Two gimbal error averaging astro-inertial navigator

A two gimbal strapdown astro-inertial navigator comprising

• a roll axis gimbal coupled to a roll gimbal torque motor for rotating said roll axis gimbal through at least 360.degree.,
• a pitch axis gimbal coupled to said roll axis gimbal and also coupled to a pitch gimbal torque motor for rotating said pitch axis gimbal through at least 360.degree.,
• a platform coupled to said pitch axis gimbal along a strapdown axis, said strapdown axis being substantially perpendicular to said platform,
• an instrument cluster coupled to said platform, said instrument cluster comprising
• an X-axis laser gyroscope,
• a Y-axis laser gyroscope,
• a Z-axis laser gyroscope,
• an X-axis accelerometer,
• a Y-axis accelerometer,
• a Z-axis accelerometer
• said instrument cluster further comprising solid state imaging means for imaging stellar objects, said solid state imaging means having an optical axis which is substantially colinear with said strapdown axis,
• means for compensating for navigation errors resulting from gyro and accelerometer errors during free inertial navigation by rotating said two gimbals, and
• means for performing three axis gyro and accelerometer error averaging based on data from rotating said two gimbals.

A method, while star tracking, of substantially cancelling fixed errors of an X-axis accelerometer and a Y-axis accelerometer both of which are coupled to a platform, comprising the steps of:

• orienting the platform in space such that an input axis of the X-axis accelerometer points along an X-axis and an input axis of the Y-axis accelerometer points along a Y-axis;
• operating the X-axis accelerometer and the Y-axis accelerometer for a first predetermined interval of time, each of the accelerometers generating an output signal which includes a first error component;
• rotating the platform in space such that the input axis of the X-axis accelerometer points along the X-axis in an opposite direction and the input axis of the Y-axis accelerometer points along the Y-axis in an opposite direction;
• operating the X-axis accelerometer and the Y-axis accelerometer for a second predetermined interval of time, each of the accelerometers generating an output signal which includes a second error component, the second error component having a sign opposite that of the first error component and a magnitude substantially equal to that of the first error component;
• and
• averaging the first and second error components such that a total error component over the first and the second predetermined intervals of time is substantially cancelled.

A method, while star tracking, of substantially compensating a boresight error of a star sensor which provides position related information to an astro-inertial navigator, the star sensor being coupled to a platform stabilized by a roll gimbal and a pitch gimbal, the platform being strapped down to the pitch gimbal along a strapdown axis which is substantially parallel with an optical axis of the star sensor, comprising the steps of:

• orienting the roll gimbal and the pitch gimbal to point the star sensor optical axis at a predetermined stellar object;
• determining a first angular misalignment between the optical axis and the stellar object, the first angular misalignment including a first component related to an attitude reference error of the navigator and a second component related to an angular misalignment of the optical axis relative to the strapdown axis;
• rotating both the roll gimbal and the pitch gimbal by 180.degree.;
• and
• determining a second angular misalignment between the optical axis and the stellar object, the second angular misalignment including a first component related to the attitude reference error of the navigator and a second component related to the angular misalignment of the optical axis relative to the strapdown axis, the second component after the step of rotating being of substantially equal magnitude and opposite sign to the second component before the step of rotating whereby the second component due to the angular misalignment of the optical axis relative to the strapdown axis is observed and compensated for.

Apparatus for substantially compensating a boresight error of a star sensor, while star tracking, which provides position related information to an astro-inertial navigator, the star sensor being coupled to a platform stabilized by a roll gimbal and a pitch gimbal, the platform being strapped down to the pitch gimbal along a strapdown axis which is substantially parallel with an optical axis of the star sensor, comprising:

• means for orienting the roll gimbal and the pitch gimbal to point the star sensor optical axis at a predetermined stellar object,
• means for determining a first angular misalignment between the optical axis and the stellar object, the first angular misalignment including a first component related to an attitude reference error of the navigator and a second component related to an angular misalignment of the optical axis relative to the strapdown axis,
• means for rotating both the roll gimbal and the pitch gimbal by 180.degree., and
• means for determining a second angular misalignment between the optical axis and the stellar object, the second angular misalignment including a first component related to the attitude reference error of the navigator and a second component related to the angular misalignment of the optical axis relative to the strapdown axis, the second component after the step of rotating being of substantially equal magnitude and opposite sign to the second component before the step of rotating whereby the second component due to the angular misalignment of the optical axis relative to the strapdown axis is observed and compensated for.

Apparatus for substantially cancelling fixed errors of an X-axis accelerometer and a Y-axis accelerometer, while star tracking, both of said accelerometers being coupled to a platform, comprising:

• means for orienting the platform in space such that an input axis of the X-axis accelerometer points along an X-axis and an input axis of the Y-axis accelerometer points along a Y-axis,
• means for operating the X-axis accelerometer and the Y-axis accelerometer for a first predetermined interval of time, each of the accelerometers generating an output signal which includes a first error component,
• means for rotating the platform in space such that the input axis of the X-axis accelerometer points along the X-axis in an opposite direction and the input axis of the Y-axis accelerometer points along the Y-axis in an opposite direction,
• means for operating the X-axis accelerometer and the Y-axis accelerometer for a second predetermined interval of time, each of the accelerometers generating an output signal which includes a second error component, the second error component having a sign opposite that of the first error component and a magnitude substantially equal to that of the first error component, and
• means for averaging the first and second error components such that a total error component over the first and the second pre-determined intervals of time is substantially cancelled.