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Data for new versions of SMAP Level-1, -2, and -3 data sets are currently available from 27 August 2020 onward. Data since 31 March 2015 will become available as they are reprocessed.
Data Set ID: 
SPL1CTB

SMAP L1C Radiometer Half-Orbit 36 km EASE-Grid Brightness Temperatures, Version 5

This Level-1C (L1C) product contains calibrated and 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 L-band Level-1B time-ordered brightness temperatures resampled to an Earth-fixed, 36 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. This L1C product is a gridded version of the SMAP time-ordered Level-1B radiometer brightness temperature product.

This is the most recent version of these data.

Version Summary: 

Changes to this version include:
- An improved calibration methodology was applied to the input Level-1B radiometer brightness temperatures.
- The data algorithms, structure, content, or processor code are otherwise unchanged from the previous version.

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Parameter(s):
  • MICROWAVE > BRIGHTNESS TEMPERATURE
Data Format(s):
  • HDF5
Spatial Coverage:
N: 85.044, 
S: -85.044, 
E: 180, 
W: -180
Platform(s):SMAP
Spatial Resolution:
  • 36 km x 36 km
Sensor(s):SMAP L-BAND RADIOMETER
Temporal Coverage:
  • 31 March 2015
Version(s):V5
Temporal Resolution49 minuteMetadata XML:View Metadata Record
Data Contributor(s):Chan, S., E. G. Njoku, and A. Colliander.

Geographic Coverage
Other Access Options

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

Chan, S., E. G. Njoku, and A. Colliander. 2020. SMAP L1C Radiometer Half-Orbit 36 km EASE-Grid Brightness Temperatures, Version 5. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/JJ5FL7FRLKJI. [Date Accessed].
Created: 
21 December 2018
Last modified: 
1 September 2020

Data Description

Parameters

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. 

This Level-1C TB data set is essentially a remapping of time-ordered swath Level-1B TB data (SMAP L1B Radiometer Half-Orbit Time-Ordered Brightness Temperatures) onto a grid, and both data sets share the same granularity (one half orbit per file). No additional geophysical processing is performed.

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

File Information

Format

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

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-1C brightness temperature product, refer to the Appendix. 

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 Appendix of this 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-looking views within a given projection.

Global Projection

In the global cylindrical EASE-Grid 2.0 projection, includes data that represent fore- and aft-looking views of the 360° antenna scan. Contains brightness temperature observations, instrument viewing geometry information, and quality bit flags. 

Corrected brightness temperatures are also provided, such as cell_tb_h_surface_corrected_aft (as opposed to cell_tb_h_aft). For these brightness temperatures, an additional correction procedure has been applied to correct fot anomalous water and land values; see the Water/Land Contamination Correction section in the SPL1BTB User Guide for details. (Level-1B brightness temperatures are used as input for this product). 

North Polar Projection

Contains the same data as the Global Projection group, but data are in the Northern Hemisphere azimuthal EASE-Grid 2.0 projection. 

South Polar Projection

Contains the same data as the Global Projection and North Polar Projection groups, but data are in the Southern Hemisphere azimuthal EASE-Grid 2.0 projection. 

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 Product Specification Document (Chan & Dunbar, 2020).

File Naming Convention

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

SMAP_L1C_TB_[Orbit#]_[A/D]_yyyymmddThhmmss_RLVvvv_NNN.[ext]

For example:

SMAP_L1C_TB_03895_D_20160113T000349_R13080_001.h5

Where:

Table 1. File Naming Conventions
Variable Description
SMAP Indicates SMAP mission data
L1C_TB Indicates specific product (L1C: Level-1C; TB: Brightness Temperature)
[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)
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, 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 a standard data product with a version of 3.242. Refer to the SMAP Data Versions page for version information.
NNN Number of times the file was generated under the same version for a particular date/time interval (002: 2nd time)
.[ext] File extensions include:
.h5 HDF5 data file
.qa Quality Assurance file
.xml XML Metadata file

Spatial Information

Coverage

Coverage spans from 180°W to 180°E, and from approximately 85.044°N and 85.044°S for the EASE-Grid, Version 2.0. The swath width is 1000 km, enabling nearly global coverage every two to three days. Figure 2 shows the spatial coverage of the SMAP L-Band Radiometer for one descending half orbit, which comprises one file of this data set.

