The Soil Moisture Active Passive (SMAP) mission is one of the first Earth observation satellites being developed by NASA in response to the National Research Council?s Decadal Survey [1]. Its mission design consists of L-band radiometer and radar instruments sharing a rotating 6-m mesh reflector antenna to provide high-resolution and high-accuracy global maps of soil moisture and freeze/thaw state every 2-3 days. The combined active/passive microwave soil moisture product will have a spatial resolution of 10 km and a mean latency of 24 hours. In addition, the SMAP surface observations will be combined with advanced modeling and data assimilation to provide deeper root zone soil moisture and net ecosystem exchange of carbon. SMAP is expected to launch in the late 2014 - early 2015 time frame

Entekhabi, Dara

Kellogg, Kent

Njoku, Eni

O'Neill, Peggy

NASA Technical Reports Server

 
 
THE NASA SOIL MOISTURE ACTIVE PASSIVE (SMAP) MISSION:   
OVERVIEW 
 
Peggy O’Neill1, Dara Entekhabi2, Eni Njoku3, and Kent Kellogg3 
 
1Hydrological Sciences Branch / Code 614.3, NASA Goddard Space Flight Center, Greenbelt, MD  20771  USA 
tel:  1-301-614-5773, fax:  1-301-614-5808,  Peggy.E.ONeill@nasa.gov 
 
2 Dept. of Civil & Environmental Engineering, Massachusetts Institute of Technology 
Cambridge, MA  02139  USA, tel: 1-617-253-9698  darae@mit.edu 
 
3Jet Propulsion Laboratory, 4800 Oak Grove Drive, 
Pasadena, CA  91109  USA, tel:  1-818-354-3693, Eni.G.Njoku@jpl.nasa.gov, 
tel:  1-818-354-5386, kkellogg@jpl.nasa.gov 
 
 
 
ABSTRACT 
The Soil Moisture Active Passive (SMAP) mission is 
one of the first Earth observation satellites being developed 
by NASA in response to the National Research Council’s 
Decadal Survey [1].  Its mission design consists of L-band 
radiometer and radar instruments sharing a rotating 6-m 
mesh reflector antenna to provide high-resolution and high-
accuracy global maps of soil moisture and freeze/thaw state 
every 2-3 days. The combined active/passive microwave 
soil moisture product will have a spatial resolution of 10 
km and a mean latency of 24 hours.  In addition, the SMAP  
surface observations will be combined with advanced 
modeling and data assimilation to provide deeper root zone 
soil moisture and net ecosystem exchange of carbon.  
SMAP is expected to launch in the late 2014 – early 2015 
time frame. 
 
Keywords (Index Terms) -- soil moisture, microwave 
radiometry, radar, freeze/thaw. 
1.  INTRODUCTION 
The National Research Council’s (NRC) 
Decadal Survey, Earth Science and Applications 
from Space: National Imperatives for the Next 
Decade and Beyond, was released in 2007 after a 
two-year study commissioned by NASA, NOAA, 
and USGS to provide consensus recommendations 
to guide the agencies’ space-based Earth 
observation programs in the coming decade [1].  
Many factors involving engineering maturity, 
scientific advances, and societal benefits of 
potential missions were considered as part of the 
NRC evaluation process.  Five of the six NRC 
Earth science discipline panels (water resources & 
the hydrologic cycle; climate; weather; human 
health & security; and land use, ecosystems, & 
biodiversity) cited numerous science and 
applications needs that could be wholly or 
partially met by a mission devoted to measuring 
surface soil moisture and its freeze/thaw state.  
Based on the NRC recommendations and on its 
own evaluation of technical readiness, NASA 
selected the Soil Moisture Active Passive (SMAP) 
mission to be the first of the Decadal Survey 
missions to be developed, with a launch date now 
in the 2014-2015 time frame.  This mission will be 
a joint effort of NASA’s Jet Propulsion 
Laboratory (JPL) and Goddard Space Flight 
Center (GSFC), with project management 
responsibilities at JPL. 
2.  SMAP SCIENCE 
Soil moisture and its freeze/thaw state are key 
variables that impact the water, energy, and 
carbon fluxes at the land-atmosphere interface.  
This interface is highly complex, and new data are 
https://ntrs.nasa.gov/search.jsp?R=20110015242 2019-08-30T17:13:38+00:00Z
required at finer space-time scales to advance 
understanding of the processes driving the Earth’s 
water, energy, and carbon cycles.  The primary 
science objectives of the SMAP mission are to 
provide high-resolution global mapping of soil 
moisture and its freeze/thaw state in order to: (1) 
estimate global water and energy fluxes at the 
Earth’s surface, (2) improve weather and climate 
forecast skill, (3) develop more accurate flood and 
drought predictions, (4) quantify carbon fluxes in 
boreal landscapes, and (5) help link together 
terrestrial water, energy, and carbon cycle 
processes.  SMAP also enables a large number of 
other science research areas and applications of 
benefit to society, including the potential to 
provide JPSS / DWSS - era soil moisture 
measurements, vegetation growth and agricultural 
productivity estimates, and information relevant to 
heat stress, human health, and national security. 
 
