Data Set ID: 
SPL3SMP

SMAP L3 Radiometer Global Daily 36 km EASE-Grid Soil Moisture, Version 3

This Level-3 (L3) soil moisture product provides a composite of daily estimates of global land surface conditions retrieved by the Soil Moisture Active Passive (SMAP) passive microwave radiometer. SMAP L-band soil moisture data are resampled to a global, cylindrical 36 km Equal-Area Scalable Earth Grid, Version 2.0 (EASE-Grid 2.0).

Version Summary: 

Changes to this version include:

  • Transitioned to Validated-Stage 2
  • Uses updated SPL2SMP V3 Validated data as input version

Geographic Coverage

Parameter(s):
  • Microwave > Brightness Temperature
  • Soils > Soil Moisture/Water Content > Soil Moisture
Spatial Coverage:
  • N: 85.044, S: -85.044, E: 180, W: -180

Spatial Resolution:
  • 36 km x 36 km
Temporal Coverage:
  • 31 March 2015
Temporal Resolution: 1 day
Data Format(s):
  • HDF5
Platform(s) SMAP Observatory
Sensor(s): SMAP L-BAND RADIOMETER
Version: V3
Data Contributor(s): O'Neill, P. E., S. Chan, E. G. Njoku, T. Jackson, and R. Bindlish.

Data Citation

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, and R. Bindlish. 2016. SMAP L3 Radiometer Global Daily 36 km EASE-Grid Soil Moisture, Version 3. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/7MINGFDCZTES. [Date Accessed].

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Detailed Data Description

Parameter Description

Surface soil moisture (0-5 cm) in cm3/cm3 derived from brightness temperatures is output on a fixed 36 km EASE-Grid 2.0.

Brightness temperature (TB) is a measure of the radiance of the microwave radiation welling upward from the top of the atmosphere to the satellite. The SMAP L-Band Radiometer measures four brightness temperature Stokes parameters: TH, TV, T3, and T4 at 1.41 GHz. TH and TV are the horizontally and vertically polarized brightness temperatures, respectively, and T3 and T4 are the third and fourth Stokes parameters, respectively.

Refer to the Data Fields document for details on all parameters.

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Format

Data are in HDF5 format. For software and more information, including an HDF5 tutorial, visit the HDF Group's HDF5 Web site.

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File Contents

As shown in Figure 1, each HDF5 file is organized into the following main groups, which contain additional groups and/or data sets:

Figure 1. Subset of File Contents
For a complete list of file contents for the SMAP Level-3 radiometer soil moisture product, refer to the Data Fields page. 

Data Fields

Each file contains the main data groups summarized in this section. For a complete list and description of all data fields within these groups, refer to the Data Fields document.

All data element arrays are one-dimensional with a size of N, with the exception of landcover_class and landcover_class_fraction, which are two-dimensional arrays with a size of N x 3; where N is the number of valid cells covered by the radiometer swath on the grid.

Soil Moisture Retrieval Data

Includes soil moisture data, ancillary data, and quality assessment flags.

Metadata Fields

Includes all metadata that describe the full content of each file. For a description of all metadata fields for this product, refer to the Metadata Fields document.

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File Naming Convention

Files are named according to the following convention, which is described in Table 1:

SMAP_L3_SM_P_yyyymmdd_RLVvvv_NNN.[ext]

For example:

SMAP_L3_SM_P_20150705_R13242_001.h5

Where:

Table 1. File Naming Conventions
Variable Description
SMAP Indicates SMAP mission data
L3_SM_P Indicates specific product (L3: Level-3; SM: Soil Moisture; P: Passive)
yyyymmdd 4-digit year, 2-digit month, 2-digit day of the first data element that appears in the product.
RLVvvv Composite Release ID, where:
R Release
L Launch Indicator (1: post-launch standard data)
V 1-Digit Major Version Number
vvv 3-Digit Minor Version Number
Example: R13242 indicates a standard data product with a version of 3.242. Refer to the SMAP Data Versions page for version information.
NNN Product Counter: Number of times the file was generated under the same version for a particular date/time interval (002: second time)
.[ext] File extensions include:
.h5 HDF5 data file
.qa Quality Assurance file
.xml XML Metadata file
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File Size

Each file is approximately 24 MB.

