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SMAP Enhanced L1C Radiometer Half-Orbit 9 km EASE-Grid Brightness Temperatures, Version 1
This enhanced Level-1C (L1C) product contains calibrated, geolocated, brightness temperatures acquired by the Soil Moisture Active Passive (SMAP) radiometer during 6:00 a.m. descending and 6:00 p.m. ascending half-orbit passes. This product is derived from SMAP Level-1B (L1B) interpolated antenna temperatures. Backus-Gilbert optimal interpolation techniques are used to extract maximum information from SMAP antenna temperatures and convert them to brightness temperatures, which are posted to a 9 km Equal-Area Scalable Earth Grid, Version 2.0 (EASE-Grid 2.0) in three projections: global cylindrical, Northern Hemisphere azimuthal, and Southern Hemisphere azimuthal.
First public data release
Geographic Coverage
Spatial Coverage: |
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Spatial Resolution: |
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Temporal Coverage: |
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Temporal Resolution: | 49 minute |
Parameter(s): |
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Platform(s) | SMAP Observatory |
Sensor(s): | SMAP L-Band Radiometer |
Data Format(s): |
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Version: | V1 |
Data Contributor(s): | Chaubell, M. J., S. Chan, R. S. Dunbar, J. Peng, and S. Yueh. |
Metadata XML: | View Metadata Record |
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.
Chaubell, M. J., S. Chan, R. S. Dunbar, J. Peng, and S. Yueh. 2016. SMAP Enhanced L1C Radiometer Half-Orbit 9 km EASE-Grid Brightness Temperatures, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: http://dx.doi.org/10.5067/2C9O9KT6JAWS. [Date Accessed].Detailed Data Description
Brightness temperatures (TBs) in kelvin derived from interpolated Level-1B antenna temperatures (TAs) are output on the EASE-Grid 2.0 at 9 km in three different equal-area projections: a global cylindrical, and a Northern and Southern Hemisphere azimuthal. Level-1B antenna temperatures, calibrated to the feedhorn after RFI detection and mitigation, were interpolated at the 9 km grid cells using the Backus-Gilbert (BG) optimal interpolation method.
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:

For a complete list of file contents for the SMAP enhanced Level-1C brightness temperature 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.
Data fields are stored as one-dimensional arrays of size N, where N is the number of valid cells covered by the radiometer swath on the grid. Note that N varies with projections, but remains the same for both fore-looking and aft-look ing views within a given projection.
Global Projection
The global EASE-Grid 2.0 projection data group contains data that represent fore- and aft-looking views of the 360° antenna scan, including enhanced brightness temperatures, instrument viewing geometry information, and quality bit flags.
North Polar Projection
The north polar EASE-Grid 2.0 projection data group contains data that represent fore- and aft-looking views of the 360° antenna scan for latitudes above zero, including enhanced brightness temperatures, instrument viewing geometry information, and quality bit flags.
South Polar Projection
The south polar EASE-Grid 2.0 projectiondata group contains data that represent fore- and aft-looking views of the 360° antenna scan for latitudes below zero, including enhanced brightness temperatures, instrument viewing geometry information, and quality bit 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.
Files are named according to the following convention, which is described in Table 1:
SMAP_L1C_TB_E_[Orbit#]_[A/D]_yyyymmddThhmmss_RLVvvv_NNN.[ext]
For example:
SMAP_L1C_TB_E_10508_A_20170119T005350_R14010_001.h5
Where:
Variable | Description | ||||||||
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SMAP |
Indicates SMAP mission data | ||||||||
L1C_TB_E |
Indicates specific product (L1C: Level-1C; TB: Brightness Temperature; E: Enhanced) | ||||||||
[Orbit#] |
5-digit sequential number of the orbit flown by the SMAP spacecraft when data were acquired. Orbit 00000 began at launch. | ||||||||
[A/D] |
Half-orbit pass of the satellite, such as: A: Ascending (where satellite moves from South to North, and 6:00 p.m. is the local solar equator crossing time) D: Descending (where satellite moves from North to South, and 6:00 a.m. is the local solar equator crossing time) |
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yyyymmddThhmmss |
Date/time in Universal Coordinated Time (UTC) of the first data element that appears in the product, where:
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RLVvvv |
Composite Release ID (CRID), where:
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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:
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Each half-orbit file is approximately 50 MB.
The daily data volume is approximately 1.5 GB.
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.
