3D Space Trajectory Tracking of Underactuated AUVs using Back-Stepping Control and Time Delay Estimation⁎This research was supported by Institute of Civil Military Technology Cooperation(ICMTC) of Agency for Defense De-velopment(ADD), funded by Defense Acuisition Program Ad-ministration(DAPA) and Ministry of Trade, Industry and En-ergy(MOTIE)(Developement of multisensor fusion based AUVs terminal guidance and docking technology, 17-CM-RB-16). Issue 16 (2021)