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Aquarius L2 Swath Single Orbit Soil Moisture, Version 4
This data set contains Level-2 global soil moisture estimates derived from the NASA Aquarius passive microwave radiometer on the Satélite de Aplicaciones Científicas (SAC-D).
Changes in the Version 4 Aquarius Soil Moisture data include: use of the most recent version (Version 4) of Aquarius Brightness Temperatures as input.
|Temporal Resolution:||98 minute|
|Data Contributor(s):||Rajat Bindlish, Thomas Jackson|
|Metadata XML:||View Metadata Record|
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.Bindlish, R. and T. Jackson. 2015. Aquarius L2 Swath Single Orbit Soil Moisture, Version 4. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: http://dx.doi.org/10.5067/Aquarius/AQ2_SM.004. [Date Accessed].
Detailed Data Description
Aquarius LeveL-2 Soil Moisture products are produced by the NASA Goddard Space Flight Center's Aquarius Data Processing Segment (ADPS) .
The data files are in Hierarchical Data Format 5 (HDF5).
Data are available on the HTTPS site in the
https://n5eil01u.ecs.nsidc.org/AQUARIUS/AQ2_SM.004/ directory. Within this directory, the folders are organized by date, for example:
Folders contain HDF5 data files and XML metadata files.
Soil moisture files are named according to the following conventions and as described in Table 1:
||Indicates Aquarius instrument|
||UTC four-digit year|
||UTC day of year|
||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.|
||Geophysical parameter: SOILM = soil moisture|
||Version, e.g. V4.0|
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.
HDF5 files range from approximately 1 MB to 16 MB.
XML files average approximately 16 KB.
Total data volume is approximately 21 GB.
Coverage spans from 180°W to 180°E, and from approximately 90°N to 90°S. The swath width is about 390 km, enabling nearly global coverage every seven days. Figure 1 shows the spatial coverage of the Aquarius Radiometer for one orbit, which comprises one granule of this data set.
Spatial Coverage Map
The Aquarius radiometer consists of 3 beams of sizes 76 x 94 km (inner beam), 84 x 120 km (middle beam) and 96 x 156 km (outer beam). The total swath width of the 3 beams is about 390 km.
Soil moisture is estimated for each individual radiometer beam footprint. The exact spatial resolution of the Level-2 product varies depending on the beam position (inner, middle or outer beam).
Figure 2 shows the position of the Aquarius beams.
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.
98 minutes per granule. Global coverage is completed every seven days.
The Single Channel Algorithm (SCA) is used to estimate soil moisture using Aquarius brightness temperature observations. The SCA is applied to the individual Aquarius footprint brightness temperature observations (L2) to produce a swath-based time-order product. Each swath is stored in a separate file. Files are divided into the groups of attributes shown in Figure 3.
The following groups of data objects (Aquarius Data, Aquarius Flags, Block Attributes, and Navigation) contain data stored as HDF5 data blocks. Each data object within these four groups contains data for each block and is therefore dimensioned by the value of the global attribute, Number of Blocks.
Aquarius Data: Attributes contained in the Aquarius Data group are described in Table 2. Each parameter in this group contains 4083 x 3 (Aquarius beams) elements.
|anc_sm||NCEP GFS GDAS soil moisture.||Volumetric Fraction (m3/m3)|
|anc_subsurf_temp||0-10 cm NCEP GFS sub-surface temperature. The subsurface temperature over the land is the NCEP GFS GDAS product for the layer (0-10 cm) because emission from this layer is most closely correlated with soil moisture.||Kelvin|
|anc_surface_temp||NCEP GFS surface temperature. The surface temperature over the ocean is the NOAA OISST (Reynolds) product. Over land, the NCEP GFS GDAS product for the surface layer is used.||Kelvin|
|anc_swe||The snow water equivalent from NCEP GFS GDAS.||Kg/m2|
|rad_TbH||Aquarius L2 brightness temperature at the Earth surface after atmospheric correction h-pol. Prior to making a correction for roughness.||Kelvin|
|rad_TbV||Aquarius L2 brightness temperature at the Earth surface after atmospheric correction v-pol. Prior to making a correction for roughness.||Kelvin|
|rad_ice_frac||Fraction of ice contamination. Gain weighted ice fraction integrated over the antenna footprint. The Integration is over the radiometer footprint with 0 = water and 0 = land and 1 = sea ice weighted by the antenna pattern. Computation is made using the NCEP GFS GDAS ice product.||Area (m2/m2)|
|rad_land_frac||Fraction of land contamination. The gain weighted land fraction: Integration over the radiometer footprint with 1 = land and 0 = non-land (water and sea ice) weighted by the antenna pattern. Computation is made using the GSFC ODPS (SeaWiFS) 1 km resolution land mask. "Land" includes ice/snow covered land.||Area (m2/m2)|
|rad_sm||Aquarius volumetric soil moisture estimates.||Volumetric Fraction (m3/m3)|
|scat_HH_toa||Top Of Atmosphere (TOA) scatterometer Normalized Radar Cross Section (NRCS) for HH (transmit horizontal, receive horizontal) polarization. Aquarius L2 normalized radar cross-section at the top of the atmosphere at HH polarization.||Decibels|
|scat_HV_toa||TOA Scatterometer NRCS for HV (transmit horizontal, receive vertical) polarization. Aquarius L2 normalized radar cross-section at the top of the atmosphere at HV polarization.||Decibels|
|scat_VH_toa||TOA Scatterometer NRCS for VH (transmit vertical, receive horizontal) polarization. Aquarius L2 normalized radar cross-section at the top of the atmosphere at VH polarization.||Decibels|
|scat_VV_toa||TOA Scatterometer NRCS for VV (transmit vertical, receive vertical) polarization. Aquarius L2 normalized radar cross-section at the top of the atmosphere at VV polarization.||Decibels|
Aquarius Flags: The Aquarius Flags group contains one set of attributes for
radiometer_flags, described in Table 3.
