Shipboard Calibration of a Diamond Nitrogen Vacancy Magnetic Field Sensor
Paul Frontera, US Naval Academy Stephen Alessandrini, Lockheed Martin Corporation John Stetson, Lockheed Martin Corporation
Accurate vessel position information in the absence of the Global Positioning System (GPS) signal is desirable for both military and civilian applications. One proposed alternative to GPS localizes geographic position by detecting magnetic field anomalies associated with unique positions in the Earth’s crust. This paper details efforts to calibrate a novel magnetic field vector sensor, a diamond nitrogen vacancy (DNV) sensor, aboard a ship for use in detecting anomalies in the Earth’s magnetic field for maritime navigation.
A shipboard magnetometer detects a magnetic field from sources both internal and external to the ship. The ship’s contribution to the detected magnetic field must be removed to produce a sufficiently accurate measurement of the external magnetic field for navigation purposes. A ship’s magnetic field is known to consist of both a dipole component and a “soft iron” or induced effect. These effects are known to significantly impact a vessel’s magnetic compasses used to determine heading and must be compensated prior to attempting to localize position.
Magnetometers based on different operating principles have been installed on ships for a variety of survey purposes. However, a practical geomagnetic navigation system remains elusive. Previously employed magnetometers include superconducting quantum interference devices (SQUID), fluxgate, and Hall effect sensors. These devices are capable of detecting magnetic field strength in a single direction with varying sensitivity. When mounted tri-axially, these magnetometers can detect both the magnitude and direction of the Earth’s magnetic field vector.
DNV magnetometers are capable of measuring the field vector using a single instrument. A controlled green light is focused on a diamond that has carefully placed nitrogen vacancies along four axes in its lattice structure. The diamond is simultaneously subjected to RF stimulation while illuminated with green light. The nitrogen vacancies produce red light photoluminescence whose brightness is dependent upon the magnetic field.
A DNV sensor was installed aboard a U.S. Navy Yard Patrol craft (YP-700) to explore the feasibility of detecting magnetic anomalies for navigation. YP-700 is a 108-foot long wooden hull-aluminum superstructure vessel propelled by two shafts, each powered by a marine diesel engines. YP-700 is capable of a maximum speed of 12 knots with normal transit speeds of 8-10 knots. Although the wooden hull minimizes the strength of its magnetic field, magnetometer calibration is necessary to allow for accurate measurement of Earth magnetic field anomalies.
A DNV sensor and a tri-axially mounted flux-gate magnetic field sensor were mounted in close proximity along with a strap-down Inertial Measuring Unit in an effort to detect the Earth’s magnetic field vector resolved in the local tangent plane. The Yard Patrol craft’s contribution to the detected magnetic field vector will be determined through a calibration process. Data was collected during October 2017 underway operations in the Chesapeake Bay and is currently being analyzed to determine a preferred approach to calibrating a DNV magnetometer for shipboard use.
YP-700 performed three different maneuvers in an effort to determine the dipole and soft iron contributions to the detected magnetic field. These maneuvers were performed in an area of low magnetic field gradients. Maneuvers consisted of a “Figure-8” conducted with approximately 10 knots of initial headway using a 20-degree rudder, 360 degree circles with the same initial conditions, and a “twist” maneuver with one shaft operating in the ahead direction with the other in the astern direction to generate the tightest possible 360 degree turn. Calibration constants generated during each of these maneuvers will be compared.
Mathematical approaches developed by other authors for fluxgate and SQUID magnetometers will be used for the DNV sensor calibration. In general, these approaches attempt to isolate the ship’s contribution to the detected magnetic field and are not specific to the magnetic field sensor. Comparison of calibration constants from the DNV sensor and the tri-axially mounted flux gate sensor should be similar given the close proximity of their shipboard installation.
The primary objective of this paper is to detail an effective approach to remove a ship’s magnetic field contribution from a DNV sensor measurement. This information is needed prior to using a DNV sensor to localize a ship’s position through the detection of Earth magnetic field anomalies. Experimental data obtained from operations in the Chesapeake Bay in the vicinity of Annapolis, MD will be analyzed as the basis of this work.