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40 Focus | Inertial measurement units data point that lies outside the range of previous readings is detected, it can be discarded and the inertial data used instead. This is particularly useful in harsh environments where there can be multi- path effects resulting from reflections of the satellite signal that are picked up at a slightly different time from the original, increasing the error of the position data. The IMU can also take in other data such as odometer signals, which are accessed via a driverless car’s existing CAN network. This signal is used as an external input to the IMU in the same way as GNSS to provide more data. Marine applications for IMUs have additional challenges, particularly in accounting for the motion of waves. Called motion reference units, they are compensated and packaged specifically for the marine market for measuring much lower acceleration, typically 0.01- 0.04 m/s 2 , and surviving in much harsher environments. The challenges include a ‘heave output’ – a measure of the instantaneous height of the craft compared to sea level, something that is required for sonar applications. While heave output can be delivered in real time, an RTOS can also be used to provide a highly accurate output. This can take a few hundred seconds to deliver but the data is time-stamped by the RTOS so that it can be integrated into a survey and mapping system to improve the quality of the mapping data. Another development is that IMUs with lower resolution but less drift are gaining interest in unmanned submersibles. The lower drift allows the craft to stay submerged for longer before surfacing to get a GNSS fix. Conclusion The next few years will see a change in the FOG market as MEMS-based IMUs become more precise, especially compared to FOG systems. Although closed-loop MEMS devices will provide higher accuracy, closed-loop FOGs will still be used for high-end compass applications. Many IMU developers are focusing on providing ever-greater performance at a specific price point and size. Also, the additional processing power of dual-core microcontrollers is allowing software such as a RTOS to add more features, at the same time as allowing more complex floating-point calculations. Advances in packaging are also key to IMU development. More cost- effective sealed packages boost the performance of the sensors, while the size of IMUs is being driven down as board-level packaging technology December/January 2017 | Unmanned Systems Technology MEMS-based IMUs and IMSs that include GNSS satellite navigation are shrinking in size (Courtesy of Inertial Sense) The processing power of dual- core controllers is allowing software to add more features while allowing complex calculations