Data Description

Parameters

The main output of this data set is 0-5 cm surface soil moisture (cm³/cm³) presented on the 9 km EASE-Grid 2.0 projection. Also included are Brightness Temperature (T_b) measurements (K), representing SMAP Level-1B brightness temperatures.

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

File Information

Format

Data are in HDF5 format. 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. 

The Soil Moisture Retrieval Data group contains soil moisture data, ancillary data, and quality assessment flags for either a descending/6:00 a.m. or ascending/6:00 p.m. half-orbit pass of the satellite. 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.

File Naming Convention

Files are named according to the following convention and as described in Table 1:
SMAP_L2_SM_P_E_[Orbit#]_[A/D]_yyyymmddThhmmss_RLVvvv_NNN.[ext]

For example:
SMAP_L2_SM_P_E_10508_A_20170119T005350_R14010_001.h5

Table 1. File Naming Conventions

Variable	Description
SMAP	Indicates SMAP mission data
L2_SM_P_E	Indicates specific product (L2: Level-2; SM: Soil Moisture; P: Passive; E: Enhanced)
[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.
A/D	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) 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)
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

File Size

Each half-orbit file ranges from approximately 10 to 15 MB.

File Volume

The daily data volume is approximately 300 MB.

Spatial Information

Coverage

Data set coverage spans from 180°W to 180°E and from approximately 85.044°N and 85.044°S. The gap in coverage at both the North and South Pole, called a pole hole, has a radius of approximately 400 km.

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. The swath width is approximately 1000 km, enabling nearly global coverage every two to three days.

Resolution

The native spatial resolution of the radiometer footprint is 36 km, but soil moisture retrieval, ancillary data, and quality flags are all interpolated, using the Backus Gilbert optimal interpolation algorithm, to a 9 km grid resolution. 

Geolocation

Data are gridded using the 9 km EASE-Grid 2.0 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
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	http://epsg.io/6933

Table 3. Grid Details for the Global EASE-Grid

Grid cell size (x, y pixel dimensions)	9,024.31 projected meters (x) 9,024.31 projected meters (y)
Number of columns	3856
Number of rows	1624
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 present.

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.

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 soil 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 additional 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: Passive Remote Sensing of Soil Moisture" of the Algorithm Theoretical Basis Document (ATBD) for the SMAP L2 & L3 Products (O'Neill et al. 2018).

Acquisition

SMAP enhanced Level-2 radiometer soil moisture data (SPL2SMP_E) are derived from SMAP Enhanced L1C Radiometer Half-Orbit 9 km EASE-Grid Brightness Temperatures, Version 2 (SPL1CTB_E)  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 Enhanced L1C Radiometer Half-Orbit 9 km EASE-Grid Brightness Temperatures, Version 2 (SPL1CTB_E) 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 an estimates of the 0-5 cm surface soil moisture (m³/m³). 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 briefly below.

For information regarding the Backus-Gilbert optimal interpolation algorithm used to enhance these data, refer to the SPL1CTB_E user guide. For more information on the soil moisture retrieval algorithms, users should refer to the O'Neill et al. 2016 and O'Neill et al. 2018 for more details.

Algorithm Inputs and Outputs

The main input to the processing algorithm is the SMAP Enhanced L1C Radiometer Half-Orbit 9 km EASE-Grid Brightness Temperatures, Version 2 (SPL1CTB_E) data set. This product contains the time-ordered, geolocated, and calibrated SMAP enhanced Level-1B radiometer Brightness Temperatures (T_b) that have been resampled to the fixed 9 km EASE-Grid 2.0. In addition to general geolocation and calibration, the enhanced Level-1B T_b data 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 T_b data are flagged accordingly and no soil moisture retrieval is attempted. Refer to the SPL1CTB_E ATBD for additional details.

The input enhanced Level-1C T_b data (SPL1CTB_E) 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 SPL1CTB_E 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.

In addition to brightness temperature observations, the SPL2SMP_E algorithm also requires ancillary data sets for the soil moisture retrieval. In order for soil moisture to be retrieved accurately, 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), permanent open water fraction, and soils 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: Ancillary Data Sets" of the SMAP L2 & L3 Products ATBD (O'Neill et al. 2018).

