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The differential GPS is equipped with the system, and the position accuracy is relatively high. Also, the accuracy of the depth gauge is 0.001 percent. Hence, the errors of GPS and depth gauge can be ignored in this article. Error sources mainly focus on position error of GAPS, position drift of PHINS and the DVL integration system, and time delay because of the sound speed in water. Position Error of GAPS GAPS has a relatively high location accuracy with the original iXBlue beacon. The range error is less than 0.2 meters. The location angle accuracy is 0.12 degrees. The major source of error is the angle-measuring error. Thus, the location error of GAPS can be written approximately as: Side scan sonar images from TUCOS-I during sea trials. Germany) S2C acoustic modems was utilized to transmit the position data to calibrate PHINS at a preset time interval. The position drifting of PHINS during the time interval was then updated using this method. When TUCOS-I was on the surface, the GPS position was used for the position calibration. The output data of PHINS, including position, attitude and velocity information, were used for navigation of TUCOS-I. Position Accuracy Analysis of the Navigation System The sources of position errors of TUCOS-I include velocity error from the DVL, depth error from depth gauge and position error from GPS, GAPS and PHINS. When the AUV works underwater, the slant range is relatively large and the time delay of the acoustic modems is considered. Where E is the location error of GAPS, L is the slant range and is the angle-measuring error. Position Drift of PHINS and DVL Integration System When PHINS and DVL were installed in TUCOS-I in June 2010, a series of strict calibrations were conducted. First, the lever arms of DVL, depth gauge, GPS antenna and GAPS beacon relative to PHINS were recorded in the PHINS. Then, sensors, except PHINS and DVL, were powered off. TUCOSI cruised for about 5 kilometers at a speed of around 4 knots. www.sea-technology.com August 2013 / st 59