6 August/September 2023 | Uncrewed Systems Technology Mission-critical info for uncrewed systems professionals Platform one Advanced Navigation has developed two digital fibre optic gyroscopes (DFOGs) with different performances (writes Nick Flaherty). The Boreas A90 and A70 are IMUs containing highly accurate DFOGs and closed-loop accelerometers. Underpinning their performance is a sensor fusion algorithm that extracts far more information from acceleration and orientation data than a Kalman algorithm. The A90 offers 0.005o roll and pitch, 0.01o latitude heading accuracy for the gyrocompass as well as 0.001o/hour bias instability and a 1000 Hz update rate. The A70 offers 0.01o roll and pitch, and the same 0.1o latitude heading accuracy, although the bias instability is lower, at 0.01o/hour. Both systems can acquire and maintain an accurate heading under all conditions without having to rely on GNSS, making them suitable for navigation in subsea, marine, land and air applications. They use a digital modulation technique that passes spread-spectrum signals through the fibre optic coil of the DFOG. This allows in-run variable errors in the coil to be measured and removed from the measurements. That makes a DFOG much more stable and reliable than traditional FOGs. It also allows for a smaller FOG with reduced coil length and still achieve the accuracy of one with a longer coil. The devices use a specially designed closed-loop fibre optic coil which has been developed to allow optimum sensing of in-run variable coil errors using the digital modulation technique. It also provides a high level of shock and vibration protection for the optical components. By integrating five sensitive optical components into a single chip and removing all the fibre splices, the size, weight and power of the devices have been reduced by 40% while significantly improving reliability and performance. The A90 and A70 also have an optional licence to add INS capabilities and enable integration with external GNSS receivers using Advanced Navigation’s range of interfaces and comms protocols. The technology is aimed at sectors such as autonomous vehicles, land surveying, and subsea navigation and mining. Navigation Digital FOGs cut errors The DFOGs use a special closed-loop fibre optic coil that allows optimum sensing of in-run variable coil errors
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