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Volume

The daily data volume is approximately 24 MB.

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Spatial Coverage

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

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Spatial Resolution

The native spatial resolution of the radiometer footprint is approximately 40 km. Data are then gridded using the 36 km EASE-Grid 2.0 global projection.

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Projection and Grid Description

EASE-Grid 2.0


These data are provided on the global cylindrical EASE-Grid 2.0 (Brodzik et al. 2012). Each grid cell has a nominal area of approximately 36 x 36 km2 regardless of longitude and latitude. Using this projection, all global data arrays have dimensions of 406 rows and 964 columns.

EASE-Grid 2.0 has a flexible formulation. By adjusting a single scaling parameter, a family of multi-resolution grids that nest within one another can be generated. The nesting can be adjusted so that smaller grid cells can be tessellated to form larger grid cells. Figure 2 shows a schematic of the nesting.

This feature of perfect nesting provides SMAP data products with a convenient common projection for both high-resolution radar observations and low-resolution radiometer observations, as well as for their derived geophysical products. For more on EASE-Grid 2.0, refer to the EASE-Grid 2.0 Format Description.

Figure 2. Perfect Nesting in EASE-Grid 2.0
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Temporal Coverage

Coverage spans from 31 March 2015 to present.

Temporal Coverage Gaps

Satellite and Processing Events

Due to instrument maneuvers, data downlink anomalies, data quality screening, and other factors, small gaps in the time series will occur. Refer to the SMAP On-Orbit Events List for Instrument Data Users page for details regarding these gaps.

Latencies

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

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Temporal Resolution

Each Level-3 file is a daily composite of half-orbit files/swaths.

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Software and Tools

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

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Data Acquisition and Processing

This section has been adapted from O'Neill et al. (2015).

Sensor or Instrument Description

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

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Data Source

SMAP Level-3 radiometer soil moisture data (SPL3SMP) are composited from SMAP L2 Radiometer Half-Orbit 36 km EASE-Grid Soil Moisture, Version 3 (SPL2SMP).

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Theory of Measurements

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), and
  • measurement is independent of solar illumination which allows for day and night observations. (O'Neill et al. 2016)

For an in-depth description of the theory of these measurements, refer to Section 2: Passive Remote Sensing of Soil Moisture in the Algorithm Theoretical Basis Document (ATBD) for this product.

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Derivation Techniques and Algorithms

The SMAP Level-3 radiometer soil moisture is a daily gridded composite of the SMAP L2 Radiometer Half-Orbit 36 km EASE-Grid Soil Moisture, Version 3 (SPL2SMP). The derivation of soil moisture from SMAP brightness temperatures occurs in the Level-2 processing.

Please refer to the Derivation Techniques section in the SPL2SMP user guide for details on algorithms and ancillary data.

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Processing Steps

The SPL3SMP product is a daily global product. To generate the product, individual SPL2SMP half-orbit files acquired over one day are composited to produce a daily multi-orbit global map of retrieved soil moisture.

The SPL2SMP swaths overlap poleward of approximately +/- 65° latitude. Where overlap occurs, three options are considered for compositing multiple data points at a given grid cell:

  1. Use the most recent (or last-in) data point
  2. Take the average of all data points within the grid cell
  3. Choose the data point observed closest to 6:00 a.m. Local Solar Time (LST)

The current approach for the SPL3SMP product is to use the nearest 6:00 a.m. LST criterion to perform Level-3 compositing for the descending passes. According to this criterion, for a given grid cell, an L2 data point acquired closest to 6:00 a.m. local solar time will make its way to the final Level-3 file; other late-coming L2 data points falling into the same grid cell will be ignored. For a given file whose time stamp (yyyy-mm-ddThh:mm:ss) is expressed in UTC, only the hh:mm:ss part is converted into local solar time. (O'Neill et al. 2015)

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Error Sources

Anthropogenic Radio Frequency Interference (RFI), principally from ground-based surveillance radars, can contaminate both radar and radiometer measurements at L-band. 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 T4 thresholds to detect and, where possible, mitigate RFI.

