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There is no major change in delineating the Chott Merouane (B-C, 3) 
according to Seasat or SIR-A data, but the floor of the Chott Melrhir (G-J, 
1-3) appears as a dark surface on Seasat data and as a bright surface on the 
SIR-A picture. The main reason could be due to flooding conditions, if any, 
existing in that region, but water records for this area are not yet 
available. It also could be due to the scattering mechanisms which give 
different returns at 50° incidence angle (SIR-A) and 20° incidence angle 
(Seasat). 
Railroads and roads north of the town Mrhaier are very well enhanced 
on the Seasat radar picture (A-B, 1-2) because they are almost perpendicular 
to the illumination direction. Those bright lines do not appear on the SIR-A 
data because they are parallel to the SIR-A illumination direction, meanwhile 
two other lines (F-G, 2) are detected in the SIR-A data and not in the Seasat 
radar picture. No explanation has been found for those two lines detected by 
SIR-A. 
The sub-areas 2a, 2b and 2c show respectively: (a) the southern 
boundary of the Chott Melrhir (F-G, 4), (b) the village of Mrhaier (B, 3), and 
(c) the northern boundary of the Chott Merouane (C-D, 2). Each of those 
subareas is displayed at full resolution and emphasize the differences between 
the two radar systems. For instance, three parameters were not the same. 
They are (1) the resolution, (2) the illumination direction and (3) the look 
angle (or depresion angle). In order to display the pictures at the same 
scale (full resolution) the SIR-A data were expanded twice as much as Seasat 
radar pictures, this also amplified the speckle noise which is evident in the 
comparison of SIR-A to Seasat (Fig. 2a). The influence of the illumination 
direction is clearly seen in Fig. 2b covering the town of Mrhaier, where 
street patterns are not detected in the same way. The difference between the 
two incidence angles (Seasat: 20° and SIR-A: 50°) may explain the main 
difference in the aspect of the Melrhir floor in the two pictures. The other 
possibility would be changes in the surface conditions, such as flooding. 
A color image of the registered data was made by displaying Seasat 
data, SIR-A data and the ratio (Seasat/SIR-A) in red, green and blue, 
respectively (Fig. 3). Filtering, to remove very low spatial frequencies, was 
applied to reduce large scale shading gradients. Brown-red areas correspond 
to sand outcrops; villages are displayed in yellow, blue and yellow colors 
mark the floor of the Chotts (those color differences may correspond to 
different extensions of*salt and mud coverages). 
SUMMARY 
A Seasat/SIR-A digital registration over Northeastern Algeria was 
undertaken. It shows characteristics of Shuttle Imaging Radar data compared 
to those obtained by Seasat Synthetic Aperture Radar. 
ACKNOWLEDGMENTS 
Thanks are expressed to Joe Fulton for the scanning of the SIR-A 
data, Sue Conrow for preparing many figures, and Tom Farr for kindly reviewing 
the manuscript. 
This paper is the result of research carried out at Jet Propulsion 
Laboratory, California Institute of Technology, under Contract NAS7-100, 
during the tenure of a postdoctoral fellowship awarded by the National 
Research Council. 
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