Investigating Tx coils and magnetic field Rx sensor configurations for underwater geo-location

In this work, new configurations of magnetic field transmitter coils (Tx) and receiver sensors (Rx) are studied for underwater (UW) geo-locations. The geo-location system, based on low frequency magnetic fields, uses measured vector magnetic fields at a given set of points in space. It contains an active pulsed direct current transmitter, tri-axial field receivers, and a global positioning system unit (GPS). The GPS is coupled with the EMI system and provides continuous geo-referencing of the UW system's position. UW geolocations are estimated using a) closed form solution, that uses the total vector magnetic field tensor's gradient, and b) nonlinear optimization technique based of the differential evolution (DE) algorithm. In this work we first investigated the advantages and disadvantages of the proposed UW low frequency magnetic field geo-location system. Namely, we present systematic studies on: a) magnetic field transmitter configurations to determine the best compromise between size, shape and practical implementation to achieve maximum transmitter range in the UW environment, b) the placements of tri-axial receiver sensors with respect to the Tx to accurately estimate the UW geo-location from the measured magnetic fields; c) different sources of noise (such as the air-water interface, coupling between targets' EMI responses and the geo-location system's signals, water conductivity), to estimate how these noises influence the system's performance and localization precision. Finally, we assessed the capabilities of the closed-form solution and the DE technique to predict the location of an underwater interrogation system by comparing their corresponding estimated results to the true value. We found that for realistic water conductivities, the frequency should be less than 100 Hz. We showed that when the primary magnetic field is contaminated with random noises due to the presence of underwater metallic targets, water conductivity/frequency changes, and finite size of the transmitter, the performance of the full vector magnetic field tensor gradient approach degrades significantly compared to that of the DE method. In addition, the number of receivers required by the vector magnetic field tensor gradient technique and its sensitivity with respect to the sensor separation prevented us from further considering this technique for UW geo-location, leaving the non-linear approach, that uses only three vector Rx, as our technique of choice for tracking the location of underwater interrogation sensors with centimeter-level accuracy.