Radiometer L3 data can contain bit errors caused by noise in communication links and memory storage devices. The CCSDS packets include error-detecting Cyclic Redundancy Checks (CRCs), which the L1A processor uses to flag errors.

More information about error sources is provided in Section 4.6: Algorithm Error Performance of the ATBD. (O'Neill et al. 2016)

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Quality Assessment

For in-depth details regarding the quality of these Version 3 Validated data, refer to the following reports:
Validated Assessment Report
Beta Assessment Report

Quality Overview

SMAP products provide multiple means to assess quality. Each product contains bit flags, uncertainty measures, and file-level metadata that provide quality information. For information regarding the specific bit flags, uncertainty measures, and file-level metadata contained in this product, refer to the Data Fields and Metadata Fields documents.

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 NSIDC. A separate metadata file with an .xml file extension is also delivered to NSIDC with the HDF5 file; it contains the same information as the file-level metadata.

A separate QA file with a .qa file extension is also associated with each data file. QA files are ASCII text files that contain statistical information in order to help users better assess the quality of the associated data file.

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.

Data Flags

Bit flags generated from input SMAP data and ancillary data are also 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 (e.g. precipitation flag). These flags will provide information as to whether the ground is frozen, snow-covered, or 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.

For a description of the data flag types and methods of flagging, refer to the Data Flags section in the SPL2SMP user guide. All flags in SPL2SMP are carried over into the SPL3SMP product.

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References and Related Publications

Contacts and Acknowledgments

Investigators

Peggy O'Neill
NASA Goddard Space Flight Center
Global Modeling and Assimilation Office
Mail Code 610.1
8800 Greenbelt Rd
Greenbelt, MD 20771 USA

Steven Chan, Eni Njoku
Jet Propulsion Laboratory
4800 Oak Grove Drive 
Pasadena, CA 91109 USA

Tom JacksonRajat Bindlish
United States Department of Agriculture/Agricultural Research Service (USDA/ARS) 
Hydrology and Remote Sensing Laboratory
104 Bldg. 007, BARC-West 
Beltsville, MD 20705 USA

Document Information

Document Creation Date

October 2015

Document Revision Date

April 2016

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. Short name Title Subsetting... read more

How To

How do I programmatically request data services such as subsetting, reformatting, and reprojection using an API?
The subsetting, reformatting, and reprojection services provided by NSIDC through NASA Earthdata Search can also be accessed programmatically as a synchronous REST interface. This programmatic access is provided via an HTTPS URL containing a series of... read more
How to import and geolocate SMAP Level-3 and Level-4 data in ENVI
The following are instructions on how to import and geolocate SMAP Level-3 Radiometer Soil Moisture HDF5 data in ENVI. Testing notes Software: ENVI Software version: 5.3 Platform: Windows 7 Data set: SMAP L3 Radiometer Global Daily 36 km EASE-Grid Soil... read more
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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 search, order, and customize SMAP data using Earthdata Search?
This video tutorial and attached PDF document provide step-by-step instructions on how to search, order, and customize SMAP data using Earthdata Search (https://search.earthdata.nasa.... 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
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How do I access data using OPeNDAP?
Data can be programmatically accessed using NSIDC’s OPeNDAP Hyrax server, allowing you to reformat and subset data based on parameter and array index. For more information on OPeNDAP, including supported data sets and known issues, please see our OPeNDAP documentation: ... read more
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This step-by-step tutorial demonstrates how to access NASA SMAP data using the Application for Extracting and Exploring Analysis Ready Samples (AppEEARS). AppEEARS allows users to obtain and display point and area data using spatial, temporal, and layer subsets. SMAP data from NSIDC that are... read more