Data Fields - SMAP L3 Radar/Radiometer Global Daily 9 km EASE-Grid Soil Moisture

This page provides a description of all data fields within the SMAP L3 Radar/Radiometer Global Daily 9 km EASE-Grid Soil Moisture (SPL3SMAP) product. The data are grouped into two main HDF5 groups:

For a description of metadata fields for this product, refer to the Metadata Fields document.

Soil_Moisture_Retrieval_Data

Table 1 describes the data fields of a typical SPL3SMAP descending half-orbit granule. 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 1. Data Fields for Soil_Moisture_Retrieval_Data
Data Field Name Shape Concept Byte Unit Min Max Fill/Gap Value
EASE_column_index LatCell_LonCell_Array integer 2 count 0 65535 65534
EASE_row_index LatCell_LonCell_Array integer 2 count 0 65535 65534
albedo LatCell_LonCell_Array real 4 normalized 0.0 1.0 -9999.0
alpha_tbh_hh LatCell_LonCell_Array real 4 Kelvins 0 350.0 -9999.0
alpha_tbv_vv LatCell_LonCell_Array real 4 Kelvins 0 350.0 -9999.0
bare_soil_roughness_retrieved LatCell_LonCell_Array real 4 meters 0.0 0.1 -9999.0
beta_tbh_hh LatCell_LonCell_Array real 4 Kelvins/dB -25 0 -9999.0
beta_tbv_vv LatCell_LonCell_Array real 4 Kelvins/dB -25 0 -9999.0
distance_from_nadir LatCell_LonCell_Array real 4 meters 0.0 500000.0 -9999.0
freeze_thaw_fraction LatCell_LonCell_Array real 4 normalized 0.0 1.0 -9999.0
gamma_hh_xpol LatCell_LonCell_Array real 4 normalized 0 2 -9999.0
gamma_vv_xpol LatCell_LonCell_Array real 4 normalized 0 2 -9999.0
landcover_class LatCell_LonCell_Array enum 1 N/A N/A N/A 254
latitude LatCell_LonCell_Array real 4 degrees_north -90.0 90.0 N/A
longitude LatCell_LonCell_Array real 4 degrees_east -180.0 180.0 N/A
radar_vegetation_index LatCell_LonCell_Array real 4 normalized 0 2 -9999.0
retrieval_qual_flag LatCell_LonCell_Array bit flag 2 N/A N/A N/A 65534
sigma0_hh_aggregated LatCell_LonCell_Array real 4 normalized 0.0 1.0 -9999.0
sigma0_vv_aggregated LatCell_LonCell_Array real 4 normalized 0.0 1.0 -9999.0
sigma0_xpol_aggregated LatCell_LonCell_Array real 4 normalized 0.0 1.0 -9999.0
soil_moisture LatCell_LonCell_Array real 4 cm3/cm3 0.02 0.5 -9999.0
soil_moisture_std_dev LatCell_LonCell_Array real 4 cm3/cm3 0.0 0.2 -9999.0
spacecraft_overpass_time_seconds LatCell_LonCell_Array real 8 seconds 0 999999.9 -9999.0
spacecraft_overpass_time_utc LatCell_LonCell_Array string 24 N/A N/A N/A N/A
surface_flag LatCell_LonCell_Array bit flag 2 N/A N/A N/A 65534
surface_temperature LatCell_LonCell_Array real 4 degrees Celsius -50.0 60.0 -9999.0
tb_h_disaggregated LatCell_LonCell_Array real 4 Kelvins 0.0 330.0 -9999.0
tb_h_disaggregated_qual_flag LatCell_LonCell_Array bit flag 2 N/A N/A N/A 65534
tb_h_disaggregated_std LatCell_LonCell_Array real 4 Kelvins 0.0 100.0 -9999.0
tb_v_disaggregated LatCell_LonCell_Array real 4 Kelvins 0.0 330.0 -9999.0
tb_v_disaggregated_qual_flag LatCell_LonCell_Array bit flag 2 N/A N/A N/A 65534
tb_v_disaggregated_std LatCell_LonCell_Array real 4 Kelvins 0.0 100.0 -9999.0
vegetation_opacity LatCell_LonCell_Array real 4 normalized 0 1 -9999.0
vegetation_water_content LatCell_LonCell_Array real 4 kg/m2 0.0 30 -9999.0
water_body_fraction LatCell_LonCell_Array real 4 normalized 0.0 1.0 -9999.0

Data Field Definitions [top]

EASE_column_index

EASE grid cell at 9 km row index on world grid in longitude direction.

