SMEX03 ENVISAT ASAR Data

Summary

This data set is comprised of browse images acquired over the regional study areas of Alabama, Georgia, and Oklahoma USA as part of the 2003 Soil Moisture Experiment (SMEX03). The experiment was conducted from March through July 2003 via the Advanced Synthetic Aperture Radar (ASAR) instrument on board the European Space Agency's (ESA) Environmental Satellite (ENVISAT). Cross-polarized C-band ASAR images were acquired in order to obtain a backscatter coefficient measurement over a range of natural vegetation types. However, the images provided here are only browse images and therefore cannot be used to derive backscatter coefficient values. Researchers interested in backscatter coefficient values must obtain the ASAR data. The total volume for this combined data set is approximately 65 megabytes. Data are available via FTP and are provided as JPEG image files and JGW georeference files. Also included and available via FTP are the following types of header files: HAN files, ASCII text files, and INI files.

These data were collected as part of a validation study for the Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E). AMSR-E is a mission instrument launched aboard NASA's Aqua Satellite on 04 May 2002. AMSR-E validation studies linked to SMEX are designed to evaluate the accuracy of AMSR-E soil moisture data. Specific validation objectives include assessing and refining soil moisture algorithm performance; verifying soil moisture estimation accuracy; investigating the effects of vegetation, surface temperature, topography, and soil texture on soil moisture accuracy; and determining the regions that are useful for AMSR-E soil moisture measurements.

Citing These Data

The following example shows how to cite the use of this data set in a publication. For more information, see our Use and Copyright Web page.

Jackson, T.J., Bindlish, R., and R. Van der Velde. 2008. SMEX03 ENVISAT ASAR Data. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media.

Overview Table

Table of Contents

1. Contacts and Acknowledgments
2. Detailed Data Description
3. Data Access and Tools
4. Data Acquisition and Processing
5. References and Related Publications
6. Document Information

1. Contacts and Acknowledgments

Investigator(s)

Thomas J. Jackson
Hydrology Remote Sensing Laboratory
US Department of Agriculture (USDA) - Agricultural Research Service (ARS)
Beltsville, Maryland 20705 USA

Rajat Bindlish
Hydrology Remote Sensing Laboratory
US Department of Agriculture (USDA) - Agricultural Research Service (ARS)
Beltsville, Maryland 20705 USA

Rogier Van der Velde
Soil Tilth Laboratory
US Department of Agriculture (USDA) - Agricultural Research Service (ARS)
Beltsville, Maryland 20705 USA

Technical Contact

2. Detailed Data Description

Format

Browse images for each regional study area are provided as JPEG image files with corresponding JGW (JPEG World) georeference files. The following types of header files are also provided:

• ASCII text (.txt extension): Output file generated by ESA's Basic ENVISAT SAR Toolbox (BEST); each ASCII text file contains an extensive annotation list for its corresponding image.
• HAN (.HAN extension): Internal BEST Header Analysis file
• INI (.ini extension): BEST configuration/initialization file generated by running the BEST Header Analysis module; contains the parameter information used to run BEST software.

One ASCII text file and one HAN file accompanies each JPEG image file in this data set, and one INI file is included for each study region. For clarification of variables for all header files, refer to the BEST User Manual (PDF file, 1.53 MB) via the Documentation link on the BEST Home Page. With the exception of .HAN files, the Sample Record section of this document provides sample data records for each type of file in this data set. HAN files, however, are internal BEST Header Analysis files containing the full product header information used for running the Basic ENVISAT SAR Toolbox (BEST) applications. BEST is a suite of executable software tools designed to facilitate the use of ESA Synthetic Aperture Radar (SAR) data.

In addition to ASCII text and HAN files, each JPEG image in this data set has an associated JGW georeference file. Refer to Table 1 for clarification of JGW georeference file values, and to Figure 2 for the Sample Data Record. The JPEG image files in this data set vary in size and orientation, and range from approximately 2800 to 8200 pixels.

