Data Set ID:

SMAPVEX08 PALS Backscatter Data, Version 1

This data set contains backscatter data obtained by the Passive Active L-band System (PALS) microwave aircraft radar instrument as part of the Soil Moisture Active Passive Validation Experiment 2008 (SMAPVEX08).

This is the most recent version of these data.

Version Summary:

initial release

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

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Data Format(s):
  • ASCII Text
Spatial Coverage:
N: 39.09, 
S: 38.93, 
E: -75.55, 
W: -76.25
Spatial Resolution:
  • 350 m to 1100 m x 350 m to 1100 m
Temporal Coverage:
  • 29 September 2008 to 13 October 2008
Temporal Resolution1 day to 3 daysMetadata XML:View Metadata Record
Data Contributor(s):Yueh, S.

Geographic Coverage

Other Access Options

Other Access Options


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.

Yueh, S. 2015. SMAPVEX08 PALS Backscatter 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

This data set contains backscatter data obtained by the Passive Active L-band System (PALS) microwave aircraft instrument collected as part of the Soil Moisture Active Passive Validation Experiment 2008 (SMAPVEX08).


Table 1 provides descriptions for each column in the ASCII data files. An associated Extensible Markup Language (XML) metadata file is also provided for each data file.

Table 1. Contents of Data Fields
Column Number Description
1 Time in seconds from PALS radar computer
2 Time in milliseconds from DADS
3 Latitude of the footprint center [°]
4 Longitude of the footprint center [°]
5 Azimuth angle of radar look direction with respect to north [°]
6 Polarization roll angle of the antenna beam [°]
7 Range to target [m]
8 Incidence angle [°]
9 VV normalized radar cross-section [dB]
10 HH normalized radar cross-section [dB]
11 HV normalized radar cross-section [dB]
12 VH normalized radar cross-section [dB]
13 Real part of cross-correlation: vvhh
14 Imaginary part of cross-correlation: vvhh
15 Real part of cross-correlation: vvhv
16 Imaginary part of cross-correlation: vvhv
17 Real part of cross-correlation: vvvh
18 Imaginary part of cross-correlation: vvvh
19 Real part of cross-correlation: hhhv
20 Imaginary part of cross-correlation: hhhv
21 Real part of cross-correlation: hhvh
22 Imaginary part of cross-correlation: hhvh
23 Real part of cross-correlation: hvvh
24 Imaginary part of cross-correlation: hvvh
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File and Directory Structure

Data files are available at:

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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
SV08PLBK Data Set ID
MM 2-Digit Month
DD 2-Digit Day
hh 2-digit hours of the start time
mm 2-digit minutes of the start time
.red Indicates file extension

Example File Name:

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File Size

Files range in size from approximately 1 to 7 MB.

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The approximate volume for this data set is 293 MB.

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Spatial Coverage

Southernmost Latitude: 38.93°N
Northernmost Latitude: 39.09°N
Westernmost Longitude: 76.25°W
Easternmost Longitude: 75.55°W

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Spatial Resolution

The 3dB spatial resolutions of the instruments at two potential altitudes are 350 m (1000 m altitude, minimum for the radar operation) and 1100 m (3000 m, maximum).

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Latitude/longitude (WGS84)

Grid Decription

No grid.

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Temporal Coverage and Resolution

Data were collected every 1 to 3 days from 29 September 2008 through 13 October 2008.

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Parameter or Variable

Parameters include normalized radar cross-section (dB), incidence angle (°), azimuth angle (°), polarization rotation angle (°), range (m), and complex cross-correlation.

Parameter Range

Valid parameter values are as follows:

Normalized radar cross-section: -40 - 0 dB
Incidence angle: 30° - 50° 
Azimuth angle: 0° - 360°
Polarization rotation angle: -90° - 90°
Range: 0 - 10000 m

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Software and Tools

Any word-processing program or Web browser is sufficient for viewing ASCII text files.

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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-band System (PALS) microwave 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, 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 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

PALS was flown in three major soil moisture experiments (SGP99, SMEX02 and CLASIC) before deployment in SMAPVEX08. 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 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 35 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.

Since the CLASIC experiment in 2007, the PALS was augmented with additional components designed to detect and mitigate Radio Frequency Interference (RFI). The demonstration and evaluation of these elements was an important consideration in the SMAPVEX08 design.

PALS was mounted at a 40° incidence angle looking to the rear of the aircraft. The 3dB spatial resolutions of the instruments at two potential altitudes are 350 m (1000 m altitude, minimum for the radar operation) and 1100 m (3000 m, 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.

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

There are no exceptional error sources for this data set.

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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. These references assure good quality of the data.

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References and Related Publications

Contacts and Acknowledgments

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

Document Information


June 2015



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