Integrated short term navigation of a towed underwater body

An underwater body (the fish) is towed by a ship at the end of a few hundred meters long cable, and uses sidescan sonar to make observations of the sea bottom. To enhance antenna resolution, a virtual antenna is built up by adding up appropriately the echoes received by the moving physical antenna. To carry out this synthetic aperture sonar experiment efficiently, it is necessary to know with an accuracy of a few cms, the trajectory of the fish relative to its otherwise unknown initial position, during a few minutes experiment, so that motion compensation can be performed during the synthetic aperture processing. For this purpose, acceleration measurements provided by an INS (inertial navigation system) located onboard the fish, can be integrated. INS measurements are known to track accurately the high frequency components in the mobile trajectory. However, even on small time intervals, integration of INS measurements can result in unacceptable drifting in position estimates. To prevent this drifting, it is common to use additional measurements which could provide good estimates of the low frequency components of the mobile trajectory. Combining the two types of measurements will improve the trajectory estimates. This can be performed by means of a Kalman filter. The paper investigates the possibility of using position estimates of the ship, provided by differential GPS. An additional difficulty is that the satellites involved in GPS cannot provide any position estimates of an underwater body such as a towed fish, but only position estimates of the surface ship. To overcome this difficulty, the authors introduce a numerical model for the tow cable, to transform GPS position measurements of the surface ship into position and velocity estimates of the towed fish, to combine these estimates with INS measurements