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

Resolution

The native spatial resolution of the radiometer footprint is approximately 36 km. Data are then gridded using using the global cylindrical, and Northern and Southern Hemisphere azimuthal EASE-Grid 2.0 projections, all at 36 km grid resolution.

Geolocation

These data are provided on the EASE-Grid 2.0 in three different equal-area projections: a global cylindrical, a Northern Hemisphere azimuthal, and Southern Hemisphere azimuthal. Each grid cell has a nominal area of approximately 36 x 36 km2 regardless of longitude and latitude. 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 EASE-Grid 2.0 projections used in this product
Global Northern Hemisphere Southern Hemisphere
Geographic coordinate system WGS 84 WGS 84 WGS 84
Projected coordinate system EASE-Grid 2.0 Global EASE-Grid 2.0 North Azimuthal EASE-Grid 2.0 South Azimuthal
Longitude of true origin 0 0 0
Standard Parallel 30° N 90° N 90° S
Scale factor at longitude of true origin N/A N/A N/A
Datum WGS 84 WGS 84 WGS 84
Ellipsoid/spheroid WGS 84 WGS 84 WGS 84
Units meter meter meter
False easting 0 0 0
False northing 0 0 0
EPSG code 6933 6931 6932
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 +proj=laea +lat_0=90 +lon_0=0 +x_0=0 +y_0=0 +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs +proj=laea +lat_0=-90 +lon_0=0 +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 http://epsg.io/6931 http://epsg.io/6932

Table 3. Grid details for the EASE-Grid 2.0 projections used in this product
Global Northern Hemisphere Southern Hemisphere
Grid cell size (x, y pixel dimensions) 36,032.22 m (x)
36,032.22 m (y)		 36,000 m (x)
36,000 m (y)		 36,000 m (x)
36,000 m (y)
Number of columns 964 500 500
Number of rows 406 500 500
Geolocated lower left point in grid 85.044° S, 180.000° W 84.634050° S, 45.000000° W 84.634050° N, 135.000000° W
Nominal gridded resolution 36 km by 36 km 36 km by 36 km 36 km by 36 km
Grid rotation N/A N/A N/A
ulxmap – x-axis map coordinate of the outer edge of the upper-left pixel -17367530.45 m -9000000.0 -9000000.0
ulymap – y-axis map coordinate of the outer edge of the upper-left pixel 7314540.83 m 9000000.0 9000000.0

Temporal Information

Coverage

Coverage spans from 31 March 2015 to the present. 

Temporary Gaps Due to Reprocessing

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

Satellite and Processing Events

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

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

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

Latencies

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

Resolution

Each Level-1C half-orbit file spans approximately 49 minutes. The SMAP orbit yields a 2-3 day average revisit frequency and repeats the exact swath every 8 days.

Data Acquisition and Processing

This section has been adapted from the Algorithm Theoretical Basis Document (ATBD) for this product (Chan et al. 2014).

Background

The Level-1C brightness temperature product is a gridded version of SMAP L1B Radiometer Half-Orbit Time-Ordered Brightness Temperatures, Version 5 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 user guide for more details.

Instrumentation

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

Acquisition

Antenna temperatures from the baseline SMAP L1B Radiometer Half-Orbit Time-Ordered Brightness Temperatures, Version 5 (SPL1BTB) are used as input to calculating this Level-1C brightness temperature product.

Derivation Techniques and Algorithms

Inverse-Distance-Squared Interpolation Algorithm

The interpolation algorithm for this product uses the Inverse-Distance-Squared (IDS) method often used in microwave radiometry applications. All brightness temperature data samples that fall within a grid cell are averaged with weights varying inversely with the square of the radial distance between the data samples and the grid cell center:

  (Equation 1)

  (Equation 2)

  (Equation 3)

and di is the great-circle distance between the data sample TBi and the grid cell center, given by:

  (Equation 4)

Here, (Φi, λi) and (Φo, λo) are the latitudes and longitudes of the data sample i and grid cell center o, respectively. RE (6378 km) is the radius of the Earth.

For more information, refer to the ATBD for this product (Chan et al. 2014).

