Magnetic guidance of autonomous vehicles (part 2)

This paper describes an extension of some magnetic guidance work reported earlier. The extension consists of determining the magnetic moment and orientation of a bottom resting magnetic anomaly as well as the vertical distance to the anomaly. This information should lower the false alarm probability of an operational magnetic guidance system. Increasing interest in underwater autonomous vehicles has resulted in a search for suitable guidance systems. Magnetic techniques could provide supplementary guidance for autonomous vehicles that need occasional precise position fixes to update their standard navigational systems, or that want to return to a particular spot on the ocean bottom with great accuracy. A small permanent magnet resting at a precisely known spot on the ocean's bottom, or naturally occurring key features in a magnetic survey of the bottom, could provide the necessary magnetic signal. An adaptive search approach to magnetic guidance is promising. In adaptive search, the location of each successive search pass is determined by information gathered on the previous pass. Computer simulation results show that three search passes are usually sufficient to locate the center of the magnetic anomaly to within several feet. The favored sensor configuration consists of a scalar magnetometer whose outputs are successively subtracted along the path of sensor motion to form an approximation to the spatial gradient of the sensor's output. Data interpretation for this technique appears simple enough to be done automatically by either algorithmic or artificial intelligence techniques. Using the adaptive search technique to accurately position the vehicle, and placing a second magnetometer directly above the first, the magnitude and orientation of the anomaly's magnetic moment can be measured. The vertical distance to the anomaly can also be determined. Thus, if an independent altimeter is available, object vertical height above or below the bottom can be measured. This information would be useful in determining that the correct object of interest has been located.