When your vessel is proceeding to the area of traffic density，（）is used to determine the exact ranges of other ships or objects in the vicinity.

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

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What does not contribute to the commercial GPS receiver position error（）.

A.Satellite clock
B.Ship's speed
C.Atmospheric/ionospheric propagation
D.Receiver

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