Ancillary data are also employed to set flags that help determine either specific aspects of the processing, such as corrections for open water and frozen ground, or the quality of the retrievals, such as the precipitation flag. Other parameters used by the SPL2SMP_E algorithm include a freeze/thaw flag, an open water fraction, and a vegetation index. Refer to the Data Flags section below for more details. 

Note: all input data to the SPL2SMP_E process are pre-mapped using the 9 km EASE-Grid 2.0 projection and then aggregated at a spatial extent that is approximately the same as the native resolution (approximately 36 km) of the SMAP radiometer.

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 T_b 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: Physics of the Problem" of this data product's ATBD (O'Neill et al. 2018); SMAP retrievals are also based on the tau-omega model. In essence, this model relates T_b (SMAP L1 observations) to soil moisture (SMAP L2 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, T_b estimates are corrected for water/land contamination (described above and in the SPL1BTB user guide). This 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. This correction reduces the dry bias seen when comparing SMAP data to in situ data from the core validation sites. At L-band frequency, the contributing soil depth of microwave emission (or penetration depth) may be slightly different from the pre-defined discrete soil depths at which the soil temperatures are available from a land surface model. The resulting discrepancy will lead to dry bias of retrieved soil moisture (i.e. retrievals 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.

The Version 3 SPL2SMP_E 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)
Modified Dual Channel Algorithm (MDCA)	soil_moisture_option3

Among the three algorithms, SCA-V delivers the best performance according to SPL2SMP_E Calibration/Validation (Cal/Val) analyses. For this reason, SCA-V is designated as the current baseline algorithm. However, all three algorithms will be continuously assessed; the choice of the operational algorithm for the release of the product will be evaluated on a regular basis as analyses of new observations and Cal/Val data become available and as algorithm parameters are tuned using a longer SMAP record.

All three algorithms operate on the same zeroth-order microwave emission model, commonly known as the tau-omega model and described briefly above. 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 O'Neill et al. 2016 for more details.

Single Channel Algorithm (SCA) 

In SCA, horizontally (SCA-H) or vertically (SCA-V) polarized brightness temperature (T_b) 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 polarized T_b 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 and polarization.

Though SCA can apply to H-pol or V-pol T_b measurements, SCA-V is the current baseline soil moisture algorithm because it performs better when compared against SMAP Cal/Val data. The SCA-H algorithm was the pre-launch baseline retrieval algorithm.

Modified Dual Channel Algorithm (MDCA)

The Modified Dual Channel Algorithm (MDCA) is an extension of the SCA and replaces the Dual Channel Algorithm implemented in previous releases. Like DCA, MDCA uses both the vertically and horizontally polarized T_b observations to solve for soil moisture and vegetation optical depth. The algorithm iteratively minimizes the cost function (Φ²) shown in Equation 1:

   (Equation 1)

In each iteration step, the soil moisture and vegetation opacity are adjusted simultaneously until the cost function attains a minimum in a least square sense. Similar to SCA-V and SCA-H, ancillary information such as effective soil temperature, surface roughness, and vegetation single scattering albedo must be known before the inversion process. MDCA permits polarization mixing of the vertically and horizontally polarized T_b observations prior to cost function minimization.

The MDCA implemented in Version 6 of this data set performs better than the Dual Channel Algorithm (DCA), Microwave Polarization Ratio Algorithm (MPRA), and Extended Dual Channel Algorithm (EDCA) used in earlier releases. While it still does not perform as well as SCA-V, the MDCA retrieves tau, the vegetation opacity, in addition to soil moisture. There is currently no independent verification of the accuracy of these 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. Consultative Committee on Space Data Systems (CCSDS) packets include error-detecting Cyclic Redundancy Checks (CRCs), which are used to flag errors.

More information about error sources is provided in "Section 4.6: Algorithm Error Performance" of the ATBD (O'Neill et al. 2018).

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 Validated Assessment Report.

6:00 p.m. Ascending / 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 beginning with Version 4 of this product. 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 33 x 33 km² inversion domain.

A brief description of data flags are provided below. For more details on all data flags, users should refer to the Appendix of this document or the Product Specification Document.

• Open Water Flag
  Open water fraction is determined by a priori information on permanent open freshwater from the Moderate Resolution Imaging Spectroradiometer (MODIS) MOD44W database. Open water are reported in Bits 0 and 1 in the surface_flag field of the SPL2SMP_E product. This 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.

