Azimuth-axis drift rate determination in an inertial navigator

• US 5,941,935 A
• Filed: 09/23/1996
• Issued: 08/24/1999
• Est. Priority Date: 09/23/1996
• Status: Expired due to Fees

• Current Assignee: Manuel Fernandez
• Original Assignee: Manuel Fernandez
• Inventors: Fernandez, Manuel
• Primary Examiner(s): Cuchlinski, Jr., William A.
• Assistant Examiner(s): ARTHUR JEANGLAUDE, GERTRUDE
• Application Number:
  US08/717,858
• Time in Patent Office: 1,065 Days
• Field of Search: 701/220, 701/221, 701/200, 701/226, 701/207, 701/224, 701/300, 73/178 R
• US Class Current: 701/220
• CPC Class Codes: G01C 21/188 for accumulated errors, e.g. by coupling inertial systems with absolute positioning systems

Abstract

A process and algorithms to determine and correct for azimuth-axis drift rate in an inertial navigator. The process and algorithms may be implemented as an automatic sequence within the system computer to determine and correct the system errors thus determined. These actions provide the system with improved directional accuracy and improved navigation performance.

Claims

1. A method for correcting an azimuth axis angular drift rate in an inertial navigator, comprising the steps of:

initializing the inertial navigator while at rest;

placing the inertial navigator in an unaided navigation mode;

measuring navigational parameters after a time interval T, said time interval T of a duration long enough to allow an error in latitude or north velocity due to azimuth-axis angular drift rate to become observable;

determining an azimuth-axis angular drift rate based upon said error; and

correcting the inertial navigator for the azimuth-axis angular drift rate and for inertial navigator errors caused by the azimuth-axis angular drift rate,

wherein said step of determining an azimuth-axis angular drift rate based upon said error is accomplished according to the formula ##EQU10## if said error comprises a latitude error, or ##EQU11## if said error comprises a north velocity error, wherein Δ

DR_GD represents the azimuth-axis angular drift rate, Lt represents a latitudinal position of the inertial navigator, Δ

Lt represents the latitude error, Δ

V_NS represents the north velocity error, Lg represents a longitude rate, Ω

represents the Earth'"'"'s angular rate in inertial space, R_EO represents a distance from the center of the Earth to the inertial navigator, t represents the time interval T, and G represents a gravitational force.

2. The method of claim 1, wherein said step of correcting the inertial navigator for the azimuth-axis angular drift rate and for the inertial navigator errors caused by the azimuth-axis angular drift rate comprises the step of correcting the inertial navigator for an azimuth-axis angular error, a north level-axis angular error with respect to the horizontal, a north velocity error, and a latitude error.

3. The method of claim 2, wherein said step of correcting the inertial navigator for an azimuth-axis angular error, a north level-axis angular error with respect to the horizontal, a north velocity error, and a latitude error is accomplished according to the formulas

4. The method of claim 1 wherein said time interval T comprises about one Shuler period.

5. The method of claim 4, wherein said error comprises a latitude error, and said step of determining an azimuth-axis angular drift rate based upon said error is accomplished according to the formula ##EQU12## wherein Δ

DR_GD represents the azimuth-axis angular drift rate, Lt represents a latitudinal position of the inertial navigator, Δ

Lt represents the latitude error, Lg represents a longitude rate, Ω

represents the Earth'"'"'s angular rate in inertial space, R_EO represents a distance from the center of the Earth to the inertial navigator, and G represents the value of gravity.

6. The method of claim 4, wherein said error comprises a north velocity error, and said step of determining an azimuth-axis angular drift rate based upon said error is accomplished according to the formula ##EQU13## wherein Δ

DR_GD represents the azimuth-axis angular drift rate, Lt represents a latitudinal position of the inertial navigator, Δ

V_NS represents the north velocity error, Lg represents a longitude rate, Ω

represents the Earth'"'"'s angular rate in inertial space, R_EO represents a distance from the center of the Earth to the inertial navigator, and G represents the value of gravity.

7. The method of claim 4, wherein said step of correcting the inertial navigator for the azimuth-axis angular drift rate and for the inertial navigator errors caused by the azimuth-axis angular drift rate comprises the step of correcting the inertial navigator for an azimuth-axis angular error, a north velocity error, and a latitude error.

8. The method of claim 7, wherein said step of correcting the inertial navigator for an azimuth-axis angular error, a north velocity error, and a latitude error is accomplished according to the formulas

9. The method of claim 1, wherein said steps of determining an azimuth-axis angular drift rate based upon said error and correcting the inertial navigator for the azimuth-axis angular drift rate and for inertial navigator errors caused by the azimuth-axis angular drift rate are carried out using an automatic sequence within a system computer.

10. The method of claim 1, wherein said step of measuring navigational parameters after a time interval T comprises the step of obtaining a reference position from at least one external navigation aid.

