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

improving to 0.02 m/s with RTK and to 0.015 m/s with post-processing. A module with this performance can’t be made as small as the kind of multi-GNSS package without integral MEMS inertial sensing, but nominal dimensions of 67 x 60 x 15 mm and a weight of about 60 g are achievable, as is power consumption of around 3.5 W at room temperature. If size and weight are not a constraint, other types of inertial sensors can provide higher degrees of accuracy and performance. GPS compassing Inertial systems are less useful for determining yaw rates and heading when the vehicle is not moving much or is stationary, but GPS compassing can provide very accurate information that is not subject to the kinds of interference that affect magnetic compasses. GPS compassing uses interferometry, exploiting the raw pseudo-range information (the range from the satellite to the receiver) and phase difference in the signal arriving at separate antennas and receivers a known distance apart. They can achieve accuracies of better than 0.1°. Monolithic integration Because MEMS inertial sensors are made from silicon, with the same photochemical etching technology used to fabricate microprocessors, they are well suited to integration at board level into electronic systems, as is the case with the survey-grade system above. The logical conclusion of this, however, is monolithic integration of electronics and the sensors into the same piece of silicon to reduce size, weight and power consumption as well as overall cost. With gyros for measuring rotation, and accelerometers to measure acceleration and therefore linear displacement, such chips could be six-axis inertial measurement units (IMUs) with additional sensors such as magnetometers and altimeters built in. All these sensors need control electronics, which are usually in the form of application-specific integrated circuits (ASICs) anyway, so putting them on the same chip is very attractive. This technology is available now for the consumer sector, following the launch of the first nine-axis device in 2012, which integrates a gyro, an accelerometer and a magnetometer, each measuring three axes for wearable motion tracking devices. For more demanding applications, however, most systems are multi-chip modules as, from a manufacturer’s point of view, there are conflicting environmental requirements as well as differences in yield to contend with. MEMS gyros need a vacuum, while accelerometers need partial atmosphere. About 98% of ASIC chips that come off the line pass inspection, but the figures for MEMS accelerometers is far lower, at about 60%, and gyros down at about 25%. Mechanical gyroscopes, including MEMS types, exploit the Coriolis effect, which induces a force when a moving body is rotated to detect and measure angular velocity. Most MEMS gyros are of the vibrating ‘tuning fork’ type with resonators in the form of beams, discs, square plates, combs, rings and other shapes, classified by whether the vibration causes them to flex, twist or expand and contract along some axis, the latter known as bulk mode. (Bulk mode resonance is the basis for a Navigation systems | Focus Unmanned Systems Technology | February/March 2016 69 A technician tends to the silicon hemisphere of a hemispherical resonator gyro during assembly (Daniel Linares photo courtesy of Sagem) This inertial sensor contains MEMS accelerometers, gyros, magnetometers, an air pressure sensor, two GNSS receivers and a microprocessor (Courtesy of VectorNav)