EASE_row_index

EASE grid cell at 9 km row index on world grid in latitude direction.

albedo

Diffuse reflecting power of the Earth's surface within the grid cell at 9 km.

alpha_tbh_hh

Alpha parameter derived for the corresponding EASE2 grid cell at the most recent prior instance when the grid cell was processed. Prior alpha is derived from the time series of brightness temperature at 36 km EASE2 grid and aggregated co-pol (hh) backscatter at 36 km EASE2 grid. The length of the time series to estimate alpha especially depends on the region and the landcover. The valid minimum and maximum below are subject to further analysis.

alpha_tbv_vv

Alpha parameter derived for the corresponding EASE2 grid cell at the most recent prior instance when the grid cell was processed. Prior alpha is derived from the time series of brightness temperature at 36 km EASE2 grid and aggregated co-pol (vv) backscatter at 36 km EASE2 grid. The length of the time series to estimate alpha especially depends on the region and the landcover. The valid minimum and maximum below are subject to further analysis.

bare_soil_roughness_retrieved

Roughness coefficient at 9-km spatial scale. Note that this parameter is the same ‘h’ coefficient in the ‘tau-omega’ model for a given polarization channel.

beta_tbh_hh

Beta parameter used in the Active/Passive retrieval algorithm for the corresponding EASE grid cell, derived using time series Tbh and sigma0_hh.

beta_tbh_vv

Beta parameter used in the Active/Passive retrieval algorithm for the corresponding EASE grid cell, derived using time series Tbv and sigma0_vv.

distance_from_nadir

The distance from the center of the 9 km EASE grid cell to the spacecraft's sub-nadir track on the Earth's surface.

freeze_thaw_fraction

Fraction of the 9 km grid cell that is denoted as frozen. Based on binary flag that specifies freeze thaw conditions in each of the component 3 km grid cells.

gamma_hh_xpol

Gamma parameter used in the Active/Passive retrieval algorithm for the corresponding EASE grid cell at the most recent prior instance when the grid cell was processed. The parameter is obtained by regression between aggregated co-pol (hh) backscatters at 9 km and cross-pol (hv) backscatters at 9 km that are contained within the respective 36 km grid cell. The valid minimum and maximum below are subject to further analysis.

gamma_vv_xpol

Gamma parameter used in the Active/Passive retrieval algorithm for the corresponding EASE grid cell at the most recent prior instance when the grid cell was processed. The parameter is obtained by regression between aggregated co-pol (vv) backscatters at 9 km and cross-pol (hv) backscatters at 9 km that are contained within the respective 36 km grid cell. The valid minimum and maximum below are subject to further analysis.

landcover_class

An enumerated type that specifies the predominant surface vegetation found in the EASE2 grid cell at 9-km.

Table 2. Landcover Classification Values
Value Description
0 Water
1 Evergreen needleleaf forest
2 Evergreen broadleaf forest
3 Deciduous needleleaf forest
4 Deciduous broadleaf forest
5 Mixed forest
6 Closed shrubland
7 Open shrubland
8 Woody savanna
9 Savanna
10 Grassland
11 Mixed forest
12 Closed shrubland
13 Open shrubland
14 Woody savanna
15 Savanna
16 Grassland
>16 TBD

latitude

Latitude of the center of the Earth based grid cell at 9 km.

longitude

Longitude of the center of the Earth based grid cell at 9 km.

radar_vegetation_index

Radar vegetation index derived from the co-pol and cross-pol radar backscatter data aggregated to 9-km mentioned in sections 4.6.10, 4.6.11, and 4.6.12.

Radar vegetation index = 8*(sigma0_hv) / (sigma0_vv + sigma0_hh + 2* sigma0_hv)

In the above equation, sigma0_hh, sigma0_vv, and sigma0_hv are from Sections 4.6.10, 4.6.11, and 4.6.12, respectively. The valid minimum and maximum below are subject to further analysis.

retrieval_qual_flag

Bit flags that record the conditions and the quality of the retrieved soil moisture. When translated to decimal representation, this parameter contains an integer indicating one of the following inversion outcomes. See Table 3.

