A、The Origins of GPS B、The Advantages of GPS C、The Future of GPS D、The Increasing Uses of GPS: Military and Civilian D掌握反映全文结构脉络

The Global Positioning System (GPS) has become indispensable for soldiers, pilots, hikers, yachtsmen, even bankers, ever since the U.S. Department of Defense started filling the sky with satellites about 25 years ago, in order to improve navigation for fighters, bombers and warships. More recently, GPS has allowed the development of precision-guided weapons, the smart bombs that home in on targets: between Serbia four years ago and this year’s Iraq war, the proportion of such bombs used rose from no more than 3 percent to at least 60 percent.
    However, the GPS is also an essential prop to countless civil applications; for every military user, there are now 100 civilian users. It provides not only satellite-navigator systems in cars and boats; it is used by Internet service providers, by banks and by surveyors. One day it might be used by air traffic control systems to permit "free flight", in which pilots of commercial aircraft find their own route and stay clear of other aircraft, without the cumbersome business of radio telephone contact with controllers on the ground.