Data Set ID:
SPL2SMP

SMAP L2 Radiometer Half-Orbit 36 km EASE-Grid Soil Moisture, Version 7

This Level-2 (L2) soil moisture product provides estimates of global land surface conditions retrieved by the Soil Moisture Active Passive (SMAP) passive microwave radiometer during 6:00 a.m. descending and 6:00 p.m. ascending half-orbit passes. SMAP L-band brightness temperatures are used to derive soil moisture data, which are then resampled to an Earth-fixed, global, cylindrical 36 km Equal-Area Scalable Earth Grid, Version 2.0 (EASE-Grid 2.0).

This is the most recent version of these data.

Version Summary:

Changes to this version include:
- Improved calibration methodology was applied to the Level-1B radiometer brightness temperatures.
- Improved land surface model outputs from the NASA Global Modeling and Assimilation Office (GMAO) were used to estimate the effective soil temperature used as input to Level-2 soil moisture geophysical inversion. This effective soil temperature is not to be confused with the physical soil temperature at a given depth (Choudhury et al., 1982).
- Improved retrieval performance of DCA (formerly known as MDCA or "the option 3" option algorithm in previous releases). DCA retrieves both soil moisture and vegetation optical depth (VOD or tau).
- Improved estimates of soil clay fraction and bulk density in most parts of the world were used in Level-2 soil moisture geophysical inversion. Work is underway to address limited spatial anomalies of these soil property estimates at high latitudes over areas rich in organic soils.
- Data quality flags were updated and corrected where faulty.
- The baseline algorithm (SCA-V) remains unchanged.

COMPREHENSIVE Level of Service

Data: Data integrity and usability verified; data customization services available for select data

Documentation: Key metadata and comprehensive user guide available

User Support: Assistance with data access and usage; guidance on use of data in tools and data customization services

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Parameter(s):
  • MICROWAVE > BRIGHTNESS TEMPERATURE
  • SOILS > SOIL MOISTURE/WATER CONTENT > SURFACE SOIL MOISTURE
Data Format(s):
  • HDF5
Spatial Coverage:
N: 85.044, 
S: -85.044, 
E: 180, 
W: -180
Platform(s):SMAP
Spatial Resolution:
  • 36 km x 36 km
Sensor(s):SMAP L-BAND RADIOMETER
Temporal Coverage:
  • 31 March 2015
Version(s):V7
Temporal Resolution49 minuteMetadata XML:View Metadata Record
Data Contributor(s):O'Neill, P. E., S. Chan, E. G. Njoku, T. Jackson, R. Bindlish, and J. Chaubell.

Geographic Coverage

Other Access Options

Other Access Options

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As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.

O'Neill, P. E., S. Chan, E. G. Njoku, T. Jackson, R. Bindlish, and J. Chaubell. 2020. SMAP L2 Radiometer Half-Orbit 36 km EASE-Grid Soil Moisture, Version 7. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/F1TZ0CBN1F5N. [Date Accessed].

Literature Citation

As a condition of using these data, we request that you acknowledge the author(s) of this data set by referencing the following peer-reviewed publication.

  • Chan, S., R. Bindlish, P. E. O'Neill, E. G. Njoku, T. Jackson, A. Colliander, F. Chen, M. Burgin, S. Dunbar, J. R. Piepmeier, S. Yueh, D. Entekhabi, M. Cosh, T. Caldwell, J. Walker, A. Berg, T. Rowlandson, A. Pacheco, H. McNairn, M. Thibeault, J. Martinez-Fernandez, A. González-Zamora, D. Bosch, P. Starks, D. Goodrich, J. Prueger, M. Palecki, E. E. Small, M. Zreda, J. Calvet, W. T. Crow, and Y. Kerr. 2016. Assessment of the SMAP passive soil moisture product, IEEE Transactions on Geoscience and Remote Sensing. 54. 4994–5007. https://doi.org/10.1109/TGRS.2016.2561938

Created: 
3 January 2019
Last modified: 
1 September 2020

Data Description

Parameters

The main output of this data set is surface soil moisture (representing the top 5 cm of the soil column, given in cm3/cm3) presented on the global 36 km EASE-Grid 2.0. Also included are brightness temperature (Tb) measurements (K), representing the weighted average of SMAP Level-1B brightness temperatures whose boresights fall within each 36 km EASE-Grid 2.0 grid cell.

Refer to the Appendix of this document for details on all parameters.

File Information

Format

Data are provided in HDF5 format (.h5). For compatible software and more information of HDF5 files, including an HDF5 tutorial, visit the HDF Group's HDF5 website.

File Contents

As shown in Figure 1, each HDF5 file is organized into two main groups, Metadata and Soil Moisture Retrieval Data, each of which contains sub-groups and/or data sets.

Sample File Image
Figure 1. Subset of File Contents
For a complete list of file contents, refer to the Appendix. 

The Soil Moisture Retrieval Data group contains soil moisture data, ancillary data, and quality assessment flags. Corrected brightness temperatures are also provided.

The Metadata Fields group includes all the metadata that describe the full content of each file. For a description of all metadata fields for this product, refer to the Product Specification Document (Chan, 2020).

All data element arrays are one-dimensional, with the exception of landcover_class and landcover_class_fraction, which are two-dimensional arrays.

File Naming Convention

Files are named according to the following convention and as described in Table 1:
SMAP_L2_SM_P_[Orbit#]_[D/A]_yyyymmddThhmmss_RLVvvv_NNN.[ext]

For example:
SMAP_L2_SM_P_10508_A_20170119T005350_R14010_001.h5 

Table 1. File Naming Conventions
Variable Description
SMAP Indicates SMAP mission data
L2_SM_P Indicates specific product (L2: Level-2; SM: Soil Moisture; P: Passive)
[Orbit#] 5-digit sequential number of the orbit flown by the SMAP spacecraft when data were acquired. Orbit 00000 began at launch. Orbit numbers increment each time the spacecraft flies over the southernmost point in the orbit path.
D/A D: Descending half-orbit pass of the satellite (where satellite moves from North to South, and 6:00 a.m. is the local solar time at the equator)
A: Ascending half-orbit pass of the satellite (where satellite moves from South to North, and 6:00 p.m. is the local solar time at the equator)
yyyymmddThhmmss Date/time in Universal Coordinated Time (UTC) of the first data element that appears in the product, where:
yyyymmdd 4-digit year, 2-digit month, 2-digit day
T Time (delineates the date from the time, i.e. yyyymmddThhmmss)
hhmmss 2-digit hour, 2-digit month, 2-digit second
RLVvvv Composite Release ID (CRID), where:
R Release
L Launch Indicator (1: post-launch standard data)
V 1-Digit Major CRID Version Number
vvv 3-Digit Minor CRID Version Number
Refer to the SMAP Data Versions page for version information.
NNN Number of times the file was generated under the same version for a particular date/time interval (002: 2nd time)
.[ext] File extensions include:
.h5 HDF5 data file
.qa Quality Assurance file
.xml XML Metadata file

Spatial Information

Coverage

Data set coverage spans from 180°W to 180°E and from approximately 85.044°N and 85.044°S. The swath width is approximately 1000 km, enabling nearly global coverage every two to three days. Figure 2 shows the spatial coverage of the SMAP L-Band Radiometer for one descending half orbit, which comprises one file of this data set. 

Figure 2. Spatial coverage map displaying one descending half orbit of the SMAP L-Band Radiometer. 

Resolution

36 km

Geolocation

These data are provided on the 36 km EASE-Grid 2.0 equal-area projection. The following tables provide information for geolocating this data set. For more on EASE-Grid 2.0, refer to the  EASE Grids website.

Table 2. Geolocation Details for the Global EASE-Grid
Geographic coordinate system
WGS 84
Projected coordinate system
EASE-Grid 2.0 Global
Longitude of true origin
0
Standard Parallel
30° N
Scale factor at longitude of true origin
N/A
Datum
WGS 84
Ellipsoid/spheroid
WGS 84
Units
meter
False easting
0
False northing
0
EPSG code
6933
PROJ4 string
+proj=cea +lon_0=0 +lat_ts=30 +x_0=0 +y_0=0 +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs
Reference

Table 3. Grid Details for the Global EASE-Grid
Grid cell size (x, y pixel dimensions) 36,032.22 m (x)
36,032.22 m (y)
Number of columns 964
Number of rows 406
Geolocated lower left point in grid 85.044° S, 180.000 ° W
Nominal gridded resolution 36 km by 36 km
Grid rotation N/A
ulxmap – x-axis map coordinate of the outer edge of the upper-left pixel -17367530.45 m
ulymap – y-axis map coordinate of the outer edge of the upper-left pixel 7314540.83 m

Temporal Information

Coverage

Coverage spans from 31 March 2015 to the present.

Temporary Gaps Due to Reprocessing

Note that coverage is currently available from 27 August 2020 onward. Data since 31 March 2015 will become available as they are reprocessed. Coverage will be continuous after reprocessing is complete.