• Precipitation Flag 
  ​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 within a 33 km inversion domain 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 T_b; corrections for participation are part of the Level-1B T_b 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
  ​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
  ​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 (T_surf) 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). For this SPL2SMP_E product, GMAO Tsurf (bit 8) is used to determine frozen ground condition. 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 latitude. While the SPL2SMP_E product contains global SMAP freeze/thaw flags, uncertainty in the flags is higher south of 45°N latitude due to small differences in the SMAP radiometer-derived reference freeze and thaw states upon which the freeze/thaw algorithm is based. More informaiton is available in the SMAP Level-3 Freeze/Thaw (SPL3FTP) Validated Assessment Report.

• Urban Area Flag 
  ​Since the T_b 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. 2016). 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.

• Mountainous Area Flag 
  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 9 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.

Instrumentation

Description

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

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
V3	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.
V2	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.
V1	December 2016	First public release

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

References

Jet Propulsion Laboratory (JPL). "SMAP Instrument." JPL SMAP Soil Moisture Active Passive. https://smap.jpl.nasa.gov/observatory/instrument/ [20 August 2015].

Carroll, M.L., C. M. DiMiceli, M. R. Wooten, A. B. Hubbard, R.A. Sohlberg, and J. R. G. Townshend. 2017. MOD44W MODIS/Terra Land Water Mask Derived from MODIS and SRTM L3 Global 250m SIN Grid V006 [Data set]. NASA EOSDIS Land Processes DAAC, USGS Earth Resources Observation and Science (EROS) Center, Sioux Falls, SD. doi: 10.5067/MODIS/MOD44W.006

O'Neill, P. E., E. G. Njoku, T. J. Jackson, S. Chan, and R. Bindlish. 2018. SMAP Algorithm Theoretical Basis Document: Level 2 & 3 Soil Moisture (Passive) Data Products, Revision D. SMAP Project, JPL D-66480, Jet Propulsion Laboratory, Pasadena, CA. (PDF, 1.7 MB; see Technical References)

O'Neill, P. E., E. G. Njoku, T. J. Jackson, S. Chan, and R. Bindlish. 2016. SMAP Algorithm Theoretical Basis Document: Level 2 & 3 Soil Moisture (Passive) Data Products, Revision C. SMAP Project, JPL D-66480, Jet Propulsion Laboratory, Pasadena, CA. (PDF, 4.9 MB; see Technical References)

Appendix - Data Fields

This appendix provides a description of all data fields within the SMAP Enhanced L2 Radiometer Half-Orbit 9 km EASE-Grid Soil Moisture (SPL2SMP_E) product. The data are grouped into two main HDF5 groups:

• Metadata
• Soil_Moisture_Retrieval_Data

For a description of metadata fields for this product, refer to the Product Specification Document. Table A1 describes the data fields of a typical half-orbit granule for this product. All data element arrays are one-dimensional with a size N, where N is the number of valid cells from the radiometer swath that appear on the grid.

