We develop an inverted long-baseline (ILBL) acoustic navigation system for determining the position of deep-towed instruments, such as controlled-source electromagnetic transmitters and receivers. The ILBL system uses a deep-tow mounted acoustic transceiver system to measure travel times to a pair of surface transponders towed on paravanes behind the survey vessel. The travel times, transponder positions and pressure depth data are inverted for the lateral position of the deep-tow vehicle, as well as the positions of any relay transponders on the antenna and receiver array that are towed horizontal behind the deep-tow vehicle. Three example applications demonstrate position accuracies of about 5 and 37 m in the inline and crossline directions for 3 km water depths and around 6 m for 1 km depth. The portability and generality of the system make it suitable for deep-tow applications for geophysical, geochemical and geological surveying purposes. We have shown that the accuracy of the ILBL system is similar to that of commercial USBL systems, but is considerably more cost effective than even portable USBL systems, and can be used on vessels lacking permanently installed USBL transceiver heads. Further, it can be used in water depths of 5,000 m or more. It could also be readily modified for further improving its position accuracy if desired.
