SMAP – Science Overview, Requirements, and Measurements

Overview

SMAP’s spaceborne Earth-observation mission will enable global mapping of soil-moisture and freeze-thaw state with unprecedented accuracy, resolution, and coverage. SMAP science objectives are to acquire space-based, hydrosphere-state measurements over a three-year period to:

  • Understand processes that link the terrestrial water, energy, and carbon cycles
  • Estimate global water and energy fluxes at the land surface
  • Quantify net carbon flux in boreal landscapes
  • Enhance weather-forecast and climate-forecast skills
  • Develop improved flood-prediction and drought-monitoring capabilities 

Water and Energy Cycles, Weather, and Climate

Recent model simulations of the effects of greenhouse gases on climate show that current models agree quite well in predicting temperature change but disagree significantly in predicting surface-moisture change and water-resource availability. Accurate soil-moisture information, such as data from SMAP, will improve the performance and enhance the predictive ability of numerical weather-prediction models and seasonal climate models. Soil moisture is a key control on evaporation and transpiration at the land-atmosphere boundary. Because vaporizing water requires large amounts of energy, soil-moisture control also has a significant impact on the surface energy flux. Soil-moisture variations affect the evolution of weather and climate, particularly over continental regions.

Carbon Cycle and Ecosystems

Soil moisture and its freeze-thaw state are also key determinants of the global carbon cycle. Carbon uptake and release in boreal landscapes is one of the major sources of uncertainty in assessing the carbon budget of the Earth system (the so-called missing carbon sink). The SMAP mission will quantify the nature, extent, timing, and duration of landscape seasonal freeze-thaw state transitions that are key to the estimation of terrestrial carbon sources and sinks. SMAP freeze-thaw state measurements will also contribute to understanding how ecosystems respond to and affect global environmental change, improving regional mapping and prediction of boreal-Arctic ecosystem processes.

SMAP Goals

The SMAP Project is designed to collect measurements of surface soil moisture and freeze-thaw state, together termed the hydrosphere state. Soil moisture is defined in terms of volume of water per unit volume of soil. Freeze-thaw state is defined as the phase of the water contained within the landscape including soil and vegetation. To meet the goals of science and applications users, SMAP must:

  • Resolve hydrometeorological water and energy flux processes and extend weather and flood forecast skill, spatial resolution of 10 km and temporal resolution of 3 days are required.
  • Resolve hydroclimatological water and energy flux processes and extend climate and drought forecast capability, spatial resolution of 40 km and temporal resolution of 3 days are required.
  • Quantify net carbon flux in boreal landscapes, spatial resolution of 3 km and temporal resolution of 2 days are required. In addition, the SMAP mission will validate a space-based measurement approach that could be used for future systematic hydrosphere state monitoring missions.

SMAP News

NASA Soil Moisture Radar Ends Operations; Mission Science Continues
NASA Focused on Sentinel as Replacement for SMAP Radar 

Earth’s water cycle involves the transfer and storage of water in the atmosphere, on the planet’s surface, underground, and by life in its many forms.

Water availability is changing as a result of global climate change. SMAP data will help researchers understand how these changes affect water supply and food production.
Credit: NASA JPL

The global carbon cycle is the complex interaction of different carbon-based gases taking place among Earth’s atmosphere, land, and oceans.

Baseline Science Requirements

  • Estimates of soil moisture in the top 5 cm of soil with an error of no greater than 0.04 cm3/cm3 (one sigma) at 10-km spatial resolution and 3-day average intervals over the global land area, excluding regions of snow and ice, frozen ground, mountainous topography, open water, urban areas, and vegetation with water content greater than 5 kg/m2 (averaged over the spatial resolution scale).
  • Estimates of surface binary freeze-thaw state in the region north of 45N latitude, which includes the boreal forest zone, with a classification accuracy of 80% at 3-km spatial resolution and 2-day average intervals.
  • Space-based measurements of soil moisture and freeze-thaw state for at least 3 years to allow seasonal and interannual variations of soil moisture and freeze-thaw to be resolved.
  • The SMAP project shall conduct a calibration and validation program to verify data delivered meets the above requirements.

SMAP applied science poster. Credit: NASA/JPL.

Measurements

Approach

The SMAP mission will validate a space-based measurement approach that could be used for future systematic hydrosphere-state monitoring missions.

  • To resolve hydrometeorological water and energy flux processes and extend weather- and flood-forecast skill, spatial resolution of 10 km, and temporal resolution of 3 days are required.
  • To resolve hydroclimatological water and energy flux processes and extend climate- and drought-forecast capability, spatial resolution of 40 km and temporal resolution of 3 days are required.
  • To quantify net carbon flux in boreal landscapes, spatial resolution of 3 km and temporal resolution of 2 days are required.

Accuracy

The science goal is to combine the attributes of the radar observations (high spatial resolution but lower soil-moisture accuracy) and radiometer observations (higher soil-moisture accuracy but coarse spatial resolution).

Joint processing of the radar and radiometer data will retrieve soil moisture at a spatial resolution of 10 km, and freeze-thaw state at a spatial resolution of 3 km.

The provision of constant incidence angle across the 1,000-km swath simplifies the data processing and enables accurate repeat-pass estimation of soil moisture and freeze-thaw.

Orbit Characteristics

The SMAP orbit is a 685-km altitude, near-polar, sun-synchronous, 6 a.m. / 6 p.m., eight-day, exact-repeat, frozen orbit.

  • Near-polar orbit provides global land coverage up to high latitudes including all freeze-thaw regions of interest.
  • Sun-synchrony provides observations of the surface close to the same local solar time each orbit throughout the mission, enhancing change-detection algorithms and scientific accuracy.
  • Consistent 6 a.m. observation time is optimal because it minimizes the effect of Faraday rotation and impact on S/C design.
  • Frozen orbit provides minimal altitude variation during an orbit, benefitting radar design and accuracy.
  • 685-km altitude is an exact 8-day repeat orbit, advantageous for radar change-detection algorithms.
  • Orbit provides optimum coverage of global land area at 3-day average intervals, and coverage of land region above 45N at 2-day average intervals.
 

SMAP spacecraft and its swath path. Credit: NASA/JPL.

SMAP antenna’s scanning pattern. Credit: NASA/JPL.

Image credit: NASA/JPL.

 Image credit: NASA/JPL. 

SMAP travels in a 98-degree 
polar orbit.

SMAP travels in a 98-degree
polar orbit.

Data Acquisition

Low-rate radiometer data and low-resolution radar data will be acquired continuously over fore and aft portions of the scan (full 360 degrees), as well as ascending and descending portions of the orbit.

High-resolution radar data will be acquired to include at a minimum:

  • 360 degrees of the antenna scan (fore and aft looks) for the morning (6 a.m. equator crossing) half-orbit over the global land region (excluding the Antarctic)
  • 180 degrees of the antenna scan (fore look) for the evening (6 p.m. equator crossing) half-orbit over the boreal land region (north of 45 degrees N latitude)
  • 180 degrees of the antenna scan (fore look) for the morning half-orbit over the coastal ocean region (within 1,000 km of continental boundaries)
  • The Science Operations Phase (SOP) begins after completion of the 90-day post-launch, in-orbit commissioning and lasts for three years. The first part of the SOP is the Calibration and Validation (Cal/Val) phase, which lasts for 12 months. Following the Cal/Val phase is the Routine Observations Phase (ROP), which lasts for 24 months.

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