SPACEBORNE IMAGING RADAR-C/
                 X-BAND SYNTHETIC APERTURE RADAR
                         (SIR-C/X-SAR)

PHOTO CAPTION                                          P-44718
                                               October 5, 1994
                                       Weddell Sea, Antarctica
                                                    L, C bands 	

This Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar 
color composite shows a portion of the Weddell Sea, which is 
adjacent to the continent of Antarctica. The image shows  
extensive coverage of first-year sea ice mixtures and patches of 
open water inside the ice margin. The image covers a 100 
kilometer by 30 kilometer (62 mile by 18.5 mile) region of the 
southern ocean, centered at approximately 57 degrees south 
latitude and 3 degrees east longitude, which was acquired on 
October 3, 1994.  Data used to create this image were obtained 
using the L-band (horizontally transmitted and vertically 
received) in red; the L-band (horizontally transmitted and 
received) in green; and the C-band (horizontally transmitted and 
received) in blue. The sea ice, which appears rust-brown in the 
image, is composed of loosely packed floes from approximately 1 
meter to 2 meters (3 feet to 6.5 feet) thick and ranging from 1 
meter to 20 meters (3 feet to 65.5 feet) in diameter. Large 
patches of open water, shown as turquoise blue, are scattered 
throughout the area, which is typical for ice margins 
experiencing off-ice winds.  The thin, well-organized lines 
clearly visible in the ice pack are caused by radar energy 
reflected by floes riding the crest of ocean swells.  The wispy, 
black features seen throughout the image represent areas where 
new ice is forming. Sea ice, because it acts as an insulator, 
reduces the loss of heat between the relatively warm ocean and 
cold atmosphere.  This interaction is an important component of 
the global climate system.   Because of the unique combination 
of winds, currents and temperatures found in this region, ice 
can extend many hundreds of kilometers north of Antarctica each 
winter, which classifies the Weddell Sea as one of nature's 
greatest ice-making engines.  During the formation of sea ice, 
great quantities of salt are expelled from the frozen water.  
The salt increases the density of the upper layer of sea water, 
which then sinks to great depths.  Oceanographers believe this 
process forms most of the oceans' deep water.  Sea ice covering 
all of the southern oceans, including the Weddell Sea, typically 
reaches its most northerly extent in about September. As periods 
of daylight become gradually longer in the Southern Hemisphere, 
ice formation stops and the ice edge retreats southward.  By 
February, most of the sea ice surrounding Antarctica disappears. 
Imaging radar is extremely useful for studying the polar regions 
because of the long periods of darkness and extensive cloud 
cover.  The multiple frequencies of the SIR-C/X-SAR instruments 
allow further study into ways of improving the separation of the 
various thickness ranges of sea ice, which are vital to 
understanding the heat balance in the ice, ocean and atmospheric 
system.
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Spaceborne Imaging Radar-C and X-band Synthetic Aperture Radar 
(SIR-C/X-SAR) is part of NASA's Mission to Planet Earth. The 
radars illuminate Earth with microwaves, allowing detailed 
observations at any time, regardless of weather or sunlight 
conditions. SIR-C/X-SAR uses three microwave wavelengths: L-band 
(24 cm), C-band (6 cm) and X-band (3 cm).  The multi-frequency 
data will be used by the international scientific community to 
better understand the global environment and how it is changing. 
The SIR-C/X-SAR data, complemented by aircraft and ground 
studies, will give scientists clearer insights into those 
environmental changes which are caused by nature and those 
changes which are induced by human activity.  SIR-C was 
developed by NASA's Jet Propulsion Laboratory. X-SAR was 
developed by the Dornier and Alenia Spazio companies for the 
German space agency, Deutsche Agentur fuer 
Raumfahrtangelegenheiten (DARA), and the Italian space agency, 
Agenzia Spaziale Italiana (ASI), with the Deutsche 
Forschungsanstalt fuer Luft und Raumfahrt e.V.(DLR), the major 
partner in science, operations and data processing of X-SAR.

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