GPS sensors perched atop two 50-story towers on Hong Kong's Tsing Ma Bridge, as well as at strategic points along 100,000 miles of suspension cables, monitor motion caused by high winds in the harbor. The bridge can withstand gusts up to 212 miles per hour.
Many people think of GPS as a convenient means for getting from one place to another without having to ask strangers for directions. Armed with inexpensive receivers no bigger than cell phones, folks who are directionally challenged can tune in to a constellation of 24 Global Positioning System satellites and find out exactly where they are on the planet at any given moment, not to mention exactly what time it is. What few casual GPS users realize, however, is that these same satellite signals can be used for more than just determining time and location. Techno-savvy scientists have discovered that GPS is a remarkable tool for detecting motion and monitoring a world in flux. If a researcher takes readings repeatedly at a fixed location with a GPS receiver, he can track patterns of movement: a volcano's flank bulging as it charges with magma, or an iceberg rotating after breaking off an Antarctic shelf. Add more receivers (additional sensors improve accuracy), reference them to a fixed GPS base station whose position is precisely known (so errors can be factored out of the readings), and suddenly movements as small as one-tenth of an inch come into clear focus and can be monitored in real time. With this new technological capability, scientists are prying into some of the most dynamic processes we know—on land, in the air and sea, and beyond.
A DANCING BRIDGE
Hong Kong's Tsing Ma Bridge—the world's longest suspension bridge that carries both road and rail traffic—is designed to sway and bend. It can tolerate high typhoon winds that send the 4,518-foot-long bridge swinging several feet from side to side and the passage of trains that can cause the main span to dip by more than two feet. Still, too much movement, say a lateral shift of more than 15 feet, would twist and buckle the bridge's steel girders and cables like Popsicle sticks and string. To guard against such a catastrophe, engineers overseeing the Tsing Ma have set up a GPS sensor array that continuously determines the exact position of the bridge in three dimensions. Fourteen GPS receivers, connected by miles of fiber-optic wire, are attached to the bridge's cables, decks, and towers. Ten times every second, the sensors relay their position to a computer at a central monitoring facility. Data from two other permanent GPS sensors are also sent to the computer, which then performs corrections to eliminate murky errors. The Tsing Ma's position in space, accurate to within four-tenths of an inch horizontally and less than eight-tenths vertically, is then displayed in real time. The computer also calculates wind speed and wind direction, and estimates stress and load on the bridge's components so engineers can plan for repair and maintenance. "No other sensors could possibly measure the motion of a long bridge like this," says civil engineer Kai-yuen Wong, head of the GPS bridge-monitoring project for Hong Kong's Highways Department. "GPS makes it possible."
THIS S ONE OF THE NEW TECHNOLOGY THAT ACTIVATES GPS BY NEXT WEEK WE CAN SEE HOW GPS S USED N DETECTING VOLCANIC ERUPTIONS AND SEA DETECTORS .
No comments:
Post a Comment