Data Set ID: 

SMAPVEX12 UAVSAR Incidence-Angle Normalized Backscatter Data, Version 1

This data set contains backscatter data obtained by the Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) instrument. The data were collected 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: 50.01, 
S: 49.32, 
E: -97.62, 
W: -98.67
Spatial Resolution:
  • 6 m x 6 m
Temporal Coverage:
  • 17 June 2012 to 17 July 2012
Temporal Resolution: 1 day to 6 days
Data Format(s):
  • Binary
Platform(s) G-III
Sensor(s): UAVSAR
Version: V1
Data Contributor(s): Xiaolan Xu

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.

Xu, X. 2014. SMAPVEX12 UAVSAR Incidence-Angle Normalized 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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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 Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) instrument. The data were collected as part of the Soil Moisture Active Passive Validation Experiment 2012 (SMAPVEX12).


The data files are provided in flat binary format on an equi-angular grid. Associated Extensible Markup Language (XML) metadata files are also provided.

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File and Directory Structure

Data files are available on the HTTPS site in the directory.

Within this directory are files containing vertically-polarized, horizontally-polarized, and cross-polarized data. An associated XML metadata file is provided for each data file.

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File Naming Convention

Files are named according to the following convention, and as described in Table 1:



Table 1. File Naming Convention
Variable Description
SV12UBK Short Name
Combined4 Indicates this file contains both vertically and horizontally polarized (V- and H-pol) data
3050 Indicates 30–50 deg and refers to the incidence angle range applied in the processing. If this number is not reported, in the file name, the full range of incidence angle is applied: 20–60 deg.
YYMMDD 2-Digit Year, 2-Digit Month, and 2-Digit Day
L L-band
ddd Steering angle
xyxy Polarization: HVHV, HHHH, or VVVV
CX Cross Polarized
vv Processing version
.ngrd Indicates this is a normalized data file
.MET.xml Indicates this is an XML metadata file

Example: SV12UBK_Combined4_120629_L090HHHH_CX_02.ngrd

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File Size

Files are approximately 1 GB each.

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The approximate volume for this data set is 11 GB.

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Spatial Coverage

Southernmost Latitude: 49.32°N
Northernmost Latitude: 50.01°N
Westernmost Longitude: 98.67°W
Easternmost Longitude: 97.62°W

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Spatial Resolution

The radar footprint size is approximately 6 m.

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

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Grid Description

Table 2 describes the grid dimensions.

Table 2. Grid Dimensions
North-West Corner Latitude 50.01050052°
North-West Corner Longitude -98.67267096°
Rows 12411
Columns 18792
Latitudinal Pixel Size 5.556e-5
Longitudinal Pixel Size 5.556e-5
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Temporal Coverage and Resolution

Data were collected every 1 to 6 days from 17 June 2012 through 17 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: -50 – 20 dB

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Software and Tools

Any software capable of reading binary files, such as MATLAB or IDL, can be used to display the data files.

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Data Acquisition and Processing

Theory of Measurements

The UAVSAR instrument collects data over a wide range of incidence angles between 25 and 65 degrees. The nominal swath is approximately 21 km. The SMAP instrument will operate at a fixed angle of 40 degrees. Since the incidence angle has a significant effect on backscattering, it is necessary to normalize the UAVSAR data to fully utilize the swath.

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Processing Steps

The incidence angle correction is based on the HIST method (Mladenova et al, 2013). The method is based on histogram matching as opposed to normalization with cumulative frequencies. As indicated in the paper, the HIST technique will preserve the natural signal variability as opposed to the cumulative distribution function (CDF) technique where it is going to be limited by and to the variability observed in the reference line. The lowest two central moments are used in the calculation, the mean and variation for every 1 degree incidence angle.

For SMAPVEX12, four diagonal flight lines covered the study domain and two horizontal lines were flown over to complement the Passive Active L-band System (PALS) instrument coverage in a day (as shown in Figure 1). To minimize the statistical error, we group the four diagonal images of the same day for the statistical (mean and variation) calculation. The other two horizontal images are calculated together but are separated from the diagonal images for determining azimuthal angle. In addition, data values lower than the noise equivalent values suggested by the UAVSAR team and greater than 5 dB were excluded for statistical calculation. Refer to the correction flow chart shown in Figure 2.

Map of UAVSAR Flight Line and Coverage
Figure 1. Map showing UAVSAR flight line and coverage.
Flowchart of the incidence angle normalization process
Figure 2. Flow chart of the incidence angle normalization process.

Based on the vegetation map from Agriculture and Agri-Food Canada (AAFC), the mean and variation for the covered 24 vegetation classes are calculated at one-degree intervals from 21° to 65°; for example, 21° represents all pixels from 20.5° to 21.5° for vertically and horizontally cross-polarized measurements (VV and HH, respectively). A more narrow range (from 21° to 50°) is used for cross-polarization due to the large error at the edge. The normalized backscattering for each pixel can be calculated as follows:

 (Equation 1)


Equation 1 (cont.)

is the radar backscattering (dB) in the raw data,


Equation 1 (cont.)

indicates the raw data with original incidence angle and corresponding vegetation class. The mean and standard deviation— σ̄ and σ̂, respectively—are computed for each vegetation class (VC) and every degree of incidence angle (21° to 65° for VV & HH, 21° to 50° for HV). In our case, the reference line is at 40 degrees.

After processing each diagonal image, the final image will average the overlapping areas and combine the four images into one. Most ground study areas are covered by the overlapping areas (across three or four images).

Data Sets

The vegetation map used in the normalization is shown in Figure 3.

To match up with the UAVSAR data, the map is re-gridded into the same equiangular coordinate system in which the normalized radar cross-section is found.

SMAPVEX Vegetation Map
Figure 3. Vegetation map used in the incidence angle normalization process.

Raw Data & Normalized Data

The ground-projected file (.grd) is taken as the raw data.

According to the Jet Propulsion Laboratory (JPL) UAVSAR Web page:

UAVSAR projects slant range images to ground range using the backward projection method. An equiangular grid is found with latitude and longitude boundaries that cover the entire slant range image. For each point on the ground range grid, the corresponding indices are calculated on the multilooked slant range image. The data value closest to the coordinates pointed by the calculated slant range indices is assigned to the point on the ground range grid.

The normalized image (.ngrd) will keep the same naming convention and format. The only difference is the .grd file is power in natural number while .ngrd is in dB.

Final Image

Each final image is an average of four diagonal images. Since there are a few missing diagonal images in the current version, the 19 June final image is a combination of two images, and the 22, 23, and 25 June final images are a combination of three images.

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Error Sources

As stated in the Final Image section above, there are a few missing diagonal images in the current version. This increases the uncertainty of the incidence-angle normalized values on these days.

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Quality Assessment

See Mladenova et al, 2013 for the quality assessment of the method.

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References and Related Publications

Contacts and Acknowledgments

Xiaolan Xu
Jet Propulsion Laboratory
California Institute of Technology
4800 Oak Grove Drive
Pasadena, CA 91109 USA
phone: +1 818.354.5097

Document Information


April 2014



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