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Some frequently asked questions:

What products are available?
How do scatterometers work?
Why scatterometers for climate studies?
Data from what instruments are available from this site?
What is the BYU .SIR file format?
How do I read and display a BYU .SIR file?
What is the "Scatterometer Image Reconstruction (SIR)" algorithm?
What is the difference between SIR and AVE?
How can I get information on the specific projection used for a BYU SIR file?
How to I convert BYU SIR images to GeoTIFF format?
How can I display a BYU SIR image in ENVI?
How can I semi automate my FTP downloads?
What tools are available to read a BYU SIR file into ArcGIS Raster format?
What tools are available for windows to read a BYU SIR file?


What products are available?

The Scatterometer Climate Record Pathfinder project develops and distributes innovative scatterometer and radiometer products designed to support climate studies. The core products are enhanced resolution radar backscatter (sigma-0) and brightness temperature (Tb) products. A brief list of available products is available.

How do scatterometers work?

Radar scatterometers are satellite instruments designed to measure winds over the ocean from space. They can also be used for land and ice studies. A scatterometer transmits pulses of microwave energy and measures the returned echo. The energy in the echo depends on the electrical properties and roughness of the surface. Over the ocean the roughness is a function of the near-surface vector wind which generates ocean waves. With measurements made from multiple azimuth directions, the wind can be estimated from the radar measurements. Over land, the return echo is a function of the land cover.

Why scatterometers for climate studies?

Radar scatterometers make frequent, very precise measurements of the globe. A variety of studies have shown that scatterometer measurements are very sensitive to key climate variables such as snowfall and melting in the polar regions and vegetation cover in the tropics, among others. A long time series of scatterometer data (dating intermittently back to 1978) has been collected. This long time series can thus be used to support climate change studies.

Data from what scatterometer instruments are available from this site?

Processed data from the Seasat Scatterometer (SASS), the NASA Scatterometer (NSCAT), the ERS-1 and ERS-2 AMI (scatterometer mode), and the SeaWinds on QuikSCAT and SeaWinds on ADEOS-II scatterometer data is available on this site. While the site emphasizes enhanced resolution image data, other data products are available.

What is the BYU .SIR file format?

The BYU .SIR image file format was developed by the Brigham Young University (BYU) Microwave Earth Remote Sensing (MERS) laboratory to store images of the earth along with the information required to earth-locate the image pixels. A "SIR" file consists of one or more 512-byte headers containing all the information required to read the remainder of the file, including projection information to map pixels to lat/lon on the Earth's surface. Image pixel values are generally stored as 2 byte (high order byte first) integers. Scale factors to convert the integer or byte pixel values to native floating point units are stored in the file header. The origin of the images is in the lower left corner of the displayed image. The earth location of a pixel is identified with its lower-left corner. The standard sir format supports a variety of image projections including: Rectangular array (no projection); A rectangular lat/lon array; Lambert equal-area; Polar stereographic; and EASE with various resolutions.

How do I read and display a BYU .SIR file?

Browse (small, reduced resolution) image files are available from the SCP web and ftp sites in GIF format for selected files; however, the best way to view a SIR file is download it and use a local program to display and analyze it. The image products are stored in the BYU MERS SIR file format in which the image is stored as a scientific (real valued) image file that includes both location and transformation information in a header. (Separate files containing the lat/lon locations of each pixel are also available.) Viewer and reader programs for the BYU MERS SIR file format are available on line from the BYU MERS web site and ftp site and the NASA Scatterometer Climate Record Pathfinder web site and ftp site.

Some options include:

  • Using matlab, IDL, PV-WAVE, ENVI, or other high level language. Reader and viewer utility routines are available for many of these.

  • Using existing ustom viewer utility programs. These include modified xv program known as xv_BYU and the sirtool program. These must downloaded and compiled on the local machine, though executables for some platforms are available.

  • Generate your own custom code. Read/write routines in c, c++, fortran, matlab, IDL, PV-WAVE are available.

  • Convert the SIR file to a plain ASCII format and reading into another program (code and sample programs are available in c, c++, and fortran. These must be downloaded and compiled on the local machine)

  • Some image processing programs (e.g., Adobe Photoshop) can be used to display file contents, though the scaling and header information are lost.

See the detailed software documentation available on the web and ftp sites under "software".

What is the "Scatterometer Image Reconstruction (SIR)" algorithm?

The Scatterometer Image Reconstruction (SIR) algorithm is a method for reconstructing images from raw scatterometer or radiometer data. The SIR algorithm is based on a multivariate form of block multiplicative algebraic recontruction. It generates enhanced resolution gridded images from irregularlly spaced measurements which may have a variable aperture function. It provides high resolution images from lower resolution images, assuming sufficiently dense sampling. Frequently mulitple satellite passes are combined to provided to ensure sufficient sampling. The theory behind the iterative SIR algorithm is described in D.S. Early and D.G. Long, "Image Reconstruction and Enhanced Resolution Imaging from Irregular Samples," IEEE Transactions on Geoscience and Remote Sensing, Vol. 39, No. 2, pp. 291-302, 2001. See also enhanced resolution imaging.

What is the difference between the SIR and AVE algorithms?

