一、GPS提供民用面临三大问题长期以来，美国全球定位系统（GPS）热点话题集中于三大关键问题：一为增加第二民用频率；二为何时中止故意降低精度的选择可用性（SA）；三为给美国空军以拒绝GPS被敌方使用的能力。这三个问题中，增加第二民用频率是美国国防部（DOD）和运输部（D First, the GPS to provide civil three major problems For a long time, the United States GPS hot topics focused on the three key issues: First, to increase the second civil frequency; Second, when to stop deliberately reduce the accuracy of the choice of availability (SA); The third is to give the USAF the ability to refuse the use of GPS by an enemy. Among the three issues, increasing the second civil frequency is the U.S. Department of Defense (DOD) and the Department of Transportation (D