Documentation for Aquarius L3 Gridded 1-Degree Soil Moisture Data, Version 4

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Detailed Data Description

The Aquarius L3 Gridded 1-Degree Soil Moisture Data are produced by NASA Goddard Space Flight Center's Aquarius Data Processing Segment (ADPS).


The data files are in Hierarchical Data Format 5 (HDF5). The values are stored as bytes, 2-byte integers, or 4-byte floats. Soil moisture data are 32-bit float, palette is 8-bit unsigned integer. Each data file is paired with an associated XML file. XML files contain file level metadata and location, platform, and campaign information.

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File and Directory Structure

Data are available on the HTTPS site in the directory. Data files are organized into directories by time period: Annual, Daily, Monthly, Seasonal, and Weekly:


Within each directory, folders are organized by date, for example:


Folders contain HDF5 and XML files.

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File Naming Convention

File names correspond to those of their parent Aquarius binned data products, indicating the binning periods as part of the names.

Soil moisture files are named according to the following conventions and as described in Table 1:




Table 1. File Naming Convention
Variable Description
Q Indicates Aquarius instrument
YYYY Year climatology start
DDD Day climatology start
yyyy Year climatology end
ddd Day climatology end
L3m Processing level

Binning period length, where:
DAY = daily
7D = weekly
MO = monthly
SNSP = seasonal spring
SNSU = seasonal summer
SNAU = seasonal autumn
SNWI = seasonal winter
YR = yearly (annual)

ppppp Geophysical parameter: SOILM = soil moisture
vvvv Data version, example: V4.0
rad_sm_1deg 1-degree radiometer soil moisture

Each data file is paired with an XML file of the same name with .XML extension. The XML file contains metadata associated with the data file.

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File Size

Data files are approximately 262 KB each.

XML files are approximately 3 KB each.

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Data Volume for Aquarius L3 Daily Soil Moisture is approximately 476 MB.

Data Volume for Aquarius L3 Weekly Soil Moisture is approximately 70 MB.

Data Volume for Aquarius L3 Monthly Soil Moisture is approximately 17 MB.

Data Volume for Aquarius L3 Seasonal Soil Moisture is approximately 5.6 MB.

Data Volume for Aquarius L3 Annual Soil Moisture is approximately 1.8 MB.

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Spatial Coverage

Spatial coverage is global.

Spatial Resolution

Spatial resolution of the L3 data is 1 degree.

Projection and Grid Description

The l3m_data object is a two-dimensional array (180 rows, 360 columns) of an Equidistant Cylindrical (also known as Plate Carrée) projection of the globe.

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Temporal Coverage

25 August 2011 to 07 June 2015.

Due to a power failure on the Satélite de Aplicaciones Científicas (SAC)-D spacecraft on 08 June 2015, data from NASA's Aquarius instrument are no longer being produced. For more information on this event, please refer to the official NASA announcement. The NASA National Snow and Ice Data Center Distributed Active Archive Center (NSIDC DAAC) will continue to distribute Aquarius soil moisture and polar-gridded data sets for the full duration of the mission, 25 August 2011 to 07 June 2015.

The weekly data start from August 20, 2011 in 7 day intervals until the present, yeilding a weekly data file every 7 days. The first weekly data have a time period of August 20 to August 26, 2011. The file name specifies the start date and end dates for Aquarius coverage. For example, the weekly file named: Q20142392014245.L3m_7D_SOILM_V4.0_rad_sm_1deg contains data from Day Of Year (DOY) 239, 2014 to DOY 245, 2014.

The month data for August 2011 is a partial coverage only, comprised of observations from August 25 to August 31, 2011.

Similarly, the yearly data for 2011 has observations after August 25, 2011 only.

The seasons are defined between equinox and solstice. The file names contain the exact DOY for each season (Autumn: March 21 to June 21; Summer: June 22 to September 22; Fall: September 23 to December 21; and Winter: December 21 to March 20). For example: the file Q20141722014263.L3m_SNSU_SOILM_V4.0_rad_sm_1deg contains the data for summer 2014 ranging from DOY 172 (June 21, 2014) to DOY 263 (September 20, 2014).

Temporal Resolution

Daily, Weekly, Monthly, Seasonal, and Annual

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Parameter or Variable

The Level-3 products are representations of binned data products generated from Aquarius data. The data object, l3m_data, represents a mean Soil Moisture at each grid point. The grid resolution is 1 degree.

Parameter Description

Each Level-3 soil moisture product contains the l3m_data object, with attributes described in Table 2.

Table 2. Level-3 Soil Moisture l3m_data Object Attributes
Name Value
Scaling linear
Scaling Equation (Slope*l3m_data) + Intercept = Parameter value
Slope 1.0
Intercept 0
_FillValue -32767.0
add_offset 0.0
scale_factor 1.0

The palette object included in the data file is the color palette used in the graphics generated from the mapped files. The user can either use this palette or any palette of their choice.

Metadata are included as global attributes with the Level-3 data files, as described for data file Q20112442011273.L3m_MO_SOILM_V4.0_rad_sm_1deg in Table 3. Values that vary from granule to granule are noted.