Processing

This 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 L1B Radiometer Half-Orbit Time-Ordered Brightness Temperatures, Version 5 product. Based on the geometry and geolocation information, the data are then remapped onto an Earth-fixed grid using the IDS gridding algorithm, which includes the following steps:

  1. Determine the 36 km EASE-Grid 2.0 row and column indices corresponding to the latitude and longitude of each observation
  2. Identify all brightness temperature data samples that are within X km of each EASE-Grid 2.0 cell center
  3. Apply averaging to these data samples
  4. Assign the computed result to the grid cell
  5. Repeat Steps 2–4 above for all other grid cells

The Level-1C processor applies the gridding algorithm to a half-orbit Level-1B brightness temperature file and converts it into a corresponding half-orbit Level-1C brightness temperature file. The Level-1C processing is essentially a remapping of time-ordered swath data onto a grid. The input Level-1B and output Level-1C data share the same granularity (one half orbit per file). There is no additional geophysical processing performed; for example, no additional brightness temperature correction is performed for fractional water within the antenna Field of View (FOV). Note that the brightness temperatures included in this product are processed/copied directly from the Level-1B product. The gridding algorithm is applied to the brightness temperatures and other applicable parameters in the Level-1B product file (latitude, longitude, azimuth angle, incidence angle, reflected sun angles, etc.). If an individual quality flag for Level-1B brightness temperature contributing to the average is set, that flag is set for the grid cell average.

Quality, Errors, and Limitations

Error Sources

This Level-1C brightness temperature product is a gridded version of the SMAP L1B Radiometer Half-Orbit Time-Ordered Brightness Temperatures, Version 5 product. Thus, the output Level-1C brightness temperature data inherit the input Level-1B error sources, primarily RFI and radiometric noise and calibration error, modified by the process of gridding the input brightness temperature data samples onto an Earth-fixed grid. The gridding process does not 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 gridding process will enlarge the effective antenna pattern footprint of the brightness temperature measurement, thereby coarsening the spatial resolution. Depending on the brightness temperature heterogeneity of the observed scene, the decrease in spatial resolution may increase the error in representing the brightness temperature of a given point on the surface.

For more information on the noise versus resolution trade-off, please refer to the ATBD for this product (Chan et al. 2014).

Quality Assessment

For in-depth details regarding the quality of these data, refer to the 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 was evaluated in a number of ways. These included:

  • 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 Appendix for details on all data flags.

Software and Tools

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

Version History

Table 4. Version History
Version Date Version Changes
V1
 		 July 2015 First public data release
V2 October 2015 No science change. CRID incremented to coincide with DOI correction for SPL2SMP and SPL3SMP. 
V3
 		 April 2016 Changes to this version include:
  • Uses updated SPL1BTB V3 Validated data as input
  • Updated processing to produce SPL1CTB data files when partial half-orbit input files are present
V4 June 2018 Updated input SPL1BTB product with water correction applied, resulting in warmer TBs over land and cooler TBs over water.
V5 August 2020

Changes to this version include:

  • An improved calibration methodology was applied to the input Level-1B radiometer brightness temperatures.
  • The data algorithms, structure, content, or processor code are otherwise unchanged from the previous version.

Related Data Sets

SMAP Data at NSIDC | Overview

SMAP Radar Data at the ASF DAAC

Related Websites

SMAP at NASA JPL

Contacts and Acknowledgments

Steven Chan
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA

References

Chan, S., Njoku, E., Colliander, A. 2014. SMAP Algorithm Theoretical Basis Document (ATBD) Level-1C Radiometer Data Product (L1C_TB). SMAP Project, Jet Propulsion Laboratory, Pasadena, CA. (see Technical References or PDF)

Chan , S. and R. S. Dunbar. 2020. SMAP Level 1C Radiometer Product Specification Document, Version 5.0, R17 Extended Mission Release. JPL D-72545, Jet Propulsion Laboratory, Pasadena, CA. (see Technical References or PDF).

Appendix - Data Fields

This page provides a description of all data fields within the SMAP L1C Radiometer Half-Orbit 36 km EASE-Grid Brightness Temperatures product. The data are grouped in the following main HDF5 groups:

  • Global_Projection
  • North_Polar_Projection
  • South_Polar_Projection
  • Metadata

For a description of metadata fields for this product, refer to the Product Specification Document (Chan & Dunbar, 2020).