Satellite and Processing Events

Due to instrument maneuvers, data downlink anomalies, data quality screening, and other factors, small gaps in the SMAP time series will occur. Details of these events are maintained on two master lists:

SMAP On-Orbit Events List for Instrument Data Users
Master List of Bad and Missing Data

A significant gap in coverage occurred between 19 June and 23 July 2019 after the SMAP satellite went into Safe Mode. A brief description of the event and its impact on data quality is available in the SMAP Post-Recovery Notice.

Latencies

FAQ: What are the latencies for SMAP radiometer data sets?

Resolution

Each Level-2 half-orbit file spans approximately 49 minutes. The SMAP orbit yields a 2-3 day average revisit frequency and repeats the exact swath every 8 days.

Data Acquisition and Processing

Background

The microwave portion of the electromagnetic spectrum, which includes wavelengths from a few centimeters to a meter, has long held the most promise for estimating surface soil moisture remotely. Passive microwave sensors measure the natural thermal emission emanating from the Earth's surface. The variation in the intensity of this radiation depends on the dielectric properties and temperature of the target medium, which for the near-surface soil layer is a function of the amount of moisture present. Low microwave frequencies, at L-band or approximately 1 GHz, offer the following advantages:

  • The atmosphere is almost completely transparent, providing all-weather sensing
  • Transmission of signals from the underlying soil is possible through sparse and moderate vegetation layers (up to at least 5 kg/m2 of vegetation water content)
  • Measurement is independent of solar illumination which allows for day and night observations

For more details, refer to Section 2 of the Algorithm Theoretical Basis Document (ATBD) for this product (O'Neill et al., 2020a), which is available as a Technical Reference.

Instrumentation

For a detailed description of the SMAP instrument, visit the SMAP Instrument page at the Jet Propulsion Laboratory (JPL) SMAP website.

Acquisition

SMAP Level-2 radiometer soil moisture data (SPL2SMP) are derived from SMAP L1C Radiometer Half-Orbit 36 km EASE-Grid Brightness Temperatures, Version 5 (SPL1CTB) and generated by the SMAP Science Data Processing System (SDS) at the Jet Propulsion Laboratory (JPL).

Processing

SDS processing software ingests the 6:00 a.m. descending and 6:00 p.m. ascending half-orbit files of the SMAP L1C Radiometer Half-Orbit 36 km EASE-Grid Brightness Temperatures, Version 5 product. The ingested data are then inspected for retrievability criteria according to input data quality, ancillary data availability, and land cover conditions. When retrievability criteria are met, the software invokes the baseline retrieval algorithm, plus two optional soil moisture algorithms, to generate soil moisture retrieval; all algorithms convert SMAP brightness temperatures into estimates of the 0-5 cm surface soil moisture (cm3/cm3). Only cells that are covered by the actual swath for a given projection are included in this data set.

The three soil moisture retrieval algorithms are described below. For more information on the soil moisture retrieval algorithms, users should refer to this data set's ATBD (O'Neill et al., 2020a).

Algorithm Inputs and Outputs

The main input to the processing algorithm is the SMAP L1C Radiometer Half-Orbit 36 km EASE-Grid Brightness Temperatures, Version 5 (SPL1CTB) data set. This product contains time-ordered, geolocated, and calibrated Level-1B brightness temperatures (Tb) that have been resampled to the fixed 36 km EASE-Grid 2.0. In addition to general geolocation and calibration, the Level-1B Tdata have also been corrected for atmospheric effects, Faraday rotation, and low-level RFI effects prior to regridding. If the RFI encountered is too large to be corrected, the Tdata are flagged accordingly and no soil moisture retrieval is attempted. Refer to the SPL1BTB and SPL1CTB ATBDs for additional details.

Starting in Version 5 of this product, the input Level-1C Tdata (SPL1CTB) have included Tb data that have been corrected for cases where a significant percentage of the grid cell contains a mix of land and open water (Water/Land Contamination Correction). This procedure corrects for anomalous soil moisture values seen near coastlines in previous versions and should result in less rejected data due to waterbody contamination. The correction is performed in the SPL1BTB product at the footprint level using the SMAP radiometer antenna gain pattern. When the antenna-gain-weighted water fraction within the antenna field of view (FOV) is less than or equal to 0.9, and when the antenna boresight falls on a land location as indicated by a static high-resolution land/water mask, the correction is applied. Conversely, when the antenna boresight falls on a water location, and when the water fraction within the antenna field of view (FOV) is greater than or equal to 0.1, the correction is applied. Over land, the resulting brightness temperatures will become warmer upon the removal of the contribution of water compared to the original uncorrected observations. Further details are provided in the Water/Land Contamination Correction section of the SPL1BTB User Guide or ATBD.

In addition to brightness temperature observations, the SPL2SMP algorithm requires ancillary data sets for soil moisture retrieval. In order for soil moisture to be accurately retrieved, a variety of global static and dynamic ancillary data are required. Static ancillary data are data which do not change during the mission, while dynamic ancillary data require periodic updates in time frames ranging from seasonally to daily. Static data include parameters such as permanent masks (land, water, forest, urban, mountain, etc.), the grid cell average elevation and slope derived from a Digital Elevation Model (DEM), and soil texture information (primarily sand and clay fraction). Dynamic ancillary data include land cover, surface roughness, precipitation, vegetation parameters, and effective soil temperatures. The specific parameters and sources of ancillary data are listed in Section 6 of the ATBD (O'Neill et al., 2020a).

Note: all input brightness temperatures and ancillary data sets are mapped to the 3-, 9-, and 36-km EASE-Grid 2.0 projection and then aggregated as applicable at a spatial extent that is approximately the same as the native resolution (~36 km) of the SMAP radiometer prior to entering the SPL2SMP processor. 

Soil Moisture Algorithms

Decades of research by the passive microwave soil moisture community have resulted in a number of viable soil moisture retrieval algorithms that can be used with SMAP brightness temperature data. The European Space Agency (ESA) Soil Moisture and Ocean Salinity Mission (SMOS) mission currently flies an aperture synthesis L-band radiometer which produces Tb data at multiple incidence angles over the same ground location. The baseline SMOS retrieval algorithm is based on the tau-omega model described in Section 2.1 of this data product's ATBD (O'Neill et al., 2020a); SMAP retrievals are also based on the tau-omega model. In essence, this model relates Tb (SMAP Level-1 observations) to soil moisture (SMAP Level-2 retrievals) through ancillary information (e.g. soil texture, soil temperature, and vegetation water content) and a soil dielectric model. 

Prior to implementing the soil moisture retrieval, Tb estimates are corrected for water/land contamination (described above and in the SPL1BTB User Guide). Beginning with Version 5 in 2018, this data product also includes an improved depth correction scheme for the effective soil temperature (i.e. the surface_temperature field), which is a critical parameter in passive soil moisture retrieval - note that the effective soil temperature is not to be confused with an actual physical temperature measured at a single depth. This correction scheme reduces the dry bias previously seen when comparing SMAP data to in situ data from the core validation sites.

At L-band frequency, the soil depth contributing to microwave emissions (or penetration depth) may be slightly different from the discrete soil depths at which the soil temperatures are available from a land surface model. The resulting discrepancy will lead to dry bias in retrieved soil moisture (i.e. retrieval lower than in situ soil moisture) if the model-based effective soil temperature is colder than the soil temperature sensed by the radiometer. Conversely, wet bias of retrieved soil moisture will occur if the model-based effective soil temperature is warmer than the soil temperature sensed by the radiometer. Since the contributing soil depth of microwave emission varies with soil moisture, the corresponding depth correction scheme for the effective soil temperature must account for soil moisture variability in Tb observations acquired between a.m./descending overpasses and p.m./ascending passes. The following modified Choudhury model (Choudhury et al., 1982) achieves this objective, resulting in good agreement between the in situ soil temperatures and modeled effective temperatures, and between the in situ soil moisture data and the retrieved soil moisture:

Teff = K × [ Tsoil2 + C (Tsoil1 - Tsoil2) ]

where:

C = 0.246 for a.m. soil moisture retrieval and C = 1.0 for p.m. soil moisture retrieval; K = 1.007 for both a.m and p.m. retrievals; Tsoil1 refers to the average soil temperature for the first soil layer (5-15 cm); and Tsoil2 refers to the average soil temperature for the second soil layer (15-35 cm) of the GMAO GEOS-5 land surface model, also known as the Global Modeling and Assimilation Office (GMAO) Goddard Earth Observing System Model, Version FP (GEOS-FP). A justification for this formulation can be found in the appendix of the latest version of the data product's ATBD (O'Neill et al., 2020a).