Table 3. Quality Bit Flag Definitions
Name Bit Position Description of Values (0:off, 1:on)
Retrieval recommended flag 0 0: Use of the soil moisture value retrieved for this pixel is recommended.
1: Use of soil moisture value retrieved for this pixel is not recommended.
Retrieval attempted flag 1 0: The algorithm attempted to retrieve soil moisture for this grid cell.
1: The algorithm did not attempt to retrieve soil moisture for this grid cell.
Retrieval success flag 2 0: Retrieval for this algorithm was successfully executed or the algorithm was not attempted.
1: The retrieval for this algorithm was attempted but failed.
Radar water body detection success flag 3 0: Radar water body detection ran successfully.
1: Unable to detect water bodies using retrieval techniques based on radar.
Freeze-thaw retrieval success flag 4 0: Freeze-thaw retrieval ran successfully.
1: Unable to ascertain freeze-thaw conditions.
Radar vegetation index retrieval success flag 5 0: Radar vegetation index retrieval ran successfully.
1: Radar vegetation index retrieval unsuccessful.
Disaggregated brightness temperature quality 6 0: Disaggregated brightness temperature retrieval ran successfully.
1: Unable to disaggregate brightness temperatures into 9 km resolution cells.

sigma0_hh_aggregated

The outcome of aggregating a set of 3-km co-pol horizontal polarization radar backscatter measurements that are encompassed within 9 km EASE2 grid cell. This aggregated radar backscatter at 9-km does not include the undesirable 3-km grid cells, mainly the grid cells having water, snow/ice and RFI contaminated pixels.

sigma0_vv_aggregated

The outcome of aggregating a set of 3-km co-pol vertical polarization radar backscatter measurements that are encompassed within 9 km EASE2 grid cell. This aggregated radar backscatter at 9-km does not include the undesirable 3-km grid cells, mainly the grid cells having water, snow/ice and RFI contaminated pixels.

sigma0_xpol_aggregated

The outcome of aggregating a set of 3-km cross-polarization (hv or vh) radar backscatter measurements that are encompassed within 9 km EASE2 grid cell. This aggregated radar backscatter at 9-km does not include the undesirable 3-km grid cells, mainly the grid cells having water, snow/ice and RFI contaminated pixels.

soil_moisture

Retrieved soil moisture estimate from the disaggregated/downscaled vertical polarization brightness temperature at 9-km grid cell.

soil_moisture_std_dev

Estimated ‘1-sigma’ error of the soil_moisture output parameter. The valid minimum and maximum below are subject to further analysis.

spacecraft_overpass_time_seconds

Number of seconds since a specified epoch that represents the spacecraft overpass relative to ground swath. The 9-km EASE2-Grid cell is assigned the UTC time of 36-km EASE2-Grid cell that is used for downscaling. The field describes the average of UTC acquisition times, in ASCII representation, of L1B_TB observations whose boresights fall within a 36-km EASE2-Grid cell.

spacecraft_overpass_time_utc

The 9-km EASE2-Grid cell is assigned the UTC time of 36-km EASE2-Grid cell that is used for downscaling. The field describes the average of UTC acquisition times, in ASCII representation, of L1B_TB observations whose boresights fall within a 36-km EASE2-Grid cell.

surface_flag

Bit flags that record ambient surface conditions for the grid cell at 9-km. See Table 4.