File and Directory Structure

Data are available on the FTP site in the /soil_moisture/SMEX03/Alabama/satellite/ASAR/ directory. Located within the ASAR directory are three regional subdirectories (AL, GA, and OK) and the readme.txt file. Each regional subdirectory contains one INI file and one or more sets of JPG, JGW, ASCII text, and HAN files. Figure 1 shows the overall directory structure.

File Naming Convention

JPEG and JGW Files

JPEG and accompanying JGW georeference files for each regional study area are named according to the following convention and as described in Table 2:

rg_ASAR_mmdd_IS#_P.jpg
rg_ASAR_mmdd_IS#_P.jgw

Where:

Examples:

al_ASAR_0701_IS2_D.jpg
al_ASAR_0701_IS2_D.jgw

ASCII Text and HAN Files

ASCII text and Header Analysis (HAN) files for each regional study area are named according to the following convention and as described in Table 3:

rg_mmdd_P.txt
rg_mmdd_P.HAN

Where:

Examples:

ok_0715_A.txt
ok_0715_A.HAN

INI Files

Also included for each regional study area is an INI file named rg_parameters.ini that contains the parameter information used to run BEST software when processing JPEG images for the region. As with all other files in this data set, rg in the INI file naming convention (rg_parameters.ini) indicates the particular study region. The INI file for Alabama, for example, is called al_parameters.ini.

File Size

Each regional study area includes JPEG image files, JGW georeference files, HAN files, ASCII text files, and one INI file. Table 4 lists the approximate file size range for each region.

Spatial Coverage

The approximate overall spatial coverage for each region is listed below. For a more detailed description, refer to Table 5.

Alabama:
Southernmost Latitude: 34.49° N
Northernmost Latitude: 35.59° N
Westernmost Longitude: 86.15° W
Easternmost Longitude: 85.02° W

Georgia:
Southernmost Latitude: 30.79° N
Northernmost Latitude: 31.91° N
Westernmost Longitude: 83.98° W
Easternmost Longitude: 82.69° W

Oklahoma:
Southernmost Latitude: 34.47° N
Northernmost Latitude: 35.61° N
Westernmost Longitude: 98.53° W
Easternmost Longitude: 97.31° W

Spatial Resolution

The ASAR instrument's radar beam viewing angle can alternate between 15 and 45 degrees resulting in swaths ranging from 56 km to 105 km wide. Table 6 lists the specifications of the different image swaths the ASAR instrument is able to collect.

All SMEX03 ASAR images have 12.5 m pixel spacing and approximately 30 m resolution. Thus, each pixel represents a 12.5 x 12.5 m area on the ground, and it is possible to discern individual objects which are approximately 30 m wide or larger.

Projection

The projection used for this data set was the Universal Transverse Mercator (UTM) with the World Geodetic System 1984 (WGS 84) datum applied.

Temporal Coverage

Data were collected between March and July 2003. Refer to Table 5 for precise temporal coverage and temporal resolution listed by region.

Temporal Resolution

Depending on the acquisition date, ASAR images over each study area were acquired once or twice during the pass of the ENVISAT satellite. Table 5 specifies whether the image was acquired during the ascending or descending pass, or both, over a given region.

Parameter or Variable

This data set consists of browse images and provides only a general quality assessment of the ASAR data used as part of the Soil Moisture Experiment 2003 (SMEX03). The measured parameter for the ASAR images was a relative radar returned signal strength, that is, the reflected radiance (measured by amplitude) for each pixel. From the measured amplitude values, the cross V-polarized (VV/VH) C-band radar backscatter coefficients were then derived. The images are a function of backscatter coefficients and are intended for browsing locations and general features. Thus, the images cannot be used to derive backscatter coefficient values; researchers interested in backscatter coefficient values must obtain the ASAR data.

Sample Data Records

With the exception of HAN files, figures 2 through 5 show sample data records for each file type included in this data set. NOTE: Figure 4 is a partial sample data record of al_0715.txt due to the size of the file; each ASCII text file contains an extensive annotation list for its corresponding image and it is therefore not shown in its entirety.

3. Data Access and Tools

Data Access

Data are available via FTP.

Volume

Total volume is approximately 65 MB for all files. Volume by region is listed in Table 7.

Software and Tools

Tools appropriate for viewing these data include any image viewing program that recognizes JPEG file format and any text editor or Web browser.