The Version 7 of the SPL2SMP product contains soil moisture retrieval fields produced by the baseline algorithm and two other optional algorithms (refer to Table 4). The operational SPL2SMP Science Production Software (SPS) produces and stores soil moisture retrieval results from all three algorithms. Within an SPL2SMP file, the soil_moisture field is linked to the retrieval result produced by the current baseline algorithm, the Single Channel Algorithm V-pol (SCA-V)

Table 4. Soil Moisture Algorithm Options
Algorithm Options Corresponding Data Field
Single Channel Algorithm H-pol (SCA-H) soil_moisture_option1
Single Channel Algorithm V-pol (SCA-V) – Current Baseline soil_moisture_option2 (internally linked to the soil_moisture field)
Dual Channel Algorithm (DCA, formerly known as Modified Dual Channel Algorithm or MDCA) soil_moisture_option3

The soil moisture retrieval performance of all three algorithms will be continuously assessed. Recent calibration/validation (cal/val) analyses show similar performance for the current SCA-V baseline and the new DCA algorithm; results from the SCA-H algorithm are somewhat worse than the other two (refer to the Assessment Report; O'Neill et al., 2020b).

All three algorithms operate on the same zeroth-order microwave emission model, commonly known as the tau-omega model. However, the algorithms differ in their approaches and solve for soil moisture under different constraints and assumptions. A brief description of each algorithm is provided below. Users should refer to the SPL2SMP ATBD (O'Neill et al., 2020a) for more details.

Single Channel Algorithm (SCA) 

In SCA, horizontally (SCA-H) or vertically (SCA-V) polarized brightness temperature (Tb) observations are converted to emissivity using a surrogate for the physical temperature of the emitting layer. The derived emissivity is corrected for vegetation and surface roughness to obtain the soil emissivity. The Fresnel equation is then used to determine the dielectric constant from the soil emissivity. Finally, a dielectric mixing model is used to solve for soil moisture given knowledge of the soil texture.

Analytically, SCA attempts to solve for one unknown variable (soil moisture) from one equation that relates the vertically or horizontally polarized Tb to soil moisture. Vegetation information is provided by a 13-year climatological data base of global Normalized Difference Vegetation Index (NDVI) and a table of parameters based on land cover class.

Though SCA can apply to vertically or horizontally polarized Tb measurements, SCA-V is the current baseline soil moisture algorithm. It outperforms the SCA-H algorithm which was the pre-launch baseline retrieval algorithm.

Dual Channel Algorithm (DCA, formerly known as Modified Dual Channel Algorithm or MDCA)

The Dual Channel Algorithm (DCA) is an extension of the SCA. DCA uses both the vertically and horizontally polarized Tb observations to solve for soil moisture and vegetation optical depth. The algorithm iteratively minimizes a cost function F that is constrained by the vegetation optical depth (VOD) climatology (τ) that is used as an ancillary input to SCA. The analytical form of this cost function is:

where  and  are the brightness temperatures modeled by the tau-omega model described in the ATBD (O'Neill et al., 2020a) and λ = 20.0. Estimates of certain model parameters (e.g., surface temperature, surface roughness, and vegetation single scattering albedo) must be provided using ancillary data sets in the inversion process. Unlike SCA, the polarization mixing factor is assumed to be linearly related to the roughness parameter h as in Q = 0.1771 h, where h (roughness_coefficient_option3, in the product) is provided to the algorithm through a pre-computed static ancillary file with global values of h over the 3-km EASE Grid 2.0 projection (see ATBD Section 6.5 for details). In addition to these differences, DCA uses different values than SCA for the vegetation single scattering albedo (albedo_option3, in the product). The new values of omega were selected based on several independent sources. Table 4 in the ATBD displays the values of albedo proposed by different independent teams (SMAP L2, SMAP L4, SMOS-I and the Multi-Temporal Dual Channel Algorithm (MTDCA)) and the resulting values used in the DCA implementation. Users should refer to the ATBD (O'Neill et al., 2020a) for more details.

The performance of the DCA implemented in Version 7 is comparable to the performance of the baseline SCA-V algorithm and therefore the SMAP team encourages the users to evaluate both algorithms and opt for the one that is the best fit for the specific application. Note that there is currently no independent verification of the accuracy of the DCA tau retrievals.

Quality, Errors, and Limitations

Error Sources

Anthropogenic RFI, principally from ground-based surveillance radars, can contaminate both radar and radiometer measurements at L-band frequencies. The SMAP radar and radiometer electronics and algorithms include design features to mitigate the effects of RFI. The SMAP radiometer implements a combination of time and frequency diversity, kurtosis detection, and use of thresholds to detect and, where possible, mitigate RFI.

Level-2 radiometer data can also contain bit errors caused by noise in communication links and memory storage devices. More information about error sources is provided in Section 4.6 of the ATBD (O'Neill et al., 2020a).

Quality Overview

Each HDF5 file contains metadata with Quality Assessment (QA) metadata flags that are set by the Science Data Processing System (SDS) at the JPL prior to delivery to the National Snow and Ice Data Center Distributed Active Archive Center (NSIDC DAAC). A separate QA file with a .qa file extension is also associated with the HDF5 file; it contains useful statistics such as the percentage of elements having various quality conditions. If a product does not fail QA, it is ready to be used for higher-level processing, browse generation, active science QA, archive, and distribution. If a product fails QA, it is never delivered to NSIDC DAAC. For in-depth details regarding the quality of these data, refer to the Assessment Report (O'Neill et al., 2020b).

6:00 p.m. Ascending / 6:00 a.m. Descending Half Orbits

Data from both 6:00 a.m. descending and 6:00 p.m. ascending half-orbit passes are used as input for soil moisture derivation. However, the radiometer soil moisture algorithm assumes that the air, vegetation, and near-surface soil are in thermal equilibrium in the early morning hours; thus, retrievals from 6:00 p.m. ascending half-orbit passes may show a slight degradation in quality. Nonetheless, ubRMSE (unbiased root mean square error) and correlation of the p.m. and a.m. retrievals are relatively close.

Data Flags

Bit flags generated from input SMAP data and ancillary data are employed to help determine the quality of the retrievals. Ancillary data help determine either specific aspects of the processing, such as corrections for transient water, or the quality of the retrievals, such as the precipitation flag. These flags provide information as to whether the ground is frozen, covered with snow, flooded, or whether it is actively precipitating at the time of the satellite overpass. Other flags will indicate whether masks for steeply sloped topography or for urban, heavily forested, or permanent snow/ice areas are in effect. Unless otherwise stated, all areal fractions defined below refer to 36 x 36 km2 inversion domain.

A brief description of data flags contained in surface_flag is provided below, including which Bit identifies them (Bit 0 being the "rightmost" bit). For more details on all data flags, users should refer to the Appendix of this User Guide and the Product Specification Document (Chan, 2020).

  • Open Water Flag (Bits 0 and 1)
    ​Open water fraction is determined by a priori information on permanent open freshwater from the Moderate Resolution Imaging Spectroradiometer (MODIS) MOD44W database. Open water fraction is reported in Bits 0 and 1 in the surface_flag field of the SPL2SMP product, with Bit 0 using the MOD44W database. Bit 1 was set to be equal to Bit 0 after the failure of the SMAP radar on July 5, 2015. This water fraction information serves as a flag to affect soil moisture retrieval processing in the following ways:
    • If water fraction is 0.00–0.05, then retrieve soil moisture, but flag for recommended quality.
    • If water fraction is 0.05–0.50, then retrieve soil moisture, and flag for uncertain quality.
    • If water fraction is 0.50–1.00, then flag, but do not retrieve soil moisture.
  • Urban Area Flag (Bit 3)
    ​Since the Tb of man-made, impervious, and urban areas cannot be estimated theoretically, the presence of urban areas in the 36 km Level-2 soil moisture grid cell cannot be corrected for during soil moisture retrieval. Thus, the presence of even a small amount of urban area in the radiometer footprint is likely to adversely bias the retrieved soil moisture. The SMAP urban flag is set based on the Columbia University Global Rural-Urban Mapping Project (GRUMP) data set (O'Neill et al., 2020a). The urban fraction affects soil moisture retrieval processing in the following ways: 
    • If urban areal fraction is 0.00–0.25, then retrieve soil moisture, but flag for recommended quality.
    • If urban areal fraction is 0.25–1.00, then flag for uncertain quality, and retrieve soil moisture.
    • If urban areal fraction is above 1.00, then flag, but do not retrieve soil moisture.
  • Precipitation Flag (Bit 4)
    ​The SMAP precipitation flag is set based on either forecasts of precipitation or using data from the Global Precipitation Mission (GPM). It is a binary precipitation/no precipitation flag which indicates the presence or absence of precipitation in the 36 km grid cell at the time of the SMAP overpass. The presence of liquid precipitation at the time of the SMAP overpass can adversely bias the retrieved soil moisture due to its large impact on Tb; corrections for precipitation are part of the Level-1B Tb processing. Unlike other flags, soil moisture retrieval will always be attempted even if precipitation is flagged. However, this flag serves as a warning to users to view the retrieved soil moisture with some skepticism if precipitation is present.
    • If precipitation is 0–1 mm/hr, then retrieve soil moisture, but flag for recommended quality.
    • If precipitation is 1–25.4 mm/hr, then flag for uncertain quality, and retrieve soil moisture.
    • If precipitation is above 25.4 mm/hr, then flag, but do not retrieve soil moisture.
  • Snow Flag (Bit 5)
    ​Although the SMAP L-Band Radiometer can theoretically see through dry snow to the soil underneath a snowpack, the snow flag is set based on the snow fraction as reported in the National Oceanic and Atmospheric Administration (NOAA) Interactive Multisensor Snow and Ice Mapping System (IMS) database. The snow flag affects soil moisture retrieval processing in the following ways: 
    • If snow areal fraction is 0.00–0.05, then retrieve soil moisture, but flag for recommended quality.
    • If snow areal fraction is 0.05–0.50, then flag for uncertain quality, and retrieve soil moisture.
    • If snow areal fraction is above 0.50, then flag, but do not retrieve soil moisture.
  • Frozen Ground Flag (Bits 7 and 8)
    ​The frozen ground flag is set from either 1) the flag passed through from the SMAP radiometer freeze/thaw algorithm, or 2) from modeled surface temperature information (Tsurf) from the Global Modeling and Assimilation Office (GMAO), which is used to determine frozen ground conditions. These two flag sources are reflected in Bits 7 and 8 of the surface_flag (Bit 7: SMAP radiometer-derived freeze/thaw state; Bit 8: GMAO Tsurf). The baseline of the Level-2 passive soil moisture processing configuration relies on the modeled surface temperature information from GMAO (Bit 8). The frozen soil flag affects soil moisture retrieval processing in the following ways:
    •    If frozen ground areal fraction is 0.00–0.05, then retrieve soil moisture, but flag for recommended quality.
    •    If frozen ground areal fraction is 0.05–0.50, then flag for uncertain quality, and retrieve soil moisture.
    •    If frozen ground areal fraction is 0.50–1.00, then flag, but do not retrieve soil moisture.

