Miniature north reference unit

Claim 2

An instrument for providing a signal from which the direction of true month may be determined, said instrument comprising: a frame; means for sensing the rate of angular motion of the earth about a sensitive axis and for providing an output signal representative of said earth rate; means for supporting said sensing means on said frame with a sensitive axis of said sensing means rigidly constrained from any rotation with respect to said frame in any one substantially horizontal first direction, said supporting means including means for isolating said sensing means from small rotations about any horizontal axis of said frame relative to the earth's surface; said output signal being indicative of the angle between said first direction and true north; and means for establishing a reference horizontal direction with respect to said first direction of the sensitive axis of said sensing means.

Claim 3

The apparatus as claimed in claim 1 further including means for selectively rotating said supporting means and said sensing means about the vertical through at least one known angle to constrain the sensitive axis of said sensing means in a substantially horizontal second direction.

Claim 4

The apparatus as claimed in claim 2 further including means for temporarily and releasably holding said sensing means in a position with the sensitive axis oriented in said first direction and for temporarily and releasably holding said sensing means in a position with the sensitive axis oriented in said second direction.

Claim 5

The apparatus as claimed in claim 3 wherein said holding means is adapted to selectively hold said sensing means with its sensitive axis oriented in any one of a plurality of horizontal directions.

Claim 6

The apparatus as claimed in claim 2 further including means for automatically computing angles which is responsive to said measuring means to determine the angle between one of said first and second directions of the sensitive axis and the direction of true north.

Claim 7

The apparatus as claimed in claim 1 wherein said sensing means comprises a gyroscope having at least a single degree of freedom and further including means for restraining rotation of said isolating means about the vertical, said gyroscope mounted on said isolating means with its spin axis in a substantially vertical direction.

Claim 8

A gyroscopic instrument for determining the direction of true north comprising: a frame; a gyroscope having two-degrees-of-freedom about two mutually orthogonal sensitive axes, said gyroscope including an inertial wheel, means for detecting angular displacement of said inertial wheel about the sensitive axes and means for torquing the inertial wheel about the sensitive axes, said torquing means responsive to said detecting means to counteract angular displacement of said detecting means about the sensitive axes resulting from at least one component of the horizontal angular rate of rotation of the earth; means for supporting said gyroscope on said frame with the spin axis of said gyroscope oriented substantially vertically and the sensitive axes oriented in a substantially horizontal first direction, said supporting means isolating said gyroscope from small horizontal rotations about any horizontal axis of said frame, relative to the earth's surface; means for rigidly restraining rotation of said isolating means and said gyroscope about the vertical with respect to said frame; means for measuring torque applied to said gyroscope to precess said gyroscope to counteract angular displacement resulting from a component of the rate of rotation of the earth; and means for establishing a reference horizontal direction with respect to the direction of at least one of the sensitive axes of said gyroscope.

Claim 9

A gyroscopic instrument for determining the direction of true north comprising: a frame; a gyroscope having two-degrees-of-freedom about two mutually orthogonal sensitive axes, said gyroscope including an inertial wheel, means for detecting angular displacement of said inertial wheel about the sensitive axes and means for torquing the inertial wheel about the sensitive axes, said torquing means responsive to said detecting means to counteract angular displacement of said detecting means about the sensitive axes resulting from at least one component of the horizontal angular rate of rotation of the earth; means for supporting said gyroscope on said frame with the spin axis of said gyroscope oriented substantially vertically and the sensitive axes oriented in a substantially horizontal first direction, said supporting means isolating said gyroscope from small horizontal rotations about any horizontal axis of said frame, relative to the earth's surface; means for rigidly restraining rotation of said isolating means and said gyroscope about the vertical with respect to said frame; means for measuring torque applied to said gyroscope to precess said gyroscope to counteract angular displacement resulting from a component of the rate of rotation of the earth; and means for establishing a reference horizontal direction with respect to the direction of at least one of the sensitive axes of said gyroscope.