Table 4. Quality Bit Flag Definitions
Name Bit Position Description of Values (0:off, 1:on)
9 km static water body flag 0 0: The fraction of the 9 km grid cell area that is over a permanent water body is less than metadata element PermanentWaterBodyThreshold. Determined by DEM.
1: The fraction of the 9 km grid cell area that is over a permanent water body is greater than or equal to metadata element PermanentWaterBodyThreshold. Determined by DEM.
9 km radar water body detection flag 1 0: Flag indicates either water less than given threshold, or water that was not detected in locations other than were permanent water is known to exist.
1: Flag indicates either water greater than given threshold, or water that was detected in locations other than were permanent water is known to exist.
9 km urban area flag 2 0: The fraction of the 9 km grid cell area that is over urban development is less than metadata element UrbanAreaThreshold.
1: The fraction of the 9 km grid cell area that is over urban development is greater than or equal to metadata element UrbanAreaThreshold.
9 km precipitation flag 3 0: No precipitation detected within the 9 km grid cell when data were being acquired.
1: Precipitation detected within the 9 km grid cell when data were being acquired
9 km snow or ice flag 4 0: No or insignificant quantities of snow or ice were detected within the 9 km cell.
1: Significant quantities of snow and/or ice were detected within the 9 km grid cell.
9 km permanent snow or ice flag 5 0: The fraction of the 9 km grid cell area that is over permanent snow or ice is less than a specified algorithmic threshold.
1: The fraction of the96 km grid cell area that is over permanent snow or ice is greater than or equal to a specified algorithmic threshold.
9 km frozen ground flag 6 0: No frozen ground detected within the 9 km grid cell.
1: Frozen ground detected within the 9 km grid cell.
9 km mountainous terrain flag 7 0: The variability of land elevation in the 9 km grid cell is less than metadata element MountainousTerrainThreshold.
1: The variability of land elevation in the 9 km grid cell is greater than or equal to metadata element MountainousTerrainThreshold.
9 km dense vegetation flag 8 0: The vegetation density within the 9 km grid cell is less than metadata element DenseVegetationThreshold.
1: The vegetation density within the 9 km grid cell area is greater than or equal to metadata element DenseVegetationThreshold.
9 km nadir region flag 9 0: Data within the grid cell were not acquired in the nadir region of the swath where sigma0s may not meet the 3 km resolution requirement.
1: A significant fraction (TBD) of the 9 km grid cell data were acquired within the nadir region of the swath where sigma0s may not meet the 3 km resolution requirement.
9 km coastal mask flag 10 0: Data within the grid cell were not acquired in the coastal region of the large water bodies where especially brightness temperature on land may get severely contaminated due to presence of water.
1: Data within the grid cell were acquired in the coastal region of the large water bodies where especially brightness temperature on land may get severely contaminated due to presence of water.

surface_temperature

Soil temperature (averaged over the top 5-cm soil layer) at 9-km spatial scale. This parameter is used as input ancillary data parameter to the SPL2SMAP processing software for the baseline algorithms. The valid minimum and maximum below are subject to further analysis on real data.

tb_h_disaggregated

Horizontal polarization brightness temperature at 9-km obtained by disaggregating/downscaling the 36 km EASE grid cells horizontal polarization brightness temperature.

tb_h_disaggregated_qual_flag

Bit flags that record the conditions and the quality of the disaggregated horizontal polarization brightness temperature generated for the grid cell. See Table 3.

tb_h_disaggregated_std

Standard deviation of the horizontal polarization brightness temperature adjusted for the presence of water bodies and disaggregated from the 36 km EASE grid cells into 9 km EASE grid cells.

tb_v_disaggregated

Vertical polarization brightness temperature at 9-km obtained by disaggregating/downscaling the 36 km EASE grid cells vertical polarization brightness temperature.

tb_v_disaggregated_qual_flag

Bit flags that record the conditions and the quality of the disaggregated vertical polarization brightness temperature generated for the grid cell. See Table 5.