    Note: SMAP radiometer freeze/thaw flags are presently validated only for all land regions north of 45°N. While the SPL2SMP_E product contains global SMAP freeze/thaw flags, uncertainty in the flags is higher south of 45°N due to small differences in the SMAP radiometer-derived reference freeze and thaw states upon which the freeze/thaw algorithm is based. More information is available in the SMAP Level-3 Freeze/Thaw (SPL3FTP) Assessment Report. 

      • Mountainous Area Flag (Bit 9) Large and highly variable slopes present in the radiometer footprint will adversely affect the retrieved soil moisture. The SMAP mountainous area flag is derived from high elevation information from a DEM coupled with a statistical threshold based on the slope variability within each 36 km grid cell.
        • If slope standard deviation is 0.0–3.0°, then retrieve soil moisture, but flag for recommended quality.
        • If slope standard deviation is 3.0°–6.0°, then flag for uncertain quality, and retrieve soil moisture.
        • If slope standard deviation is above 6.0°, then flag, but do not retrieve soil moisture.
      As with any satellite retrieval data product, proper data usage is encouraged. The following two simple practices are recommended for using SMAP soil moisture retrievals with maximum scientific benefits:
      • Use the retrieval_qual_flag field to identify retrievals in the soil_moisture field estimated to be of recommended quality. A retrieval_qual_flag value of either 0 or 8 indicates high-quality retrievals (8 because a failed F/T retrieval does not affect soil moisture retrieval). Proper use of the retrieval_qual_flag field is an effective way to ensure that only retrievals of recommended quality will be used in data analyses.
      • For further investigation, use the surface_flag field and the associated definition described above to determine why the retrieval_qual_flag field did not report recommended quality at a given grid cell.

      Software and Tools

      For tools that work with SMAP data, see the Tools web page.

      Version History

      Table 5. Summary of Version Changes
      Version
      Date
      Description of Changes
      V1 September 2015 First public release 
      V2 October 2015

      Changes to this version include:

      • Uses SPL1CTB V2 Validated-Stage 1 data as input
      • Corrects the retrieval quality flag error
        V3
        April 2016

        Changes to this version include:

        • Transitioned to Validated-Stage 2
        • Uses updated SPL1CTB V3 Validated data as input
        V4
        December 2016

        Changes to this version include:

        • Added 6:00 p.m. ascending half orbits, which provide:
          • More frequent regional/global coverage (critical in flood monitoring)
          • Soil moisture diurnal variability information (useful in data assimilation systems)
          • Consistency with other similar satellite-based soil moisture products
        • Contains frozen ground flag (bit 7 of surface_flag) derived using Normalized Polarization Ration (NPR)-based SMAP passive freeze-thaw retrieval; replaces former SMAP radar-based freeze/thaw flag
        V5
        June 2018

        Changes to this version include:

        • Level-1B water-corrected brightness temperatures are used in passive soil moisture retrieval. This procedure corrects for anomalous soil moisture values seen near coastlines in the previous version and should result in less rejected data due to waterbody contamination. Five new data fields accommodate this correction: grid_surface_status, surface_water_fraction_mb_h, surface_water_fraction_mb_v, tb_h_uncorrected, and tb_v_uncorrected.
        • Improved depth correction for effective soil temperature used in passive soil moisture retrieval; new results are captured in the surface_temperature data field. This correction reduces the dry bias seen when comparing SMAP data to in situ data from the core validation sites.
        • Frozen ground flag updated to reflect improved freeze/thaw detection algorithm, providing better accuracy; new results are captured in bit 7 of the surface_flag.

          V6
          August 2019

          Changes to this version include:

          • The Dual Channel Algorithm (DCA) has been replaced by the Modified Dual Channel Algorithm (MDCA). MDCA achieves better retrieval performance through the modeling of polarization mixing between the vertically and horizontally polarized brightness temperature channels, as well as new estimates of single-scattering albedo and roughness coefficients. MDCA supersedes optional algorithms MPRA (option 4) and E-DCA (option 5). 
            • As part of the option algorithm changes, the following data fields were removed: soil_moisture_option4, vegetation_opacity_option4, retrieval_qual_flag_option4,  soil_moisture_option5, vegetation_opacity_option5, retrieval_qual_flag_option5.
            • As part of the option algorithm changes, the following data fields were added: albedo_option3, roughness_coefficient_option3, bulk_density, clay_fraction.
          • The baseline algorithm (SCA-V) remains unchanged.
          • Improved aggregation of values in input ancillary data, e.g. roughness, soil texture, NDVI. The fix has negligible impacts on retrievals estimated to be of recommended quality. 
          V7 August 2020

          Changes to this version include:

          • Improved calibration methodology was applied to the Level-1B radiometer brightness temperatures.
          • Improved land surface model outputs from the NASA Global Modeling and Assimilation Office (GMAO) were used to estimate the effective soil temperature used as input to Level-2 soil moisture geophysical inversion. This effective soil temperature is not to be confused with the physical soil temperature at a given depth (Choudhury et al., 1982).
          • Improved retrieval performance of DCA (formerly known as MDCA or "the option 3" option algorithm in previous releases). DCA retrieves both soil moisture and vegetation optical depth (VOD or tau).
          • Improved estimates of soil clay fraction and bulk density in most parts of the world were used in Level-2 soil moisture geophysical inversion. Work is underway to address limited spatial anomalies of these soil property estimates at high latitudes over areas rich in organic soils.
          • Data quality flags were updated and corrected where faulty.
          • The baseline algorithm (SCA-V) remains unchanged.

          Related Data Sets

          SMAP Data at NSIDC | Overview
          SMAP Radar Data at the ASF DAAC

          Related Websites

          SMAP at NASA JPL

          Contacts and Acknowledgments

          Peggy O’Neill and Rajat Bindlish
          NASA Goddard Space Flight Center
          Greenbelt, MD

          Steven Chan, Eni Njoku, and Julian Chaubell
          Jet Propulsion Laboratory
          California Institute of Technology
          Pasadena, CA

          Tom Jackson
          USDA Agricultural Research Service
          Beltsville, MD

          References

          Chan, S. 2020. SMAP Level 2 Passive Soil Moisture Product Specification Document, Version 7.0, R17 Extended Mission Release. JPL D-72547, Jet Propulsion Laboratory, Pasadena, CA. (see Technical References or PDF).

          Choudhury, B. J., T. J. Schmugge, and T. Mo. 1982. A Parameterization of Effective Soil Temperature for Microwave Emission. Journal of Geophysical Research. 87(C2):1301-1304.

          O'Neill, P. E., R. Bindlish, S. Chan, J. Chaubell, E. Njoku, and T. Jackson. 2020a. SMAP Algorithm Theoretical Basis Document: Level 2 & 3 Soil Moisture (Passive) Data Products, Revision F, August 31, 2020, SMAP Project, JPL D-66480, Jet Propulsion Laboratory, Pasadena, CA. (see Technical References or PDF).