Table A1. Data Fields for Soil_Moisture_Retrieval_Data

Data Field Name	Type	Byte	Unit	Valid Min	Valid Max	Fill/Gap Values	Dimensions (Number of grid cells covered by swath)	Derivation Method(s)*
EASE_column_index	Uint16	2	N/A	0	1623	65534	N	2
EASE_row_index	Uint16	2	N/A	0	3855	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	Degree	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
grid_surface_status	Uint16	2	N/A	0	1	65534	N	7
latitude	Float32	4	Degree	-90.0	+90.0	-9999.0	N	2
latitude_centroid	Float32	4	Degree	-90.0	+90.0	-9999.0	N	1
longitude	Float32	4	Degree	-180.0	+180.0	-9999.0	N	2
longitude_centroid	Float32	4	Degree	-180.0	+180.0	-9999.0	N	1
radar_water_body_fraction	Float32	4	N/A	0.0	1.0	-9999.0	N	6
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	3.0	-9999.0	N	6
roughness_coefficient_option3	Float32	4	N/A	0.0	3.0	-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	Kelvin	253.15	313.15	-9999.0	N	6
surface_water_fraction_mb_h	Float32	4	Kelvin	0.0	1.0	-9999.0	N	1
surface_water_fraction_mb_v	Float32	4	Kelvin	0.0	1.0	-9999.0	N	1
tb_3_corrected	Float32	4	Kelvin	-50.0	50.0	-9999.0	N	1
tb_4_corrected	Float32	4	Kelvin	-50.0	50.0	-9999.0	N	1
tb_h_corrected	Float32	4	Kelvin	0.0	340.0	-9999.0	N	1
tb_h_uncorrected	Float32	4	Kelvin	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	Second	0	N/A	-9999.0	N	1
tb_time_utc	Char24	24	N/A	2015-01-31T 00:00:00.000Z	N/A	16777214	N	1
tb_v_corrected	Float32	4	Kelvin	0.0	340.0	-9999.0	N	1
tb_v_uncorrected	Float32	4	Kelvin	0.0	340.0	-9999.0	N	1
vegetation_opacity	Float32	4	N/A	0.01	5.00	-9999.0	N	6
vegetation_opacity_option1	Float32	4	N/A	0.01	5.00	-9999.0	N	6
vegetation_opacity_option2	Float32	4	N/A	0.01	5.00	-9999.0	N	6
vegetation_opacity_option3	Float32	4	N/A	0.01	5.00	-9999.0	N	5
vegetation_water_content	Float32	4	kg/m2	0	30.0	-9999.0	N	6
* Derivation methods are: From Level-1C brightness temperature data From 9-km EASE-Grid 2.0 array definition 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. Determined by Level-2 radiometer soil moisture processing software Available only with option algorithms that use two polarization channels From external ancillary data whose location and time stamp coincide with those of the input data Nearest-neighbor interpolation

Data Field Definitions

Column index of a 9-km EASE 2.0 grid cell. In most grid cells, both fore-looking L1C_TB_E data and aft-looking L1C_TB_E 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 (e.g., aft-looking group) will be written into this parameter.

Row index of a 9-km EASE 2.0 grid cell. In most grid cells, both fore-looking L1C_TB_E data and aft-looking L1C_TB_E data are available for soil moisture retrieval. But when one group (e.g., fore-looking group) is not available, the GridRow parameter of the other group (e.g., aft- looking group) will be written into this parameter.

Single-scattering albedo at 9-km grid posting. 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 9-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 'tau-omega' model when used in MDCA.

Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input L1C_TB_E granule. The resulting parameter thus describes the weighted average of incidence angles of L1B_TB_E observations whose boresights fall within a 9-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 9-km grid posting.

clay_fraction
Clay fraction at 9-km grid posting.

Freeze/thaw areal fraction at 9-km grid posting. 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 9-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 determined by the antenna boresight location.

Latitude of the center of the Earth based grid cell.

Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input L1C_TB_E granule. The resulting parameter thus describes the weighted average of latitudes of L1B_TB_E observations whose boresights fall within a 9-km EASE-Grid 2.0 cell.

Longitude of the center of the Earth based grid cell.

Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input L1C_TB_E granule. The resulting parameter thus describes the weighted average of latitudes of L1B_TB_E observations whose boresights fall within a 9-km EASE-Grid 2.0 cell.

Radar-derived water body areal fraction at 9-km grid posting. 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 9-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.

A 16-bit binary string of 1’s and 0’s that indicate whether retrieval was performed or not at a given grid cell. When retrieval was 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 A2. 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 A3.

Table A2. 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 at 9-km grid posting. Note that this parameter is the same 'h' coefficient int he 'tau-omega' model for a given polarization channel.

roughness_coefficient_option3
Roughness coefficient at 9-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.

The estimated ‘1-sigma’ error of the soil_moisture output parameter. The valid minimum and maximum below are subject to further analysis on real data. This data field is currently filled with FillValue.

The estimated soil moisture at 9-km grid posting, as returned by the L2_SM_P_E processing software. The soil_moisture field is internally linked to the output produced by the baseline algorithm (option 2 as of now). At present, the operational SPL2SMP Science Production Software (SPS) produces and stores soil moisture retrieval results from the three algorithms listed in Table A3; retrieval quality flags that correspond to each of these algorithms are also listed in Table A3.

Table A3. Soil Moisture Algorithm Options and Corresponding Data Fields

Algorithm Option	Corresponding 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 areal fraction at 9-km grid posting. 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 9-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 when the original data were acquired.

A 16-bit binary string of 1’s and 0’s that indicate the presence or absence of certain surface conditions at a grid cell. In Table A4, 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 A4. The surface_flag field is internally linked to the output produced by the baseline algorithm (option 2 as of now).