Spatial Coverage Map
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 native spatial resolution of the radiometer footprint is 36 km. Data are then interpolated using the Backus-Gilbert optimal interpolation algorithm into the global cylindrical, and Northern and Southern Hemisphere azimuthal EASE-Grid 2.0 projections with 9 km spacing.
EASE-Grid 2.0
These data are provided on the EASE-Grid 2.0 (Brodzik et al. 2012) in three different equal-area projections: a global cylindrical, and both a Northern and Southern Hemisphere azimuthal. Each grid cell has a nominal area of approximately 9 x 9 km2 regardless of longitude and latitude.
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 3 shows a schematic of the nesting to a resolution of 3 km (4872 rows x 11568 columns on global coverage), 9 km (1624 rows x 3856 columns on global coverage), and 36 km (406 rows x 964 columns on global coverage).
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.
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
Each enhanced Level-1C half-orbit file spans approximately 49 minutes.
Software and Tools
For tools that work with SMAP data, refer to the Tools Web page.
Data Acquisition and Processing
This section has been adapted from Chaubell et al. (2016).
For a detailed description of the SMAP instrument, visit the SMAP Instrument page at Jet Propulsion Laboratory (JPL) SMAP Web site.
Antenna temperatures from the baseline SMAP L1B Radiometer Half-Orbit Time-Ordered Brightness Temperatures, Version 3 (SPL1BTB) product are used as input to calculating this enhanced Level-1C brightness temperature product, SPL1CTB_E.
The enhanced Level-1C brightness temperature product is an interpolated and gridded version of SMAP L1B Radiometer Half-Orbit Time-Ordered Brightness Temperatures, Version 3 and thus shares most of the same major output data fields, data granularity (one half-orbit per file), and theory of measurements. Refer to the Level-1B Theory of Measurements for more details.
Backus-Gilbert Optimal Interpolation Algorithm
The baseline SMAP Level-1B brightness temperature product (SPL1BTB) contains global surface brightness temperature estimates over a 36 km regular global grid. The aim of the SMAP enhanced Level-1B brightness temperature product (SPL1BTB_E) is to provide an optimal interpolation of the radiometer measurements onto a global 9 km grid. The SMAP sampling pattern results in overlapping measurements which, together with optimal interpolation, results in more accurate estimation of brightness temperature.
There are a number of algorithms directed towards the goal of image reconstruction and interpolation. A long-standing approach and one with extensive heritage in microwave radiometry is the Backus-Gilbert (BG) interpolation (Backus and Gilbert, 1970). This technique has been applied to the Special Sensor Microwave/Imager (SSM/I) measurements (Stogryn, 1978; Poe, 1990; Robison et al., 1992; Farrar and Smith, 1992; Sethmann et al., 1994; Long and Daum, 1998; Migliaccio and Gambardella, 2005) and the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) measurements (Chakraborty et al., 2008). A unique feature of the BG interpolation is that it is optimal in the sense that the resulting interpolated data is closest to what would have been measured had the radiometer actually made the measurements with the interpolation point as its bore-sight center (Poe, 1983). In this sense and in this respect, it is superior to ad hoc or empirical interpolation techniques. According to Long and Brodzik (2016), BG provides higher spatial resolution surface brightness temperature images with smaller total error compared with conventional drop-in-the-bucket gridded image formation. The SPL1CTB_E algorithm uses the polarimetric implementation of the BG optimal interpolation algorithm derived by Dr. Simon Yueh to interpolate basline SMAP Level-1B antenna temperatures on the EASE-Grid 2.0 points within the boundaries of the orbit path.
For details regarding the BG theory and implementation, refer to the enhanced Level-1B ATBD, Section 2: Optimal Interpolation of Polarimetric Brightness Temperatures.
Gridding Algorithm
As mentioned previously, the SPL1CTB_E algorithm uses BG optimal interpolation to interpolate SMAP Level-1B antenna temperatures on the EASE-Grid 2.0 points within the boundaries of the orbit path.
In other words, calling ρd a point on the EASE-Grid 2.0, we compute the antenna temperature at ρd as
where TAi are the antenna temperatures at the SMAP footprint locations ρd, i=1…N.
The coefficients are given by
(Equation 2)
where the elements of the matrix g are
and the vectors ν and μ are given by
and
These equations are the bases for the direct evaluation of the vector u and v and the matrix g, necessary to obtain the coefficients a. These calculations can be computationally very expensive. In order to make our algorithm more computationally efficient, we implemented the some approximations. Details of this approximation and its error evaluation can be found in the SPL1BTB_E ATBD.