          O’Neill, P. E., S. Chan, R. Bindlish, M. Chaubell, A. Colliander, F. Chen, S. Dunbar, T. Jackson, J. Peng, M. Cosh, T. Bongiovanni, J. Walker, X. Wu, A. Berg, H. McNairn, M. Thibeault, J. Martínez- Fernández, Á. González-Zamora, E. Lopez-Baeza, K. Jensen, M. Seyfried, D. Bosch, P. Starks, C. Holifield Collins, J. Prueger, Z. Su, R. van der Velde, J. Asanuma, M. Palecki, E. Small, M. Zreda, J. Calvet, W. Crow, Y. Kerr, S. Yueh, and D. Entekhabi. 2020b. Calibration and Validation for the L2/3_SM_P Version 7 and L2/3_SM_P_E Version 4 Data Products, SMAP Project, JPL D-56297, Jet Propulsion Laboratory, Pasadena, CA.(see Technical References or PDF).

          Appendix - Data Fields

          This appendix provides a description of all data fields within the SMAP L2 Radiometer Half-Orbit 36 km EASE-Grid Soil Moisture (SPL2SMP) product. The data are grouped into two main HDF5 groups:

          • Metadata
          • Soil_Moisture_Retrieval_Data

          For a description of the metadata fields associated with this product, refer to the Product Specification Document (Chan, 2020). Table A1 describes the Soil_Moisture_Retrieval_Data groups associated with this product, with a more detailed description of each data field below.

          Table A1. Data Fields for Soil_Moisture_Retrieval_Data
          Data Field Name Type Byte Unit Valid Min Valid Max Fill/Gap Value Dimensions
          (Number of grid cells covered by swath)
          Derivation Method(s)*
          EASE_column_index Uint16 2 N/A 0 963 65534 N 2
          EASE_row_index Uint16 2 N/A 0 405 65534 N 2
          albedo Float32 4 N/A 0.0 1.0 -9999.0 N 6
          albedo_option3 Float32 4 N/A 0.0 1.0 -9999.0 N 6
          boresight_incidence Float32 4 degrees 0.0 90.0 -9999.0 N 1
          bulk_density Float32 4 N/A 0.0 3.0 -9999.0 N 6
          clay_fraction Float32 4 N/A 0.0 1.0 -9999.0 N 6
          freeze_thaw_fraction Float32 4 N/A 0.0 1.0 -9999.0 N 6, 7
          grid_surface_status Uint16 2 N/A 0 1 65534 N 8
          landcover_class Uint8 1 N/A 0 16 254 N x 3 6
          landcover_class_fraction Float32 4 N/A 0 1.0 -9999.0 N x 3 6
          latitude Float32 4 degrees -90.0 +90.0 -9999.0 N 2
          latitude_centroid Float32 4 degrees -90.0 +90.0 -9999.0 N 1
          longitude Float32 4 degrees -180.0 +180.0 -9999.0 N 2
          longitude_centroid Float32 4 degrees -180.0 +180.0 -9999.0 N 1
          radar_water_body_fraction Float32 4 N/A 0.0 1.0 -9999.0 N 7
          retrieval_qual_flag Uint16 2 N/A 0 65,536 65534 N 4
          retrieval_qual_flag_option1 Uint16 2 N/A 0 65,536 65534 N 4
          retrieval_qual_flag_option2 Uint16 2 N/A 0 65,536 65534 N 4
          retrieval_qual_flag_option3 Uint16 2 N/A 0 65,536 65534 N 4
          roughness_coefficient Float32 4 N/A 0.0 1.0 -9999.0 N 6
          roughness_coefficient_option3 Float32 4 N/A 0.0 1.0 -9999.0 N 6
          soil_moisture Float32 4 cm3/cm3 0.02 0.5 -9999.0 N 4
          soil_moisture_error Float32 4 cm3/cm3 0.0 0.2 -9999.0 N 4 or 6
          soil_moisture_option1 Float32 4 cm3/cm3 0.02 0.5 -9999.0 N 4
          soil_moisture_option2 Float32 4 cm3/cm3 0.02 0.5 -9999.0 N 4
          soil_moisture_option3 Float32 4 cm3/cm3 0.02 0.5 -9999.0 N 4
          static_water_body_fraction Float32 4 N/A 0.0 1.0 -9999.0 N 6
          surface_flag Uint16 2 N/A 0 65,536 65534 N 4
          surface_temperature Float32 4 K 0 350 -9999.0 N 6
          surface_water_fraction_mb_h Float32 4 N/A 0 1 -9999.0 N 1
          surface_water_fraction_mb_v Float32 4 N/A 0 1 -9999.0 N 1
          tb_3_corrected Float32 4 K -50 50 -9999.0 N 1
          tb_4_corrected Float32 4 K -50 50 N N/A 1
          tb_h_corrected Float32 4 K 0.0 330.0 -9999.0 N 1
          tb_h_uncorrected Float32 4 K 0.0 340.0 -9999.0 N 1
          tb_qual_flag_3 Uint16 2 N/A 0 65,536 65534 N 4
          tb_qual_flag_4 Uint16 2 N/A 0 65,536 65534 N 4
          tb_qual_flag_h Uint16 2 N/A 0 65,536 65534 N 4
          tb_qual_flag_v Uint16 2 N/A 0 65,536 65534 N 4
          tb_time_seconds Float64 8 seconds 0 N/A -9999.0 N 1
          tb_time_utc Char24 24 N/A 2014-10-31T
          00:00:00.000Z
          N/A 16777214 N 1
          tb_v_corrected Float32 4 K 0.0 330.0 -9999.0 N 1
          tb_v_uncorrected Float32 4 K 0.0 330.0 -9999.0 N 1
          vegetation_opacity Float32 4 N/A 0.0 10.00 -9999.0 N 6
          vegetation_opacity_option1 Float32 4 N/A 0.0 10.00 -9999.0 N 6
          vegetation_opacity_option2 Float32 4 N/A 0.0 10.00 -9999.0 N 6
          vegetation_opacity_option3 Float32 4 N/A 0.0 10.00 -9999.0 N 5
          vegetation_water_content Float32 4 kg/m2 0 20.0 -9999.0 N 6

          Derivation methods are:

          1. From Level-1C brightness temperature data
          2. From 36 km EASE-Grid 2.0 array definition
          3. Value corrected for the presence of water wherever water/land areal fraction is below a threshold; when the fraction is zero, no correction is performed
          4. Determined by Level-2 radiometer soil moisture processing software
          5. Available only with option algorithms that use two polarization channels
          6. From external ancillary data whose location and time stamp coincide with those of the input data
          7. From Level-2 radar soil moisture data
          8. Nearest-neighbor interpolation

          Data Field Definitions

          EASE_col_index

          Zero-based column index of a 36 km EASE-Grid 2.0 cell. In most grid cells, both fore-looking Level-1C brightness temperature data and aft-looking Level-1C brightness temperature data are available for soil moisture retrieval. But when one group (e.g. fore-looking group) is not available, the GridCol parameter of the other group (i.e. aft-looking group) will be written into this parameter.

          EASE_row_index

          Zero-based row index of a 36 km EASE-Grid 2.0 cell. In most grid cells, both fore-looking Level-1C brightness temperature data and aft-looking Level-1C brightness temperature data are available for soil moisture retrieval. But when one group (e.g. the fore-looking group) is not available, the GridRow parameter of the other group (i.e. the aft-looking group) will be written into this parameter.

          albedo

          Single-scattering albedo at 36 km spatial scale. Note that this parameter is the same 'omega' parameter in the 'tau-omega' model for a given polarization channel.

          albedo_option3
          Single-scattering albedo at 36-km grid posting derived from landcover-based table used for the Modified Dual-Channel Algorithm (MDCA). Note that this parameter is the same ‘omega’ parameter in the ‘tau-omega’ model when used in MDCA.

          boresight_incidence

          Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input Level-1C brightness temperature granule. The resulting parameter thus describes the weighted average of incidence angles of Level-1B brightness temperature observations whose boresights fall within a 36 km EASE-Grid 2.0 cell. The incidence angle is defined as the included angle between the antenna boresight vector and the normal to the Earth's surface.

          bulk_density
          Bulk density at 36 km spatial scale.

          clay_fraction
          Clay fraction at 36 km spatial scale.

          freeze_thaw_fraction

          Freeze/thaw fraction at 36 km spatial scale. The fraction is computed based on the number of frozen land pixels and thawed land pixels reported on the 3-km global cylindrical EASE-Grid 2.0 projection in the SMAP Level 2 Active Soil Moisture Product (L2_SM_A). If there are NF frozen ground pixels and NT thawed land pixels within a 36-km grid cell, this parameter refers to the fraction of NF / (NF + NT). At present the L2_SM_P processing software can be configured to provide this parameter from a dynamic ancillary data database or from the SMAP L2_SM_A product. Since the failure of the SMAP radar this field has been derived from external soil temperature ancillary data.

          grid_surface_status

          Surface type (land or water) as determine by the antenna boresight location. Indicates if the grid point lies on land (0) or water (1).

          landcover_class

          The first three most dominant land cover classes according to the MODIS International Geosphere-Biosphere Programme (IGBP) land cover map. The relative dominance is determined based on ranking among land cover classes using statistical mode.

