Abstract: A terrain-referenced navigation system for an aircraft comprises: a stored digital terrain map; a position calculation unit arranged to calculate aircraft position relative to the stored digital terrain map to determine a terrain-referenced aircraft position; a fall line calculation unit arranged to calculate a fall line for a projectile starting from the terrain-referenced aircraft position as a launch point; and an impact point calculation unit arranged to directly compare the fall line with the digital terrain map, by incrementally comparing a height of the projectile along the fall line with a height of the terrain according to the stored digital terrain map in order to find an expected impact point on the terrain.

Abstract: A method of determining a reference direction for an angular measurement device, comprising: providing a rigid structure having an antenna for a global navigation satellite system (GNSS) fixed at a first point thereof; fixing the angular measurement device to a second point on the rigid structure, separated from the first point by at least 0.5 meters; while rotating the rigid structure so as to cause rotational movement of the antenna around the sensitive axis, acquiring velocity measurement data from the GNSS and angular velocity measurement data from the angular measurement device; and using the velocity measurement data and the angular velocity measurement data to determine a reference direction for the angular measurement device.

Abstract: A capacitive accelerometer includes a proof mass, first and second fixed capacitive electrodes, and a DC biasing element arranged to apply a DC voltage (VB) to the proof mass based on a threshold acceleration value. A first closed loop circuit is arranged to detect a signal resulting from displacement of the proof mass and control the pulse width modulation signal generator to apply the first and second drive signals V1, V2 with a variable mark:space ratio. A second closed loop circuit keeps the mark:space ratio constant and to change the magnitude, VB, of the DC voltage applied to the proof mass by the DC biasing element so as to provide a net electrostatic restoring force on the proof mass for balancing the inertial force of the applied acceleration and maintaining the proof mass at a null position, when the applied acceleration is greater than a threshold acceleration value.

Abstract: A vibrating structure gyroscope comprises a resonant structure arranged to vibrate under stimulation from a primary drive electrode. A drive system is arranged to vibrate the vibrating structure at a resonance frequency. An automatic gain control unit varies an amplitude of a primary drive signal (PD). A controller operates the gyroscope such that in a first mode of operation, the automatic gain control unit varies an amplitude of the drive signal (PD) between an operating range defined by upper and lower bounds and in a second mode operation, in which the automatic gain control unit sets the amplitude of the drive signal (PD) to a predetermined level outside of the operating range. In the second mode of operation an amplitude of a primary sense signal (PP) is measured after a predetermined time period to determine an oscillation cycle count during said predetermined time period.

Abstract: An inertial navigation system includes a first inertial measurement unit with at least a first sensor and a second inertial measurement unit with at least a second sensor corresponding in type to the first sensor. The first inertial measurement unit is rotatably mounted relative to the second inertial measurement unit, The inertial navigation system further include a controller arranged to: acquire a first set of measurements simultaneously from both the first inertial measurement unit and the second inertial measurement unit; rotate the first inertial measurement unit relative to the second inertial measurement unit; acquire a second set of measurements simultaneously from both the first inertial measurement unit and the second inertial measurement unit; and calculate from the first set of measurements and the second set of measurements at least one error characteristic of the first sensor and/or the second sensor.

Abstract: A sensor package includes a sensor, at least one external wall, and an interposer, arranged between the sensor and the at least one external wall. The sensor is wire bonded to the interposer and the interposer is wire bonded to the at least one external wall. Using an interposer, wire bonded to both the sensor and the at least one external wall, is an improved approach to electrically connecting a sensor and a sensor package. The interposer allows for short wire bonds from the sensor and the at least one external wall to the interposer, replacing the single, long wire bond from the sensor to the at least one external wall in the prior art. This provides improved resilience of the sensor package under high stress. Furthermore, it allows an existing sensor and package combination to be improved without needing to redesign either component.

Abstract: A sensor package comprising: a sensor, wherein the sensor comprises a sensing structure formed in a material layer and one or more further material layers arranged to seal the sensing structure to form a hermetically sealed sensor unit; a support structure; one or more springs flexibly fixing the hermetically sealed sensor unit to the support structure; wherein the one or more springs are formed in the same material layer as the sensing structure of the sensor unit; and one or more external package wall(s) encapsulating the sensor unit, the support structure, and the one or more springs, wherein the support structure is fixed to at least one of the package wall(s). The springs decouple mechanical stresses between the sensor unit and the external package wall(s) so as to reduce the long term drift of scale factor and bias.

