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Aquarius L3 Gridded 1-Degree Soil Moisture, Version 2

This document covers five data sets derived from the NASA Aquarius passive microwave instrument on the Satélite de Aplicaciones Científicas (SAC-D):

  • AQ3_DYSM: Aquarius L3 Gridded 1-Degree Daily Soil Moisture
  • AQ3_WKSM: Aquarius L3 Gridded 1-Degree Weekly Soil Moisture
  • AQ3_MOSM: Aquarius L3 Gridded 1-Degree Monthly Soil Moisture
  • AQ3_SNSM: Aquarius L3 Gridded 1-Degree Seasonal Soil Moisture
  • AQ3_ANSM: Aquarius L3 Gridded 1-Degree Annual Soil Moisture

Table of Contents

  1. Contacts and Acknowledgments
  2. Detailed Data Description
  3. Data Access and Tools
  4. Data Acquisition and Processing
  5. References and Related Publications
  6. Document Information

Citing These Data

We kindly request that you cite the use of this data set in a publication using the following citation example. For more information, see our Use and Copyright Web page.

Bindlish, Rajat and Thomas Jackson. 2013. Aquarius L3 Gridded 1-Degree Daily Soil Moisture, Version 2, [indicate subset used]. Boulder, Colorado USA: NASA DAAC at the National Snow and Ice Data Center. http://dx.doi.org/10.5067/Aquarius/AQ3_DYSM.002.

Bindlish, Rajat and Thomas Jackson. 2013. Aquarius L3 Gridded 1-Degree Weekly Soil Moisture, Version 2, [indicate subset used]. Boulder, Colorado USA: NASA DAAC at the National Snow and Ice Data Center. http://dx.doi.org/10.5067/Aquarius/AQ3_WKSM.002.

Bindlish, Rajat and Thomas Jackson. 2013. Aquarius L3 Gridded 1-Degree Monthly Soil Moisture, Version 2, [indicate subset used]. Boulder, Colorado USA: NASA DAAC at the National Snow and Ice Data Center. http://dx.doi.org/10.5067/Aquarius/AQ3_MOSM.002.

Bindlish, Rajat and Thomas Jackson. 2013. Aquarius L3 Gridded 1-Degree Seasonal Soil Moisture, Version 2, [indicate subset used]. Boulder, Colorado USA: NASA DAAC at the National Snow and Ice Data Center. http://dx.doi.org/10.5067/Aquarius/AQ3_SNSM.002.

Bindlish, Rajat and Thomas Jackson. 2013. Aquarius L3 Gridded 1-Degree Annual Soil Moisture, Version 2, [indicate subset used]. Boulder, Colorado USA: NASA DAAC at the National Snow and Ice Data Center. http://dx.doi.org/10.5067/Aquarius/AQ3_ANSM.002.

Overview

Platform

SAC-D

Sensor

Aquarius

Spatial Coverage

Global

Spatial Resolution

1 degree

Temporal Coverage

25 August 2011 to present

Temporal Resolution

Daily, Weekly, Monthly, Seasonal, Annual

Parameters

Soil moisture

Data Format

HDF5

Metadata Access

View Metadata Record
AQ3_DYSM Daily Soil Moisture
AQ3_WKSM Weekly Soil Moisture
AQ3_MOSM Monthly Soil Moisture
AQ3_SNSM Seasonal Soil Moisture
AQ3_ANSM Annual Soil Moisture

Version

V2. See the Version History section of this document for previous version information.

Get Data

Daily
Weekly
Monthly
Seasonal
Annual

1. Contacts and Acknowledgments

Investigator(s) Name and Title

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

Technical Contact

NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449  USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
e-mail: nsidc@nsidc.org

Acknowledgements

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 System group in the implementation of the Aquarius Soil moisture algorithm.

2. Detailed Data Description

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

The weekly data start from August 20, 2011 in 7 day duration until the present. The first weekly data have a time period of August 20 to August 26, 2011. Note: Although the weekly periods start from August 20, 2011 there is no Aquarius data before August 25, 2011.