          Table A2 provides a description of MODIS IGBP classes and the percentage of each land type.

          Class Description Percentage
          Table A2. MODIS_IGBP Land Classification and Percentage
          0 Water -
          1 Evergreen Needleleaf Forest 3.96
          2 Evergreen Broadleaf Forest 10.04
          3 Deciduous Needleleaf Forest 0.63
          4 Deciduous Broadleaf Forest 1.59
          5 Mixed Forests 4.69
          6 Closed Shrublands 0.55
          7 Open Shrublands 18.26
          8 Woody Savannas 7.52
          9 Savannas 6.97
          10 Grasslands 9.27
          11 Permanent Wetlands 0.22
          12 Croplands 8.95
          13 Urban and Built-Up 0.50
          14 Cropland/Natural Vegetation Mosaic 2.10
          15 Snow and Ice 11.04
          16 Barren or Sparsely Vegetated 13.70
          landcover_class_fraction

          The areal fractions of the first three most dominant land cover classes according to a 500-meter MODIS IGBP land cover map. The relative dominance is determined based on ranking among all land cover classes using statistical mode. For example, if there are N1 pixels that correspond to first class and there are NT pixels comprising all land cover classes within a 36-km grid cells, the corresponding percentage refers to (N1 / NT).

          latitude

          Latitude of the center of a 36 km EASE-Grid 2.0 cell.

          latitude_centroid

          Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input Level-1C brightness temperature granule. The resulting parameter thus describes the weighted average of latitudes of Level-1B brightness temperature observations whose boresights fall within a 36 km EASE-Grid 2.0 cell.

          longitude

          Longitude of the center of a 36 km EASE-Grid 2.0 cell.

          longitude_centroid

          Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input Level-1C brightness temperature granule. The resulting parameter thus describes the weighted average of longitudes of Level-1B brightness temperature observations whose boresights fall within a 36 km EASE-Grid 2.0 cell.

          radar_water_body_fraction

          Radar-derived water body fraction at 36 km spatial scale. The fraction is computed based on the number of water pixels and land pixels reported on the 3-km global cylindrical EASE-Grid 2.0 projection in the SMAP Level 2 Active Soil Moisture Product (L2_SM_A). If there are NW water pixels and NL land pixels within a 36 km grid cell, this parameter refers to the fraction of NW / (NW + NL). Note that NW is the number of water pixels regardless of their temporal span – NW captures both static water pixels and transient water pixels.Since the failure of the SMAP radar, this field has been set to the static_water_body_fraction field.

          retrieval_qual_flag, retrieval_qual_flag_option[1-5]

          A 16-bit bingary string of 1's and 0's that indicate whether retrieval was performed or not at a given grid cell. When retrieval is performed, it contains additional bits to further indicate the exit status and quality of the retrieval. A summary of bit definition of the retrieval_qual_flag field is listed in Table A3.
          The retrieval_qual_flag field is internally linked to the retrieval_qual_flag_option2 field produced by the baseline algorithm. All soil moisture algorithm options, soil moisture data fields, and corresponding retrieval quality flags are listed in Table A4.

          Table A3. Retrieval Quality Flag Definition
          Bit Retrieval Information Bit Value and Interpretation
          0 Recommended Quality 0: Soil moisture retrieval has recommended quality
          1: Soil moisture retrieval doesn't have recommended quality
          1 Retrieval Attempted 0: Soil moisture retrieval was attempted
          1: Soil moisture retrieval was skipped
          2 Retrieval Successful 0: Soil moisture retrieval was successful
          1: Soil moisture retrieval was not successful
          3* Retrieval Successful 0: Freeze/thaw state retrieval was successful
          1: Freeze/thaw state retrieval was not successful
          4-15 Undefined 0 (not used)
          roughness_coefficient

          Roughness coefficient at 36 km spatial scale. Note that this parameter is the same 'h' coefficient in the 'tau-omega' model for a given polarization channel.

          roughness_coefficient_option3​
          Roughness coefficient at 36-km grid posting derived from 3 km global map of ‘h’ created by special dual-channel retrieval. Note that this parameter is the same ‘h’ coefficient in the ‘tau-omega’ model when used in MDCA.

          soil_moisture_error

          Estimated '1-sigma' error of the soil_moisture output parameter. The valid minimum (0.00) and maximum (soil porosity) are subject to further analysis on real data. This data field is currently filled with FillValue (-9999.0).

          soil_moisture, soil_moisture_option[1-3]

          Estimated soil moisture at 36 km grid posting, as returned by the L2_SM_P processing software. The soil_moisture field is internally linked to the soil_moisture_option2 field produced by the baseline algorithm. At present, the operational SPL2SMP Science Production Software (SPS) produces and stores soil moisture retrieval results from the three algorithms listed in Table A4; retrieval quality flags that correspond to each of these algorithms are also listed in Table A4.

          Table A4. Soil Moisture Algorithm Options and Corresponding Data Fields
          Soil Moisture Algorithm Option
          Corresponding Soil Moisture Data Field
          Corresponding Retrieval Quality Flag Data Field
          Single Channel Algorithm H-pol (SCA-H)
          soil_moisture_option1
          retrieval_qual_flag_option1
          Single Channel Algorithm V-pol (SCA-V) – Current Baseline
          soil_moisture_option2 (Internally linked to the soil_moisture field)
          retrieval_qual_flag_option2 (Internally linked to the retrieval_qual_flag field)
          Modified Dual Channel Algorithm (MDCA)
          soil_moisture_option3
          retrieval_qual_flag_option3
          static_water_body_fraction

          Static water body fraction at 36 km spatial scale. The fraction is computed based on the number of water pixels and land pixels reported on a 250-meter grid. If there are NW water pixels and NL land pixels within a 36 km grid cell, this parameter refers to the fraction of NW / (NW + NL). Note that NW is the number of water pixels regardless of their temporal span – NW captures both static water pixels and transient water pixels from when the original data were acquired.

          surface_flag

          A 16-bit integer field whose binary representation consists of bits that indicate the presence or absence of certain surface conditions at a grid cell. In Table A5, a ‘0’ indicates the presence of a surface condition favorable to soil moisture retrieval. Each surface condition is numerically compared against two non-negative thresholds: T1 and T2, where T1 < T2. In most cases, when a surface condition is found to be below T1, retrieval is attempted and flagged for recommended quality. Between T1 and T2, retrieval is still attempted but flagged for uncertain quality. Above T2, retrieval is skipped. A summary of surface conditions and their thresholds are listed in Table 10. The surface_flag field is internally linked to the surface_flag field associated with the baseline algorithm.

          Note: Bit position '0' refers to the least-significant bit. Final bit positions and definitions are subject to future revision and expansion as needed.

          Table A5. Surface Condition Bit Flag Definition
          Bit
          Surface Condition
          T1
          T2
          Bit Value and Interpretation
          0
          Static Water
          0.05
          0.50
          0: Water areal fraction ≤ T1 and IGBP wetland fraction < 0.50:
          • Retrieval attempted for fraction ≤ T2
          1: Otherwise:
          • Retrieval skipped for fraction > T2
          1
          Radar-derived Water Fraction (no longer available and now defaults to match Bit 0)
          0.05
          0.50
          0: Water areal fraction ≤ T1 and IGBP wetland fraction < 0.50:
          • Retrieval attempted for fraction ≤ T2
          1: Otherwise:
          • Retrieval skipped for fraction > T2
          2
          Coastal Proximity
          N/A
          1.0
          0: Distance to nearby significant water bodies > T2 (number of 36-km grid cells)
          1: Otherwise
          3
          Urban Area
          0.25
          1.00
          0: Urban areal fraction ≤ T1:
          • Retrieval attempted for fraction ≤ T2
          1: Otherwise:
          • Retrieval skipped for fraction > T2
          4
          Precipitation
          2.78e-04 (equivalent to
          1.0 mm/hr)
          7.06e-03 (equivalent to
          25.4 mm/hr)
          0: Precipitation fraction ≤ T1:
          • Retrieval attempted for fraction ≤ T2
          1: Otherwise:
          • Retrieval skipped for fraction > T2
          5
          Snow
          0.05
          0.50
          0: Snow areal fraction ≤ T1:
          • Retrieval attempted for fraction ≤ T2
          1: Otherwise:
          • Retrieval skipped for fraction > T2
          6
          Permanent Ice
          0.05
          0.50
          0: Ice areal fraction ≤ T1:
          • Retrieval attempted for fraction ≤ T2
          1: Otherwise:
          • Retrieval skipped for fraction > T2
          7
          Frozen Ground (from radiometer-derived FT state)
          0.05
          0.50
          0: Freeze ground areal fraction ≤ T1:
          • Retrieval attempted for fraction ≤ T2
          1: Otherwise:
          • Retrieval skipped for fraction > T2
          8
          Frozen Ground (from modeled effective soil temperature)

           
          0.05
          0.50
          0: Freeze ground areal fraction ≤ T1:
          • Retrieval attempted for fraction ≤ T2
          1: Otherwise:
          • Retrieval skipped for fraction > T2
          9
          Mountainous Terrain
          0: Slope standard deviation ≤ T1
          1: Otherwise
          10
          Dense Vegetation
          5.0
          30.0
          0: Vegetation Water Content (VWC) ≤ T1:
          • Retrieval attemtped for VWC ≤ T2
          1: Otherwise:
          • Retrieval skipped for VWC > T2
          11
          Nadir Region / Undefined
          0 (not used in SPL2SMP)
          12-15
          Undefined
          0
          surface_temperature

          Effective soil temperature (average for the top 5-cm soil layer) at 36-km grid spacing. This calculated model parameter is used as an input ancillary data parameter to the L2_SM_P processing software for both baseline and option algorithms. The valid minimum and maximum below are subject to further analysis on real data.

