which may have better diurnal and tidal sampling, would also meet WaCM observation requirements.”^g

Additional notes:

• One of WaCM’s cost drivers is the radar radio frequency source, because power drives the size and complexity of the spacecraft.^g
• The notional platform’s equator crossing time could affect its suitability as a host for WaCM, particularly if it is envisioned as part of a global constellation.

Snow depth may be inferred from lidar measurements (snow-on minus snow-off) and change in SWE from L-band InSAR (or possibly C-, X-, and Ku-band SAR instruments that can do InSAR). SWE can be estimated from C-, X-, and Ku-band SAR instruments at high-spatial resolution on the order of tens of meters; P-band SAR may also be useful as it can penetrate forests to provide subcanopy SWE. Model integration with SAR has been effective in very steep watersheds (without vegetation). The optimal orbit for such measurements is a Sun-synchronous/near-polar orbit with a 6:00 am crossing.

In general, meeting the need to map SWE at watershed scales in mountain regions, including where forests are present, and mapping SWE across large spatial extent globally, will require a constellation of radars. Measurement needs:

• Crossing time: 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 snowstorm frequency and snow accumulation and ablation.
• Pointing precision and fixed viewing geometry are critical for SAR revisits as time-series is essential to detect change.
• Any L-band reflectometry should also be able to map changes in SWE as, for instance, from GNSS-R.

Spatial Resolution for Snow Depth and SWE: In mountain watersheds where snowmelt runoff is critical, snow depth and changes in SWE are needed at a spatial resolution of 20–100 m in order to capture the spatial variability in forested and complex topography.

Synergistic measurement approaches will likely be required, thus limiting the notional large platform to meet this goal:

Active techniques:

• Lidar: 3D winds can be retrieved using Doppler wind lidar, which provides line-of-sight winds with good vertical resolution by tracking aerosols which are always present. However, the 600–800 km altitude of the notional platform does not support this measurement. ESA’s Aeolus mission uses a direct detection UV laser lidar, which required placement in a very low-altitude (~320 km) orbit (SSO with an early morning crossing time (dawn-dusk) to provide maximum illumination from the Sun and a stable thermal environment). Observations of the wind will be taken from the night side of the satellite to avoid the solar background. While orbiting over the hemisphere experiencing winter, the satellite will enter Earth’s shadow for up

to 20 minutes per orbit. This means that the satellite will be subjected to huge temperature changes as it passes from day to night. The thermal design is, therefore, robust.^h

• Ku-band scatterometer (e.g., SeaWinds on QuickScat, which is in a SSO at ~809 km with a crossing time of ~6:00 am). Provides ocean surface wind speeds (horizontal) and direction.

Passive:

• Imagery: MISR on EOS Terra (SSO, 705 km, 10:30 am) acquires imagery using cloud or water vapor tracers (daylight only due to wavelengths used) at 275 m resolution at nine angles ranging from 0° (nadir) to 70° off-nadir. This multi-angle capability facilitates the stereoscopic retrieval of heights and AMVs for clouds and aerosol plumes.
• Radiometry: Microwave emission from the ocean surface varies based on wind speed and wind direction. Polarimetric microwave observations made by WindSat (SSO, 840 km, 98.7°) and COWVR, which is operating on the ISS, have demonstrated this technique for measuring ocean surface wind vectors. However, the presence of significant cloud liquid water presents significant challenges for the passive polarimetric technique and thus limits its utility in supporting operational marine weather forecasting and warning.ⁱ

STV requirements from the ESAS 2017 decadal survey are in the Incubation category. The requirements, and possible observational strategies, are currently being defined by the NASA STV study team.^j There are five areas of observations needed: solid Earth topography and change; vegetation structure and change; cryosphere topography and change; hydrology topography/shallow water bathymetry and change; and coastal processes and change. As shown in the STV report’s Science and Applications Traceability Matrix, the requirements for spatial resolution, vertical accuracy, revisit time, and event latency are heterogeneous. Moreover, three types of instruments will contribute to this Targeted Observable, including lidar, radar, and stereogrammetry. The required host platforms range from drones to low- and high-flying aircraft to spacecraft (mostly commercial). However, a conclusion from the STV report is that the commercial sector does not provide the global products needed to address this Targeted Observable.

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. In addition, the data volumes from these instruments are much larger than most downlink capabilities. 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).

Climate sensitivity

Inter-calibration of inflight radiometers

Noting heritage to CLARREO, ESAS 2017 decadal survey states, “Best achieved by seeking lower-cost options, such as Venture-Continuity, to enable multi-decade continuity of this important measurement.” The committee notes the potential utility of the notional platform as a host for instruments selected as part of NASA’s Earth Venture, with the EV-C strand being of particular interest. EV-C is directed toward long-term acquisition of key continuity observations through innovative technology infusion.

Many of the factors that would determine whether the notional platform is a viable option for continuity measurements are beyond the scope of this study. The recommendation from ESAS 2017—one the committee fully supports—is that NASA ESD, “lead the development of a more formal continuity decision process (as in NASEM, 2015^l) to determine which satellite measurements have the highest priority for continuation, then work with U.S. and international partners to develop an international strategy for obtaining and sharing those measurements.”^b