Raw data processing of the Africa data was performed by the NASDA Earth Observation Center. Post-processing (generation of low resolution frames, additional radiometric correction) and image mosaicking were performed by the Space Applications Institute/Global Vegetation Monitoring Unit of the JRC.
Over 1400 full resolution JERS-1 SAR images were used to produce the mosaic tiles contained on this CD-ROM. There are 15 mosaic tiles, each tile typically 5 deg x 5 deg in latitude and longitude. This CD-ROM contains both the binary data (pixel spacing = 100 meters ) and lower resolution (pixel spacing = 500 meters) GIF images that are linked from a clickable image map or from a table. These mosaic tiles should exactly fit together.
The images on this CD-ROM are actually mosaics of 100 or so JERS-1 images.
The 8 bit binary image files are raw byte files where each pixel represents the brightness of the radar reflections. The pixel spacing of these images is 100 meters, and was obtained by block averaging of full resolution 12.5 pixel spacing data. The dimensions of these files vary with mosaic tile. Each 8 bit pixel can be converted to sigma0 by the formula given below.Typically, these binary image files are around 40 megabytes in size.
These binary image files are located in the binaries subdirectory within the top level data folder of this CD-ROM. These files have the name ###.dat, where ### is the three digit tile number. To find out the tile number, please view either the clickable image map or the table.
The GIF images of each mosaic tile are linked from these pages, either via a clickable image map, or via a table . These images have a pixel spacing of 500 m (exactly 5 times the pixel spacing of the corresponding binary file), and are typically about 1 Mbyte in size.
The mosaic projection used is Mercator, which projects the Earth's surface onto a cylinder which tangents the Equator (to be distinguished from the Universal Transverse Mercator - UTM - where the cylinder tangents the Earth along a meridian). Mercator yields small deformations in areas relatively close to the Equator. For a conformal projection, the pixel size is always equal in the x and y directions, and it a function of the latitude. For a spherical Earth, the pixel size can be calculated as:
Pixel size = 100*cos(phi) [metres]
where phi is the geodetic latitude.Hence, at 10 degrees latitude, the pixel size becomes 98.48 metres (or 98.49 m using e.g. the Clarke 1866 ellipsoid).
The upper left and lower right hand corner coordinates are are listed in the table.
sigma0 = 20 log10(DN*7+250)+F
where DN is the DN value of each pixel (between 0 and 255), and F is the calibration factor. For the Central Africa data on this CD-ROM, the calibration factor F = -68.2 . The result will be in dB.
Unfortunately, Murphy's law (what can go wrong, will go wrong) dictates that the area you wish to study is at the intersection of two or more mosaic tiles. Therefore, we include on this CD-ROM both a 500 meter (27 Mbytes) and a 2 km (1.5 Mbytes) pixel spacing complete mosaic. However, if you are interested in mosaicking the 100 meter binary images, you will need software (e.g. Adobe Photoshop) that allows you to manipulate images.For the 100 meter mosaics, adjacent tiles fit exactly along side one another. If the mosaic tiles don't have the same width where they are adjacent, or if they are offset from each other, it may be necessary to "slide" one image along until they match.