| 摘要 |
<P>PROBLEM TO BE SOLVED: To improve reliability in the connection of communication link. Ž<P>SOLUTION: A control unit determines that the strength of a receiving signal, received from an antenna 12a installed in travelling direction side, is higher than a first predetermined threshold and, thereafter, determines that the communication link is good under the condition that the received signal strength, received from another antenna 12b installed at the rear side of the travelling direction, is higher than a second threshold and when the communication link is determined so as to be good, the communication link between a base station is established. Ž<P>COPYRIGHT: (C)2010,JPO&INPIT Ž
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