The monthly data for August 2011 is comprised of observations from August 25 to August 31, 2011. The month data for August 2011 is a partial coverage only. Similarly, the yearly data for 2011 has observations after August 25, 2011 only. Therefore, the yearly 2011 data does not cover observations from the entire calendar year of 2011.

Format

The data files are in Hierarchical Data Format 5 (HDF5). The values can be stored as bytes, 2-byte integers, or 4-byte floats. Soil moisture data are 32-bit float, palette is 8-bit unsigned integer. The first two are scaled real values and may be converted to geophysical values using the global attributes Scaling, Scaling Equation, Base, Slope, and Intercept.

File and Directory Structure

Data files are organized into directories by time period, Annual, Daily, Monthly, Seasonal, and Weekly, and within time periods organized by date:
/AQ3_ANSM.002/
/AQ3_DYSM.002/
/AQ3_MOSM.002/
/AQ3_SNSM.002/
/AQ3_WKSM.002/
 /2013.09.03/
 /2013.09.10/
 /2013.09.17/
 /2013.09.24/

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:

Q20130012013031.L3m_MO_SOILM_V2.0_rad_sm_1deg

Qyyyydddhhmmsss.L3m_ttt_ppppp_vvvv_rad_sm_1deg

Where:

Table 1. File Naming Convention
Variable Description
Q Indicates Aquarius instrument
yyyy UTC year
ddd UTC day of year
hhmmsss UTC hours, minutes, and seconds of the first sample block in the product "Sample block" is defined as the first set of observations from the three Aquarius beams.
L3m Processing level
ttt

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: V2.0
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.

File Size

Data files are approximately 262 KB each.

Volume

Data Volume for Aquarius L3 Daily Soil Moisture is approximately 262 MB.

Data Volume for Aquarius L3 Weekly Soil Moisture is approximately 39 MB.

Data Volume for Aquarius L3 Monthly Soil Moisture is approximately 9 MB.

Data Volume for Aquarius L3 Seasonal Soil Moisture is approximately 3 MB.

Data Volume for Aquarius L3 Annual Soil Moisture is approximately 688 KB.

Spatial Coverage

Spatial coverage is global.

Spatial Resolution

Spatial resolution of the L3 data is 1 degree.

Projection and Grid Description

The l2m_data object is a two-dimensional array (180 rows, 360 columns) of an Equidistant Cylindrical (also known as Plate Carrée) projection of the globe.

Temporal Coverage

25 August 2011 to present

Temporal Resolution

Daily, Weekly, Monthly, Seasonal, and Annual

The weekly periods start from August 20, 2011 to August 26, 2011. The file name specifies the start date and end dates for Aquarius coverage. For example, the weekly file named: Q20120012012007.L3m_7D_SOILM_V2.0_rad_sm_1deg contains data from Day Of Year (DOY) 1, 2012 to DOY 7, 2012.

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 Q20121732012264.L3m_SNSU_SOILM_V2.0_rad_sm_1deg contains the data for summer 2012 ranging from DOY 173 (June 22, 2012) to DOY 264 (September 22, 2012).

Parameter or Variable

The Level-3 standard mapped image (SMI) 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.

NetCDF metadata are included as global attributes with the Level-3 data files, as described in Table 3.

