When navigating using DGPS（Differential Global Positioning System）you may expect your position to be accurate to within a radius of（）.

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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To obtain accuracy in fixing by DF，（）and three stations should be used.

A.Parallel bearings
B.Long-range bearings
C.Short-range bearings
D.Cross bearings

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