North, South, and Global Projections

Table A1 describes the data fields within the HDF5 group for each projection. Data are provided in three EASE-Grid 2.0 projections:

  • Global (M36, or Midlatitude 36 km)
  • North (N36, or North 36 km)
  • South (S36, or South 36 km)

Within each group, the data are provided in fore-looking and aft-looking views. Data from the fore- and aft-look portions of the 360° antenna scan are provided separately in order to benefit radiometric analyses over regions where there is strong dependence of brightness temperature (TB) on viewing azimuth. Each set of looks contains brightness temperature observations, instrument viewing geometry information, and quality bit flags. The fore-looking set refers to information derived from the Level-1B brightness temperature observations acquired in the forward-looking portion of the scans when the antenna angle falls between -90 degrees and 90 degrees; the aft-looking set refers to information derived from the Level-1B brightness temperature observations acquired in the backward-looking portion of the scans. Only those cells that are covered by the swath for a given projection receive a valid data value; the remainder are populated with the fill value.

Table A1. Data Fields for North_Polar_Projection, South_Polar_Projection, and Global_Projection
Azimuth
Direction		 Data Field Name Type Byte Valid Min Valid Max Unit Fill/Gap Values Derivation Method*
N/A cell_column Uint16 2 0 963  N/A 65534 2
cell_grid_surface_status Uint16 2 0 1 N/A 65534 2
cell_lat Float32 4 -90.0 90.0 degree 65534 2
cell_lon Float32 4 -180.0 180.0 degree 65534 2
cell_row Uint16 2 0 405  N/A -9999.0 2
Fore-
Looking
Data Arrays		 cell_antenna_scan_angle_fore Float32 4 0.0 359.999 degree -9999.0 1
cell_boresight_incidence_fore Float32 4 0.0 180.0 degree -9999.0 1
cell_centroid_lat_fore Float32 4 -90.0 90.0 degree -9999.0 1
cell_centroid_lon_fore Float32 4 -180.0 180.0 degree -9999.0 1
cell_number_measurements_3_fore Uint16 2 0 65535 N/A 65534 1
cell_number_measurements_4_fore Uint16 2 0 65535 N/A 65534 1
cell_number_measurements_h_fore Uint16 2 0 65535 N/A 65534 1
cell_number_measurements_v_fore Uint16 2 0 65535 N/A 65534 1
cell_solar_specular_phi_fore Float32 4 0.0 359.999 degree 65534 1
cell_solar_specular_theta_fore Float32 4 0.0 99.9 degree -9999.0 1
cell_surface_water_fraction_mb_h_fore Float32 4 0.0 1.0 N/A -9999.0 1
cell_surface_water_fraction_mb_v_fore Float32 4 0.0 1.0 N/A -9999.0 1
cell_tb_3_fore Float32 4 -50.0 50.0 K -9999.0 1
cell_tb_4_fore Float32 4 -50.0 50.0 K -9999.0 1
cell_tb_error_3_fore Float32 4 -50.0 50.0 K -9999.0 1
cell_tb_error_4_fore Float32 4 -50.0 50.0 K -9999.0 1
cell_tb_error_h_fore Float32 4 0.0 330.0 K -9999.0 1
cell_tb_error_v_fore Float32 4 0.0 330.0 K -9999.0 1
cell_tb_h_surface_corrected_fore Float32 4 0.0 330.0 K -9999.0 1
cell_tb_qual_flag_3_fore Uint16 2 0 65,536 N/A 65534 1
cell_tb_qual_flag_4_fore Uint16 2 0 65,536 N/A 65534 1
cell_tb_qual_flag_h_fore Uint16 2 0 65,536 N/A 65534 1
cell_tb_qual_flag_v_fore Uint16 2 0 65,536 N/A 65534 1
cell_tb_time_seconds_fore Uint16 2 -999999.9 9.4E8 seconds -9999.0 1
cell_tb_time_utc_fore String 24 2014-10-31
T00:00:00.000Z		 N/A N/A N/A 1
cell_tb_v_fore Float32 4 0.0 330.0 K -9999.0 1
cell_tb_v_surface_corrected_fore Float32 4 0.0 330.0 K -9999.0 1
Aft-
Looking
Data Arrays
cell_antenna_scan_angle_aft Float32 4 0.0 359.999 degree -9999.0 1
cell_boresight_incidence_aft Float32 4 0.0 180.0 degree -9999.0 1
cell_centroid_lat_aft Float32 4 -90.0 90.0 degree -9999.0 1
cell_centroid_lon_aft Float32 4 -180.0 179.999 degree -9999.0 1
cell_number_measurements_3_aft Uint16 2 0 65535 N/A 65534 1
cell_number_measurements_4_aft Uint16 2 0 65535 N/A 65534 1
cell_number_measurements_h_aft Uint16 2 0 65535 N/A 65534 1
cell_number_measurements_v_aft Uint16 2 0 65535 N/A 65534 1
cell_solar_specular_phi_aft Float32 4 0.0 359.999 degree -9999.0 1
cell_solar_specular_theta_aft Float32 4 0.0 99.9 degree -9999.0 1
cell_surface_water_fraction_mb_h_aft Float32 4 0.0 1.0 N/A -9999.0 1
cell_surface_water_fraction_mb_v_aft Float32 4 0.0 1.0 N/A -9999.0 1
cell_tb_3_aft Float32 4 -50.0 50.0 K -9999.0 1
cell_tb_4_aft Float32 4 -50.0 50.0 K -9999.0 1
cell_tb_error_3_aft Float32 4 -50.0 50.0 K -9999.0 1
cell_tb_error_4_aft Float32 4 -50.0 50.0 K -9999.0 1
cell_tb_error_h_aft Float32 4 0.0 330.0 K -9999.0 1
cell_tb_error_v_aft Float32 4 0.0 330.0 K -9999.0 1
cell_tb_h_surface_corrected_aft Float32 4 0.0 330.0 K -9999.0 1
cell_tb_qual_flag_3_aft Uint16 2 0 65,536 N/A -9999.0 1
cell_tb_qual_flag_4_aft Uint16 2 0 65,536 N/A -9999.0 1
cell_tb_qual_flag_h_aft Uint16 2 0 65,536 N/A -9999.0 1
cell_tb_qual_flag_v_aft Uint16 2 0 65,536 N/A -9999.0 1
cell_tb_time_seconds_aft Float64 4 -999999.9 9.4E8 seconds -9999.0 1
cell_tb_time_utc_aft Uint16 2 -9999.0 1
cell_tb_v_aft Float32 4 0.0 330.0 K -9999.0 1
cell_tb_v_surface_corrected_aft Float32 4 0.0 330.0 K -9999.0 1