Table 3. Level-3 Soil Moisture Metadata General Attributes
Name Value
Product Name Q20112442011273.L3m_MO_SOILM_V4.0_rad_sm_1deg
Sensor Name Aquarius
Sensor Aquarius
Title Aquarius Level-3 Standard Mapped Image
Mission SAC-D Aquarius
Mission Characteristics Nominal orbit: inclination=98.0 (Sun-synchronous); node=6PM (ascending); eccentricity=<0.002; altitude=657 km; 
ground speed=6.825 km/sec
Sensor Characteristics Number of beams=3; channels per receiver=4; frequency 1.413 GHz; bits per sample=16; instatntaneous field of view=6.5 degrees; 
science data block period=1.44 sec
Product Type MO
Processing Version V4.0
Software Name smigen
Software Version 5.04
Processing Time 2015166163059000 (varies)
Input Files Q20112442011273.L3b_MO_SOILM_V4.0.main (varies)
Processing Control smigen par=Q20112442011273.L3m_MO_SOILM_V4.0_rad_sm_1deg.param (varies)
Input Parameters ifile = Q20112442011273.L3b_MO_SOILM_V4.0.main|ofile = Q20112442011273.L3m_MO_SOILM_V4.0_rad_sm_1deg|prod = rad_sm|palfile =
/sdps/sdpsoper/Science/OCSSW/V2015.2/data/common/palette/sm.pal|processing version = V4.0|meas = 1|stype = 1|datamin = 0.000000|datamax =
0.400000|lonwest = -180.000000|loneast = 180.000000|latsouth = -90.000000|latnorth = 90.000000|resolution = 1deg|projection = RECT|gap_fill = 0|seam_lon = 
-180.000000|minobs = 0|deflate = 4|oformat = HDF5|precision = F|
Period Start Year 2011
Period Start Day 244 (varies)
Period End Year 2011 (varies)
Period End Day 274 (varies)
Start Time 2011244021854088 (varies)
End Time 2011274002912490 (varies)
Start Year 2011 (varies)
Start Day 244 (varies)
Start Millisec 1752490 (varies)
End Year 2011 (varies)
End Day 274 (varies)
End Millisec 1752490 (varies)
Start Orbit 1214
End Orbit 1654
Map Projection Equidistant Cylindrical
Latitude Units degrees North
Longitude Units degrees East
Northernmost Latitude 90.0
Southernmost Latitude -90.0
Westernmost Longitude -180.0
Easternmost Longitude 180.0
Latitude Step 1.0
Longitude Step 1.0
SW Point Latitude -89.5
SW Point Longitude -179.5
Data Bins 10875
Number of Lines 180
Number of Columns 360
Parameter Soil Moisture
Measure Mean
Units m3/m3
Scaling linear
Scaling Equation (Slope*l3m_data) + Intercept = Parameter value
Slope 1.0
Intercept 0.0
Data Minimum 0.02
Data Maximum 0.85605
Suggested Image Scaling Minimum 0.0
Suggested Image Scaling Maximum 0.4
Suggested Image Scaling Type LINEAR
Suggested Image Scaling Applied No

Sample Data Record

Below is a sample of the l3m_data soil moisture data array from the Level-3 Aquarius soil moisture file: Q20112442011273.L3m_MO_SOILM_V4.0_rad_sm_1deg

Figure 1 shows the average soil moisture estimates for the month of July 2012.

Figure 1. Aquarius soil moisture estimates using all three beams for the month of July 2012.

For additional images, see: NASA Aquarius Gallery: Soil Moisture - monthly soil moisture map images.

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Software and Tools

HDF-aware software must be used to read the Aquarius soil moisture files. The following external links provide access to software for reading and viewing HDF5 data files. Please be sure to review instructions on installing and running the programs.

HDFView: Visual tool for browsing and editing HDF4 and HDF5 files.

Panoply netCDF, HDF and GRIB Data Viewer: Cross-platform application. Plots geo-gridded arrays from netCDF, HDF and GRIB datasets.

For additional tools, see the HDF5 Tools and Software Web site.

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Data Acquisition and Processing

Theory of Measurements

The Aquarius SCA algorithm uses the L-band horizontally polarized (h-pol) brightness temperature observations due to the higher sensitivity of this channel to soil moisture. The Aquarius SCA approach is based on the simplified radiative transfer model developed under the assumption that the canopy and soil temperatures are the same (Jackson 1993). The SCA is applied to the individual Aquarius footprint Level-2 brightness temperature observations to produce a swath-based time-order product. (Bindlish and Jackson, 2013Bindlish et al, 2013). Details on these steps are provided in the Aquarius L2 Soil Moisture documentation.

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Data Acquisition Methods

The Version 4 Aquarius Level-3 Soil Moisture product is generated from measurements derived from the NASA Aquarius Level-2 Sea Surface Salinity & Wind Speed Data V4.0 product. Each Level-3 product contains data from one time period (daily, weekly, monthly, seasonally or yearly) of Aquarius data. The best quality data are selected for each orbit during Level-0 to Level-1A data processing and are then used to create the Level-2 file that is input to the Level-3 science file.