Related Data Collections

• AMSR-E Validation Data: Soil Moisture Data Web site: Data sets from SMEX campaigns available at NSIDC.
• AMSR-E/Aqua Data at NSIDC Web site: AMSR-E standard products available at NSIDC.
• RADARSAT Data Web site: RADARSAT products available at NSIDC.

4. Data Acquisition and Processing

Theory of Measurements

A Synthetic Aperture Radar (SAR) is an active microwave sensor that transmits a focused pulse of radar energy and receives reflected energy from the target (in this case, the Earth) in a form that can be converted into a high-resolution image. A SAR uses the sweep of a relatively small antenna through space to mimic the resolution attainable by a much larger stationary antenna. SAR images of a surface give a different view than visible/near infrared imagery due to the differences in interactions with surface materials in the microwave band. SAR images show the great variability in radar backscattering properties of a wet versus dry surface. In general, dry surfaces are very good radar absorbers, while wet surfaces are bright radar reflectors.

However, the interplay between a radar signal and the ground surface depends upon many factors, including vegetation cover, the density and dielectric properties of surface materials, surface roughness with regard to the signal's wavelength, the topography of a region, and the instrument's viewing angle and signal polarization. The resolution of a radar image is particularly affected by signal strength and bandwidth, chirp pulse length and the time between pulses, and the return signal integration time.

For more information, refer to the European Space Agency's (ESA) ASAR FAQ Web page.

Sensor or Instrument Description

The Advanced Synthetic Aperture Radar (ASAR) instrument onboard the European Environmental Satellite (ENVISAT) is the first dual-polarized radar instrument mounted on a satellite platform. The ASAR antenna operates at 5.33 GHz in the microwave C-band. In its alternating polarization mode, ASAR can acquire backscatter coefficients in one of the following three polarization combinations: HH/VV (co-polarized mode), HH/HV (cross H-polarized mode) and VV/VH (cross V-polarized mode). All radar images used for the SMEX03 regional study areas were acquired in the cross V-polarized mode, or VV/VH. The spatial resolution of the sensor is approximately 30 meters, although the pixel spacing is 12.5 meters.

Data Acquisition Methods

During the SMEX03 field campaign ASAR acquired one radar image over the Alabama study area, three images over Georgia, and six over the Oklahoma, USA study area. Table 5 lists these ASAR acquisitions with their spatial coverage and image swath.

Data Source

ENVISAT is a polar-orbiting, sun-synchronous advanced Earth observation satellite developed by the European Space Agency (ESA) to monitor environmental change. The ENVISAT platform flies at approximately 800 km altitude and has a repeat orbit cycle of 35 days. Its wide-swath instruments, such as ASAR, provide complete coverage of the globe within one to three days.

Derivation Techniques and Algorithms

All SMEX03 ASAR images were acquired in VV/VH mode for different image swaths. The ASAR images were then processed at various ESA stations to Level-1B products. Level-1B data consist of geocoded amplitude values. Geocoding is the transformation of the sensor-specific geometry of an image into a cartographic representation with well-defined scale and angular distortions. In this case, the geocoded format is defined by the Universal Transverse Mercator projection. Backscatter coefficients can be derived from the amplitude values using equation 2.27 from the ASAR Product Handbook, which is formulated as follows:

(Equation 1)

Where is the backscatter coefficient (in power), A² is the amplitude, k is the calibration coefficient from the product header, and α is the viewing angle.

Processing Steps

Images were acquired from the ASAR instrument then processed as Level-1B data at ESA stations. Headers were generated using BEST software. Since ENVISAT/ASAR data is not public domain and needs to be acquired by individual research groups, images were converted to JPEG image files for browsing purposes. Refer to ESA's Specific Instrument Acquisition Web page for information regarding ESA data acquisition.

5. References and Related Publications

For more information, related resources for this data set are provided in Table 8.

6. Document Information

Acronyms and Abbreviations

Table 9 lists the acronyms and abbreviations used in this document.

Document Creation Date

January 2009

Document URL

http://nsidc.org/data/docs/daac/nsidc0357_smex03_envisat_asar/index.html