Issue 54 Uncrewed Sytems Technology Feb/Mar 2024 uWare uOne UUV l Radio and telemetry l Rheinmetall Canada medevacs l UUVs insight DelltaHawk engine l IMU focus l Skygauge in operation l CES 2024 report l Blueflite l Hypersonic flight

84 Focus | IMUs, gyros and accelerometers IMUs, it is more effective at eliminating discrepancies in mechanical sensitivity, by virtue of all three gyros sitting at 120° to the Y-axis, rather than each having a different angle to that axis (and similar factors of that nature). Further configurations are being conceived to enable higher production volumes and lower costs of FOG IMUs for mass-market applications. One is simply making lower-end devices to fill the perceived middle ground between MEMS and FOG systems; for instance, by using smaller coil sizes of 300-400 m fibre lengths. FOG-on-chip tech This leads naturally to smaller FOGs that better fit the space constraints of most uncrewed vehicles. To an extent, this is being achieved through FOG-onchip (FoC) technology, also referred to as photonic gyro technology, in which the various components of a FOG are integrated into a planar optical chip, rather than involving a PCB and the inside of an electronics enclosure. A small variety of such FOG products are commercially available, although, arguably, the condensed nature of FoCs brings a far bigger benefit in terms of manufacturing simplicity than the size reduction that one might first notice. Engineers are also developing higherend systems to cater for the similarly broadening ecosystem of uncrewed vehicle types. Some readers may be familiar with the industry accepted distinction between tactical-grade IMUs, which require external aiding and are suitable for dead reckoning during brief GNSS outages of up to 10 minutes at a time before drift becomes problematic, and navigation-grade IMUs, which can empower uncrewed vehicles to navigate safely without GNSS for up to an hour at a time and can be used for gyrocompassing (finding true north). FOG-based systems often meet the standards defining navigation-grade solutions, while MEMS are typically used as tactical-grade devices (as well as industrial-grade systems, which can be relied on for one minute of dead reckoning). But some FOG designers are now eyeing what some refer to as the ‘strategic–’ or ‘sovereign-grade’ level, at which drift and positional error are between a tenth and a hundredth of that in navigation-grade systems. Usage cases for strategic-grade IMUs are presently few, with just submarines and intercontinental ballistic missiles (ICBMs) as the generally accepted examples, but as more new uncrewed systems go from concept to product, there are likely to be some high-end military UUVs, uncrewed fighter aircraft, autonomous air-freight carriers and other applications requiring strategic-grade navigation systems. Strategic-grade FOG IMUs are unlikely to be created or demanded in high volume any time soon, although producing them is estimated to require around 5 km of optical fibre, as well as an advanced form of accelerometer known as a quantum gravimeter. February/March 2024 | Uncrewed Systems Technology FOG manufacturers are expanding their production facilities to output their gyros and IMUs in higher volumes at lower cost (Image courtesy of Fiberpro) Some FOG IMUs are prized for their gyrocompassing ability, finding true north cost-effectively (Image courtesy of Exail)