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Countdown | Overview | Data Products
| EarthKAM | Benefits |Antenna | Mast | Objectives
From the Maldeives Islands to Flinders Australia, or How to Map the World in Nine Days Six Hours and 23 Minutes
By: Philip Chien
February 22 - On a globe the Maldive Islands in the Indian Ocean and Flinders Island off
the south coast of Australia are just 8,254 km. (5,159 statute miles)
apart. The Space Shuttle Endeavour could travel that distance in under 18
minutes if it was flying directly from the Maldive to Flinders. But it
took the long way around - 222 hours to be exact, and 149.5 orbits of the
Earth. During that period the shuttle was almost continuously mapping the
Earth's surface whenever it was over land, covering virtually every piece
of land between 60 degrees North and 55 degrees South.
By anybody's standards this mission has been an incredible success. In
just under 10 days the SRTM payload has mapped almost every piece of land
underneath the shuttle's flight path. Only 50,000 square kilometers was
missed - less than .042 percent. And that tiny percentage is primarily in
the United States which has already been mapped.
5.46% of the area was only mapped once. Two mapping passes eliminates any
'radar shadows' which can occur under certain geometries and reduces the
amount of errors. But the data returned so far has been of such high
quality that the scientists who are going to process the data aren't
concerned about any lost data in those areas.
Global Data Coverage
The other statistics are just as amazing. With the shuttle traveling at a
stately 7.7 kilometers per second it mapped the world at a rate of 1,747
square kilometers each second, the equivalent of mapping an area the size
of the state of Florida once every 97.5 seconds. Multiply that by 99.2
hours of data and you've got an incredible amount of information. NASA
estimates it will take the equivalent of 144 desktop computers running for
70 days around the clock to process the data.
The shuttle's communications systems was not up to returning data at such a
high rate, but could return a tiny fraction of the data for 'quick look'
purposes. The data was downlinked primarily so radar specialists could
assure themselves that the hardware was working properly, and if so how
well, and if not give clues for possible on-orbit repairs. It turns out
that the data was far better than anticipated before launch with much
higher accuracy than planned.
Only fifteen 30 second passes of C-Band radar were returned during the
mission - just 7.5 minutes out of almost a hundred hours of data. That
works out to 10.125 Gigabytes of data downlinked - the storage capacity of
a typical hard drive.
Each 30 second 'snapshot' consisted of a 225 by 231 km. scene. Most
downlinks were over coastlines and consisted of about half land / half
water. Eleven sets of data was processed by the radar imaging team at
NASA's Jet Propulsion Laboratories in Pasadena California, producing
stunning three-D images. Showing the versatility of digital data many of
the SRTM radar images were combined with natural color data from Landsat
earth resource satellites and images from aerial digital photograph
libraries. Most of these images are available for viewing in our Image Gallery.
It's important to note that these images were created with just the raw
radar data downlinked from the space shuttle, without any of the
calibration information collected by other sensors on the shuttle. It
turns out that the mast was much steadier than the specifications required,
the shuttle was more stable than needed, and other critical systems all
worked better than their specifications. It's too early to say just how
much better the data will be than the planned specifications, but little
doubt that it will exceed the requirements.
About the only significant hardware problem was with a small thruster which
failed to generate enough pressure. But shuttle engineers were able to
save enough shuttle propellant through other methods to make up for most of
the difference. 10 hours of data were lost due to the thruster problem,
resulting in the slightly imperfect map of the world, but SRTM scientists
are confident that the missing small pieces won't affect the overall map.
The SRTM system is designed to generate data with 30 meters horizontal
resolution and vertical resolution of 16 meters with 90% certainty. In
some cases the quick-look images were sampled at lower resolutions to
reduce the calculation times required to generate the daily images.
In addition to the U.S. C-Band system a complimentary German X-Band radar
performed simultaneous mapping. The X-Band system has higher resolution,
but a smaller swath so it isn't a contiguous complete map like the C-Band
system. It also generated incredible results.
The shuttle's returning with 325 high density cartridges, with 11.7
Terabytes of data, the equivalent of all of the books and information in
the Library of Congress.
There was one piece of drama early Monday morning when the mast was
retracted. It took three tries before the mast would finally close
properly. Had it not closed it's probable that shuttle managers would have
been forced to order the astronauts to toss it overboard to permit the
shuttle to safely return to Earth.
The shuttle Endeavour is scheduled to return to Earth on Tuesday afternoon,
weather permitting.
Project Scientist Mike Kobrick's estimating it will take about six months
to a year before the first calibrated images are released at scientific
conferences. And it will take about a year or two for the computers to
generate the full three-D map of the world.
Note, the author is a full-time aerospace writer covering the space shuttle and other
aspects of the space program with over 15 years of experience.
Please contact the Editor for additional details.
Exclusive - Shuttle Mission Begins Its Ambitious Map of the World (Feb. 12, 2000)
Exclusive - A Sneak Peek at the First Digital Three-D Map of the World (Feb 14, 2000 - 10:30 PM)
Exclusive - NASA Managers Decide to Give Nine Hours of Additional Mapping to Shuttle Mission (Feb 18, 2000 - 10:30 PM)
Return to SRTM Main Page
The contents of this article are the property of GeoComm International Corp. (c)2000 and may not be reposted, copied,
or retransmitted in any way without the consent of GeoComm International Corp.
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