More exact assessment of visibility when（）is used to determine the range of vessel or other objects in the vicinity.

A.While a fix can be generated by signals received from two satellites，three satellites are necessary for reliable accuracy
B.The satellites are in equatorial orbits around the earth about 60°of longitude apart
C.The satellite navigation system determines a fix by measuring the doppler shift of the radio signals from the satellite
D.The ship's receiver cannot begin processing data until the antenna locks onto the satellite and starts continuous tracking