Unmanned Systems Technology 006 | ECA Inspector Mk2 USV | Antenna systems | Northwest UAV NW-44 | Unmanned ground vehicles | Navigation systems | Lunar X challenge

72 for example, the required accuracy is 100 ppb or better, and bias stability 0.1°/h. Aircraft navigation sets the bar still higher, at 10 ppb and 0.01°/h, while strategic submarines pose the greatest challenge at 1 ppb and 0.001°/h. These days, MEMS sensor performance is considered to be limited by architecture and manufacturing methods, and is unlikely to move beyond tactical grade without a technology shift. That means the more demanding requirements are likely to remain the domain of other technologies such as ring laser gyros (RLGs), fibre-optic gyros (FOGs) and hemispherical resonator gyros (HRGs). FOGs and HRGs are serious rivals for high-performance applications. Optical gyros use light beams and interferometry to measure rotation, exploiting the Sagnac effect, which is a rotation-induced phase shift between light beams propagating in clockwise and counter-clockwise directions. RLGs send the lasers around a triangular path with a mirror at each vertex, while FOGs use coils of optical fibre. FOGs can be of solid-state, all-fibre or hybrid construction; moderately priced, they are typically used for industrial and high-end tactical applications. In a FOG, for example, the counter- propagating beams meet in an interferometer that contains a diffraction grating, the slits in which generate interference patterns that can yield precise angular rotation measurements. The effect can be compared to ripples on water refracting around a small island to create a distinctive interference pattern downstream. If the entire apparatus – light path, light source and detector – is stationary, then both beams will reach the detector at the same time and recombine constructively in a distinctive interference fringe pattern. If, however, the apparatus is rotated about the axis around which the two beams are travelling in opposite directions, then the beams will travel different distances before recombining. The beam moving around the loop in the direction of rotation will have a longer path, while the beam travelling against the direction of rotation will have a shorter path. In effect, the finish line is moved closer to the counter-rotating beam, making its trip slightly shorter. The result is another distinctive interference fringe pattern as the two out-of-phase light beams come together. There are no moving parts in a FOG, and it can exploit the Sagnac effect via a laser diode or super-luminescent diode as the light source, along with couplers, polarisers and a light detector. Given that the data provided by a FOG or FOG-based IMU is inherently dependent on the speed of light travelling through the FOG’s fibre coil, a lot of information can be collected and transmitted in a very short time, which bestows performance and accuracy characteristics that far exceed the capabilities of a MEMS device. The trade- offs in obtaining FOG-level accuracy though are usually in terms of size and cost: a navigation-grade FOG IMU is about the size of a standard coffee cup and can cost anywhere from $10,000 to more than $60,000. An HRG meanwhile consists of a vibrating hemisphere that exploits the Coriolis force – as MEMS gyros do – and comes in a ‘wine glass’ or ‘mushroom’ configuration because of their resemblance to these familiar objects. Like MEMS gyros, HRGs are excited into resonance (in this case flexural resonance), which changes in proportion to rotation about its axis of symmetry, and the output signal read by electrostatic transducers. Unlike MEMS sensors though, HRGs are made from fused quartz, of February/March 2016 | Unmanned Systems Technology This fibre-optic gyro-based inertial system provides advanced performance with minimal weight and low power consumption. (Courtesy of KVH Industries) Silicon hemispherical resonator gyro ‘mushroom’ resonators in production (Daniel Linares photo Courtesy of Sagem)