* Derivation methods are:

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

Data Field Definitions 

All data field definitions below are valid for fore-looking and aft-looking groups, as well as ascending and descending half-orbit files.

cell_column 
EASE grid column index of cell on world grid in longitude direction.

cell_grid_surface_status
Indicates if the grid point lies on land (0) or water (1).

cell_lat
Latitude of the center of the Earth-based grid cell.

cell_lon

Longitude of the center of the Earth-based grid cell.

cell_row

EASE grid row index of cell on world grid in latitude direction.

cell_antenna_scan_angle fore|aft

Representative scan angle of the SMAP antenna on the spacecraft for all forward-looking footprints within the cell.

cell_boresight_incidence fore|aft

Representative angle between the antenna boresight vector and the normal to the Earth's surface for all forward-looking footprints within the cell. Representative scan angle of the SMAP antenna on the spacecraft for all aft-looking footprints within the cell.

cell_lat_centroid fore|aft

Weighted average of the latitude of the center of the forward-looking brightness temperature footprints that fall within the EASE-Grid cell. Representative angle between the antenna boresight vector and the normal to the Earth's surface for all aft-looking footprints within the cell.

cell_lon_centroid fore|aft

Weighted average of the longitude of the center of the forward-looking brightness temperature footprints that fall within the EASE-Grid cell. Weighted average of the latitude of the center of the aft-looking brightness temperature footprints that fall within the EASE-Grid cell.

cell_number_measurements_3 fore|aft
Number of forward-looking TB3 samples within the grid cell that were used to calculate the representative value in this product. Weighted average of the longitude of the center of the aft-looking brightness temperature footprints that fall within the EASE-Grid cell. Number of aft-looking TB3 samples within the grid cell that were used to calculate the representative value in this product.