11. The method of claim 10, wherein said at least one external navigation aid comprises a global positioning system (GPS).

12. The method of claim 1, further comprising the steps of:

estimating functions (Ω

-Lg)·

cos Lt and R_EO from a system model, wherein Ω

represents the Earth'"'"'s angular rate in inertial space, Lg represents a longitudinal rate, Lt represents a latitudinal position of the inertial navigator, and R_EO represents a distance from the center of the Earth to the inertial navigator;

selecting a test value for the azimuth-axis angular drift rate;

deriving an estimated north velocity error or latitude error based upon said test value for the azimuth-axis angular drift rate; and

calculating an actual azimuth-axis angular drift rate based upon said estimated north velocity error or latitude error and the north velocity error or latitude error obtained by the step of measuring said navigational parameters.

13. The method of claim 12, wherein said step of calculating the actual azimuth-axis angular drift rate based upon said estimated north velocity error or latitude error and the north velocity error or latitude error obtained by the step of measuring said navigational parameters is accomplished according either one of the formulas

14. The method of claim 1, wherein said steps of measuring navigational parameters, determining an azimuth-axis angular drift rate based upon said error, and correcting the inertial navigator for the azimuth-axis angular drift rate and for inertial navigator errors caused by the azimuth-axis angular drift rate, are repeated so as to periodically correct the inertial navigator for the azimuth-axis angular drift rate and for inertial navigator errors caused by the azimuth-axis angular drift rate.

15. A method for correcting an azimuth axis angular drift rate in an inertial navigator, comprising the steps of:

initializing the inertial navigator while at rest,

placing the inertial navigator in an unaided navigation mode,

measuring navigational parameters after a time interval T, said time interval T of a duration long enough to allow an error in latitude or north velocity due to azimuth-axis angular drift rate to become observable,

determining an azimuth-axis angular drift rate based upon said error;

correcting the inertial navigator for the azimuth-axis angular drift rate and for inertial navigator errors caused by the azimuth-axis angular drift rate;

measuring a north velocity error while the inertial navigator is at rest;

measuring an east velocity error while the inertial navigator is at rest;

determining drift rates about an east axis and a north axis;

correcting the inertial navigator for the drift rates about the east axis and the north axis;

correcting the inertial navigator for the east velocity error and the north velocity error;

determining cross-axis position and velocity errors in navigational data output by the inertial navigator after the inertial navigator has traveled from an original location to a new location, said cross-axis position and velocity errors caused at least in part by an initial azimuth misalignment;

correcting the inertial navigator for the cross-axis position and velocity errors;

determining initial azimuth misalignment from at least one of said cross-axis errors; and

correcting the inertial navigator for an initial azimuth misalignment and an east gyro drift rate that caused the initial azimuth misalignment.

16. A method of navigating using an inertial navigator, comprising the steps of:

initializing the inertial navigator while at rest, said inertial navigator thereby being self-aligned in azimuth;

placing the inertial navigator in an unaided navigation mode;

measuring, during an at-rest measurement interval, a north velocity error while the inertial navigator is at rest;

measuring, during said at-rest measurement interval, an east velocity error while the inertial navigator is at rest;

determining drift rates about an east axis and a north axis;

correcting the inertial navigator for the drift rates about the east axis and the north axis;

correcting the inertial navigator for the north velocity error and the east velocity error;

placing the inertial navigator in motion;

determining cross-axis portion and velocity errors in navigational data output by the inertial navigator after the inertial navigator has traveled from an original location to a new location, said cross-axis position and velocity errors caused at least in part by an initial azimuth misalignment;

determining an initial azimuth misalignment and an east gyro drift rate from at least one of said cross-axis position and velocity errors;

correcting the inertial navigator for the cross-axis position and velocity errors;

correcting the inertial navigator for an initial azimuth misalignment and an east gyro drift rate that caused the initial azimuth misalignment;

measuring navigational parameters after a time interval T from placing the inertial navigator in the unaided navigation mode, said time interval T of a duration long enough to allow an error in latitude or north velocity due to azimuth-axis angular drift rate to become observable;

determining an azimuth-axis angular drift rate based upon said error;

correcting the inertial navigator for the azimuth-axis angular drift rate and for the inertial navigator errors caused by the azimuth-axis angular drift rate.

17. The method of claim 16, wherein said inertial navigator travels from said original location to said new location on a vehicle of flight.

18. The method of claim 16, wherein said time interval T comprises about one Schuler period.

19. The method of claim 16, wherein said step of correcting the inertial navigator for the azimuth-axis angular drift rate and for the inertial navigator errors caused by the azimuth-axis angular drift rate comprises the step of correcting the inertial navigator for an azimuth-axis angular error, a north level-axis angular error with respect to the horizontal, a north velocity error, and a latitude error.