Data Set ID: 

SMAPVEX12 PALS Soil Moisture Data, Version 1

This data set contains soil moisture data obtained by the Passive Active L-band System (PALS) aircraft instrument. The data were collected as part of SMAPVEX12, the Soil Moisture Active Passive Validation Experiment 2012.

Version Summary: 

First public data release

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Geographic Coverage

STANDARD Level of Service

Data: Data integrity and usability verified

Documentation: Key metadata and user guide available

User Support: Assistance with data access and usage; guidance on use of data in tools

See All Level of Service Details

  • Microwave > Brightness Temperature
  • Soils > Soil Texture > Clay Fraction
  • Land Use/Land Cover > Land Cover > Land Cover Classification
  • Soils > Soil Texture > Sand Fraction
  • Soils > Soil Moisture/Water Content > Soil Moisture
  • Soils > Soil Temperature
  • Vegetation > Vegetation Cover > Vegetation Temperature
  • Vegetation > Plant Characteristics > Vegetation Water Content
Spatial Coverage:
N: 49.96, 
S: 49.44, 
E: -97.85, 
W: -98.51
Spatial Resolution:
  • 1500 m x 1500 m
Temporal Coverage:
  • 12 June 2012 to 19 July 2012
Temporal Resolution: 1 day to 4 days
Data Format(s):
  • ASCII Text
Platform(s) AIRCRAFT
Sensor(s): PALS
Version: V1
Data Contributor(s): Andreas Colliander

Data Citation

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.

Colliander, A. 2017. SMAPVEX12 PALS Soil Moisture Data, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: [Date Accessed].

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Detailed Data Description

Parameter Description

The retrieved parameter for this data set is soil moisture [m3/m3]. Parameters used as input to the algorithm are also provided, including brightness temperature [K], estimated soil temperature [°C], estimated vegetation temperature [°C], vegetation water content [kg/m2], land cover class, sand fraction [%], and clay fraction [%].

Parameter Range

Valid parameter values are as follows:

Soil moisture: 0-1 m3/m3
Brightness temperature: 50-350 K
Temperature: 0-40°C
Vegetation water content: 0-40 kg/m2
Sand and clay fractions: 0-100 %

Table 1 describes the land cover classes, which range from 1 to 11.

Table 1. Land Cover Classes
Class Number Description
1 Unclassified
2 Water
3 Urban
4 Shrub
5 Wetlands
6 Pasture
7 Cereals
8 Corn
9 Canola
10 Soybean
11 Broadleaf
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Data are provided in ASCII comma-delimited text files. Table 2 provides descriptions for each column in the ASCII files.

An associated Extensible Markup Language (XML) metadata file is also provided for each data file.

Table 2. Description of Columns in Data Files
Column Number Column Heading Column Description
1 Date Date in YYYYMMDD format [4-digit year, 2-digit month, two-digit day]
2 Row Row number
3 Col Column number
4 X (UTM) Universal Transverse Mercator X (easting) coordinate of the boresight [m]
5 Y (UTM) Universal Transverse Mercator Y (northing) coordinate of the boresight [m]
6 VSM Volumetric soil moisture [m3/m3]
7 TaH Horizontally polarized brightness temperature [K]
8 TaV Vertically polarized brightness temperature [K]
9 Tsoil Estimate of the soil temperature [°C]
10 Tveg Estimate of the vegetation temperature [°C]
11 VWC Average vegetation water content in the pixel [kg/m2]
12 LC Dominant land cover class in the pixel [-]
13 S% Average sand fraction in the pixel [%]
14 C%: Average clay fraction in the pixel [%]
Note: The fill value NaN, for Not a Number, indicates when no data are available.
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File and Directory Structure

Data are available via HTTPS in the following directory:

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File Naming Convention

Files are named according to the following convention:



Table 3. File Naming Convention
Variable Description
SV12PLSM Product Short Name
PALS Passive Active L- and S-band (PALS) Data
VSM Volumetric Soil Moisture
2012 2012 (indicates year of SMAPVEX12 campaign)
MM 2-Digit Month
DD 2-Digit Day
HiAlt High-Altitude Data
vXX Data Version (v01: version 1.0)
.txt Indicates ASCII text file format

Example: SV12PLSM_PALS_VSM_20120629_HiAlt_v01.txt

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File Size

Files are approximately 0.2 MB each.

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File Volume

The approximate volume for this data set is 3.2 MB.