cell_number_measurements_4 fore|aft
Number of forward-looking TB4 samples within the grid cell that were used to calculate the representative value in this product. Number of aft-looking TB4 samples within the grid cell that were used to calculate the representative value in this product.

cell_number_measurements_h fore|aft
Number of forward-looking TBH samples within grid cell that were used to calculate the representative value in this product. Number of aft-looking TBH samples within grid cell that were used to calculate the representative value in this product.

cell_number_measurements_v fore|aft
Number of forward-looking TBV samples within grid cell that were used to calculate the representative value in this product. Number of aft-looking TBV samples within grid cell that were used to calculate the representative value in this product.

cell_solar_specular_phi fore|aft
Weighted average of the azimuthal component of a spatial angle defined by the vector that extends from the sun to the solar glint spot and the antenna boresight vector for forward-looking footprints within the cell. Weighted average of the azimuthal component of a spatial angle defined by the vector that extends from the sun to the solar glint spot and the antenna boresight vector for aft-looking footprints within the cell.

cell_solar_specular_theta fore|aft 
Weighted average of the elevation component of the spatial angle defined by the vector that extends from the sun to the solar glint spot and the antenna boresight vector for forward-looking footprints within the cell. Weighted average of the elevation component of the spatial angle defined by the vector that extends from the sun to the solar glint spot and the antenna boresight vector for aft-looking footprints within the cell.

cell_surface_water_fraction_mb_h fore|aft
Gain-weighted fraction of static water within the forward-looking horizontally polarized antenna pattern. Gain-weighted fraction of static water within the aft-looking horizontally polarized antenna pattern.

cell_surface_water_fraction_mb_v fore|aft
Gain-weighted fraction of static water within the forward-looking vertically polarized antenna pattern. Gain-weighted fraction of static water within the aft-looking vertically polarized antenna pattern.

cell_tb_3 fore|aft
Representative TB3 for all forward-looking samples that fall within the grid cell. Representative TB3 for all aft-looking samples that fall within the grid cell.

cell_tb_4 fore|aft
Representative TB4 for all forward-looking samples that fall within the grid cell. Representative TB4 for all aft-looking samples that fall within the grid cell.

cell_tb_error_3 fore|aft
Error measure of the representative TB3 for all forward-looking samples that fall within the grid cell. Error measure of the representative TB3 for all aft-looking samples that fall within the grid cell.

cell_tb_error_4 fore|aft
Error measure of the representative TB4 for all forward-looking samples that fall within the grid cell. Error measure of the representative TB4 for all aft-looking samples that fall within the grid cell.

cell_tb_error_h fore|aft
Error measure of the representative TBH for all forward-looking samples that fall within the grid cell. Error measure of the representative TBH for all aft-looking samples that fall within the grid cell. 

cell_tb_error_v fore|aft
Error measure of the representative TBV for all forward-looking samples that fall within the grid cell. Error measure of the representative TBV for all aft-looking samples that fall within the grid cell.

cell_tb_h fore|aft
Representative TBH for all forward-looking samples that fall within the grid cell. Representative TBH for all aft-looking samples that fall within the grid cell.

cell_tb_h_surface_corrected fore|aft
Water/land contamination corrected forward-looking horizontally polarized brightness temperature at the surface after RFI filtering within the grid cell. Water/land contamination corrected aft-looking horizontally polarized brightness temperature at the surface after RFI filtering within the grid cell.

cell_tb_qual_flag_3 fore|aft
Bit flags that represent the quality of the fore/aft-looking 3rd Stokes brightness temperature for each grid cell.

cell_tb_qual_flag_4 fore|aft
Bit flags that represent the quality of the fore/aft- looking 4th Stokes brightness temperature for each grid cell.

cell_tb_qual_flag_h fore|aft
Bit flags that represent the quality of the fore/aft- looking horizontal polarization brightness temperature within each grid cell.

cell_tb_qual_flag_v fore|aft
Bit flags that represent the quality of the fore/aft- looking vertical polarization brightness temperature within each grid cell.

cell_tb_time_seconds fore|aft
Weighted average of the acquisition time of all of the forward-looking brightness temperature footprints with a center that falls within the EASE-Grid cell in seconds since noon on January 1, 2000 UTC. Weighted average of the acquisition time of all of the aft-looking brightness temperature footprints with a center that falls within the EASE-Grid cell in seconds since noon on January 1, 2000 UTC.

