PURPOSE:To attempt at safety in guiding a ship in approach to a pier and improve reliability by judging abnormality if change in the acceleration of the ship approaching a pier based on measured distances by means of an ultrasonic wave or others is more than the specified acceleration and estimating the distance of the ship from the immediately preceding measurement value. CONSTITUTION:The distance to a ship 1 approaching a pier and the speed of ship by the change in the distance to the ship are measured by means of an ultrasonic wave sending and receiving device 2 that is immersed in the water at a dolphine pier 3, and measured distances are stored in a distance memory 18 through a counter 17 and the speed is written in a memory 21 through a velocity counter 20. An abnormal value judging circuit 30 calculates the approaching speed of the ship to the pier by the difference of the contents of the counter 20 and memory 21. If the change in the acceleration is more than a specified acceleration, the abnormality judgement signal is input to a distance estimating circuit 40. The circuit 40 upon receiving the signal calculates for estimation the distance of the ship 1 based on the memory contents immediately before that are stored in the memories 18 and 21. Accordingly misjudgement of approaching to the pier due to noises developed by the measurement of, for instance, a tug boat in the vicinity can be avoided and safety in the guiding of the approach to the pier and increased reliability can be thereby attempted.
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