|0||No SM Retrieval||SMRET||No Soil Moisture retrieval performed|
|1||Brightness Temp||TB||TB < 0 or Tb > 320|
|2||Orbit Maneuver||ORBIT||ACS mode = 5|
|3||RFI||RFI||Tbh > Tbv; Tb > 320|
|4||Surface Temp||TSURF||Tb > Tsurf|
|5||Frozen Ground||FROZ||NCEP surface or sub-surface temperature below 273.15|
|6||Snow||SNOW||NCEP SWE > 10 kg/m2|
|7||Ice||ICE||NCEP ice fraction > 0.1|
|8||NDVI||NDVI||MODIS NDVI climatology flag|
|9||Dense Vegetation||VEG||Vegetation Water Content > 5 kg/m2|
|10||Urban||URBAN||IGBP Land Cover|
|11||Soil||SOIL||Invalid Soil Texture data|
|12||Water||WATER||Land Fraction < 0.99|
Block Attributes: Parameters contained in the Block Attributes group are described in Table 4. Each parameter in this group contains 4083 x 1 elements.
|sec||Mid-block times of Aquarius physical parameter values in seconds of day.||Seconds|
|secGPS||GPS time; block times of Aquarius physical parameter values in seconds since the GPS epoch (0 hours UTC on 6 January 1980).||Seconds|
Navigation: Parameters contained in the Navigation group are described in Table 5.Each parameter in this group contains 4083 x 3 elements, except zang which is 4083 x 1.
|att_ang||Spacecraft roll, pitch, yaw||Degrees|
|beam_clat||Beam Center Latitude||Degrees|
|beam_clon||Beam Center Longitude||Degrees|
Metadata are included as global attributes with the Level-2 data files. Table 6 lists attribute names and values for data file
Q2011237000100.L2_SOILM_V4.0 Values that vary from granule to granule are noted.
|Title||Aquarius Level-2 Soil Moisture Data|
|Data Center||NASA/GSFC Aquarius Data Processing Center|
|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; instantaneous field of view=6.5 degrees; science data block period=1.44 sec.|
|Processing Time||2015167000645000 (varies)|
|Input Files||Q2011237000100_L2_SM.txt (varies)|
|Processing Control||ifile=Q2011237000100_L2_SM.txt ofile=Q2011237000100.L2_SOILM_V4.0 pversion=V4.0 (varies)|
|Start Time||2011237000101 (varies)|
|Start Year||2011 (varies)|
|Start Day||237 (varies)|
|Start Millisec||60001 (varies)|
|End Time||2011237013859 (varies)|
|End Year||2011 (varies)|
|End Day||237 (varies)|
|End Millisec||60001 (varies)|
|Node Crossing Time||2011237002530000 (varies)|
|Orbit Node Longitude||-95.19445 (varies)|
|Latitude Units||degrees North|
|Longitude Units||degrees East|
|Orbit Number||1110 (varies)|
|Cycle Number||1 (varies)|
|Pass Number||1 (varies)|
|Ancillary Files||Aquarius_sand_p05d.bin,Aquarius_clay_p05d.bin,Aquarius_bd_p05d.bin,Aquarius_land_cover_5km.dat, ndvi_max.dat,ndvi_min.dat,ndvi_cmg_08_3.dat,flag_cmg_08_3.dat,ndvi_cmg_08_3.dat|
Sample Data Record
Below is a sample of data records from the rad_sm parameter within the Aquarius Data group in the Aquarius soil moisture file:
Figure 4 shows the Aquarius soil moisture estimates using all three beams for July 1, 2012.
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 data sets.
For additional tools, see the HDF5 Tools and Software Web site.
Data Acquisition and Processing
Aquarius L2 Swath Single Orbit Soil Moisture Data products are produced by the NASA Goddard Space Flight Center's Aquarius Data Processing Segment (ADPS).
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 and land surface than the v-pol observations. 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 and Meissner et al, 2014). Details on these steps are provided in the Derivation Techniques and Algorithms section.
