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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).
Changes to this version include:
| Parameter(s): |
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|---|---|
| Spatial Coverage: |
N: 85.044, S: -85.044, E: 180, W: -180 |
| Spatial Resolution: |
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| Temporal Coverage: |
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| Temporal Resolution: | 1 day |
| Data Format(s): |
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| Platform(s) | SMAP Observatory |
| Sensor(s): | SMAP L-BAND RADIOMETER |
| Version: | V4 |
| Data Contributor(s): | O'Neill, P. E., S. Chan, E. G. Njoku, T. Jackson, and R. Bindlish. |
| Metadata XML: | View Metadata Record |
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 4. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/OBBHQ5W22HME. [Date Accessed].Surface soil moisture (0-5 cm) in m3/m3 derived from brightness temperatures (TBs) is output on a fixed global 36 km EASE-Grid 2.0. Also included are brightness temperatures in kelvin representing the weighted average of Level-1B brightness temperatures whose boresights fall within a 36 km EASE-Grid 2.0 cell.
Refer to the Data Fields document for details on all parameters.
Data are in HDF5 format. For software and more information, including an HDF5 tutorial, visit the HDF Group's HDF5 Web site.
As shown in Figure 1, each HDF5 file is organized into the following main groups, which contain additional groups and/or data sets:
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 two-dimensional, with the exception of landcover_class and landcover_class_fraction, which are three-dimensional arrays.
Includes soil moisture data, ancillary data, and quality assessment flags for each descending half-orbit pass of the satellite (where the satellite moves from North to South and 6:00 a.m. is the Local Solar Time (LST) at the equator.
Includes soil moisture data, ancillary data, and quality assessment flags for each ascending half-orbit pass of the satellite (where the satellite moves from South to North and 6:00 p.m. is the LST at the equator.
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.
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_20170117_R14010_001.h5
Where:
| 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 (CRID), where:
|
||||||||
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:
|
Each file is approximately 24 MB.
The daily data volume is approximately 24 MB.
Coverage spans from 180°W to 180°E, and from approximately 85.044°N and 85.044°S for the global EASE-Grid 2.0 projection. 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 approximately 1000 km, enabling nearly global coverage every three days.
The native spatial resolution of the radiometer footprint is 36 km. Data are then gridded using the 36 km EASE-Grid 2.0 projection.
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.
Coverage spans from 31 March 2015 to present.
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
Each Level-3 file is a daily composite of half-orbit files/swaths. Note that data from descending passes (a.m.) and ascending passes (p.m.) are stored separately in the same daily composite granule.
For tools that work with SMAP data, see the Tools Web page.
This section has been adapted from O'Neill et al. (2012).
For a detailed description of the SMAP instrument, visit the SMAP Instrument page at the Jet Propulsion Laboratory (JPL) SMAP Web site.
SMAP Level-3 radiometer soil moisture data (SPL3SMP) are composited from SMAP L2 Radiometer Half-Orbit 36 km EASE-Grid Soil Moisture, Version 4 (SPL2SMP).
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:
For an in-depth description of the theory of these measurements, refer to Passive Remote Sensing of Soil Moisture in the Algorithm Theoretical Basis Document ATBD for this product. (O'Neill et al. 2015)
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 4 (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.
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:
The current approach for the SPL3SMP product is to use the nearest 6:00 a.m. LST and nearest 6:00 p.m. LST criteria to perform Level-3 compositing separately for the descending and ascending passes. According to these criteria, for a given grid cell, an L2 data point acquired closest to 6:00 a.m. LST or closest to 6:00 p.m. LST 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. 2012)
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. 2015)
For in-depth details regarding the quality of these Version 4 Validated data, refer to the following reports:
Validated Assessment Report
Beta Assessment Report
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 from both 6:00 a.m. descending and 6:00 p.m. ascending half-orbit passes are used as input for soil moisture derivation in this Version 4 Validated 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 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.
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.
Peggy O'Neill
NASA Goddard Space Flight Center
Global Modeling and Assimilation Office
Mail Code 610.1
8800 Greenbelt Rd.
Greenbelt, MD 20771 USA
Rajat Bindlish
NASA Goddard Space Flight Center
Hydrological Sciences Laboratory
Code 617, Bldg 33, G216
Greenbelt, MD 20771 USA
Steven Chan
Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, CA 91109 USA
Tom Jackson
United States Department of Agriculture/Agricultural Research Service (USDA/ARS)
Hydrology and Remote Sensing Laboratory
104 Bldg. 007, BARC-West
Beltsville, MD 20705 USA
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