North-Finding System Using Multi-Position Method With a Two-Axis Rotary Table for a Mortar

North-finding techniques involve detecting and calculating the azimuth angle, thus determining north using various kinds of sensors. North identification can be accomplished through the use of a digital magnetic compass. However, the accuracy of a magnetic sensor is easily degraded by spatial and temporal distortions due to a ferrous material or electromagnetic interference. Moreover, GPS-based north-finding systems (NFSs) can accurately determine the azimuth by two separated GPS antennas. However, this still has a practical limitation in jamming or indoor environments. Therefore, an inertial NFS has been developed to replace the classic equipment used in estimating the fire direction of hand-carried projectile weapons such as with mortars. To improve the precision of seeking north with a middle grade gyro, a multi-position method is implemented. Using only a single fiber-optic gyroscope and two MEMS accelerometers on a rotary platform, the north orientation is accurately found by observing the horizontal component of the earth's rotation vector. With a minimum number of inertial sensors on a manual rotary platform, the NFS can be compact, light-weight, low-cost, and robust enough for easily portable mortar. Using additional auxiliary information, which can be obtained by magnetic sensors, the time for locating north can be dramatically reduced. The test results compared with the navigation grade inertial navigation system show that a single digit mrad heading accuracy can be achieved. The operability of NFS mounted beside a mortar was tested in the field, in which the reference positions were acquired by differential GPS. To insure the durability in gun-fire shock, the drop-tower test was performed based on the shock profile measured in real mortar fire.