PC Media 4/PC MEDIA CD04.iso/nasa/sc-safsa.txt

Text File | 1995-02-23 | 5KB | 82 lines

PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA 91109.  TELEPHONE [phone]

PHOTO CAPTION                                             P-43920
                                                   April 16, 1994
                                                      Safsaf, L&C

This is a false-color image of the uninhabited Safsaf Oasis in
southern Egypt near the Egypt/Sudan border.  It was produced from
data obtained from the L-band and C-band radars that are part of
the Spaceborne Imaging Radar C/X-Band Synthetic Aperture Radar
(SIR-C/X-SAR) onboard space shuttle Endeavour on April 9, 1994.
The image is centered at 22 degree north latitude, 29 degrees
east longitude.  It shows detailed structures of bedrock; the
dark blue sinuous lines are braided channels that occupy part of
an old broad river valley.  On the ground and in optical
photographs, this big valley and the channels in it are invisible
because they are entirely covered by windblown sand.  Some of
these same channels were observed in SIR-A images in 1981.  It is
hypothesized that the large valley was carved by one of several
ancient predecessor rivers that crossed this part of North
Africa, flowing westward, tens of millions of years before the
Nile River existed.  The Nile flows north about 300 kilometers
(200 miles) to the east.  The small channels are younger, and
probably formed during relatively wet climatic periods within the
past few hundred thousand years.  This image shows that the
channels are in a river valley located in an area where U.S.
Geological Survey geologists and archeologists discovered an
unsual concentration of handaxes (stone tools) used by Early Man
(Homo erectus) hundreds of thousands of years ago.  The image
clearly shows that in wetter times, the valley would have
supported game animals and vegetation.  Today, as a result of
climate change,the area in uninhabited and lacks water except for
a few scattered oases.  This color composite image was produced
from C-band and L-band horizontal polarization images.  The C-
band image was assigned red, the L-band (HH) polarization image
is shown in green, and the ratio of these two images (LHH/CHH)
appears in blue.  The primary and composite colors on the image
indicate the degree to which the C-band, H-band, their ratio --
or some combination of all three -- respond to the roughness of
the radar backscattering surface.  Using this coloring scheme,
areas that appear bright at both L-band and C-band are colored
yellow, while areas that appear brighter at L-band than C-band
appear more blue.  Detailed analysis of this scene indicates that
the separate C-band and L-band images used to produce this color
composite have a very similar overall appearance.  This suggests
that the C- band and the L-band signals are both easily
penetrating the thin 1- to 12-centimeter (0.5- to 5-inch)
"average" surface cover of loose windblown sand, and are commonly
"seeing" similar interfaces just below that cover.  This radar
interface may be at the scattered rocky outcrops on the ground
surface, but is more likely to be the shallow underlying surfaces
of river gravel or bedrock, which are generally covered by only a
few inches of windblown sand.  Virtually everything visible on
this radar composite image cannot be seen, either when standing
on the ground or when viewing photographs or satellite images
such as the United States' Landsat or the French SPOT satellite.

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Spaceborne Imaging Radar-C and X-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).

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