Table 3. Level-3 Soil Moisture netCDF Metadata General Attributes
Name Value
Product Name Q20130012013031.L3m_MO_SOILM_V2.0_rad_sm_1deg
Sensor Name 0B
Sensor Aquarius
Title Level-3 Standard Mapped Image
Data Center 0B
Station Name 0B
Station Latitude 0.0f
Station Longitude 0.0f
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 0B
Product Type 0B
Processing Version V2.0
Software Name smigen
Software Version 4.3
Processing Time 201312702000500
Input Files Q20130012013031.L3b_MO_SOILM_V2.0.main
Processing Control smigen par=Q20130012013031.L3m_MO_SOILM_V2.0_rad_sm_1deg.param
Input Parameters IFILE = /data3/sdpsoper/vdc/vpu10/workbuf/Q20130012013031.L3b_MO_SOILM_V2.0.main|OFILE = Q20130012013031.L3m_MO_SOILM_V2.0_rad_sm_1deg|PFILE = |PROD = rad_sm|PALFILE = DEFAULT|PROCESSING VERSION = V2.0|MEAS = 1|STYPE = 0|DATAMIN = 0.000000|DATAMAX = 0.000000|LONWEST = -180.000000|LONEAST = 180.000000|LATSOUTH = -90.000000|LATNORTH = 90.000000|RESOLUTION = 1deg|PROJECTION = RECT|GAP_FILL = 0|SEAM_LON = -180.000000|PRECISION=F
L2 Flag Names 0B
Period Start Year 0S
Period Start Day 0S
Period End Year 0S
Period End Day 0S
Start Time 2013001011415303
End Time 2013002004408124
Start Year 2013S
Start Day 1S
Start Millisec 4455303
End Year 2013S
End Day 2S
End Millisec 2648124
Start Orbit 0
End Orbit 0
Orbit 0
Map Projection Equidistant Cylindrical
Latitude Units degrees North
Longitude Units degrees East
Northernmost Latitude 90.0f
Southernmost Latitude -90.0f
Westernmost Longitude -180.0f
Easternmost Longitude 180.0f
Latitude Step 1.0f
Longitude Step 1.0f
SW Point Latitude -89.5f
SW Point Longitude -179.5f
Data Bins 0
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 1.0f
Intercept 0.0f
Data Minimum 0.02f
Data Maximum 0.60225f
Suggested Image Scaling Minimum 0.0f
Suggested Image Scaling Maximum 0.4f
Suggested Image Scaling Type LINEAR
Suggested Image Scaling Applied No
_lastModified 2013127020005000
Conventions CF-1.6
institution NASA/GSFC OBPG

Sample Data Record

Below is a sample of the l3m_data soil moisture data array from the Level-3 Aquarius soil moisture file: Q20130012013031.L3m_MO_SOILM_V2.0_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.

3. Data Access and Tools

Get Data

See the NSIDC Aquarius Soil Moisture Order Data page for a list of order options.

Software and Tools

HDF 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 HDF-EOS Tools and Information Center.

4. 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, 2013; Bindlish et al, 2013). Details on these steps are provided in the Aquarius L2 Soil Moisture documentation.

Data Acquisition Methods

An Aquarius Level-3 Soil Moisture product is generated from physical measurements computed from the Aquarius Level-2 Soil Moisture 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.

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, 1996; Lilly 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, 1996; Lilly 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 Level-2 Soil Moisture product.

Version History

V2.0 is the first version of Aquarius Level-3 Soil Moisture data available from NSIDC. Previous versions are available via the NASA Aquarius Web site (http://aquarius.nasa.gov/).

Error Sources

The current Version 2.0 Aquarius calibration is valid over a narrow range of TB (oceans only) (Piepmeier et al. 2013). The Aquarius brightness temperatures are re-calibrated for each beam using co-located and concurrent Soil Moisture Ocean Salinity (SMOS) observations. The re-calibrated brightness temperatures are used in version 2.0 of the soil moisture algorithm. The Aquarius mission is planning to address the full range calibration in the next couple of data releases. These calibration coefficients will be evaluated after each Aquarius data release.

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 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.
Scatterometer
  • 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

5. References and Related Publications

Bindlish, Rajat, and Thomas J. Jackson. 2013. Aquarius Soil Moisture ATBD Users Guide, Version 2.0. Beltsville, Maryland USA: USDA Hydrology and Remote Sensing Lab. (PDF file, 315 KB)

Bindlish, Rajat, Thomas Jackson, Michael Cosh, Tianjie Zhao and Peggy O'Neill. 2013. Global Soil Moisture from the Aquarius Satellite: Description and Initial Assessment. IEEE Geosciences and Remote Sensing Letters (in review).

