Unmanned Systems Technology 023 I Milrem Multiscope I Wireless charging I Logistics insight I InterGeo, CUAV London & USA show reports I VideoRay Defender I OS Engines GR400U-FI I Ultrabeam Hydrographic Ultra-2 I IMUs

86 Focus | IMUs, gyros and accelerometers than 0.001 º /h, and scale factor error of less than 0.001%. However, that can actually far exceed what is necessary for most unmanned vehicles, and the low-volume production combined with high expense and weight has caused them to be largely replaced by FOGs in most very high-precision unmanned applications (one exception being high-grade UUVs, some of which still integrate LRGs). FOGs work using what is called the Sagnac Interferometer Model. A diode built into the gyro emits a beam of light that is split and routed in opposite directions around a fibre optic coil (which can extend from a few hundred metres to several kilometres in length) held in loops or ‘turns’ inside the gyro housing. When the coil is stationary, the clockwise and anti-clockwise beams travel through the fibre in the same amount of time, and arrive back at the source simultaneously. However, when the coil is rotating clockwise, for example, the clockwise beam will arrive slightly late and the anti- clockwise beam will arrive slightly early – a phenomenon called the Sagnac effect. As a rule, rotation of the gyroscope causes both light paths to face ‘interference’ in the form of unequal distances, and the extra time taken by the signal travelling ‘with’ the direction of the spin compared with the one travelling ‘against’ the direction of spin is measured by a photodiode to determine the angular rate. LRGs also operate using the Sagnac effect, but rather than sending a laser through a fibre optic coil, they typically feature three hollow channels arranged triangularly inside a glass block, and send two lasers ‘bouncing’ across mirrors placed at the three corners of the channels to gauge the interferometry and calculate angular rate. The strength of the Sagnac effect depends on the effective area of the optical path, which is a combination of the geometric area of the coil and the number of turns it has inside the FOG housing. Thus the more powerful the FOG is in terms of bandwidth, sensitivity, stability and other factors, the larger and heavier it will be. Despite the seeming inevitability of MEMS gyros catching up to FOGs in power and stability characteristics, the need to couple IMUs with GNSS for drift correction and providing position and heading means FOGs will remain superior for INSs in GNSS-denied environments for some time. That is because vehicles working in such environments are at less risk of position estimates drifting dangerously off-course when using the more powerful FOGs. As alluded to earlier, UUVs may always be better off using FOGs than MEMS gyros (or LRGs rather than FOGs or MEMs gyros, as some still do), since GNSS signals are rapidly attenuated below water. Successful operations are more likely when the UUV does not have to resurface repeatedly to regain a GPS fix for its survey measurements. The superior dead reckoning of FOGs compared with MEMS IMUs is also in high demand in defence applications, where reconnaissance UAVs in particular are increasingly at risk from GNSS- jamming signals. Their improved vibration tolerance compared with MEMS gyroscopes also makes them better suited to the surveillance gimbals and navigation systems of turbine-powered medium-to- high altitude UAVs, such as those for the world’s major navies and air forces. It must be noted though that the extra performance and reliability still comes at a cost. Estimates put the price of FOGs at around 10 times that of IMUs, with around four times the power consumption and more than 10 times the weight, meaning a generally similar ratio of SWaP-C (size, weight, power and cost) to performance for FOGs as well as MEMS gyros. December/January 2019 | Unmanned Systems Technology Fibre optic gyros use a diode to send light through the optical circuit of a coiled fibre, and detect rotation through the Sagnac effect (Courtesy of KVH)