Sigma-0 Browse Product FAQ

Why a sigma-0 browse product?

The sigma-0 browse products are intended to help users to identify features of interest directly in the Qscat sigma-0 measurements by providing a spatial and temporal (over one day) average view of the Qscat sigma-0 measurements. The resulting iamges can be used to identify areas of potential interst over land, ice, and ocean.

As a cautionary note, experience suggests that the sigma-0 images contain many features which can be difficult to explain and can be misleading, in part because forward and aft-looking measurements are combined. Comparison with the corresponding wind maps is strongly recommended.

What are the available products?

Sigma-o browse products are produced on three grid productions: 1) a global rectangular lat/lon grid, 2) a southern hemisphere polar stereographic projection, and 3) a northern hemisphere polar stereographic projection.

The global projection product is best suited for equatorial and mid-latitude views, while the polar stereographic projections are best suited for high-latitude views. While the global browse product includes the entire globe, the grid distortion in the global projection results in very poor sampling in the polar regions; hence the production of separate polar products. The polar stereographic views include only measurements above a high latitude cutoff of 52 deg.

One set of image products is produced per mission Julian day. For each of the three projections, three separate image products are produced: the mean sigma-0 value in each pixel, the normalized standard deviation of the measurement in each pixel, and the count of the number measurements used in each pixel. Each image browse product is stored in a separate file.

What is the resolution of the images?

The grid resolution for the global projection is 5 pixels/deg, or about 22.5 km/pixel at the equator. The polar stereographic projections use a 70 deg reference latitude with a nominal grid size of about 22.5 km.

How are the browse products produced?

The center of each L1B egg sigma-0 measurement is located and averaged into the grid element which contains the center of the measurement. Thus, the effective resolution of the image products is less than the pixel resolution, or approximately 55-60 km at the equator. The resulting images are temporal averages over a one day period of all the sigma-0 measurements whose centers fall within each image pixel area.

Computation of the average sigma-0 value is done in normal space (not in dB) with negative values included. If the resulting average is negative, the sign of the corresponding count image pixel is set to negative. The count images are also useful for creating multi-day averages and as a quality control.

Note that due to the rotation of the antenna, the cross-track density of sigma-0 measurements varies with more measurements at the outer edges of the swath than at the swath center. As a result, swath edges are clearly visible in the count images. Note that some areas may not be covered in a single day, resulting in diamond shaped regions of no coverage in mid-latitude and equatorial averages.

What data is used to make the sigma-0 browse products?

The sigma-0 browse products are produced from L1B data. Only measurements flagged as `usable' are included in the browse products.

What about time/azimuth variations?

All \sigmao measurements (from a single beam) falling within a single pixel are averaged and thus forward-looking and aft-looking measurements are averaged. The resulting average is over the various azimuth angles of the measurements. The azimuth angle sampling varies with pixel location and the Julian day and may be affected by missing or low-quality data. Swath edge discontinuities may result in areas of significant azimuth modulation of sigma-0 at surface. The normalized standard deviation (K_p) images can be used to evaluate temporal and azimuth variation in the sigma-0 measurements.

What is the file format used for the products?

The browse image products are stored in the BYU MERS SIR file format in which the image is stored as a scientific (real valued) image that includes both location and transformation information in a header. Viewer and reader programs for the BYU MERS SIR file format are available on line from the BYU MERS web site and MERS ftp site.

A SIR format file consists of one or more 512 byte headers followed by the image data and additional zero padding to insure that the file is a multiple of 512 bytes long. The file header record contains all of the information required to read the remainder of the file and the map projection information required to map pixels to lat/long on the Earth surface. The image pixel values may be stored in one of three ways. The primary way is as 2 byte integers (with the high order byte first), though the pixels may be stored as single bytes or IEEE floating point values. Scale factors are stored in the header to convert the integer or byte pixel values to native floating point units.

The sir file header contains other numerical values and strings which describe the image contents. For example, a no-data flag value is set in the header as well as a nominal display range and the minimum and maximum representable value.

The image is stored in row-scanned (left to right) order from the lower left corner (the origin of the image) up through the upper right corner. By default, the location of a pixel is identified with its lower-left corner. The origin of pixel (1,1) is the lower left corner of the image. The array index $n$ of the $(i,j)^{th}$ pixel where $i$ is horizontal and $j$ is vertical is given by $n=(j-1)*N_x+i$ where $N_x$ is the horizontal dimension of the image.

What is the product naming scheme?

A standardized naming schem is used for the brose products. The file name is prefaced by "QS_Xb". A three character string follows which denotes the image type and location. This is follwed by "S3C" and a four digiti year and 3 digit Julian data of the data contained in the file. The file extension is the year, day, and time of the file production. For example: QS_XbvaGS3C1999240.1999145010233

The three character image type/location scheme is:

• First character: the beam polarization code, "h"=inner beam, "v"=outer beam.

• Second character: the image type code, "a"=mean sigma-0, "C"=count of measurements, "K"=normalized standard deviation.

• Third character: the region/projection code, "G"=global, "N"=northern hemisphere, "S"=southern hemisphere.

Where is the full documentation?

Documentation is available in either postscript (965K) or pdf (208K) form. Further information is available on line from the BYU MERS web site

Send suggestions or comments to mers-info@ee.byu.edu