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How to Map the Earth (or what was the shuttle for) was the title of an excellent talk to the IET in Southern California by Dr Kobrick, a scientist from the Jet Propulsion Laboratory in Pasadena. The first manned mission for the space shuttle, and it’s second flight overall, STS-2 had a Synthetic Aperture Radar (SAR) loaded in the payload bay to capture images of earth. To create a SAR image, signal processing of the echoes of the pulses transmitted by the single beam forming antenna is combined with the position of shuttle in orbiting to allow the generation of topographic information.
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Since a synthetic aperture radar captures detail by moving over the target it is possible to get a higher level of resolution compared to a beam scanning radar and as the wavelengths are not blocked by clouds or vegetation, differences in surface roughness and terrain attitude can be used to delineate such geological features as faults, anticlines, folds and domes, drainage patterns, and stratification. By comparing the SAR captured images against other images it is possible to see for example the rock formations under the deserts of Egypt, or the true elevations of land obscured by vegetation as the sand and vegetation are both transparent to the SAR.


The STS-2 mission was cut short, so although the mission was able to capture a swath width of 50 km and a total of about 10 million square kilometers of earth at a resolution of 40 m there was a desire to capture more of the planet, and several more missions were planned to take advantage in advances in technology and processing capability resulting in the STS-99 mission flown in February 2000 known as the Shuttle Radar Topography Mission (SRTM). As the scientists involved wanted to capture as big a resolution model as possible it was decided to use an Interferometric synthetic aperture radar which combines the returns from two SAR's using the phase change in the returns to give more accurate 3D information in a single pass, rather than relying to flying the exact same path twice with a single radar.
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To achieve this one radar was in the shuttle payload bay, with a second set of receiving antennae extended from the payload bay on the end of a 60m mast. The mast is a truss structure that consists of 87 cube-shaped bays with unique latches on the diagonal members of the truss allowing the mechanism to be deployed or recovered bay-by-bay out of the mast canister through a motor driven nut. This process can be seen in this mission video at the 4 minute mark.



The size of the swath captured on a single orbit was increased to 225km, and as Dr Kobrick observed every condition required to make the mission a success aligned perfectly. The limit to a Shuttle mission is about 10 days, the same amount of time it would take to overfly the entire planet with the swath captured greater than the crosstrack on each orbit so the ‘strips’ of data would overlap and not have gaps between them. 10 days of data capture was also the same limit on the number of data tapes that could be carried to capture all the data, meaning that the Astronauts would have very little to do on the mission except change the tapes, which in turn resulted in some interesting video of the imaginative maneuvers undertaken in zero gravity during tape changing to relieve boredom. The optimum attitude of the Shuttle for the mapping to take place also allowed one the navigation star trackers to be always orientated correctly resulting in no time lost in re-orientating the shuttle to fix its position periodically and the default location of the Canadarm did not interfere.


Once all the data was processed, the net result is a Digital Elevation Model, in a near-global scale from 56° S to 60° N, which covers nearly all the land mass on earth. These Digital Elevation Models have many uses from Civil Engineering (where to build a dam), Aviation flight planning (to reduce controlled flight into terrain), Archeology (to pick up structures covered by soil), Communications (where to place antenna's for best line of sight), Military mission planning, as well as Geology and Hydrology determining fault lines in the earth's crust or the impact of rising water levels.


One of the really interesting aspects of this data set that was captured was on the Yucutan Peninsula now called the Chicxulub crater. Most scientists now are of the opinion that this is the smoking gun that caused the extinction of the dinosaurs and the large majority of species of life on earth about 65 million years ago.
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As this image shows the resolution of the data captured is so good that it is possible to see something called the line of cenotes which are limestone sinkholes that have chained together to form sort of a moat. The data shows a semi-circular depression in the surface of the earth, which is so very subtle which is why you can't see it if you are out there on the surface, but it stands right out on the STRM mission captured data.


As the data is free for use, a data tile of about 3 arc seconds in size is downloaded about once a second, and has been since it was released. The data set also acts as a complete record of Earth over a period of 10 days, which could be referred to in hundreds of years from now, though quite what future generations will determine with all of the Los Angeles major sports stadia in existence at the time located on a bearing of 34 degrees from north, with Los Angeles at 34 degrees latitude…


If you are interested in much more technical description of the mission and the science and engineering involved, the JPL STRM website has many great images and technical descriptions of the components used. There have been over 10,000 peer reviewed scientific papers using the STRM data and if you are in the Washinton DC area, visit the Udvar-Hazy Center to see both the Shuttle and the mast and antenna displayed.
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