layover is strong on the Seasat image and it is reduced on the SIR-A image 
  
d : which is more usefull for mapping in a mountaineous region. 
= 
=~ 2) Volcanic subarea: This area is located in the southern part of the 
Ee mosaic and was surveyed by SIR-A (Fig.4b) and two Seasat orbits (Fig.4a and 
e c). 
iio 
3 
This area is characterized by two high volcanoes (lpala and Suchitan), and 
Two calderas (Retana and Ayarza).  Seasat rev 759 covers the left part of the 
SIR-A image displayed in Fig.4 and Seasat rev 1211 overlaps the right part of 
the SIR-A data. The only common features to each image are the Suchitan 
Volcano and the Retana caldera. 
On the two Seasat radar images the volcanoes appear highly distorted due to 
layover and are tilted toward the radar antenna (Fig.4a and c). With a dual 
coverage, the backslope in one illumination direction corresponds to the 
foreslope in the opposite illumination direction. For the reason previously 
explained, the SIR-A image lacks such a distortion and presents less data 
loss. 
  
  
The town of Monjas is the bright spot surrounded by specular reflection in 
the center of the SIR-A image. The black reflection corresponds to quaternary 
alluviums. The town and the alluviums are more apparent on the SIR-A image 
(fig.4b) than on the Seasat image. Seasat SAR emphasizes relief variations 
and SIR-A data emphasize lithologic variations. A sharp fault running N150 
is only seen on the Seasat rev 1211, because it is oriented perpendicular to 
the radar illumination; on the SIR-A image it is not visible because it is 
parallel to the illumination direction. This illustrate the utility and need 
of multi-look angles and mul ti-illumination directions missions. 
ACKNOWLEDGEMENT 
  
Thanks are expressed to Sue Conrow for drawing many of the figures. 
  
  
This work 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. 
REFERENCES 
Elachi C., Shuttle Imaging Radar-A (SIR-A) experiment, preliminary report, 
JPL, 1982. 
Molnar P., Sykes L.R., Tectonics of the Caribbean and middle America regions 
from focal mechanisms and seismicity. GSA Bull., v80, pp 1639-1684. 
Muehl berger W.R., Richtie A.W., Caribbean-Americas Plate boundary in Guatemala 
and southern Mexico as seen on Skylab IV orbital photography. Geology, v3, pp 
232-235, 
  
Weyl R., Geology of Central America, 2nd ed, Beitrage zur regionalen Geologie 
Est 100 der Erde, Gebruder Burntraeger ed, Stuttgart, 1980. 
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