The Version 4 Aquarius L2 Swath Single Orbit Soil Moisture Data product is generated from measurements derived from the NASA Aquarius Level-2 Sea Surface Salinity & Wind Speed Data V4.0 product. The soil moisture product contains measurements at the observed surface locations, along with coordinates of viewed locations and navigation data. This product is stored as one physical HDF file. Each product contains data from one orbit of Aquarius data. An orbit begins and ends as the SAC-D spacecraft crosses the South Pole. 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 1A file that is input to the Level-2 science file (Pratt 2013).
The derivation techniques and algorithms in this section are from the Aquarius Soil Moisture ATBD Users Guide (Bindlish and Jackson, 2013 and Meissner et al, 2014). The entire ATBD Users Guide may be viewed in PDF format.
Brightness temperatures are converted to emissivity using a surrogate for the effective physical temperature (T) of the emitting layer. The observed emissivity (eobs) is corrected for vegetation and surface roughness to obtain the smooth soil emissivity (esoil). The Fresnel equation is then used to determine the dielectric constant of the soil-water mixture (k). Finally, a dielectric mixing model is used to obtain the soil moisture (SM).
Brightness temperature of the land surface is equivalent to its emissivity (eobs, where eobs = 1 - r) (r = Reflectivity) multiplied by its physical temperature (T). It is assumed that the temperatures of the soil and the vegetation are the same.
Based upon the above, the complete radiative transfer model can be simplified yielding the following expression for the observed TB in Equation 1:
Ancillary surface temperature data from the Numerical Weather Prediction model of the National Centers for Environmental Prediction Global Forecast System (NCEP GFS) is used as the effective physical temperature of the emitting medium.
The emissivity retrieved above is that of the soil as modified by any overlying vegetation and surface roughness. In the presence of vegetation, the observed emissivity is a composite of the soil and vegetation. To retrieve soil water content, it is necessary to isolate the soil surface emissivity (esurf). First, the correction for the presence of vegetation is done based on Jackson and Schmugge (1991), as in Equation 2:
||single scattering albedo|
||one-way transmissivity of the canopy|
||soil surface emissivity|
Both the single scattering albedo
(ω) and the one-way transmissivity of the canopy
(γ) are dependent upon the vegetation structure, polarization and frequency. The transmissivity is a function of the optical depth
(τ) of the vegetation canopy:
||optical depth of vegetation canopy|
||system incidence angle|
A constant value of the single scattering albedo is used in the Aquarius formulation (
ω=0.05). Re-arranging equation 2 yields:
The vegetation optical depth is a function of the Vegetation Water Content (VWC). In studies reported in Jackson and Schmugge (1991), it was found that the following functional relationship between the optical depth and vegetation water content could be applied:
||Proportionality value. Depends on vegetation structure and microwave frequency|
||Vegetation Water Content|
The algorithm uses a default global constant value of b = 0.8 for all vegetation classes. The vegetation water content can be estimated using several ancillary data sources. The baseline approach utilizes a set of land cover-based equations to estimate VWC from values of the Moderate Resolution Imaging Spectroradiometer (MODIS) derived Normalized Difference Vegetation Index (NDVI), an index derived from visible-near infrared reflectance data. The approach uses a MODIS NDVI climatology that was derived based on observations from 2001-2010.
The emissivity that results from the vegetation correction is that of the soil surface, including any effects of surface roughness. These effects are removed in order to determine the smooth surface soil emissivity (esoil), which is required for the Fresnel equation inversion. One approach to removing this effect is a model described in Choudhury et al. (1979) that yields the bare smooth soil emissivity:
The cos2 Θ term is often dropped to avoid overcorrecting for roughness. The Aquarius soil moisture algorithm does not drop this term.
Emissivity is related to the dielectric properties (ε) of the soil and the viewing or incidence angle. For ease of computational inversion, it is assumed that the real component (εr) of the dielectric constant provides a good approximation of the complex dielectric constant. However, this assumption can be modified if additional evidence is found to support the use of this more complex formulation. The Fresnel equations link the dielectric constant to emissivity. For horizontal polarization:
||real component of the dielectric constant|
The dielectric constant of soil is a composite of the values of its components air, soil, and water, which have greatly different values. A dielectric mixing model is used to relate the estimated dielectric constant to the amount of soil moisture. The Aquarius SCA uses Wang and Schmugge (1980) dielectric mixing model to estimate soil moisture.
Changes in the Version 4 Aquarius Soil Moisture data include: use of the most recent version (Version 4) of Aquarius Brightness Temperatures as input.
Changes in the Version 3 Aquarius Soil Moisture data include: 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). For more information, see: Summary of Aquarius V3.0 Algorithm and Processing Changes from V2.0
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 13 summarizes instrument characteristics.
|3 radiometers in push-broom alignment||
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: 7-day, 103 orbits
References and Related Publications
Contacts and Acknowledgments
NASA Goddard Space Flight Center
Hydrological Sciences Laboratory
Code 617, Bldg 33, G216
Greenbelt, MD 20771 USA
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 CREATION DATE
02 December 2013
DOCUMENT REVISION DATE
03 June 2014
12 November 2014
01 October 2015