# US 8,751,161 B2 · Method of determining a heading in the geographical north direction by means of an inertial unit > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 8,751,161 B2 - Document ID: US8751161B2 - Status: Granted - Kind: B2 - Application number: 12/396,246 - Filed: 2009-03-02 - Published: 2010-06-24 - Granted: 2014-06-10 - Earliest claimed: 2008-12-22 - Assignees: Sagem Defense Securite - Applicants: Yves Becheret - Inventors: Yves Becheret - Web page: https://patentgrep.com/patent/US8751161B2 ## CPC - G01C 19/38: with north-seeking action by other than magnetic means, e.g. gyrocompasses using earth's rotation - G01C 21/166: {Mechanical, construction or arrangement details of inertial navigation systems} - G01C 25/005: {initial alignment, calibration or starting-up of inertial devices} - G01C: MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY (measuring liquid level G01F; radio navigation, determining distance or velocity by use of propagation effects, e.g. Doppler effects, propagation time, of radio waves, analogous arrangements using other waves G01S) ## Abstract A method of determining a heading in the geographical North direction by means of an inertial sensor module having three rate gyro measurement axes and three accelerometer measurement axes, the method comprising the steps of: using data from the inertial sensor module in a North-seeking mode to obtain a first heading value; using data from the inertial sensor module in a gyro-compass mode to obtain a second heading value; and determining the heading in the North direction by using the first heading value and the second heading value. ## Claims ### Claim 1 A method of determining a heading in the geographical North direction of a platform by means of an inertial sensor module mounted on the platform having three rate gyro measurement axes and three accelerometer measurement axes with outputs which are received by a computer performing the method, the method comprising the steps of: using data from the inertial sensor module in a North-seeking mode to obtain a first heading value of the platform; using data from the inertial sensor module in a gyro-compass mode to obtain a second heading value; and determining the heading in the North direction by using the first heading value and the second heading value of the platform; wherein, in the North-seeking mode, the first heading value is calculated from gyro data, data from the accelerometers not being used when calculating the first heading value, but being used to determine a mean horizontal plane relative to the sensor module; and in the gyro-compass mode, the second heading value is calculated from accelerometer data, with gyro data being used only for stabilizing the platform. ### Claim 2 (depends on 1) A method according to claim 1, wherein the heading calculation includes the steps of: calculating a difference between the first heading value and the second heading value; and using as the heading in the North direction either the first heading value or the second heading value, taking account of the calculated difference. ### Claim 3 (depends on 2) A method according to claim 2, including the step of estimating consistency between the calculated difference and the assumption that there is no movement of the inertial unit relative to the ground; and if the difference is consistent with that assumption, taking the first heading value as the heading; and if the difference is inconsistent with that assumption, taking the second heading value as the heading in the North direction. ### Claim 4 (depends on 3) A method according to claim 3, wherein estimating consistency includes the step of comparing a weighted difference of a latitude cosine with at least one threshold. ### Claim 5 (depends on 4) A method according to claim 4, wherein the threshold is equal to the square root of the sum of the square of a standard deviation of a mean East drift component of the gyros during the measurement time plus the square of a standard deviation of a mean bias variation of the accelerometers in the South direction during the measurement time. ### Claim 6 (depends on 1) A method according to claim 1, wherein the heading calculation includes the steps of: weighting the first heading value and the second heading value; and adding together the first and second heading values as weighted in this way to obtain the heading in the North direction, the weighting being determined so as to minimize a standard deviation of error concerning the heading in the North direction. ### Claim 7 (depends on 1) A method according to claim 1, including the step of detecting movement of the sensor module by means of a detector external to the inertial sensor module and, if such movement is detected, using the second heading value as the heading in the North direction. ### Claim 8 A method of determining a heading in the geographical North direction of a platform by means of an inertial sensor module mounted on the platform having three rate gyro measurement axes and three accelerometer measurement axes with outputs which are received by a computer performing the method, the method comprising the steps of: using data from the inertial sensor module in a North-seeking mode to obtain a first heading value of the platform; using data from the inertial sensor module in a gyro-compass mode to obtain a second heading value; and determining the heading in the North direction by using the first heading value and the second heading value of the platform; wherein the heading calculation includes the steps of: calculating a difference between the first heading value and the second heading value; and using as the heading in the North direction either the first heading value or the second heading value, taking account of the calculated difference. ### Claim 9 (depends on 8) A method according to claim 8, including the step of estimating consistency between the calculated difference and the assumption that there is no movement of the inertial unit relative to the ground; and if the difference is consistent with that assumption, taking the first heading value as the heading; and if the difference is inconsistent with that assumption, taking the second heading value as the heading in the North direction. ### Claim 10 (depends on 9) A method according to claim 9, wherein estimating consistency includes the step of comparing a weighted difference of a latitude cosine with at least one threshold. ### Claim 11 (depends on 10) A method according to claim 10, wherein the threshold is equal to the square root of the sum of the square of a standard deviation of a mean East drift component of the gyros during the measurement time plus the square of a standard deviation of a mean bias variation of the accelerometers in the South direction during the measurement time.