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 A4. 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	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	3°	6°	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 this product)
12-15	Undefined			0

Effective soil temperature (average for the top 5-cm soil layer) at 9-km grid spacing. This calculated model parameter is used as an input ancillary data parameter to the L2_SM_P_E 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.

Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input L1C_TB_E granule. The resulting parameter thus describes the weighted average of L1B_TB_E 3^rd Stokes polarized brightness temperatures whose boresights fall within a 9-km EASE-Grid 2.0 cell.

Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input L1C_TB_E granule. The resulting parameter thus describes the weighted average of L1B_TB_E 4^th Stokes polarized brightness temperatures whose boresights fall within a 9-km EASE-Grid 2.0 cell.

Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input L1C_TB_E granule. The resulting parameter thus describes the weighted average of L1B_TB_E horizontally polarized brightness temperatures whose boresights fall within a 9-km EASE-Grid 2.0 cell. Wherever water fraction is below a threshold, water brightness temperature correction is applied to this parameter prior to L2_SM_P_E inversion.

tb_h_uncorrected
Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input L1C_TB_E granule. The resulting parameter thus describes the weighted average of L1B_TB_E horizontally polarized brightness temperatures prior to water correction whose boresights fall within a 9-km EASE-Grid 2.0 cell.

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 L1C_TB_E granule. A ‘0’ indicates that both the fore-looking and aft-looking L1C_TB_E observations satisfy a given quality criterion described in L1B_TB_E’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) L1C_TB_E observations. Bit position '0' refers to the least significant digit. Bit definitions are shown in Table A5.

Table A5. tb_qual_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 had 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 = Direction 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 = Observation 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 = Observation was free of RFI
	1 = Observation was RFI contaminated

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 L1C_TB_E granule. A ‘0’ indicates that both the fore-looking and aft-looking L1C_TB_E observations satisfy a given quality criterion described in L1B_TB_E’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) L1C_TB_E observations. Bit position '0' refers to the least significant digit. Bit definitions are shown in Table A5.

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 L1C_TB_E granule. A ‘0’ indicates that both the fore-looking and aft-looking L1C_TB_E observations satisfy a given quality criterion described in L1B_TB_E’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) L1C_TB_E observations. Bit position '0' refers to the least significant digit. Bit definitions are provided in Table A6.

Table A6. tb_qual_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 had 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 = Direction 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 = Observation was a null value
13	0 = Water correcition 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 = Observation was free of RFI
	1 = Observation was RFI contaminated

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 L1C_TB_E granule. A ‘0’ indicates that both the fore-looking and aft-looking L1C_TB_E observations satisfy a given quality criterion described in L1B_TB_E’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) L1C_TB_E observations. Bit position '0' refers to the least significant digit. Bit definitions are provided in Table A6.

Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input L1C_TB_E granule. The resulting parameter thus describes the average of UTC acquisition times of L1B_TB_E observations whose boresights fall within a 9-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)).

Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input L1C_TB_E granule. The resulting parameter thus describes the average of UTC acquisition times, in ASCII representation, of L1B_TB_E observations whose boresights fall within a 9-km EASE-Grid 2.0 cell.

Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input L1C_TB_E granule. The resulting parameter thus describes the weighted average of L1B_TB_E vertically polarized brightness temperatures whose boresights fall within a 9-km EASE-Grid 2.0 cell. Wherever water fraction is below a threshold, water brightness temperature correction is applied to this parameter prior to L2_SM_P_E inversion.

tb_v_uncorrected
Arithmetic average of the same parameters found in the fore- and aft-looking groups in the input L1C_TB_E granule. The resulting parameter thus describes the weighted average of L1B_TB_E vertically polarized brightness temperatures prior to water correction whose boresights fall within a 9-km EASE-Grid 2.0 cell.

where b is a landcover-based parameter described in the SPL2SMP/SPL3SMP ATBD, VWC is vegatation water content in kg/m² dervied from NDVI climatology, and θ is the incidence angle (= 40 degrees) for SMAP. The valid minimum and maximum 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_opion3 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 at 9-km grid posting. This parameter is used as input ancillary data parameter to the L2_SM_P_E processing software when the baseline algorithm is used. The valid minimum and maximum below 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 A7 lists the acronyms and abbreviations used in this document.

Table A7. 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_E	SMAP Enhanced 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