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

73 high purity, rather than silicon, because of its very low damping characteristics that preserve the resonance. Operating in a vacuum, HRGs achieve very high Q factors, which is a measure of how quickly the vibration fades due to damping forces. A good resonator will carry on vibrating for a long time after being excited, the way a well-made bell will continue to ring after the strike of the clapper. HRGs also differ from other, non- MEMS gyros in that their accuracy is not related to their size, so high-performance HRGs can potentially be made smaller and lighter than their optical equivalents, although they are limited by current manufacturing technology. As they are formed from quartz, the shell must then be machined to extremely fine tolerances, using techniques such as ion-beam micro-erosion or laser ablation, on both the inner and outer surfaces of the hemisphere to achieve the accuracy required, and current machinery can only manage this down to a diameter of about 20 mm. HRGs are best known in spacecraft applications, where they have been used in missions lasting more than 15 years without failure – in fact it is claimed that no HRG has ever failed in space. With costs coming down, they are now being developed for airliner applications and are already offered for submarines, ships, airborne precision weapons and land vehicle positioning and navigation applications, so migration to unmanned vehicle systems can’t be far behind. In a representative strap-down attitude and heading reference system for surface ships for example, a typical package measuring 20.8 x 27.5 x 13.6 cm and weighing 4.5 kg can achieve a heading dynamic accuracy of 0.1° root mean square (RMS), a roll and pitch accuracy of 0.05° RMS, a heave accuracy of 5 cm or 5% (whichever is greater) and a position drift of 1 nautical mile per hour unaided. An alignment time of less than six minutes to a heading accuracy of better than 2° is also claimed, along with an MTBF of around 100,000 h. There is also a ground vehicle package available measuring 8.8 x 11 x 11 cm, weighing less than 1.5 kg and drawing under 13 W that is claimed to achieve horizontal and vertical position accuracies better than 1% of distance travelled expressed as a circular error probable. The manufacturer also claims a heading accuracy of 1° RMS and 0.6° RMS in pitch and roll, and an MTBF of 75,000 h. Magnetic options Inertial sensors are not the only GNSS- and radio-independent navigation sources available. The magnetic compass is one of the oldest of all, and it retains an important place in integrated multi-sensor navigation systems because the Earth’s magnetic field is available everywhere and always points to Magnetic North, although a declination angle should be applied to allow for the difference between Magnetic and Geographic North. The magnetic field is not subject to short-term drift but it does move over longer timescales. It is also subject to disturbances and distortion from ferrous metal objects, so modern digital magnetic compasses (DMCs) are packaged increasingly with other sensors and firmware to compensate. Direction is calculated from the displacement angles between the two horizontal axes of the magnetic field, the x and y components. There is also a vertical ( z ) component that is often ignored but can be used for tilt compensation and calculating pitch and roll angles, for example. Most DMCs are based on solid-state magneto-resistive sensors that can resolve rotation angles of less than 0.1°. The sensor’s electrical resistance varies with magnetic field strength, causing a proportional analogue change in the voltage of its output current, which is fed through an A-to-D converter to generate a digital signal. Three-axis sensors are available as surface-mount chips as small as 3 x 3.5 x 0.9 mm; however integrated systems with inertial compensation come in larger packages. For example, a multi-sensor module containing three magneto-resistive sensors, three MEMS accelerometers and a MEMS yaw-rate gyro could measure 25.4 x 26.4 x 13.7 mm. Such systems can offer better Navigation systems | Focus Unmanned Systems Technology | February/March 2016 The Earth’s magnetic field has been used for navigation for centuries, and is still exploited by digital magnetic compasses (Courtesy of the ESA)