Table 5. Quality Bit Flag Definitions
Name Bit Position Description of Values (0:off, 1:on)
Disaggregated brightness temperature v-pol quality 0 0: Disaggregated vertical polarization brightness temperature has acceptable quality.
1: Unable to disaggregate vertical polarization brightness temperatures into 9 km resolution cells.
Sigma0_vv quality flag 1 0: All vertical polarization sigma0 input that contributed to disaggregation of vertical polarization brightness temperatures were deemed as good quality.
1: Some vertical polarization sigma0 input that contributed to disaggregation of vertical polarization brightness temperatures was of questionable or poor quality.
Sigma0_xpol quality flag 2 0: All cross polarized sigma0 input that contributed to disaggregation of vertical polarization brightness temperatures were deemed as good quality.
1: Some cross polarized sigma0 input that contributed to disaggregation of vertical polarization brightness temperatures was of questionable or poor quality.
Brightness temperature v-pol quality flag 3 0: Vertical polarization brightness temperature input that was used for disaggregation was deemed as good quality.
1: Some vertical polarization brightness temperature input that was used for soil moisture retrieval was of questionable or poor quality.
Brightness temperature v-pol RFI detected flag 4 0: Insignificant levels of RFI detected in the vertical polarization radiometer brightness temperature input.
1: Significant levels of RFI were detected in the vertical polarization radiometer brightness temperature input.
Brightness temperature v-pol RFI corrected flag 5 0: The vertical polarization radiometer brightness temperature input is based on data that were repaired for the effects of RFI.
1: Unable to repair the vertical polarization radiometer brightness temperature input for the effects of RFI.
Sigma0_vv RFI detected flag 6 0: Insignificant levels of RFI detected in the vertical polarization radar sigma0 input.
1: Significant levels of RFI were detected in the vertical polarization radar sigma0 input.
Sigma0_vv RFI corrected flag 7 0: The input for retrieval is based on vertical polarization radar sigma0s that were repaired for the effects of RFI.
1: Unable to repair the vertical polarization radar sigma0 input for the effects of RFI.
Sigma0_xpol RFI detected flag 8 0: Insignificant levels of RFI detected in the cross polarized radar sigma0 input.
1: Significant levels of RFI were detected in the cross polarized radar sigma0 input.
Sigma0_xpol RFI corrected flag 9 0: The input for retrieval is based on cross polarized radar sigma0s that were repaired for the effects of RFI.
1: Unable to repair the cross polarized radar sigma0 input for the effects of RFI.
Negative sigma0_vv flag 10 0: The input for retrieval is based on vertical polarization radar sigma0s that are greater than zero.
1: The input for retrieval is based on vertical polarization radar sigma0s that are less than or equal to zero.
Negative sigma0_xpol flag 11 0: The input for retrieval is based on cross polarized radar sigma0s that are greater than zero.
1: The input for retrieval is based on cross polarized radar sigma0s that are less than or equal to zero.

tb_v_disaggregated_std

Standard deviation of the vertical polarization brightness temperature adjusted for the presence of water bodies and disaggregated from the 36 km EASE grid cells into 9 km EASE grid cells.

vegetation_opacity

Estimated vegetation opacity at 9-km spatial scale. Note that this parameter is the same ‘tau’ parameter normalized by the cosine of the incidence angle in the ‘tau-omega’ model, where:

tau-omega equation

The valid minimum and maximum below are subject to further analysis.

vegetation_water_content

Vegetation water content at 9-km spatial scale. This parameter is used as input ancillary data parameter to the SPL2SMAP processing software when the baseline algorithm is used. The valid minimum and maximum below are subject to further analysis.

water_body_fraction

Water body fraction at 9-km spatial scale. 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. At present the SPL2SMAP processing software can be configured to provide this parameter from a static water fraction database or from the SPL2SMA product.

Metadata [top]

For a description of metadata fields for this product, refer to the Metadata Fields document.

Fill/Gap Values [top]

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 SPL3SMAP product when the Level 3_SM_AP 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 Science Production Software (SPS) executables that generate the SPL3SMAP 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 SPL3SMAP 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 L1B_TB product. If only some of the input that contributes to a particular grid cell is fill data, the SPL3SMAP SPS will most likely be able to generate some output. However, some portion of the SPL3SMAP 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 SPL3SMAP product is equal to the values that represent fill. If any exceptions should exist in the future, the SPL3SMAP 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 SPL3SMAP 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 SPL3SMAP 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 inOrbitMeasuredLocation/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.

Notations [top]

Table 6 lists the notations used in this document.

Table 6. Notation Definitions
Notation Definition
Int8 8-bit (1-byte) signed integer
Int16 16-bit (2-byte) signed integer
Int32 32-bit (4-byte) signed integer
Uint8 8-bit (1-byte) unsigned integer
Uint16 16-bit (2-byte) unsigned integer
Float32 32-bit (4-byte) floating-point integer
Float64 64-bit (8-byte) floating-point integer
Char 8-bit character
H-pol Horizontally polarized
V-pol Vertically polarized