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SMEX04 ENVISAT ASAR Data: Arizona

Summary

This data set is comprised of browse images acquired over the regional study area of Arizona, USA for the 2004 Soil Moisture Experiment (SMEX04). The experiment was conducted using 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 for 09 September 2003 and 14 October 2003 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 data set is approximately 8.3 megabytes. Data are available via FTP and are provided as JPEG image files. Also included and available via FTP are the following types of header files: HAN files, ASCII text files, and one INI file.

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

We kindly request that you cite the use of this data set in a publication using the following citation example. For more information, see our Use and Copyright Web page.

Jackson, T., R. Bindlish, and R. Van der Velde. 2009. SMEX04 ENVISAT ASAR Data: Arizona. [indicate subset used]. Boulder, Colorado USA: NASA DAAC at the National Snow and Ice Data Center.

Overview Table

Category Description
Data format JPEG image files

Header Files:
ASCII text files

HAN files
INI file
Spatial coverage and resolution Southernmost Latitude: 31.41° N
Northernmost Latitude: 32.57° N
Westernmost Longitude: 110.31° W
Easternmost Longitude: 109.01° W

Spatial resolution: 30 m
Pixel spacing: 12.5 m
Temporal coverage and resolution 09 September 2003
14 October 2003
Tools for accessing data JPEG image files: any image viewing program
ASCII text files: any text editor or Web browser
HAN files: N/A [Internal Basic ENVISAT SAR Toolbox (BEST) files]
INI file: any text editor or Web browser
Grid/projection description UTM, WGS84
File naming convention rg_ASAR_mmdd_IS#_P.jpg
rg_mmdd_P.txt
rg_mmdd_P.HAN
rg_parameter.ini
File size ~1 KB - 3 GB
Parameter(s) Cross V-polarized (VV/VH) C-band radar backscatter browse images
Procedures for obtaining data Data are available via FTP.

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

NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449  USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
e-mail: nsidc@nsidc.org


2. Detailed Data Description

Format

Browse images for each regional study area are provided as JPEG image files. The following types of header files are also provided:

One ASCII text file and one HAN file accompanies each JPEG image file in this data set, and a single INI file is included for the data set. 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.

File and Directory Structure

Data are available on the FTP site in the /soil_moisture/SMEX04/Arizona/satellite/ASAR/ directory. The ASAR directory also contains the 00README.txt file, one INI file, and two sets each of JPEG, ASCII text, and HAN files.

File Naming Convention

JPEG and JGW Files

The JPEG image files are named according to the following conventions and as described in Table 1:

az_ASAR_mmdd_IS#_D.jpg

Where:

Table 1. Description of File Name Variables
Variable Description
az Arizona
ASAR Advanced Synthetic Aperture Radar (ASAR)
mm Two-digit month
dd Two-digit day
IS# Image Swath #: 1, 2, 3, 4, 5, 6, 7
D Descending ENVISAT Pass
.jpg Indicates that this is a JPEG file

Example:

az_ASAR_0909_IS2_D.jpg

 

ASCII Text and HAN Files

ASCII text and Header Analysis (HAN) files are named according to the following convention and as described in Table 2:

az_mmdd_D.txt
az_mmdd_D.HAN

Where:

Table 2. Description of File Name Variables
Variable Description
az Arizona
mm Two-digit month
dd Two-digit day
D Descending ENVISAT Pass
.txt Indicates that this is a text file
.HAN Indicates that this is a HAN file

Examples:

az_0909_D.txt
az_0909_D.HAN


INI File

Also included for each regional study area is an INI file named az_parameters.ini that contains the parameter information used to run BEST software when processing JPEG images for the region.

File Size

The files in this data set, including JPEG image files, HAN files, ASCII text files, and the INI file, range in size from approximately 1 kilobyte to 3 gigabyte.

Spatial Coverage

The approximate overall spatial coverage for each region is listed below. For a more detailed description, refer to Table 3.

Southernmost Latitude: 31.41° N
Northernmost Latitude: 32.57° N
Westernmost Longitude: 110.31° W
Easternmost Longitude: 109.01° W

 


Table 3.
Specifications of SMEX04 Arizona ASAR Images
Study Area Date
(mm-dd-yyyy)
ENVISAT Pass Image Swath Spatial Coverage Coordinates:
Upper Left Corner
Spatial Coverage Coordinates:
Lower Right Corner
Latitude Longitude Latitude Longitude
Arizona 09-09-2003 Descending IS2
32.581 -110.310 31.412 -109.014
Arizona 14-10-2003 Descending IS2 32.567 -110.312 31.413 -109.019

 

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 4 lists the specifications of the different image swaths the ASAR instrument is able to collect.

