单项选择题
A.High frequency pulses
B.Combinations of high and low frequency pulses
C.The same frequency for all stations in a chain
D.Varied long and short pulses
单项选择题 The time interval between the transmission of signals from a pair of Loran-C stations is very closely controlled and operates with（）.
单项选择题 The receiver uses（）satellites to compute latitude，longitude，altitude，and velocity．
单项选择题 The operator of the ship’s radiotelephone，if the radiotelephone is carried voluntarily，must hold at least a（）.