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Soil Moisture Active Passive (SMAP)


Launched on 31 January 2015, the NASA Soil Moisture Active Passive (SMAP) mission is designed to principally measure soil moisture and freeze/thaw state from space for all non-liquid water surfaces globally within the top layer of the Earth. The mission additionally provides a value-added Level 4 terrestrial carbon dataset derived from SMAP observations.

SMAP is now also building upon the legacy of Aquarius/SAC-D mission in delivering both soil moisture and derived sea surface salinity (SSS) observations for the world’s oceans. With the loss of the Aquarius mission on 7 June 2015 it became critical also to continue the time series of global salinity observations important to studies of the Earth’s water cycle.  Because both Aquarius and SMAP shared a L-band feed-horn configuration, lessons learned from the algorithm development under the Aquarius mission could be applied to SMAP to retrieve SSS via SMAP. However, because of the larger swath coverage, spatial resolutions under SMAP are approximately 40 km instead of 100 km with Aquarius. The increased spatial coverage provides opportunities for applying SMAP data for higher resolution studies than Aquarius. With the initiation of SMAP science operations and data flows in April of 2015, the approximate 3-month overlap period between SMAP and Aquarius also allows for inter-calibration and comparative studies.

The primary SMAP salinity datasets include a Level 2 orbital dataset, in which data granules contain both the ascending and descending arcs of the orbit, and two Level 3 gridded datasets: an 8-day running average (linked to the day repeat cycle of SMAP) and monthly average. SMAP SSS data are archived and distributed via the PO.DAAC. SMAP soil moisture and L4 carbon datasets are available from the National Snow and Ice Data Center (NSIDC), with Level 1 SMAP radar data distributed by the Alaska Satellite Facility (ASF). These DAACs are the official NASA repositories for SMAP mission data.

  • Radiometer – L-band microwave radiometer to measure the brightness temperature of microwave emissions from the ocean surface with a center frequency of 1.41 GHz.
  • Radar – L-band (1.26 GHz) radar to measure the echoes of very short radio frequency (RF) pulses that bounce ("backscatter") off the Earth's surface. The radar uses a special technique known as "synthetic aperture" to resolve the RF backscatter over much smaller surface areas that would otherwise be possible.
News and Announcements (chronological order)

Remote Sensing Systems (RSS) SMAP Sea Surface Salinity (SSS) V4.0 Datasets Release

Thursday, August 29, 2019

DOI:  10.5067/SMP40-2SOCS

The PO.DAAC is pleased to announce the release of the version 4.0 SMAP Sea Surface Salinity (SSS) data from Remote Sensing Systems (RSS).  Enhancements over the previous release include:

  • Improved land correction allows for salinity retrievals within 30 – 40km of the coast. The land correction uses a land correction table at 0.125 degree spatial resolution.  The land emission that is used in the land correction is based on SMAP TB.
  • Replacement of the previous NCEP sea-ice mask with one based on RSS AMSR-2 and implementation of a sea-ice threshold of 0.3% (gain weighted sea-ice fraction).
  • Revised solar flagging that depends on glint angle and wind speed.
  • Inclusion of estimated SSS-uncertainty in the L3 products.

Datasets comprising this release include Level 2C and L3 mapped monthly and 8-day running mean SMAP-SSS. All products are in the netCDF4 file format and are CF/ACDD metadata standards compliant. Note that unlike in prior releases, the 40-km and 70-km fields have been consolidated as variables within a single data product rather than being made available as distinct datasets.  The 70-km data are based on a smoothing of the 40-km product, which is retrieved directly from the geophysical model function (Meissner et al. 2018). The 70-km version should be considered the official product with the 40-km included for experimental purposes.  For most open ocean applications, the 70-km fields are the best used since they have significantly lower noise than the 40-km data.  We also recommend the use of the V4.0 forward stream data over the prior version, which is deprecated and will be retired within the next 6 months.

The RSS SMAP-SSS V4.0 datasets are described, discoverable, and accessible via the PO.DAAC data portal, as are the associated technical documentation. General information regarding the SMAP mission is available from the mission website  and also via PO.DAAC’s SMAP  and salinity webpages.

Reference: Meissner, T., F.J. Wentz, and D.M. Le Vine, 2019, The Salinity Retrieval Algorithms for the NASA Aquarius Version 5 and SMAP Version 3 Releases, Remote Sensing 10, 1121, doi:10.3390/rs10071121.

Citation:  Meissner, T., F. J. Wentz, A. Manaster, R. Lindsley, 2019:  Remote Sensing Systems SMAP Ocean Surface Salinities [Level 2C, Level 3 Running 8-day, Level 3 Monthly], Version 4.0 validated release. Remote Sensing Systems, Santa Rosa, CA, USA. Available online at, doi: 10.5067/SMP40-2SOCS,10.5067/SMP40-3SPCS, 10.5067/SMP40-3SMCS

JPL SMAP Sea Surface Salinity (SSS) CAP V4.2 Dataset released

Friday, January 25, 2019

The PO.DAAC is pleased to announce the availability of the PI-produced JPL V4.2 SMAP Sea Surface Salinity (SSS) and extreme winds data, which includes Level-2B (L2B) and Level 3 (L3) standard datasets based on the JPL Combined Active-Passive (CAP) algorithm applied to data from the NASA Soil Moisture Active Passive (SMAP) observatory.  Included as part of this release is also a near real-time L2B dataset, available with a latency of about 4.3 hours as a 8-day rolling store.  For science applications, use of the standard products is strongly advised.  We also recommend the use of the V4.2 forward stream data over the prior version, which is deprecated and will be retired within the next 6 months.

SMAP SSS V4.2 accommodates changes in the underlying Level 1B-TB data resulting from the SMAP mission data reprocessing in June 2018. Updates with version 4.2 include: improvement in the brightness temperature (TB) calibration using an adjusted reflector emissivity, inclusion of a Sea Surface Temperature (SST) dependent flat surface emissivity model, use of updated land correction tables, and inclusion of averaged ice concentration ancillary data in the V4.2 datasets.  Data begins on 1 April 2015 and is ongoing.  Data sets comprising this release include the L2B orbital data and two L3 mapped, global coverage salinity datasets: an 8-day running mean dataset based on the repeat orbit of the SMAP mission, along with a monthly average dataset.  The L2B data are provided on a 25 km swath grid while the L3 data are gridded at 0.25o x 0.25o. The spatial resolution of all four datasets is approximately 60 km.  The L2B dataset is in HDF5 format, and the L3 datasets are in netCDF4 file format.  All datasets are CF/ACDD metadata standards compliant, and include estimated SSS uncertainty fields.

See all Official NASA SMAP Mission News
See all PO.DAAC Announcements


Sea Surface Salinity from SMAP RSS V4 (2015-2019)

Animation of sea surface salinity from 27 March 2015 to 13 August 2019 based on the 8-day running mean version 4.0 Level 3 NASA Soil Moisture Active Passive (SMAP) dataset from Remote Sensing Systems (RSS). The dataset can be accessed from the PO.DAAC Portal at (DOI:10.5067/SMP40-3SPCS).