 
Figure 1.  The SMAP mission concept consists of an 
L-band radar and radiometer sharing a single 
spinning 6-m mesh antenna in a sun-synchronous 
dawn/dusk orbit.   
3.  SMAP MISSION DESIGN 
The SMAP mission design is driven by the 
temporal and spatial resolution requirements of 
the hydrometeorology, hydroclimatology, and 
carbon cycle communities [2].  SMAP will consist 
of a polarimetric 1.4 GHz radiometer and a 1.26 
GHz (HH, VV, HV pol) radar sharing a single L-
band feed and a 6-meter spinning deployable 
mesh reflector antenna.  The antenna rotates about 
the nadir axis at 14.6 rpm, producing a conically 
scanned antenna beam with a surface incidence 
angle of ~ 40⁰  (Figure 1).  SMAP will be 
launched into a sun-synchronous dawn-dusk orbit, 
and its wide swath of 1000 km will enable global 
mapping of surface soil moisture and freeze/thaw 
every 2-3 days.  The SMAP radiometer will 
produce calibrated brightness temperatures at a 
coarse spatial resolution of 40 km, while the 
SMAP radar will produce calibrated backscatter at 
1-3 km resolution over the outer 70% of the swath 
(radar resolution degrades to 30 km at center of 
swath).  By combining the high soil moisture 
retrieval accuracy but coarse resolution radiometer 
data with the high resolution but lower accuracy 
radar data, an intermediate resolution 10 km soil 
moisture product will be generated.  Baseline 
mission duration is three years in order to capture 
positive and negative seasonal anomalies in the 
water, energy, and carbon cycles across 
environmentally diverse regions of the globe. 
 
4.  SMAP GEOPHYSICAL DATA 
PRODUCTS 
 
The planned SMAP data products are listed in 
Table 1.  Level 1B and 1C data products are 
calibrated and geolocated instrument 
measurements of radar backscatter and brightness 
temperature.  Level 2 products are geophysical 
retrievals of soil moisture on a fixed Earth grid 
based on Level 1 products and ancillary 
information; the Level 2 products are output on a 
half-orbit basis.  Level 3 products are daily 
composites of Level 2 surface soil moisture and 
freeze/thaw state.  Retrieval algorithms for 
these geophysical products are currently being  
refined and tested, and vetted in open community  
Table 1.  SMAP DATA PRODUCTS 
 
  
workshops [3].  The SMAP Project is also 
generating two model-derived value-added Level 
4 data products (root zone soil moisture and net 
ecosystem exchange of carbon) that support key 
SMAP applications and more directly address 
driving science questions.  The baseline mission 
requirements are to provide estimates of soil 
moisture in the top 5 cm of soil with an error no 
greater than 0.04 cm3/cm3 volumetric (excluding 
regions of snow and ice, frozen ground, 
mountainous topography, open water, urban areas, 
and vegetation with water content greater than 5 kg 
m-2), and a binary freeze/thaw classification 
accuracy of 80%.  These levels of performance 
will enable SMAP to meet the needs of the 
hydroclimatology and hydrometeorology 
applications identified in the NRC report [1], and 
to provide the new global data sets necessary to 
tackle hydrologically-relevant societal issues. 
The latency of SMAP mission products is 
important to a variety of end users.  As a non-
operational research mission, SMAP is 
constrained by budget and other logistical issues, 
and will make mission data available to the public 
archive (to be selected by NASA HQ) as quickly 
as possible on a best-effort basis. Mean latencies 
for the standard mission products are listed in 
Table 1.  These products will be verified during an 
extended calibration/validation period during the 
first year SMAP is in orbit.  Calibration/validation 
activities will include comparisons of SMAP 
products with data from in situ instrument 
networks, dedicated field campaigns, aircraft 
underflights, models, and other missions. 
 