The Scatterometer Image Reconstruction (SIR) algorithm is an iterative recontruction algorithm at creates high resolution images from irregullarly spaced measurements. The AVE algorithm is the first iteration of SIR and is computed by defining a high resolution pixel grid. A value for each pixel is created by weighted averaging all of the measurements that have a non-zero spatial response function for the pixel. The weighting function is the normalized value of each measurement's spatial response function for the particular pixel, see D.G. Long, P. Hardin, and P. Whiting, "Resolution Enhancement of Spaceborne Scatterometer Data," IEEE Transactions Geoscience Remote Sensing, vol. 31, pp. 700-715, 1993 and D.S. Early and D.G. Long, "Image Reconstruction and Enhanced Resolution Imaging from Irregular Samples," IEEE Transactions on Geoscience and Remote Sensing, Vol. 39, No. 2, pp. 291-302, 2001. The Scatterometer Image Reconstruction (SIR) algorithm is an iterative recontruction algorithm at creates high resolution images from irregullarly spaced measurements. The AVE algorithm is the first iteration of SIR and is computed by defining a high resolution pixel grid. A value for each pixel is created by weighted averaging all of the measurements that have a non-zero spatial response function for the pixel. The weighting function is the normalized value of each measurement's spatial response function for the particular pixel, see D.G. Long, P. Hardin, and P. Whiting, "Resolution Enhancement of Spaceborne Scatterometer Data," IEEE Transactions Geoscience Remote Sensing, vol. 31, pp. 700-715, 1993 and D.S. Early and D.G. Long, "Image Reconstruction and Enhanced Resolution Imaging from Irregular Samples," IEEE Transactions on Geoscience and Remote Sensing, Vol. 39, No. 2, pp. 291-302, 2001.

How can I get information on the specific projection used for a BYU SIR file?

The BYU .SIR file format includes all of the information needed to geolocate individual pixels. Software in various computer languages is available on the ftp/web site to read the file header and compute the forward and reverse map projections. In addition, the interactive C program sir_util can display individual pixel values and locations, as well as dump each pixel location to a text file. sir_util can print detailed map projection information and provide strings to use with standard proj4 and gdal_translate utilities. (Note: sir_util.exe is available as a precompiled windows executable. Source code [in tools] can be compiled for other platforms.).

How do I convert BYU .SIR image files to GeoTIFF format?

The BYU .SIR file format includes all of the information needed to geolocate individual pixels. To convert these images to GeoTIFF format can be bit tricky due to the complexities of the GeoTIFF format. Some simple utilities have been written to simplify the conversion process. These are described in the page Conversion of BYU .SIR files to GeoTIFF format. The latest version of the sir_util2 utility includes geotiff conversion capability.

How can I display a BYU .SIR image file in ENVI?

There are several ways to display BYU. SIR files in ENVI. One is to convert the image to a GEOTIFF first. Second, the IDL file readers can be used. Alternatnately for just displying the image, follow theses steps after starting ENVI: you have to know the file size (X by Y) in pixels. This can be determined using the IDL reader code.

  • From the File menu select, "Open image file", and select the file to read after gunzipping it.

  • A pop-up widow appears. Enter the X file size in "samples".

  • Enter Y file size in "lines".

  • Enter 1 in the "bands" and 512 in the "offset"

  • Select "Unknown" for "File type" and "Network (IEEE)" for the "Byte Order".

  • Select "Integer" for the "Data Type" (leave "BSQ" for "Interleave").

  • Hit OK.

  • Select the "Gray Scale" radio button and press "Load Band".

Note: the file will be displayed flipped vertically since ENVI defines the image origin (pixel 1,1) at the upper-left rather than lower-left..

How can I semi automate my FTP downloads?

FTP download automation can be easily semi automated depending upon how comfortable you are with scripting. Create a simple text file, ftp.script, which contains the ftp paths of the files you wish to download. example:

anonymous

get data/qscat/1999/sir/qush/Arc/202/a/qush-a-Arc99-202-202.sir.gz
get data/qscat/1999/sir/qush/Arc/203/a/qush-a-Arc99-203-203.sir.gz
get data/qscat/1999/sir/qush/Arc/204/a/qush-a-Arc99-204-204.sir.gz

For *nix systems pipe this file to your ftp command
ftp.scp.byu.edu < ftp.script
and type in the password at the prompt.

For Microsoft Windows XP and similiar systems bring up a command line.
1. Click the Start Menu.
2. Select Run.
3. Type "cmd", and click run.
4. In the black command line window that appears type:
ftp -i -s:C:\ftp.script ftp.scp.byu.edu
4a. "C:\ftp.script" needs to be replaced with the path of your script.
5. Type in the password at the prompt.

This simple semi automation can be further modified as the directory structure of scp.byu.edu is very predictable and consistent.

What tools are available to read a BYU SIR file into ArcGIS Raster format?

The BYU .SIR file format includes all of the information needed to geolocate individual pixels. The Marine Geospatial Ecology Tools (MGET) project (also known as GeoECho Python) has developed an open source geoprocessing tool box that enables conversion of BYU SIR files into ArcGIS raster format files.

What tools are available for Windows to read a BYU SIR file?

In reponse to requests, a simple program that reads BYU SIR files is available from the SCP web/ftp site. This pre-compiled program, sir_util.exe, can print individual pixel and location values or dump the contents of the file to a text file. Geometric transformations are supported as is conversion of a SIR file to a BMP or GIF image file. Source code for the program is included in the regular c code reader distribution in the tools subdirectory. Source code for the latest version (sir_util2.c) is now available as is a windows executable sir_util2.exe. The new version includes color tables and conversion options to TIFF and GeoTIFF files. Note: The binary executable on the web site is compiled for Windows XP. It should run in Windows 7. After downloading, right click on the code and click the "Properties" tab. On the "Properties" sheet, click on the "Compatibility" tab, then click on the box "Run this program in compatibility mode for:" and select "Windows XP SP3" from the dropdown list.

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