Fan, J. and I. Gijbels. 1996. Local Polynomial Modelling and its Applications, Chapman and Hall, 1996.

Jackson, T. J. 1993. Measuring Surface Soil Moisture Using Passive Microwave Remote Sensing. Hydrological Processes 7:139–152.

Lilly, Jonathan and Gary Lagerloef. 2008. Aquarius Level 3 Processing Algorithm Theoretical Basis Document. ftp://podaac-ftp.jpl.nasa.gov/allData/aquarius/docs/v2/AquariusLevel3_GriddingSmoothingPaper_Lilly&Lagerloef2008.pdf

Piepmeier, Jeffrey, Shannon Brown, Joel Gales, Liang Hong, Gary Lagerloef, David Le Vine, Paolo de Matthaeis, Thomas Meissner, Rajat Bindlish, and Thomas Jackson. 2013. Aquarius Radiometer Post-Launch Calibration for Product Version 2.0, Aquarius Project Document: AQ-014-PS-0015. ftp://podaac-ftp.jpl.nasa.gov/allData/aquarius/docs/v2/AQ-014-PS-0015_AquariusInstrumentCalibratrionDescriptionDocument.pdf.

Related Data Collections

Related Web Sites

  • Aquarius Web site at NASA Goddard Space Flight Center (http://aquarius.nasa.gov/)
  • Aquarius Data Web Site at NSIDC (http://nsidc.org//data/aquarius/index.html)
  • Aquarius Web Site at PODAAC (http://podaac.jpl.nasa.gov/aquarius)
  • SMAP Web Site at JPL (http://smap.jpl.nasa.gov)
  • SMOS Website at ESA (http://www.esa.int/Our_Activities/Observing_the_Earth/The_Living_Planet_Programme/Earth_Explorers/SMOS/ESA_s_water_mission_SMOS)

6. Document Information

Acronyms and Abbreviations

The acronyms used in this document are listed in Table 5.

Table 5. Acronyms and Abbreviations
Acronym Description
ACS Attitude Control System
ADPS Aquarius Data Processing System
ATBD Algorithm Theoretical Basis Document
CONAE Comisión Nacional de Actividades Espaciales
GPS Global Positioning System
GSFC Goddard Space Flight Center
HDF5 Hierarchical Data Format 5
HH transmit Horizontal, receive Horizontal
HV transmit Horizontal, receive Vertical
IGBP International Geosphere-Biosphere Programme
L3 Level-3 processing
MODIS Moderate Resolution Imaging Spectroradiometer
NASA National Aeronautics and Space Administration
NCEP GFS National Centers for Environmental Prediction Global Forecast System
NCEP GFS GDAS National Centers for Environmental Prediction Global Forecast System Global Data Assimilation System
NDVI Normalized Difference Vegetation Index
NOAA National Oceanic and Atmospheric Administration
NRCS Normalized Radar Cross Section
ODPS Ocean Data Processing System (ODPS)
OISST Optimum Interpolation (OI) Sea Surface Temperature (SST) V2
RFI Radio Frequency Interference
SAC-D Satélite de Aplicaciones Científicas
SCA Single Channel Algorithm
SeaWiFS Sea-viewing Wide Field-of-view Sensor
SM Soil Moisture
SMOS Soil Moisture Ocean Salinity
SWE Snow Water Equivalent
TB Brightness Temperature
TOA Top Of Atmosphere
UTC Coordinated Universal Time
VH transmit Vertical, receive Horizontal)
VV transmit Vertical, receive Vertical)
VWC Vegetation Water Content

Document Creation Date

02 December 2013

Document Revision Date

03 June 2014

Document URL

http://nsidc.org/data/docs/daac/aquarius/aq3_sm/index.html