5.  SMAP APPLICATIONS OUTREACH 
A key challenge for SMAP as the first of 
NASA’s Earth Science Decadal Survey missions 
is to foster applications and applied sciences given 
(1) budget constraints, (2) long development 
horizons, and (3) the primacy of science over 
*  Mean latency under normal operating conditions.  Latency defined as time from data acquisition by instrument to  
    availability to designated archive.  The SMAP project will make a best effort to reduce these latencies. 
applications in driving the mission measurement 
requirements.  A primary goal of the SMAP 
Mission is to engage SMAP end users early in the 
pre-launch phase and begin to prepare them to 
take advantage of SMAP data as soon as SMAP is 
launched.  Broad support for SMAP applications 
is sought through a transparent and inclusive 
process.  Toward this goal, the SMAP Project has 
formed a SMAP Applications Working Group, 
which is open to all interested members of the 
community (register at http://smap.jpl.nasa.gov/ 
science/applicWG).  [Information about other 
SMAP working groups on Algorithms, RFI, and 
Calibration/Validation can be found at 
http://smap.jpl.nasa.gov/science/wgroups/.] 
 The first SMAP Applications Workshop was 
held at NOAA (National Oceanographic and 
Atmospheric Administration) facilities in Silver 
Spring, MD on September 9-10, 2009 [4].  
Discussions during this meeting formed the basis 
for the SMAP Applications Plan, currently in draft 
form.  
 
6.   SUMMARY 
SMAP is scheduled to be the first of NASA’s 
Earth Science Decadal Survey missions, with a 
launch date in the late 2014 – early 2015 time 
frame.  It will provide high resolution and 
frequent revisit global mapping of soil moisture 
and freeze/thaw state, utilizing enhanced Radio 
Frequency Interference (RFI) mitigation 
approaches to collect new measurements of the 
hydrological condition of the Earth’s surface.    
These observations will (1) improve our 
understanding of linkages between the Earth’s 
water, energy, and carbon cycles, (2) benefit many 
application areas including numerical weather and 
climate prediction, flood and drought monitoring, 
agricultural productivity, human health, and 
national security, (3) help to address priority 
questions on climate change, and (4) potentially 
provide continuity with brightness temperature 
and soil moisture measurements from ESA’s 
SMOS (Soil Moisture Ocean Salinity) and 
NASA’s Aquarius missions.   
 
 
 
 
Figure 2.  1st SMAP Applications Workshop, Silver 
Spring, MD, September, 2009. 
 
 
 
REFERENCES 
[1]  National Research Council, “Earth Science 
and Applications from Space: National 
Imperatives for the Next Decade and 
Beyond,” pp. 400, 2007. 
 
[2]  Entekhabi, D., E, Njoku, P. O’Neill, K. 
Kellogg,  plus 19 others, “The Soil Moisture 
Active Passive (SMAP) Mission,”  
Proceedings of the IEEE
 
, Vol. 98, No. 5, 
May, 2010. 
 [3]   O’Neill, P., M. Moghaddam, and T. Jackson, 
“SMAP Algorithms and Calibration / 
Validation Workshop,” The Earth Observer, 
NASA, 21 (5), October, 2009, pp. 40-43, 
49. 
 
[4]   Moran, S., P. O’Neill, D. Entekhabi, E. 
Njoku, and K. Kellogg, “Fostering 
Applications Opportunities for the NASA 
Soil Moisture Active Passive (SMAP) 
Mission,”  Proceedings of IGARSS’10, 
Honolulu, HI, July 25-30, 2010. 
 
 


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