INERTIAL NAVIGATION SYSTEM

• US 20170160306A1
• Filed: 12/07/2016
• Published: 06/08/2017
• Est. Priority Date: 12/07/2015
• Status: Active Grant

• Current Assignee: Atlantic Inertial Systems Ltd (Rtx Corporation)
• Original Assignee: Atlantic Inertial Systems Ltd (Rtx Corporation)
• Inventors: Sheard, John Keith, Faulkner, Nicholas Mark
• Granted Patent: US 11,112,422 B2
• Time in Patent Office: Days
• CPC Class Codes: F41G 7/36 using inertial references
• CPC Class Codes: F42B 15/01 Arrangements thereon for guidance or control steering arrangements F42B10/60; aircraft flight control B64C; guidance systems other than those installed aboard F41G7/00, F41G9/00; locating by use of radio or other waves G01S; flight control in general G05D1/00; computer aspects G06]
• CPC Class Codes: G01C 21/183 Compensation of inertial measurements, e.g. for temperature effects
• CPC Class Codes: G01C 25/005 initial alignment, calibration or starting-up of inertial devices
• CPC Class Codes: G01P 13/025 indicating air data, i.e. flight variables of an aircraft, e.g. angle of attack, side slip, shear, yaw
• CPC Class Codes: G05D 1/108 animated with a rolling movement

Abstract

An inertial measurement system for a spinning projectile comprising: 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 such that they define a three dimensional coordinate system; 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 based on the difference between the computed pitch and yaw angles and expected pitch and yaw angles; 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; wherein the Kalman filter models roll angle error as a function of roll rate and one or more wind variables. The system provides improved calibration of the roll axis rate gyro scale factor, e.g. of an IMU fitted to a rolling projectile. A separate process (an Euler angle filter) is used to calculate an estimate of the roll angle error without the use of the Kalman filter and is then provided as an input to the Kalman filter which can then operate in a stable manner. The filter can be configured to estimate and correct for crosswind effects which would otherwise significantly degrade performance.

Claims

1. An inertial measurement system for a spinning projectile comprising:

a first, roll gyro to be oriented substantially parallel to a spin axis of the projectile;

a second gyro and a third gyro with axes arranged with respect to the roll gyro such that they define a three dimensional coordinate system;

the first gyro, the second gyro and the third gyro each being arranged to produce an output;

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 based on a difference between the computed pitch and yaw angles and expected pitch and yaw angles;

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;

wherein the Kalman filter models roll angle error as a function of roll rate and one or more wind variables.

2. The inertial measurement system as claimed in claim 1, wherein the Kalman filter models the roll angle error as having a component correlated with roll rate and a separate component correlated with wind effects.

3. The inertial measurement system as claimed in claim 1, wherein the Kalman filter is configured such that in use it iteratively updates the model parameters.

4. The inertial measurement system as claimed in claim 1, wherein the controller is arranged to supply meteorological data to the filter as an initial condition for the Kalman filter.

5. The inertial measurement system as claimed in claim 1, wherein the controller is arranged to apply the roll rate scale factor correction directly to the roll gyro output, and is arranged to apply the roll angle correction to an attitude integration unit.

6. The inertial measurement system as claimed in claim 1, wherein the roll gyro is a MEMS gyro.

7. The inertial measurement system as claimed in claim 1, wherein the expected pitch and yaw angles as a function of flight time correspond to those expected from planar ballistic flight.

8. The inertial measurement system as claimed in claim 7, wherein the roll angle error is calculated as the angle whose tangent is the ratio of the rate of change of the calculated yaw angle to the rate of change of the calculated pitch angle.

9. The inertial measurement system as claimed in claim 8, further comprising a low pass filter arranged to filter out high frequency components of changes in the yaw angle and the pitch angle before calculation of the rate of change of the yaw angle and the rate of change of the pitch angle.

10. The inertial measurement system as claimed in claim 1, wherein the expected values for pitch and yaw angles as a function of flight time are taken from a pre-computed flight trajectory which may be non-planar.

11. The inertial measurement system as claimed in claim 1, wherein the system has no linear accelerometers.

12. The inertial measurement system as claimed in claim 1, wherein the roll angle correction and roll scale factor correction are only applied prior to any guidance action being initiated.

13. The inertial measurement system as claimed in claim 1, wherein the controller is arranged to calculate the roll angle error based on the difference between the computed pitch angle rate and yaw angle rate and expected pitch and yaw angle rates;

14. A method of correcting roll angle in an inertial measurement system for a spinning projectile, comprising carrying out the following steps on a controller:

computing a current projectile attitude comprising a roll angle, a pitch angle and a yaw angle;

calculating a roll angle error based on the difference between the computed pitch and yaw angles and expected pitch and yaw angles;

providing 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

applying the calculated roll angle correction and roll rate scale factor correction to the output of a roll gyro;

wherein the Kalman filter models roll angle error as a function of roll rate and one or more wind variables.

15. A method of correcting roll angle in an inertial navigation system for a longitudinal projectile, comprising carrying out the following steps on a controller:

computing a current projectile attitude comprising a roll angle, a pitch angle and a yaw angle;

calculating the rates of change of the pitch angle and yaw angle, using these rates to calculate a roll angle error and providing this as an input to a Kalman filter that outputs a roll angle correction and a roll rate scale factor correction; and

applying the calculated roll angle correction to the computed attitude and the roll rate scale factor correction to the roll gyro output;

wherein the Kalman filter models roll angle error as a function of roll rate and one or more wind variables.