The U. S. Navy has demonstrated that total-field magnetometers and gradiometers as well as tensor fluxgate and superconducting magnetic gradiometers can be used in towed underwater platform environments. In these applications, the principal platform noise issue has been that caused by the rotation of onboard magnetic materials in the large background earth's magnetic field. The associated induced magnetic polarization changes and eddy currents cause secondary magnetic fields to be generated at the onboard sensor, and these overwhelm the magnetic signature of objects to be detected and localized.In these applications it has been established that knowledge of the earth's field vector time history in the platform reference frame can be used in a relatively simple fixed-parameter filter model to estimate the "motion noise" contribution to the measured signal, and to effectively remove it. The associated model has been modified to contain up to three magnetic dipole sources, and measured, time-windowed data has been used to simultaneously remove platform motion noise and localize magnetic targets.To operate a magnetic sensor onboard an AUV, it is necessary to deal with the problems encountered on a towed platform, that is, motion noise in the earth's field, but now there are numerous additional magnetic sources that are independent of the external field. These include onboard current loops and magnetic materials that move relative to the AUV platform. Examples are control surface mechanisms, motors and controllers, onhoard processors, and switch magnets.To model the noise from all magnetic sources, it is still necessary to monitor the earth's magnetic field onhoard the platform. It also is necessary to add measurements of onboard currents and magnetic field measurements made very near any sources that move relative to the platform. The basic issue here is that all onboard magnetic sensors will respond to the large earth's magnetic field changes due to rotation of the AUV in the earth's field. The additional magnetic field sensors should be positioned such that the additional field changes due to the local sources are measurable relative to the rotational changes.We describe both frequency-domain and time-domain filter models that incorporate onhoard AUV reference sensor measurements to cancel platform noise, and apply the models to data collected as described in companion papers. We evaluate the relative performance of the time-domain and frequencydomain models, and show that it depends on the measurement sets used to perform the modeling. We conclude that windowed time-domain filters hold much promise for future AUV-resident magnetic sensor systems
In keeping with the Navy's policy to remove humans from hanns way, the Autonomous Underwater Vehicle (AUV) is replacing human divers for many missions. The Advanced Marine Systems Lab at Florida Atlantic University (FAll) has developed a small, magnetically friendly, modular plastic AUV called Morpheus designed for coastal applications and especially suited for very shallow water (VSW) mine reconnaissance. Currently employed sensor technologies on AUVs have certain deficiencies and limitations when used across the wide gamut of naval targets and environments, and a strong requirement exists for a sensor or sensors to fill these niches. The Real-time Tracking Gradiometer (RTG) selected for this integration is truly such a niche sensor because its capabilities are not degraded by media interfaces or environmental conditions. It is an experimental prototype fiuxgate magnetometer array developed by Quantum Magnetics for the Coastal Systems Station (CSS) and was designed to be man portable and self contained. While limited by physics in detection range, it is capable of detecting ferrous targets under the worst environmental conditions, even when the target is buried. While not having the range of sonar, the RTG does not respond to the false alarms that are indicated by sonar, and since it is capable of also providing range and bearing information, it provides an invaluable niche filling classification tool.The placing of any magnetic sensing system on a conventional AUV is a non-trivial problem. The standard AUV is designed around materials and components that were selected to maximize performance without regard to the magnetic properties of the materials used in its fabrication. To minimize the degradation of sensor performance caused by the platform, several steps must be taken. These include; the substitution of nonferrous components for ferrous, maximizing the separation between the sensor and magnetic field sources, minimizing current loops and using auxiliary current and field sensors capable ofgenerating noise canceling signals. To maximize utility, the magnetic sensor system should also provide range, bearing and magnetic target strength. While all data and results contained in this paper have been obtained with land-based testing, they are easily adapted to the underwater environment of the AUV. The RTG was recently attached to the Morpheus, and data were collected with the unmodified Morpheus powered and undergoing simulated sea motions on a three-axis motion table. These tests indicate that integration, while not trivial, is indeed feasible, and work is continuing toward mounting the sensor internal to the AUV and implementing the required noise mitigation solutions.
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