Position information and course information are usually captured to confirm the underwater robots.The distance information is measured by ultrasonic sensors and the position information can be obtained by drawing arcs to seek the point of intersection.Two position arithmetics are advanced.The course information of robots is hard to acquire for the complex underwater environment.So an estimate head model is created to obtain the course information.The model is a way that is set by calculating curve derivation which is fit the robot history position information to substitute the course information.The orientation arithmetic is validated in the experiment and is proved feasible and credible.The position absolute error is less than 20 cm and the course absolute error is mostly under 10°that can satisfy the demand.Then the course arithmetic is corrected by analyzing the source of orientation errors.