ION GNSS 2012
Session F3: MEMS Technology

Title: Three Axis Magnetometer Navigation in Suburban Areas
Author(s): J.A. Shockley and J.F. Raquet, Air Force Institute of Technology
Date/Time: Thursday, September 20, 2012, 8:35 a.m.
Room: 206 (NCC)

Alternative navigation technologies often seek to exploit naturally occurring measurement sources such as magnetic fields. For large open areas, the Earth´s magnetic field is consistent, slowly-changing, and well-modeled. However, the same is not true near the surface of the Earth in urban or suburban environments. Buildings, underground pipes, and other substantial ferrous materials perturb the Earth´s magnetic field creating unique, location-specific magnetic field measurements. Magnetic field sensors, commonly known as magnetometers, can provide measurements in one, two, or three axes. While the compass has been a popular single-axis magnetometer for centuries, two and three-axis magnetometers are prevalent in electronic devices and navigation systems. Exploiting the unique magnetic field information due to perturbations can be a valuable navigation source. Unfortunately, the collection platform may interfere with the magnetometer measurements causing severe degradation or rendering them completely useless. Rigorous magnetometer measurements in vehicle environments can be difficult due to the large quantity of ferrous materials and changing magnetic fields caused by onboard systems (engine components, electrical devices, other navigation equipment). The oft used method of distancing the magnetometer from the vehicle platform to reduce or eliminate magnetic interference is not conducive to full integration of the magnetometer into the navigation system. For utility, mounting the magnetometer in the vehicle along with other navigation sensors is feasible, provided some mitigation is conducted to reduce spurious magnetic interference from the vehicle. Furthermore, different vehicle types affect the magnetometer in different ways due to composition of the vehicle and mounting locations. Magnetic interference of a vehicle and in different vehicles manifests as bias, scale-factor, and distortion in the magnetometer measurements. The first objective is to make these measurements relevant and useful, which requires a thorough assessment of the measurements themselves and results in the conditions required for utility. An examination of how uncalibrated and/or calibrated measurements can be used is presented. Additionally, three-axis magnetometer measurements can be paired with coordinate information to create a magnetic map for navigation. The magnetic map can then be used to determine location based on single or multiple magnetic measurements. Multiple magnetic measurements can describe magnetic features, distinct to a particular location. The magnetic map can also be constructed based on distance, rather than time between measurements. This allows for variability in the magnetic map based on utilization of the measurements, size, memory constraints, or typical feature length. Multiple magnetic maps of the same area can also provide insight on the consistency of features over time or between different vehicle platforms. A discussion of magnetic map creation and utility outlines how the information can be used, and how this supports navigation efforts. Results of navigation performance are presented to evaluate the potential benefits of magnetic field measurements in different vehicles. Magnetic maps created in one vehicle can be used in different vehicle platforms with varying results based on estimation technique. Highlights of different techniques are presented along with advantages and disadvantages of each technique to demonstrate the trade-offs that may be necessary in a navigation system using magnetic field measurements. The techniques include a simple maximum likelihood method, a nominal likelihood method, and a complete particle filter for navigation. Magnetic field measurements present a viable source of navigation information. Using those measurements requires knowledge of the Earth´s magnetic field, collection requirements, utility after collection, and techniques to process the collected measurements. This paper provides knowledge and tools as well as initial navigation performance for a localized set of magnetic measurements.

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