Claim 10

The apparatus as claimed in claim 7 further including means for automatically computing angles responsive to said torquing means to determine the angle between the direction of at least one of the sensitive axes of said gyroscope and the direction of true north.

Claim 11

The device as claimed in claim 7 further including means for selectively rotating said isolating means said gyroscope through at least one known angle.

Claim 12

The apparatus as claimed in claim 9 wherein the magnitude of said known angle is 180°.

Claim 13

The apparatus as claimed in claim 7 wherein said detecting means comprises a first and a second pick-off means, each pick-off means sensing angular displacement of said gyroscope about one of the two sensitive axes of the gyroscope, said first and second pick-off means generating first and second pick-off signals, respectively, in response to angular displacement of said gyroscope about a sensitive axis.

Claim 14

The apparatus as claimed in claim 11 wherein said torquer means comprises first and second torquer means, each of said torquer means adapted to precess said gyroscope about one of the two sensitive axes of said gyroscope.

Claim 15

The apparatus as claimed in claim 12 further including first and second rebalance circuits, said first rebalance circuit connected between said first pick-off means and said first torquer means for coupling said first pick-off signal to said first torquer means, said torquer means responsive to said first pick-off signal to torque said gyroscope in a direction to null said first pick-off signal, said second rebalance circuit connected between said second pick-off means and said second torquer means for coupling said second pick-off signal to said second torquer means, said second torquer means responsive to said second pick-off signal to torque said gyroscope in a direction to null said second pick-off signal.

Claim 16

The apparatus as claimed in claim 13 wherein said first and second rebalancing circuits are digital circuits and said first and second pick-off signals each comprise a pulse train.

Claim 17

The apparatus as claimed in claim 14 wherein said automatic computing means comprises means for determining the arc tangent of an angle the tangent of which is the one vector component of earth rate in the direction of one sensitive axis of said gyroscope divided by the other vector component of earth rate in the direction of the other sensitive axis of said gyroscope.

Claim 18

The apparatus as claimed in claim 7 wherein said supporting means comprises an inner housing, a flotation fluid contained in said inner housing and a float floating in said fluid wherein said gyroscope is mounted on said float with its spin axis in a substantially vertical orientation.

Claim 19

The device as claimed in claim 16 wherein said restraining means is connected between said inner housing and said float.

Claim 20

The apparatus as claimed in claim 17 wherein said restraining means comprises at least one diaphragm, said diaphragm connected between said float and said frame to provide high compliance about all axes in a substantially horizontal plane and to provide rigidity about an axis perpendicular to the substantially horizontal plane.

Claim 21

The apparatus as claimed in claim 7 wherein said means for establishing a reference direction comprises a mirror mounted on said frame, the plane of said mirror being perpendicular to the direction of one of said sensitive axes.

Claim 22

The apparatus as claimed in claim 8 wherein said automatic computing means determines the magnitude of an angle α the tangent of which is equal to one vector component of the horizontal rate of rotation of the earth one sensitive axis of said gyroscope divided by an orthogonal vector component of the horizontal rate of rotation of the earth about the other sensitive axis of said gyroscope.

Claim 23

The apparatus as claimed in claim 20 wherein said computing means calculates an arc tangent to determine the angle α.

Claim 24

The apparatus as claimed in claim 20 wherein said automatic computing means determines an arc cotangent to determine the angle α.

Claim 27

The method as claimed in claim 23 wherein said gyroscope has at least a single degree of freedom and further comprising the step of rotating said gyroscope about its substantially vertical spin axis through an angle of 90° which additional step is performed after the step of torquing said gyroscope about a sensitive axis in a first direction.