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Spatial Coverage

Southernmost Latitude: 49.44°N
Northernmost Latitude: 49.96°N
Westernmost Longitude: 98.51°W
Easternmost Longitude: 97.85°W

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Spatial Resolution

The high-altitude footprint size is approximately 1500 m, with a flight overpass sampling area of apporximately 46 km x 47 km.

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Data are provided in Universal Transverse Mercator (UTM) World Geodetic System 1984 (WGS84) coordinates, Zone 14 U.

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Temporal Coverage and Resolution

Data were collected every one to four days from 12 June 2012 through 19 July 2012.

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Software and Tools

No special tools are required to view these data. Any word-processing program or Web browser will display the data.

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Data Acquisition and Processing

A description of SMAPVEX12 experiment domain and data acquisition regime is given in McNairn et al. (2015).

Theory of Measurements

Soil moisture was retrieved using PALS brightness temperature observations. The algorithm uses a variant of the traditional t-w-h algorithm (Njoku and Entekhabi, 1996). A detailed description of the algorithm and processing is given in Colliander et al. (2016).

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Sensor or Instrument Description

With NASA support, the Jet Propulsion Laboratory (JPL) designed, built, and tested the Passive Active L-band System (PALS) microwave aircraft instrument and deployed the instrument for measurements of soil moisture and ocean salinity (Wilson et al. 2001). PALS provides radiometer products, such as vertically and horizontally polarized brightness temperatures, and radar products, such as normalized radar backscatter cross-section for V-transmit/V-receive, V-transmit/H-receive, H-transmit/H-receive, and H-transmit/V-receive. In addition, it can also provide the polarimetric third Stokes parameter measurement for the radiometer and the complex correlation between any two of the polarized radar echoes (VV, HH, HV and VH). The following table provides the key characteristics of PALS:

Table 4. Description of the PALS instrument
Passive Frequency 1.413 GHz
Polarization V, H, +45, -45
Calibration stability 1 K (bias); 0.2 K (stability)
Active Frequency 1.26 GHz
Polarization VV, HH, VH, HV
Calibration accuracy <2 dB (bias); 0.2 dB (stability)
Antenna Half Power Beamwidth 20° (passive); 23°(active)
Beam Efficiency 94%
Directivity 18.5 dB
Polarization isolation > 35 dB

The PALS instrument was flown in four major soil moisture experiments (SGP99, SMEX02, CLASIC07 and SMAPVEX08 [Colliander et al. 2012]) before deployment in SMAPVEX12. Beginning with CLASIC07, a new flat-panel antenna array was substituted for the large horns. The planar antenna consists of 16 stacked-patch microstrip elements arranged in a four by- four array configurations. Each stacked-patch element uses a honeycomb structure with extremely low dielectric loss at L-band to support the ground plane and radiating patches. The measured antenna pattern shows better than 33 dB polarization isolation, far exceeding the need for the polarimetric measurement capability. This compact, lightweight antenna has enabled PALS to transition to operating on small aircraft, such as the Twin Otter (Yueh et al. 2008).

PALS was mounted at a 40° incidence angle looking to the rear of the aircraft. The 3 dB spatial resolutions of the instruments at two potential altitudes are 500 m (due to 1000 m altitude, minimum for the radar operation) and 1500 m (due to 3000 m altitude, maximum). It is important to note that PALS provides a single beam of data along a flight track and that any mapping must rely upon multiple flight lines at a spacing of the footprint width.

The calibration of the PALS brightness temperature during SMAPVEX12 is described in Colliander et al. (2015).

Figure 1. Images of Three Different Aircraft Installations of the PALS Combined Active and Passive L-band Instrument
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Error Sources

Refer to Colliander et al. (2016) for a detailed discussion on error sources impacting the soil moisture retrieval.

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Quality Assessment

Metrics for the soil moisture retrieval performance in non-forested areas based on in situ measurements was found to be as follows: RMSD (root mean square difference) 0.058 m3/m3, bias -0.015 m3/m3, ubRMSD (unbiased RMSD) 0.056 m3/m3, and Pearson correlation 0.87. The performance is not defined for forested areas and usage of the retrievals over areas which are marked as broadleaf is not recommended for any scientific purposes. Refer to Colliander et al. (2016) for details.

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References and Related Publications

Contacts and Acknowledgments

Andreas Colliander
Jet Propulsion Laboratory
California Institute of Technology
4800 Oak Grove Dr, Pasadena, CA 91109 USA

Document Information


June 2017



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