          It has come to our attention that this parameter is often mistaken for the physical temperature of the top soil layer. The designation “effective” signifies an attempt to capture the soil integrated temperature and canopy temperature in a single parameter, as is widely reported in the literature.  Depending on the actual emission sensing depth (which varies with soil moisture), this parameter usually does not coincide with a thermal physical temperature at a fixed depth (e.g. 5 cm or 10 cm).

          surface_water_fraction_mb_h

          Water fraction with the SMAP radiometer main-beam (mb) IFOV weighted by antenna gain pattern at the horizontal polarization.

          surface_water_fraction_mb_v

          Water fraction with the SMAP radiometer main-beam (mb) IFOV weighted by antenna gain pattern at the vertical polarization.

          tb_3_corrected

          Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input Level-1C brightness temperature granule. The resulting parameter thus describes the weighted average of Level-1B brightness temperature 3rd Stokes polarized brightness temperatures whose boresights fall within a 36 km EASE-Grid 2.0 cell. 

          tb_4_corrected

          Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input Level-1C brightness temperature granule. The resulting parameter thus describes the weighted average of Level-1B brightness temperature 4th Stokes vertically polarized brightness temperatures whose boresights fall within a 36 km EASE-Grid 2.0 cell. 

          tb_h_corrected

          Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input Level-1C brightness temperature granule. The resulting parameter thus describes the weighted average of Level-1B brightness temperature horizontally polarized brightness temperatures whose boresights fall within a 36 km EASE-Grid 2.0 cell. Wherever water fraction is below a threshold, water brightness temperature correction is applied to this parameter prior to SPL2SMP inversion.

          tb_h_uncorrected

          Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input L1C_TB granule. The resulting parameter describes the weighted average of the L1B_TB horizontally polarized brightness temperatures prior to water correction whose boresights fall within a 36 km EASE-Grid 2.0 cell.

          tb_qual_flag_3

          A 16-bit or two-byte binary number formed by applying a Boolean 'AND' operation between the same parameters from both fore- and aft-looking groups in the input Level-1C brightness temperature granule. A '0' indicates that both the fore-looking and aft-looking Level-1C brightness temperature observations satisfy a given quality criterion described in Level-1B brightness temperature's tb_qual_flag_3 output parameter; a '1' indicates that the same criterion is violated by either fore-looking or aft-looking (or both) Level-1C brightness temperature observations. Bit position '0' refers to the least-significant digit. The possible values for each bit position are shown in Table A6.

          Table A6. Brightness Temperature Quality Flag Bit Definitions
          Bit Position Bit Value and Interpretation
          0 0 = Observation has acceptable quality
          1 = Observation does not have acceptable quality
          1 0 = Observation within physical range
          1 = Observation beyond physical range
          2 0 = RFI was not detected in the observation
          1 = RFI was detected in the observation
          3 0 = RFI was detected and corrected in the observation
          1 = RFI was detected but not correctable in the observation
          4 0 = Observation has acceptable NEDT
          1 = Observation did not have acceptable NEDT
          5 0 = Direct sun correction was successful
          1 = Direct sun correction was not successful
          6 0 = Reflected sun correction was successful
          1 = Reflected sun correction was not successful
          7 0 = Reflected moon correction was successful
          1 = Reflected moon correction was not successful
          8 0 = Direct galaxy correction was successful
          1 = Direct galaxy correction was not successful
          9 0 = Reflected galaxy correction was successful
          1 = Reflected galaxy correction was not successful
          10 0 = Atmosphere correction was successful
          1 = Atmosphere correction was not successful
          11 Intentionally left undefined
          12 0 = Observation was a valid value
          1 = Observaiton was a null value
          13 0 = Observation was within half orbit
          1 = Observation was outside half orbit
          14 0 = TA minus TA_FILTERED was less than a threshold
          1 = TA minus TA_FILTERED was greater than a threshold
          15 0 = Observationw as free of RFI
          1 = Observation was RFI contaminated
          tb_qual_flag_4

          A 16-bit or two-byte binary number formed by applying a Boolean 'AND' operation between the same parameters from both fore- and aft-looking groups in the input Level-1C brightness temperature granule. A '0' indicates that both the fore-looking and aft-looking Level-1C brightness temperature observations satisfy a given quality criterion described in Level-1B brightness temperature's tb_qual_flag_4 output parameter; a '1' indicates that the same criterion is violated by either fore-looking or aft-looking (or both) Level-1C brightness temperature observations. Bit position '0' refers to the least significant digit. The possible values for each bit position are shown in Table A6.

          tb_qual_flag_h

          A 16-bit or two-byte binary number formed by applying a Boolean 'AND' operation between the same parameters from both fore- and aft-looking groups in the input Level-1C brightness temperature granule. A '0' indicates that both the fore-looking and aft-looking Level-1C brightness temperature observations satisfy a given quality criterion described in Level-1B brightness temperature's tb_qual_flag_h output parameter; a '1' indicates that the same criterion is violated by either fore-looking or aft-looking (or both) Level-1C brightness temperature observations. Bit position '0' refers to the least significant digit. The possible values for each bit position are shown in Table A7.

          Table A7. Brightness Temperature Quality Flag Bit Definitions
          Bit Position Bit Value and Interpretation
          0 0 = Observation has acceptable quality
          1 = Observation does not have acceptable quality
          1 0 = Observation within physical range
          1 = Observation beyond physical range
          2 0 = RFI was not detected in the observation
          1 = RFI was detected in the observation
          3 0 = RFI was detected and corrected in the observation
          1 = RFI was detected but not correctable in the observation
          4 0 = Observation has acceptable NEDT
          1 = Observation did not have acceptable NEDT
          5 0 = Direct sun correction was successful
          1 = Direct sun correction was not successful
          6 0 = Reflected sun correction was successful
          1 = Reflected sun correction was not successful
          7 0 = Reflected moon correction was successful
          1 = Reflected moon correction was not successful
          8 0 = Direct galaxy correction was successful
          1 = Direct galaxy correction was not successful
          9 0 = Reflected galaxy correction was successful
          1 = Reflected galaxy correction was not successful
          10 0 = Atmosphere correction was successful
          1 = Atmosphere correction was not successful
          11 0 = Faraday rotation correction was successful
          1 = Faraday rotation correction was not successful
          12 0 = Observation was a valid value
          1 = Observaiton was a null value
          13 0 = Water correction was not performed
          1 = Water correction was performed
          14 0 = TA minus TA_FILTERED was less than a threshold
          1 = TA minus TA_FILTERED was greater than a threshold
          15 0 = Observationw as free of RFI
          1 = Observation was RFI contaminated
          tb_qual_flag_v

          A 16-bit or two-byte binary number formed by applying a Boolean 'AND' operation between the same parameters from both fore- and aft-looking groups in the input Level-1C brightness temperature granule. A '0' indicates that both the fore-looking and aft-looking Level-1C brightness temperature observations satisfy a given quality criterion described in Level-1B brightness temperature's tb_qual_flag_v output parameter; a '1' indicates that the same criterion is violated by either fore-looking or aft-looking (or both) Level-1C brightness temperature observations. Bit position '0' refers to the least significant digit. The possible values for each bit position are shown in Table A7.

          tb_time_seconds

          Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input Level-1C brightness temperature granule. The resulting parameter thus describes the average of UTC acquisition times of Level-1B brightness temperature observations whose boresights fall within a 36 km EASE-Grid 2.0 cell. The result is then expressed in J2000 seconds (the number of seconds since 12:00:00.000 on January 1, 2000 Barycentric Dynamical Time (TDB)).