Abstract: A method of demodulating a MEMS sensor pickoff signal from a vibrating resonator of said sensor, the method comprising: sampling the pickoff signal with an asynchronous ADC at a sampling rate of at least 50 times the resonant frequency of the resonator to generate a stream of samples; generating a first value by combining samples from said stream of samples according to a selected operation, said operation being selected in dependence on a synchronous clock signal that is synchronous to the resonant frequency of the resonator, said synchronous clock signal having a frequency at least twice the resonant frequency of the resonator; and counting the number of samples contributing to the first value. The increased sampling rate of the pickoff signal allows a much higher number of samples to be taken into account, thereby reducing noise. However, the ADC asynchronously from the resonator of the MEMS sensor.

Abstract: A vibrating structure gyroscope includes a permanent magnet, a structure arranged in a magnetic field of the permanent magnet and arranged to vibrate under stimulation from at least one primary drive electrode and a drive system that includes: one primary drive electrode arranged at least one primary sense electrode arranged to sense motion in the vibrating structure and a drive control loop controlling the primary drive electrode dependent on the primary sense electrode. The structure also includes a compensation unit arranged to receive a signal from the drive system representative of a gain in the drive control loop and arranged to output a scale factor correction based on that signal. As the magnet degrades (e.g. naturally over time as the material ages), the magnetic field weakens. To compensate for this, the primary drive control loop will automatically increase the gain.

Abstract: A vibrating structure angular rate sensor comprises a MEMS structure includes a mount, a plurality of supporting structures fixed to the mount, and a vibrating planar ring structure flexibly supported by the plurality of supporting structures to move elastically relative to the mount. At least one primary drive transducer is arranged to cause the ring structure to oscillate in a primary mode at the resonant frequency of the primary mode. At least one primary pick-off transducer arranged to detect oscillation of the ring structure in the primary mode. At least three secondary pick-off transducers are arranged to detect oscillation of the ring structure in a secondary mode induced by Coriolis force when an angular rate is applied around an axis substantially perpendicular to the ring structure. At least one secondary drive transducer is arranged to null the induced oscillation in the secondary mode.

Abstract: An angular velocity sensor comprises: an insulative support layer (10); a substrate layer (8) formed of a silica-based material and comprising a planar ring structure (2) mounted to vibrate in-plane; and a plurality of conductive electrodes (14), each comprising a first set of moveable conductive electrode tracks (14a) formed on a surface of the planar ring and a second set of fixed conductive electrode tracks (14b) formed on a surface of the insulative support layer axially spaced from the surface of the planar ring. The first and second sets of conductive electrode tracks are interdigitated with a lateral spacing between them in a radial direction. Each moveable conductive electrode track has a radial offset from a median line between adjacent fixed conductive electrode tracks such that each moveable conductive electrode track has a different lateral spacing from two different adjacent fixed conductive electrode tracks in opposite radial directions.

Abstract: A method of manufacturing an inertial measurement unit (IMU) comprises fabricating a plurality of individual MEMS inertial sensor packages at a package level as sealed packages containing a MEMS inertial sensor chip and an integrated circuit electrically connected together. Fabricating the individual MEMS inertial sensor packages comprises forming mechanical interconnect features in each package and assembling the IMU by mechanically interconnecting each individual MEMS inertial sensor package with another individual MEMS inertial sensor package in a mutually orthogonal orientation.

Abstract: An inertial measurement system for a spinning projectile includes: a first, roll gyro to be oriented substantially parallel to the spin axis of the projectile; a second gyro and a third gyro with axes arranged with respect to the roll gyro; a controller, arranged to: compute a current projectile attitude from the outputs of the first, second and third gyros, the computed attitude comprising a roll angle, a pitch angle and a yaw angle; calculate a roll angle error; provide the roll angle error as an input to a Kalman filter that outputs a roll angle correction and a roll rate scale factor correction; and apply the calculated roll angle correction and roll rate scale factor correction to the output of the roll gyro.

