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Data Set ID: 

SMAPVEX12 PALS Backscatter Data, Version 1

This data set contains backscatter data obtained by the Passive Active L-band System (PALS) microwave aircraft instrument as part of the Soil Moisture Active Passive Validation Experiment 2012 (SMAPVEX12).

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Geographic Coverage

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  • Radar > Radar Cross-section > Radar Backscatter
  • Radar > Radar Backscatter
Spatial Coverage:
N: 49.96, 
S: 49.44, 
E: -97.85, 
W: -98.51
Spatial Resolution:
  • 500 m to 1500 m x 500 m to 1500 m
Temporal Coverage:
  • 7 June 2012 to 19 July 2012
Temporal Resolution: 1 day to 5 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. 2014. SMAPVEX12 PALS Backscatter Data, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/KQC0KOL4DK1I. [Date Accessed].

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The documentation for this data set was provided solely by the Principal Investigator(s), and was not further developed, thoroughly reviewed, or edited by NSIDC. Thus, support for this data set may be limited.

Detailed Data Description

This data set contains backscatter data obtained by the Passive Active L- and S-band (PALS) microwave aircraft instrument. The data were collected as part of the Soil Moisture Active Passive Validation Experiment 2012 (SMAPVEX12).


Table 1 provides descriptions for each column in the data files.

Table 1. Contents of Data Fields
Column Number Description
1 UTC time in seconds
2 Latitude of the boresight (footprint center) [°]
3 Longitude of the boresight [°]
4 UTM x-coordinate of the boresight [m]
5 UTM y-coordinate of the boresight [m]
6 VV normalized radar cross-section [dB]
7 HH normalized radar cross-section [dB]
8 HV normalized radar cross-section [dB]
9 VH normalized radar cross-section [dB]
10 Heading uncertainty flag [0/1]
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File and Directory Structure

Data files are available on the HTTPS site in the https://n5eil01u.ecs.nsidc.org/SMAP_VAL/SV12PLBK.001/ directory.

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File Naming Convention

Files are named according to the following convention, and as described in Table 2:



Table 2. Contents of Data Fields
Variable Description
SV12PLBK Short Name
PALS Passive Active L- and S-band (PALS) data
S0 Sigma Nought (dB)
2012 2012 (representing SMAPVEX12 campaign)
MM 2-Digit Month
DD 2-Digit Day
[Hi/Lo]Alt Indicates whether this is a high- or low-altitude file
v101 Data version
.txt Indicates this is an ASCII text file

Example: SV12PLBK_PALS_S0_20120629_LoAlt_v101.txt

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File Size

Files range in size from approximately 3 to 39 MB.

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The approximate volume for this data set is 514 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 low-altitude radiometer footprint size is approximately 500 m, and the high-altitude radiometer footprint size is approximately 1500 m.

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

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Temporal Coverage and Resolution

Data were collected every 1 to 5 days from 07 June 2012 through 19 July 2012.

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Parameter or Variable

The parameter for this data set is normalized radar cross-section (dB).

Parameter Range

Valid parameter values are as follows:
Normalized radar cross-section: -40 – 0 dB

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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

Theory of Measurements

Current microwave models and retrieval algorithms have significant limitations in their treatment of different vegetation types and heterogeneous scenes (mixtures of grass, crops, trees, streams, lakes) and quantitative treatment of algorithm scaling and error analysis for such heterogeneous scenes. Measurements over wide varieties of terrain are needed, with joint active and passive sensors, to develop algorithms and parameterizations that can work across all terrain types, and extract optimum information from the combined data. This will have direct impact on the design of dedicated soil moisture missions and development of methods to assimilate such data into land surface models.

Microwave radiometry and radar are well-established techniques for surface remote sensing. Combining passive and active sensors provides complementary information contained in the surface emissivity and backscatter signatures, which can improve the accuracy of retrieval of geophysical parameters. Over land, it has been demonstrated that the radiometer and the radar both provide information for estimating soil moisture and vegetation water content (Bolten et al. 2003, Njoku et al. 2002, Narayan et al. 2004).

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Sensor or Instrument Description

The campaign deployed the Jet Propulsion Laboratory (JPL), with NASA support, designed, built and tested a precision Passive/Active L/S-band (PALS) aircraft instrument for measurements of soil moisture and ocean salinity (Wilson et al. 2001). PALS provides radiometer products, vertically and horizontally polarized brightness temperatures, and radar products, including 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). Table 3 provides the key characteristics of PALS.

Table 3. 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, CLASIC and SMAPVEX08 [Colliander et al. 2012]) before deployment in SMAPVEX12. Beginning with CLASIC, 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 3dB spatial resolutions of the instrument at the minimum and maximum altitudes are 500 m (1000 m altitude, minimum for the radar operation) and 1500 m (3000 m altitude, maximum for Twin Otter operation without oxygen supply). 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.

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

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Error Sources

When the aircraft heading uncertainty flag is set to 1 the uncertainty of the boresight geolocation exceeds the nominal. The uncertainty in the heading of the aircraft was caused by drifting navigation unit. Except for the very first days of the campaign this drift was compensated for.

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Quality Assessment

The quality of the normalized radar cross-section relies on internal calibration utilizing a calibration loop. The external calibration utilizes predetermined coefficients of the antenna and front-end and comparisons to concurrent UAVSAR measurements. These references assure generally good quality of the data.

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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


October 2013



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