Table 4. ASAR Image Swath Specifications
ASAR Image Swath Swath Width
(km)
Incidence Angle Range
(degrees)
IS1 105 15.0 - 22.9
IS2 105 19.2 - 26.7
IS3 82 26.0 - 31.4
IS4 88 31.0 - 36.4
IS5 64 35.8 - 39.4
IS6 70 39.1 - 42.8
IS7 56 42.5 - 45.2

Both SMEX04 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 on 09 September 2003 and 14 October 2003.

Temporal Resolution

The ASAR images were acquired once during the pass of the ENVISAT satellite.

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 2004 (SMEX04). 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 1 through 3 show sample data records for each file type included in this data set. NOTE: Figure 2 is a partial sample data record of az_0909_D.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.


Fig 2. ASAR Sample Image
Figure 1. Sample ASAR Browse Image
Sample of the az_ASAR_0909_IS2_D.jpg file, a false-color C-band image.

 

======================================================================================
BEST - ESA / Telespazio - ANNOTATION LIST
======================================================================================
Processing time............: 21-Jan-2004 09:18:40.000
Product type...............: gec
Sensor Mode................: Alternating Polarization
Desired AP dataset.........: 1
Source.....................: ASAR
Data format................: MPH-SPH
Facility id................: esp
Format descriptor record...: C:\BEST\\cfg\gec3eespap
--------------------------------------------------------------------------------------
File name..................: PDF - PRODUCT_DATA_FILE
Record name................: Main Product Header Record
Pos Esa field name Value Units Tag
--------------------------------------------------------------------------------------
1 dummy PRODUCT=" - -
--------------------------------------------------------------------------------------
2 Product Tag ASA_APG_1PNUPA20030909_17 - product_name
2742_000000162019_00413_0
7982_0084.N1
--------------------------------------------------------------------------------------
3 Product ID ASA_APG_1P - envisat_prod_id
--------------------------------------------------------------------------------------

Figure 2. Partial Sample Data Record of az_0909_D.txt.

 

[HEADER ANALYSIS]
Input Media Path = "d:\ASA_APG_1PNUPA20030909_172742_000000162019_00413_07982_0084.N1"
Input Media Type = "CDROM"
Sensor Id = "ASAR"
Sensor Mode = "Alternating Polarization"
AP Dataset = 1
Product Type = "GEC"
Data Format = "ENVISAT"
Source Id = "esp"
Number Of Volumes = 1
Output Dir = "C:\ASAR\AZ\"
Annotation File = "AZ_0909"
Header Analysis File = "AZ_0909"
Acknowledge Mount = 'Y'

Figure 3. Full Sample Data Record of az_parameters.ini.

 

3. Data Access and Tools

Data Access

Data are available via FTP.

Volume

The total volume of this data set is approximately 8.3 megabytes.

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

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 SMEX04 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

For the SMEX04 field campaign, ASAR acquired two radar images over the Arizona, USA study area. Table 3 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

The SMEX04 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. ASAR Backscatter Coefficient Equation
(Equation 1)

Where sigma 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 and 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 5.

Table 5. Related Documents
Document Description URL
USDA SMEX04 Web Site More details on the SMEX04 field campaign http://hydrolab.arsusda.gov/smex04/
European Space Agency (ESA) ENVISAT Web Site Additional information regarding the ENVISAT platform http://envisat.esa.int/
ASAR Product Handbook The ASAR Product Handbook is an online resource available at the ESA Earthnet Online Web site. The handbook includes the ASAR Products User Guide, information regarding data formats and algorithms, details on the ASAR Instrument, Frequently Asked Questions (FAQs), a glossary, and additional resources. http://envisat.esa.int/dataproducts/asar/CNTR.htm

6. Document Information

Acronyms and Abbreviations

Table 6 lists the acronyms and abbreviations used in this document.

Table 6. Acronyms and Abbreviations
AMSR-E Advanced Microwave Scanning Radiometer - Earth Observing System
ARS Agricultural Research Service
ASAR Advanced Synthetic Aperture Radar
ASCII American Standard Code for Information Interchange
AZ Arizona
BEST Basic ENVISAT SAR Toolbox
DAAC Distributed Active Archive Center
ENVISAT Environmental Satellite
ESA European Space Agency
FAQ Frequently Asked Questions
FTP File Transfer Protocol
HAN BEST Header Analysis file (.HAN extension)
INI Initialization file (.ini extension)
JPEG Joint Photograph Experts Group file (.jpg extension)
N/A Not Applicable
NASA National Aeronautics and Space Administration
NSIDC National Snow and Ice Data Center
SAR Synthetic Aperture Radar
SCAN Soil Climate Analysis Network
SMEX04 Soil Moisture Experiment 2004
UTM Universal Transverse Mercator
URL Uniform Resource Locator

Document Creation Date

August 2009

Document URL

http://nsidc.org/data/docs/daac/nsidc0417_smex04_envisat_asar/index.html