单项选择题
When navigating using DGPS（Differential Global Positioning System）you may expect your position to be accurate to within a radius of（）.
A.10 meters B.20 meters C.50 meters D.100 meters
单项选择题 What does not contribute to the commercial GPS receiver position error（）.
单项选择题 To obtain accuracy in fixing by DF，（）and three stations should be used.
单项选择题 The use of pulse groups and extremely precise timing at each Loran-C station makes possible the use of（）.