This enhanced product is generated by the SMAP Science Data Processing System (SDS) at JPL in Pasadena, California USA. To generate the product, the processing software ingests a half-orbit file of the SMAP enhanced Level-1B radiometer brightness temperature data set (L1B_TB_E) to extract and transfer key data fields to the SPL1CTB product. Only cells that are covered by the actual swath for a given projection are written in the product.
Prior to the production of the SPL1CTB_E product, the L1B_TB_E processor reads from the SPL1BTB product the baseline Level-1B antenna temperatures, which have been calibrated (by removing sun/moon/galactic contributions and applying reflector emissivity corrections) and processed by radio frequency interference detection and mitigation algorithms. The L1B_TB_E algorithm applies the Backus-Gilbert interpolation theory to interpolate Level-1B antenna temperatures on the EASE-Grid 2.0 points within the boundaries of the orbit path. The algorithm uses six SMAP footprints from the baseline SPL1BTB product to perform the interpolation. The selection of those points is explained in the SPL1BTB_E ATBD. If one of those selected points is a fill value, then the value assigned to the antenna temperature is a fill value. The interpolated antenna temperatures are further processed to remove the effects of the antenna sidelobes outside the radiometer antenna main beam, cross-polarizations, Faraday rotation, and atmospheric effects (excluding rain). The resulting L1B_TB_E data represent enhanced surface-referenced brightness temperatures.
This enhanced Level-1C brightness temperature product (SPL1CTB_E) contains a subset of data fields of the input L1B_TB_E data set. Two-dimensional arrays that are transferred from L1B_TB_E are reformatted as one-dimensional arrays for compactness and improved I/O speed in Level 2 processing. In terms of noise performance, SPL1CTB_E inherits the same Error Sources that affect SPL1BTB. These error sources include RFI, radiometric noise and calibration error, modified by the process of Backus-Gilbert interpolation in SPL1BTB_E. The interpolation process is not expected to affect the calibration errors, such as biases and drifts, but will reduce the radiometric noise, such as the random component of the brightness temperature error. Conversely, the interpolation process may enlarge the effective antenna pattern footprint of the brightness temperature measurement.
In addition, because image reconstruction includes a trade-off between noise and resolution, estimated noise variances in the interpolated fields are reported in the SPL1BTB_E ATBD. However, the noise levels obtained for SPL1BTB_E and thus SPL1CTB_E measurements are improved over the baseline SPL1BTB single footprint measurements due to the interpolation performed, and are similar to the noise levels of the baseline SPL1CTB product, which also performs an interpolation of single footprint measurements in mapping to a 36 km grid.
For more information, please refer to the ATBD for this product.
For in-depth details regarding the quality of these Version 1 data, refer to the following reports:
Validated Assessment Report
Beta Assessment Report
Quality Overview
Each HDF5 file contains metadata with Quality Assessment (QA) metadata flags that are set by the SDS at the JPL prior to delivery to the National Snow and Ice Data Center Distributed Active Archive Center (NSIDC DAAC). A separate metadata file with an .xml file extension is also delivered to NSIDC DAAC with the HDF5 file; it contains the same information as the HDF5 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.
Various levels of QA are conducted with Level-1C data. 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.
In addition, during the post-launch Calibration/Validation period, the performance of the Level-1C brightness temperature product relative to the Level-1B brightness temperature product are evaluated in a number of ways. These include:
- Comparing images and examining differences between the two products over coastlines and other discrete boundaries, and heterogeneous terrain (lakes, mountains, rivers).
- Comparing TB and TB-gradient histograms of the two products over regions of varying heterogeneity.
Refer to the Data Fields document for details on all data flags.
References and Related Publications
Contacts and Acknowledgments
Investigators
M. Julian Chaubell, Steven Chan,
R. Scott Dunbar, Simon Yueh
Jet Propulsion Laboratory
California Institute of Technology
4800 Oak Grove Dr.
Pasadena, CA 91109 USA
Jinzheng Peng
NASA Goddard Space Flight Center
8800 Greenbelt Rd.
Greenbelt, MD 20771 USA
Document Information
Document Creation Date
December 2016
Document Revision Date
N/A
FAQ
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.
... read moreThe following table describes the data subsetting, reformatting, and reprojection services that are currently available for SMAP data via the NASA Earthdata Search tool.