DESIGN OF A LOW COST ORIENTATION MEASUREMENT UNIT FOR MILITARY VEHICLES

The OMU (orientation measurement unit) a combination of inertial (accelerometer, gyroscope) magnetometer and GPS/GNSS sensors, can play a significant role in the stabilization, orientation, navigation and munitions guidance applications performed in ground based military vehicles. The raw data measured by the OMU's sensor array includes angular rate, acceleration, magnetic field strength as well as position. By blending these sensor measurements with the use of software algorithms (a.k.a. sensor fusion) the data can be transformed into orientation data (pitch, roll yaw) commonly referred to as Euler Angles. OMUs have a wide range of price that depends on the quality of its individual device sensors, environmental packaging, standards met and the sophistication of the device firmware used to filter, correct and smooth the inertial inputs used in the computation of application output data. In the ground based military vehicle industry, applications supported by the OMU could range from a simple rollover warning/prevention system to a sophisticated slew to cue director. The success of these applications depends greatly on the OMU's individual sensor quality/grade, which are tightly coupled with the sensor's technology type, cost and size. This paper focuses on how the OMU can be designed to achieve/approach the operational performance of the more costly and well known Strategic/Navigation grade military navigation sensor product offerings, for a fraction of the price. The OMU's expected applications, operational objectives and the resulting performance requirements, which lead to component selection, will be explained. The paper will also highlight the results of the effort to develop an OMU and provide a discussion of recent work related to device testing and integration.  An OMU (orientation measurement unit) is a sensor system used in military vehicles to determine attitude (pitch, roll) and heading (yaw) otherwise known as rotational orientation data. An OMU utilizes an inertial sensor array which measures inertial data (acceleration, angular rate) and computes rotational orientation data (pitch, roll yaw) also known as Euler Angles, using sensor fusion algorithms. Inertial sensor arrays can be found in a wide range of applications/systems spanning commercial, industrial and defense products. Basic OMUs are usually comprised of a 9 DOF (degrees of freedom) sensor array, often consisting of a 3 axis Gyroscope, 3 axis Accelerometer and 3 axis Magnetometer. However, more advanced systems will sometimes incorporate measurement data such as barometric pressure, temperature and GPS (position, velocity, time) The OMU can be designed and implemented in a variety of ways, but the implementations discussed in this paper primarily focus on COTS based design solutions that incorporate the use of either AHRS (attitude heading reference system) or INS (inertial navigation system) hardware.  Inertial sensors are found in the aerospace and defense industries, where they are traditionally used for avionics, flight analysis and UAV (unmanned aerial vehicle) guidance/control purposes. Additionally, these sensors are found in unmanned/manned ground vehicle military systems, where applications such as positioning, orientation, navigation and munitions/guidance are important. Outside of the aerospace/defense industry, inertial sensors play an important role in industries such as construction, precision agriculture and mining, where measurement data such as position, orientation are important to machine control, especially for unmanned equipment. Inertial systems are used in car motion analysis to help study/characterize the acceleration, trajectory and steering of high speed vehicles. Inertial systems are also found in unique/niche vehicle areas such as Image Georeferencing as well as mobile mapping. In the marine environment, inertial sensors are utilized for offshore drilling platforms, buoys and ship vessels. Each of these marine systems requires stabilization and knowledge of the platform's position, velocity and linear orientation data (sway, heave surge) Lastly, inertial sensors play a role in the measurement and data collection related to objects or people for indoor positioning systems found in manufacturing/assembly plants as well as virtual reality and gaming systems. Regardless of industry, the underlying relationship of the various system platforms involving their dependence on inertial sensors is that they require knowledge of their position, linear/rotational orientation and the rates at which the system moves. As stated previously, the inertial sensor is a major component of the OMU and can be found in multiple industries, serving a large variety of applications. However, the focus of this paper will be on the impact/application of the inertial sensors within the groundbased military vehicle domain. OMU PERFORMANCE The OMU's applications dictate the performance required of the OMU's individual sensors. The OMU applications specific to this paper include orientation (pitch, roll, yaw) measurement and heading estimate. Effectiveness and quality of the OMU is determined by how accurate the OMU measures orientation angles and estimates heading under static/dynamic conditions. The OMU's overall performance is attributed to the performance of its individual components  and is directly impacted by its mounting provisions, operating environment and the design specifications of its associated components. The inertial sensors found in the OMU (and similar platforms) measure an object's angular (rotational) rate as well as its acceleration for the purposes of determining orientation and position. A device which measures angular/rotational rate is known as a gyroscope. Said differently, the gyroscope measures how quickly an object turns or measures the rate of change of orientation. This ability enables the gyroscope to detect the orientation deviation of an object from an inertial reference frame. Gyroscopes can be divided into two main categories, depending on whether the angular velocity or orientation is being measured  Rate gyroscopes measure the angular velocity, or the rate of rotation of an object. Angle gyroscopes, also called Rate Integrating gyroscopes, measure the angular position or orientation of an object directly.  High level OMU architecture. A device which measures/senses body acceleration is known as an accelerometer. Given the ability to measure the acceleration of an object (vehicle) enables the OMU (and similar systems) to determine the object's velocity and position by performing successive mathematical integrations of the acceleration with respect to time. The magnetometer and GPN/GNSS sensors play a key role in assisting the OMU with the determination/calculation of the heading angle. For the purposes of navigation the magnetometer is primarily used to measure magnetic field strength (G) and/or the direction of the magnet field at some point on earth. A magnetometer, like a compass, will point towards the earth's magnetic north pole, which can be used to estimate an objects' heading angle relative to magnetic north. However, direction finding based solely on the use of a magnetometer may be unreliable due to the magnetometer's possible performance degradation caused by exposure to static or dynamic magnetic fields or proximity to ferrous metal. A physical phenomenon which also hampers the direct use of magnetometer readings by navigation equipment is magnetic declination, which is the horizontal angle (variation) or difference between True North and the magnetic north pole at some point on earth. To overcome the limitations of magnetometers, navigation systems are integrated with GPS/GNSS sensors. The GPS/GNSS sensors, while moving, are able to receive Position (latitude/longitude) Velocity,  Unit for Military Vehicles, Ronnie L. Wright Ph.D. et al.  Heading updates from the satellites within their view. However, use of the GPS/GNSS receiver measurement data requires vehicle movement. To provide a True North heading estimate when the vehicle is not moving, navigation systems blend together their magnetometer readings with the GPS/GNSS receiver position to determine the magnetic variation at that position, using a World Magnetic Model (WMM) or other geographic magnetic models. The magnetometer's reading is then augmented with the WMM (or another) magnetic declination value to arrive at a True North heading estimate.  There are many design parameters associated with inertial sensor performance. However, there are four key design parameters which directly contribute to inertial sensor measurement accuracy and are commonly used for gyroscope/accelerometer performance comparison: i. Angular/Velocity Random Walk (deg/hr, mg/Hz) ii. Bias (In Run) Stability (deg/hr, m/s) iii. Bias Repeatability (deg/hr, m/s) iv. Scale Factor Error or ppm) The Angular/Velocity Random Walk is a measure of the noise inherent to the device, resulting from electrical or mechanical (friction/vibrations) disturbances. The Bias (Inrun) Stability is a measure of how stable the output of the sensor is over a specified time period. For the OMU's sensor fusion