Abstract: An micro electro mechanical sensor comprising: a substrate; and a sensor element movably mounted to a first side of said substrate; wherein a second side of said substrate has a pattern formed in relief thereon. The pattern formed in relief on the second side of the substrate provides a reduced surface area for contact with the die bond layer. The reduced surface area reduces the amount of stress that is transmitted from the die bond layer to the substrate (and hence reduces the amount of transmitted stress reaching the MEMS sensor element). Because the substrate relief pattern provides a certain amount of stress decoupling, the die bond layer does not need to decouple the stress to the same extent as in previous designs. Therefore a thinner die bond layer can be used, which in turn allows the whole package to be slightly thinner.

Abstract: A navigation system comprising: an inertial navigation system arranged to output a first position estimate; a terrain based navigation unit arranged to output a second position estimate; a gravity based navigation unit arranged to output a third position estimate; a stored gravity map arranged to receive a position and to output gravity information for that position; and an iterative algorithm unit arranged to determine an INS error state in each iteration; wherein in each iteration the iterative algorithm unit is arranged to: receive the first position estimate, the second position estimate, and the third position estimate; determine a gravity corrected position estimate based on the first position estimate, the INS error state and the gravity information; and update the INS error state for the next iteration based on the INS error state, the gravity corrected position estimate, the second position estimate and the third position estimate.

Abstract: A terrain-based navigation system include at least three laser range finders, each fixedly mounted to a vehicle body, each pointing in a different direction and arranged such that they can be used to calculate terrain gradient in two dimensions. Existing terrain-based navigation systems for aircraft that use a radar altimeter to determine the distance of the vehicle from the ground make use of the large field of view of the radar altimeter. The first return signal from the radar altimeter may not be from directly below the aircraft, but will be interpreted as being directly below the aircraft, thereby impairing the chances of obtaining a terrain match, or impairing the accuracy of a terrain match. The use of a plurality of laser range finders each fixedly mounted to the vehicle body allows more terrain information to be obtained as the terrain can be detected from the plurality of different directions.

Abstract: A navigation system comprising: an inertial navigation system arranged to output a first position estimate; a terrain based navigation unit arranged to output a second position estimate; a stored gravity map arranged to receive a position and to output gravity information for that position; and an iterative algorithm unit arranged to determine an INS error state in each iteration; wherein in each iteration the iterative algorithm unit is arranged to: receive the first position estimate and the second position estimate; determine a gravity corrected position estimate based on the first position estimate, the INS error state and the gravity information; and update the INS error state for the next iteration based on the INS error state, the gravity corrected position estimate and the second position estimate.

Abstract: A vibrating structure angular rate sensor is provided which comprises a substrate; a plurality of flexible supporting structures fixed to the substrate; an annular member which is flexibly supported by the plurality of supporting structures to move elastically relative to the substrate; and an electrical drive system configured to drive the annular member to oscillate in a primary mode of oscillation with a resonant frequency f1. The plurality of supporting structures comprises at least one active supporting structure which carries an active electrical connection from the annular member to the drive system; and at least one passive supporting structure which does not carry an active electrical connection from the annular member to the drive system.

Abstract: An accelerometer closed loop control system comprising: a capacitive accelerometer comprising a proof mass moveable relative to first and second fixed capacitor electrodes; a PWM generator to generate in-phase and anti-phase PWM drive signals with an adjustable mark/space ratio, wherein said drive signals are applied to the first and second electrodes such that they are charged alternately; an output signal detector to detect a pick-off signal from the accelerometer representing a displacement of the proof mass from a null position to provide an error signal, wherein the null position is the position of the proof mass relative to the fixed electrodes when no acceleration is applied; a PWM servo operating in closed loop to vary the mark/space ratio of said PWM drive signals in response to the error signal so that mechanical inertial forces are balanced by electrostatic forces.

Abstract: A sensing structure for an accelerometer includes a support and a proof mass mounted thereto by flexible legs. The proof mass has moveable electrode fingers perpendicular to the sensing direction and at least four fixed capacitor electrodes, with fixed capacitor electrode fingers perpendicular to the sensing direction. The fixed capacitor electrode fingers interdigitate with the movable electrode fingers and the proof mass is mounted to the support by an anchor on a centre line of the proof mass. The proof mass has an outer frame surrounding the fixed capacitor electrodes and the flexible legs extend laterally inwardly from the proof mass to the anchor. The fixed capacitor electrodes comprise two inner electrodes, one on each side of the proof mass centre line, and two outer electrodes, one on each side of the proof mass centre line.