cell_tb_time_utc fore|aft
ASCII representation of the weighted average of the acquisition time of all of the forward-looking brightness temperature footprints with a center that falls within the EASE-Grid cell in UTC. ASCII representation of the weighted average of the acquisition time of all of the aft-looking brightness temperature footprints with a center that falls within the EASE-Grid cell in UTC.

cell_tb_v fore|aft
Representative TBV for all forward-looking samples that fall within the grid cell. Representative TBV for all aft-looking samples that fall within the grid cell.

cell_tb_v_surface_corrected fore|aft
Water/land contamination corrected forward- and aft-looking vertically polarized brightness temperature at the surface after RFI filtering within the grid cell. Water/land contamination corrected aft-looking vertically polarized brightness temperature at the surface after RFI filtering within the grid cell.

Table A2. Bit Values for all Cell TB Quality Flags (TH, TV, T3, and T4)
Bit Position Bit Value and Interpretation
0 0 = Has acceptable quality
1 = Does not have acceptable quality
1 0 = Within expected range
1 = Beyond expected range
2 0 = RFI not detected
1 = RFI detected
3 0 = RFI detected and corrected (RFI algorithm functioned correctly; however, does not indicate that resultant bit is RFI free)
1 = RFI was detected but not correctable in the observation
4 0 = Has acceptable NEDT
1 = Has unacceptable NEDT
5 0 = Solar direct TB correction successful
1 = Solar direct TB correction not successful
6 0 = Solar specular TB correction successful
1 = Solar specular TB correction not successful
7 0 = Lunar specular TB correction successful
1 = Lunar specular TB correction not successful
8 0 = Galactic specular TB correction successful
1 = Galactic specular TB correction not successful
9 0 = Atmospheric correction successful
1 = Atmospheric correction not successful
10 0 = Faraday rotation correction successful
1 = Faraday rotation correction not successful
11 0 = Faraday rotation correction was successful
1 = Faraday rotation correction was not successful
12 0 = The corresponding brightness temperature element contains a calculated value
1 = The corresponding brightness temperature element is null
13 0 = Observation was within half orbit
1 = Observation was outside half orbit
14 0 = The difference between TA filtered and TA is less than the threshold
1 = The difference between TA filtered and TA is not less than the threshold
15

0 = Radiometer processor declares a pixel is free of RFI by incorporating the following factors:

  • The data are in range (Bit 1)
  • The RFI removal algorithm worked (Bit 3)
  • The data do not display high NEDT (Bit 4)
  • The difference between TA filtered and TA is less than a specific threshold (Bit 14)
1 = Radiometer processor does not declare a pixel is free of RFI

Fill/Gap Values

SMAP data products employ fill and gap 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 SMAP Level-1C brightness temperature product when the Level-1C brightness temperature 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 Level-1C brightness temperature 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 Level-1C brightness temperature 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 the Level-1B brightness temperature product. If only some of the input that contributes to a particular grid cell is fill data, the Level-1C brightness temperature SPS will most likely be able to generate some output. However, some portion of the Level-1C brightness temperature output for that grid cell may appear as fill values.

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 Level-1C brightness temperature product is equal to the values that represent fill. If any exceptions should exist in the future, the Level-1C brightness temperature 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 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 elementOrbitMeasuredLocation/halfOrbitStartDateTime.
  • The character string stored in metadata element Extent/rangeEndingDateTime will match the character string stored in metadata elementOrbitMeasuredLocation/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 A3. Acronyms and Abbreviations
Abbreviation Definition
ABFCS Antenna Beam Frame Coordinate System
Char 8-bit character
Int8 8-bit (1-byte) signed integer
Int16 16-bit (2-byte) signed integer
Int32 32-bit (4-byte) signed integer
ECR Earth Centered Rotating
ET Ephemeris Time
Float32 32-bit (4-byte) floating-point integer
Float64 64-bit (8-byte) floating-point integer
H-pol Horizontally polarized
N/A Not Applicable
NEDT Noise Equivalent Delta Temperature
PRI Pulse Repetition Interval
RFI Radio Frequency Interference
SI International System of Units
SPS Science Production Software
SRF Science Orbit Reference Frame
TA Antenna Temperature
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

How To

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

FAQ

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