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Derivation Techniques and Algorithms

The Aquarius Level-3 gridding algorithm uses local polynomial fitting to grid the Level-2 soil moisture retrievals on a 1 degree grid (Fan and Gijbels, 1996Lilly and Lagerloef, 2008). The Level-3 processing of Aquarius satellite data takes measurements at the boresight locations of the three radiometer beams, which have been already converted into physical units of soil moisture, and maps these onto a 1 degree grid.

This method fits a Pth-order polynomial at each grid point xm. For data values gn observed at locations xn, n=1, 2,…N, this corresponds to minimizing

Equation 1

at every grid point x=xm, where

decaying weighting function

is a decaying weighting function which depends upon the bandwidth h, with K(x) being a probability distribution function.

The regression coefficients

regression coefficients

p=1, 2,…P vary with spatial location, and are estimated at all grid point locations.

The function g(x) is estimated by the lowest order coefficient,

lowest order co-efficient

while higher-order regression coefficients estimate the derivatives of the field through

higher order regression co-efficients

The above discussion focuses on a 1-dimensional application, but can be extended to a 2-dimensional application. A complete description for the 2-dimensional problem is available in Fan and Gijbels, 1996Lilly and Lagerloef, 2008.

Processing Steps

Each product represents data binned over the period covered by the original Aquarius product. The mean for the observation period is used to obtain the values for the grid points from the binned data products. Each product contains one soil moisture image and is stored in one physical HDF file. The data are not filtered during the gridding process. The user is advised to refer to the flags in the Aquarius L2 Swath Single Orbit Soil Moisture Data.

Version History

Changes in the Version 4 Aquarius L3 Gridded 1-Degree Soil Moisture Data data include: use of the most recent version (Version 4) of Aquarius Brightness Temperatures as input.

The Aquarius L3 Gridded 1-Degree Soil Moisture Data, Version 3 are processed from the Aquarius L2 Swath Single Orbit Soil Moisture Data, Version 3. Changes in the Aquarius L2 Swath Single Orbit Soil Moisture Data, Version 3 data included: use of the most recent version (Version 3) of Aquarius Brightness Temperatures as input; Aquarius Brightness Temperatures are no longer re-calibrated before soil moisture retrievals as was done for Version 2 data; soil moisture observations are valid over a wider range of brightness temperatures compared to Version 2 data; updates to the soil moisture model parameters (b and ω).

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Sensor or Instrument Description

Aquarius/SAC-D is a collaboration between NASA and Argentina's space agency, Comisión Nacional de Actividades Espaciales (CONAE), with participation from Brazil, Canada, France and Italy. The Aquarius instrument was built jointly by NASA's Jet Propulsion Laboratory and NASA's Goddard Space Flight Center.

The Aquarius instrument includes three radiometers and one scatterometer. The soil moisture data are collected by the radiometers. The radiometers measure brightness temperature at 1.414 GHz in the horizontal and vertical polarizations (TH and TV). The scatterometer is a microwave radar sensor that measures backscatter for surface roughness corrections. Table 4 summarizes instrument characteristics.

Table 4. Aquarius Instrument Characteristics
Instrument Characteristics
3 radiometers in push-broom alignment
  • Frequency: 1.413 GHz
  • Band width: less than or equal to 26 MHz
  • Swath Width: 390 km
  • Science data block period: 1.44 sec
  • Footprints for the beams are: 74 km along track x 94 km cross track, 84 x 120 km, and 96 x 156 km, yielding a total cross track of 390 km.
  • Beam incidence angles of 29.36, 38.49, and 46.29 degrees incident to the surface. Beams point away from the sun.
  • Frequency: 1.26 GHz
  • Band Width: 4 MHz
  • Swath Width: 390 km
  • Science data block period: 1.44 sec

SAC-D spacecraft Orbit Parameters:

  • 98 minute sun-synchronous
  • 6 PM ascending orbit, 6 AM descending orbit
  • 657 km equatorial altitude (655 km minimum, 685 km maximum over the orbit)
  • Ground-track repeat interval: Weekly, 103 orbits
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References and Related Publications

Contacts and Acknowledgments

Rajat Bindlish
NASA Goddard Space Flight Center
Hydrological Sciences Laboratory
Code 617, Bldg 33, G216
Greenbelt, MD 20771 USA

Thomas Jackson
United States Department of Agriculture
Agricultural Research Service
Hydrology and Remote Sensing Laboratory
Beltsville, MD 20705 USA


This work was funded by NASA under the Interagency agreement NNH10AN10I. Tianjie Zhao helped with development of the soil moisture algorithm. The support provided by Michael Cosh, Peggy O'Neill, Thomas Holmes and Wade Crow is acknowledged. We acknowledge the support provided by Gary Lagerloef, David Le Vine, Gene Feldman and the Aquarius Data Processing Segment (ADPS) group in the implementation of the Aquarius Soil moisture algorithm.

Document Information


02 December 2013


03 June 2014

14 November 2014

01 October 2015