Claim 28

The method as claimed in claim 24 wherein said first direction and said second direction differ by 90° and further including after the step of torquing the the gyroscope about a sensitive axis in said second direction the additional steps of: rotating at least the pickoff and torquer of said gyroscope about the vertical through an angle of substantially 90° to extend said first sensitive axis in a substantially horizontal third direction which differs by substantially 180° from said first direction; torquing said gyroscope about said first sensitive axis to counteract the angular displacement resulting from a vector component of the horizontal rate of rotation of the earth about said first sensitive axis extending in said third direction and to cancel any errors in the torquing about said first sensitive axis extending in said first direction due to a gyro bias error and to nonorthogonality between the spin and sensitive axes and to any tilt angle of the spin axis from the vertical or to any tilt angle of the sensitive axis from the horizontal; rotating at least the pick-off and torquer of said gyroscope about its spin axis through an angle of substantially 90° to extend said first sensitive axis in a substantially horizontal fourth direction which differs by substantially 180° from said second direction; torquing said gyroscope about said first sensitive axis to counteract the angular displacement resulting from a vector component of the horizontal rate of rotation of the earth about said second sensitive axis extending in said fourth direction and to cancel any errors in the torquing about said first sensitive axis extending in said second direction due to a gyro bias error and to nonorthogonality between the spin and sensitive axes and to any tilt angle of the spin axis from the vertical or to any tilt angle of the spin axis from the horizontal; and wherein the step of calculating comprises calculating the ratio of the algebraic difference of the counteracting torques applied to said gyroscope about said first sensitive axis in said first and third directions divided by the algebraic difference of the counteracting torques applied to said gyroscope about said first sensitive axis extending in second and fourth directions.

Claim 29

The method of claim 25 wherein the step of calculating comprises calculating an arc tangent.

Claim 30

The method of claim 25 wherein the step of calculating comprises calculating an arc cotangent.

Claim 31

The method as claimed in claim 23 wherein said gyroscope has two degrees of freedom with mutually orthogonal sensitive axes in said first and second directions and wherein the steps of torquing said gyroscope about said first and second sensitive axes extending in said first and second directions are simultaneously performed.

Claim 32

The method as claimed in claim 23 wherein said gyroscope has two degrees of freedom and two mutually orthogonal first and second sensitive axes and further including after the steps of torquing said gyroscope about sensitive axes in said first and second directions the additional steps of: rotating said gyroscope about the vertical through an angle of substantially 180° so that said first sensitive axis extends in a substantially horizontal third direction and said second sensitive axis extends in a substantially horizontal fourth direction, said third direction differing from said first direction by substantially 180° and said fourth direction differing from said second direction by substantially 180°; torquing said gyroscope about said first sensitive axis extending in said third direction to counteract angular displacement resulting from a vector component of the horizontal rate of rotation of the earth about said first sensitive axis in said third direction and to cancel any errors in the torquing about a sensitive axis extending in said first direction due to a gyro bias error and to nonorthogonality between the spin and sensitive axis and to any tilt angle of the spin axis from the vertical or to any tilt angle of the sensitive axis from the horizontal; torquing said gyroscope about said second sensitive axis extending in said fourth direction to counteract angular displacement resulting from a vector component of the horizontal rate of rotation of the earth about said second sensitive axis in said fourth direction and to cancel any errors in the torquing about said second sensitive axis extending in said second direction due to a gyro bias error and to any nonorthogonality between the spin and sensitive axes and to any tilt angle of the spin axis from the vertical or to any tilt angle of the sensitive axis from the horizontal; and wherein the step of calculating the ratio of the algebraic difference of the counteracting torques applied to said gyroscope about said first sensitive axis extending in said first and third divided by the algebraic difference of the counteracting torques applied to said gyroscope about said second sensitive axis extending in said second and fourth directions.

Claim 33

The method as claimed in claim 29 wherein the steps of torquing said gyroscope about sensitive axes extending in said third and fourth directions are simultaneously performed.

Claim 34

The method as claimed in claim 23 further including the step of calculating the latitude of said gyroscope from a function of the ratio of the rate of rotation of the earth divided by the horizontal rate of rotation of the earth experienced by said gyroscope.