          tb_time_utc

          Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input Level-1C brightness temperature granule. The resulting parameter thus describes the average of UTC acquisition times, in ASCII representation, of Level-1B brightness temperature observations whose boresights fall within a 36 km EASE-Grid 2.0 cell.

          tb_v_corrected

          Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input Level-1C brightness temperature granule. The resulting parameter thus describes the weighted average of Level-1B brightness temperature vertically polarized brightness temperatures whose boresights fall within a 36 km EASE-Grid 2.0 cell. Wherever water fraction is below a threshold, water brightness temperature correction is applied to this parameter prior to SPL2SMP inversion.

          tb_v_uncorrected

          Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input L1C_TB granule. The resulting parameter describes the weighted average of the L1B_TB vertically polarized brightness temperatures prior to water correction whose boresights fall within a 36 km EASE-Grid 2.0 cell.

          vegetation_opacity, vegetation_opacity_option[1-3]
          Daily global composite of the estimated vegetation opacity at 36-km grid posting, as returned by the L2_SM_P processing software. Note that this parameter is the same ‘tau’ parameter normalized by the cosine of the incidence angle in the ‘tau-omega’ model:

          tau-omega equation

          where b is a landcover-based parameter described in the SMAP Level 2/3 Passive Soil Moisture Product ATBD, VWC is vegetation water content in kg/m2 derived from NDVI climatology, and θ is the incidence angle (= 40 deg) for SMAP. The valid minimum (0.0) and maximum (5.0) are subject to further analysis on real data. The vegetation_opacity field is internally linked to the vegetation_opacity_option2 field produced by the baseline algorithm.
          vegetation_opacity_option3 is not calculated by the equation above, but rather is retrieved directly along with soil moisture from the Modified Dual Channel Algorithm.

          vegetation_water_content

          Vegetation water content at 36 km grid posting. This parameter is used as input ancillary data parameter to the L2_SM_P processing software when the baseline algorithm is used. The valid minimum (0.0) and maximum (30.0) are subject to further analysis on real data.

          Fill/Gap Values 

          SMAP data products employ fill values to indicate when no valid data appear in a particular data element. Fill values ensure that data elements retain the correct shape. Gap values locate portions of a data stream that do not appear in the output data file.

          Fill values appear in the SPL2SMP product when the SPL2SMP Science Production Software (SPS) can process some, but not all, of the input data for a particular swath grid cell. Fill data may appear in the product in any of the following circumstances:

          • One of SPS executables that generate the SMAP SPL2SMP product is unable to calculate a particular science or engineering data value. The algorithm encounters an error. The error disables generation of valid output. The SPS reports a fill value instead.
          • Some of the required science or engineering algorithmic input are missing. Data over the region that contributes to particular grid cell may appear in only some of the input data streams. Since data are valuable, the SPL2SMP product records any outcome that can be calculated with the available input. Missing data appear as fill values.
          • Non-essential information is missing from the input data stream. The lack of non-essential information does not impair the algorithm from generating needed output. The missing data appear as fill values.
          • Fill values appear in the input radiometer Level-1C brightness temperature product.

          SMAP data products employ a specific set of data values to connote that an element is fill. The selected values that represent fill are dependent on the data type. 

          No valid value in the SPL2SMP product is equal to the values that represent fill. If any exceptions should exist in the future, the SPL2SMP content will provide a means for users to discern between elements that contain fill and elements that contain genuine data values. This document will also contain a description of the method used to ascertain which elements are fill and which elements are genuine.

          The SPL2SMP product records gaps when entire frames within the time span of a particular data granule do not appear. Gaps can occur under one of two conditions:

          • One or more complete frames of data are missing from all data streams.
          • The subset of input data that is available for a particular frame is not sufficient to process any frame output.

          The Level-1C brightness temperature product records gaps in the product level metadata. The following conditions will indicate that no gaps appear in the data product:

          • Only one instance of the attributes Extent/rangeBeginningDateTime and Extent/rangeEndingDateTime will appear in the product metadata.
          • The character string stored in metadata element Extent/rangeBeginningDateTime will match the character string stored in metadata element OrbitMeasuredLocation/halfOrbitStartDateTime.
          • The character string stored in metadata element Extent/rangeEndingDateTime will match the character string stored in metadata element OrbitMeasuredLocation/halfOrbitStopDateTime.

          One of two conditions will indicate that gaps appear in the data product:

          • The time period covered between Extent/rangeBeginningDateTime and Extent/RangeEndingDateTime does not cover the entire half orbit as specified in OrbitMeasuredLocation/halfOrbitStartDateTime and OrbitMeasuredLocation/halfOrbitStartDateTime.
          • More than one pair of Extent/rangeBeginningDateTime and Extent/rangeEndingDateTime appears in the data product. Time periods within the time span of the half orbit that do not fall within the sets of Extent/rangeBeginningDateTime and Extent/rangeEndingDateTime constitute data gaps.

          Acronyms and Abbreviations

          Table 13. Acronyms and Abbreviations
          Abbreviation Definition
          Char 8-bit character
          IGBP International Geosphere-Biosphere Programme
          Int8 8-bit (1-byte) signed integer
          Int16 16-bit (2-byte) signed integer
          Int32 32-bit (4-byte) signed integer
          Float32 32-bit (4-byte) floating-point integer
          Float64 64-bit (8-byte) floating-point integer
          H-pol Horizontally polarized
          N/A Not Applicable
          NF Number of frozen ground pixels
          NL Number of land pixels
          NT Number of thawed land pixels
          NW Number of water pixels
          SI International System of Units
          SPL2SMP SMAP L2 Radiometer Half-Orbit 36 km EASE-Grid Soil Moisture
          SPS Science Production Software
          T1, T2 Threshold 1, Threshold 2
          TB Brightness Temperature
          Uint8 8-bit (1-byte) unsigned integer
          Uint16 16-bit (2-byte) unsigned integer
          UTC Universal Coordinated Time
          V-pol Vertically polarized
          VWC Vegetation Water Content

          How To

          Programmatic Data Access Guide
          Data from the NASA National Snow and Ice Data Center Distributed Active Archive Center (NSIDC DAAC) can be accessed directly from our HTTPS file system or through our Application Programming Interface (API). Our API offers you the ability to order data using specific temporal and spatial filters... read more
          How to import and geolocate SMAP Level-1C and Level-2 data in ENVI
          The following are instructions on how to import and geolocate SMAP Level-1C HDF5 data in ENVI. Testing notes Software: ENVI Software version: 5.3 and above. If using version 5.3, service pack 5.3.1 is needed.  Platform: Windows 7 Data set: SMAP L1C... read more
          How do I visualize SMAP data in Worldview?
          This video tutorial provides step-by-step instructions on how to visualize SMAP data in Worldview (http://worldview.earthdata.nasa.gov/). NASA Worldview is a map-based application... read more
          How do I interpret the surface and quality flag information in the Level-2 and -3 passive soil moisture products?
          SMAP data files contain rich quality information that can be useful for many data users. The retrieval quality flag and surface flag bit values and interpretations are documented in the respective product Data Fields pages: Level-2 soil moisture product (SPL2SMP)... read more
          How to learn more about SMAP ancillary data
          SMAP Ancillary data sets are used to produce SMAP Level-1, -2, -3, and -4 standard data products. Several of these ancillary data sets are produced by external organizations, such as NOAA, the NASA Global Modeling and Assimilation... read more
          Visualize NSIDC data as WMS layers with ArcGIS and Google Earth
          NASA's Global Imagery Browse Services (GIBS) provides up to date, full resolution imagery for selected NSIDC DAAC data sets. ... read more
          Search, order, and customize NSIDC DAAC data with NASA Earthdata Search
          NASA Earthdata Search is a map-based interface where a user can search for Earth science data, filter results based on spatial and temporal constraints, and order data with customizations including re-formatting, re-projecting, and spatial and parameter subsetting. Thousands of Earth science data... read more
          Filter and order from a data set web page
          Many NSIDC data set web pages provide the ability to search and filter data with spatial and temporal contstraints using a map-based interface. This article outlines how to order NSIDC DAAC data using advanced searching and filtering.  Step 1: Go to a data set web page This article will use the... read more

          FAQ

          What are the latencies for SMAP radiometer data sets?
          The following table describes both the required and actual latencies for the different SMAP radiometer data sets. Latency is defined as the time (# days, hh:mm:ss) from data acquisition to product generation. Short name Title Latency Required Actual (mean1) SPL1AP SMAP L1A... read more
          What data subsetting, reformatting, and reprojection services are available for SMAP data?
          The following table describes the data subsetting, reformatting, and reprojection services that are currently available for SMAP data via the NASA Earthdata Search tool and a Data Subscription... read more
          How do I convert an HDF5/HDF-EOS5 file into binary format?
          To convert HDF5 files into binary format you will need to use the h5dump utility, which is part of the HDF5 distribution available from the HDF Group. How you install HDF5 depends on your operating system. Full instructions for installing and using h5dump on Mac/Unix and... read more