Introduction to the Soil Moisture Active Passive (SMAP) Mission

NASA’s Soil Moisture Active Passive (SMAP) mission, initiated in February 2008 and planned for launch in January 2015, will deliver a global map of soil moisture and freeze/thaw conditions with extraordinary accuracy, high-resolution, and coverage over a three-year period

NASA’s Soil Moisture Active Passive (SMAP) mission, initiated in February 2008 and planned for launch in January 2015, will deliver a global map of soil moisture and freeze/thaw conditions with extraordinary accuracy, high-resolution, and coverage over a three-year period.

SMAP will gather data on soil moisture states and freeze/thaw states. Image Credit: NASA.

SMAP will gather data on soil moisture states and freeze/thaw states. Image Credit: NASA.

In response to the 2007 National Research Council (NRC) Decadal Survey report recommendations, the SMAP mission strategy aims to support applications research and broad user development. The NRC report suggested that NASA be responsible for ensuring that “emerging scientific knowledge is actively applied to obtain societal benefits” and that there be early and continued interaction among the Earth science community and broader social science organizations and individuals.

According to NASA, SMAP data will enable interpreters across several disciplines to better: (1) understand how terrestrial water, energy and carbon cycles work as a system, (2) estimate global water and energy fluxes at the land surface, (3) quantify net carbon flux in boreal landscapes, (4) enhance weather and climate forecast skill, and (5) develop improved flood prediction and drought monitoring capability. The SMAP science team is working to make the data user-friendly and accessible so that all potential users may benefit from the mission data.

In an interview with Earthzine, Vanessa Escobar, deputy coordinator of SMAP Mission Applications, pointed to the importance of data utilization. “Data does not become useful information unless someone is putting it to use,” Escobar explained, “and it does not have value unless somebody is using it to quantify some improvement.”

Several strategies were put in place to ensure that the SMAP mission overcomes this data issue early on. More than 30 national and international businesses, government agencies, and organizations including U.S. Forest Service, National Drought Mitigation Center, United Nations World Food Programme, and National Agricultural Statistical Service were identified as potential data users and collected to form the Early Adopters group. The Early Adopters group was given a set of synthetic data that mimicked SMAP data to identify and fix possible user issues. Also, the SMAP science team worked with the group to understand how the SMAP data may be formatted and manipulated to better fit their existing models.

“There is no point in NASA creating all of this great data if it doesn’t fit with the user’s models, Escobar said. “We know with the right fit, the data can make a difference in a lot of areas.”

Wade Crow, one of the Early Adopters and research physical scientist at the U.S. Department of Agriculture, said much of the mission’s initial success had to do with interaction between NASA and the Early Adopters in efforts to improve the data format and instrument development.
”The decision to bring in users into the instrument data development early on provided the Early Adopters with more input and weight in the mission, which hasn’t really been seen before,” Crow said.

With feedback from the Early Adopters, the SMAP science team was able to manipulate its original algorithms into a more user-friendly format that appeals to a broader community of end-users and decision-makers with varying plans of application. The wide range of applications will be used in military mobility, oceanography, space environment monitoring, disaster management for flood and drought planning and crop yield estimation for evaluating food security.

With available SMAP data, users can better understand several climate processes. For example, evaporation and transpiration occurring at the land-atmosphere boundary are highly dependent on the soil moisture state. And because significant amounts of energy are required to vaporize water, soil moisture states significantly influence the surface energy flux, which impact the overall energy cycles. It is important to improve understanding of energy cycles and vaporization processes as these processes play a major role in continental weather and climate patterns. SMAP will go beyond the current temperature prediction models that fail to accurately predict surface moisture change and water resource availability. SMAP data will improve the performance of quantitative weather prediction models and heighten their predictive skill of seasonal climate models.

The carbon flux in boreal landscapes is another process of interest to data users. According to NASA, the carbon uptake and release in boreal landscapes is not well understood and poses a knowledge gap in the overall global carbon budget system. SMAP will help fill this gap by providing data on the nature, extent, timing and duration of landscape seasonal freeze/thaw state transitions that are essential for estimating land carbon sources and sinks. These measurements also will improve researchers’ understanding of how ecosystems respond to and affect global environmental change, improving regional mapping and prediction of boreal-arctic ecosystem processes.

After launch, SMAP will fly a near-polar, sun synchronous orbit, crossing the equator at 6:00 a.m. and 6:00 p.m. SMAP has active and passive remote sensing capabilities, using an L-band radar and an L-band radiometer oriented on one feedhorn and deployable lightweight mesh parabolic reflector.

The reflector is offset from nadir and rotates about the nadir axis at 14.6 rpm to provide a conically scanned antenna beam with a surface incidence angle of 40 degrees and a swath that spans 1,000 kilometers. Combining the active and passive remote sensors will provide more accurate estimates of soil moisture and freeze/thaw states at spatial scales that are geared toward climate and weather analysis. Multiple polarizations enable accurate soil moisture estimates to be made with corrections for vegetation, surface roughness, Faraday rotation, and other disruptive barriers. At three-day intervals, SMAP will deliver estimated soil moisture in the top 5 centimeters of soil with an accuracy of 0.04 cm3/cm3 volumetric soil moisture, at 10-kilometer resolution.

Previous L-band instruments have picked up disruptive interference from transmitters operating near the same band. The SMAP team developed a sophisticated microwave radiometer to find and remove radio frequency interference (RFI) so that data measurements are not skewed.

Jeff Piepmeier, associate branch head of the Microwave Instrument and Technology Branch, said, “SMAP includes the most advanced RFI mitigating radiometer to ever go up in space,” adding that “developing such high-level RFI required as much time and effort as developing the actual SMAP instrument,” which has taken about seven years.

After 90 days of orbit, a three- to six-month calibration and validation phase will occur to ensure the completeness and accuracy of the data. Subsequent calibration and validation phases will take place more quickly. The data will be available to the public through two NASA-designated Earth science data centers, the Alaska Satellite Facility and the National Snow & Ice Data Center.