The present invention relates to an apparatus to estimate an entrance-angle of an underwater vehicle in an environment with a severe magnetic field disturbance and a method thereof, which are capable of estimating the entrance-angle of the underwater vehicle by using a reference landmark and sonar to correct an error in the entrance-angle. According to the present invention, the apparatus to estimate an entrance-angle comprises: a work frame unit wherein a plurality of reference landmarks are formed in an underwater work area; and an entrance-angle estimation unit which comprises: a navigation unit mounted on an underwater vehicle; a sonar transmission/reception unit which transmits sound waves and then receives the sound waves reflected from the reference landmarks; an entrance-angle calculation unit which calculates the position of the reference landmarks to calculate an entrance-angle; and a correction unit which corrects an error of the navigation unit. The present invention allows the estimation of an entrance-angle of an underwater vehicle by using an underwater work environment as the coordinate system, thereby estimating the precise entrance-angle even when a low-priced navigation apparatus is used.
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