# US 2024/0200948 A1 · Terrain referenced navigation system > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 2024/0200948 A1 - Document ID: US20240200948A1 - Status: Published application - Kind: A1 - Application number: 18/538,225 - Filed: 2023-12-13 - Published: 2024-06-20 - Earliest claimed: 2022-12-16 - Assignees: Atlantic Inertial Systems Limited - Applicants: Atlantic Inertial Systems Limited - Inventors: Geoffrey Thomas Henderson - Web page: https://patentgrep.com/patent/US20240200948A1 ## CPC - G01C 21/1656: {with passive imaging devices, e.g. cameras} - G01C 5/005: {altimeters for aircraft (G01C5/02, G01C5/06 take precedence)} - G01C 21/005: {with correlation of navigation data from several sources, e.g. map or contour matching (G01C21/30 takes precedence)} - G01C 23/00: Combined instruments indicating more than one navigational value, e.g. for aircraft; Combined measuring devices for measuring two or more variables of movement, e.g. distance, speed or acceleration - G06V 20/17: taken from planes or by drones - G08G 5/50: Navigation or guidance aids - G08G 5/55: for a single aircraft - G08G 5/74: for monitoring terrain - 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) - G06V: IMAGE OR VIDEO RECOGNITION OR UNDERSTANDING - G08G: TRAFFIC CONTROL SYSTEMS (guiding railway traffic, ensuring the safety of railway traffic B61L; arrangement of road signs or traffic signals E01F9/00; radar or analogous systems, sonar systems, lidar systems specially adapted for traffic control G01S13/91, G01S15/88, G01S17/88; {radar or analogous systems, sonar systems, lidar systems specially adapted for anti-collision purposes G01S13/93, G01S15/93, G01S17/93}) ## Abstract A navigation system for an aircraft includes a microelectromechanical systems inertial measurement unit (MEMS-IMU), a camera that captures, over time, a series of images of the terrain below the aircraft, a terrain map comprising terrain elevation data, an inertial navigation system (INS) and an optical terrain-referenced navigation unit (O-TRN). The INS generates a first position estimate based on one or more signals output by the MEMS-IMU. The O-TRN identifies a terrain feature in an image captured by the camera, derives a terrain height estimate of the terrain feature, and generates a second position estimate based on a comparison between the terrain height estimate and terrain elevation data extracted from the terrain map. The navigation system generates a third position estimate based on the first position estimate and the second position estimate. ## Claims ### Claim 1 A navigation system for an aircraft, comprising: a microelectromechanical systems inertial measurement unit (MEMS-IMU); a camera arranged to capture, over time, a series of images of the terrain below the aircraft; a terrain map comprising terrain elevation data; an inertial navigation system (INS) arranged to generate a first position estimate based on one or more signals output by the MEMS-IMU; an optical terrain-referenced navigation unit (O-TRN) configured to: identify a terrain feature in an image captured by the camera; derive a terrain height estimate of the terrain feature; generate a second position estimate based on a comparison between the terrain height estimate and terrain elevation data extracted from the terrain map; and wherein the navigation system is configured to generate a third position estimate based on the first position estimate and the second position estimate. ### Claim 2 (depends on 1) A navigation system as claimed in claim 1, wherein the navigation system further includes: an iterative algorithm unit (IAU) arranged to determine a system state array in each iteration, the system state array comprising a plurality of state variables, wherein the IAU is configured, in each iteration, to: update the system state array for the next iteration based on the current system state array, the first position estimate and the second position estimate; and generate the third position estimate based on the updated system state array. ### Claim 3 (depends on 2) A navigation system as claimed in claim 2, wherein: each state variable comprises an estimate of an error associated with a corresponding system characteristic; and the IAU is arranged, in each iteration, to generate the third position estimate by applying an error correction to the first position estimate based on the updated system state array. ### Claim 4 (depends on 2) The navigation system as claimed in claim 2, wherein: the MEMS-IMU comprises one or more accelerometers and/or gyroscopes; and the system state array comprises at least one state variable corresponding to an alignment error, scale factor error, and/or a bias error of an accelerometer and/or gyroscope. ### Claim 5 (depends on 2) The navigation system as claimed in claim 2, wherein: the INS comprises an integrator; and the system state array comprises at least one state variable associated with an operational parameter of the integrator. ### Claim 6 (depends on 5) The navigation system as claimed in claim 5, wherein the navigation system is arranged to control the integrator based on the current state variable associated with the operational parameter of the integrator in the system state array. ### Claim 7 (depends on 1) The navigation system as claimed in claim 1, wherein the system state array comprises state variables corresponding to one or more of: up to three angle or tilt errors; up to three velocity errors; up to three position errors; up to three gyroscope bias errors; up to three accelerometer bias errors; up to four, or more, operational parameters or coefficients of integrators; up to three accelerometer scale factor errors; up to three gyroscope scale factor errors; up to six accelerometer alignment errors; up to six gyroscope alignment errors; and up to nine gyroscope acceleration- or g-sensitivity errors. ### Claim 8 (depends on 2) The navigation system as claimed in claim 2, wherein the IAU comprises a Kalman filter. ### Claim 9 (depends on 1) The navigation system as claimed in claim 1, wherein the O-TRN is configured to: identify the terrain feature in two or more images captured by the camera; calculate a flow rate at which the terrain feature passes below the aircraft based on said two or more images; and derive the terrain height estimate based on the calculated flow rate. ### Claim 10 (depends on 9) A navigation system as claimed in claim 9, wherein: the INS is arranged to output a first velocity estimate and a first altitude estimate based on one or more signals output by the MEMS-IMU; and the O-TRN is arranged to derive the terrain height estimate further based on the first velocity estimate and the first altitude estimate. ### Claim 11 (depends on 9) A navigation system as claimed in claim 9, further comprising: a barometric altimeter; wherein the O-TRN is arranged to derive the terrain height estimate further based on a second altitude estimate derived from an output of the barometric altimeter. ### Claim 12 (depends on 1) The navigation system as claimed in claim 1, wherein the O-TRN is configured to: identify a plurality of terrain features in a plurality of images captured by the camera; derive a respective terrain height estimate of each of the plurality of terrain features; and output the second position estimate based on a correlation between the plurality of terrain height estimates and terrain elevation data extracted from the terrain map. ### Claim 13 (depends on 1) The navigation system as claimed in claim 1, wherein: the system state array comprises greater than fifteen state variables, optionally at least twenty state variables, optionally at least thirty state variables, optionally at least forty state variables, optionally at least forty-five state variables. ### Claim 14 (depends on 1) The navigation system as claimed in claim 1, wherein the camera comprises a short-wave infra-red camera. ### Claim 15 A method of navigating an aircraft, the method comprising: deriving a first position estimate based on one or more signals output by a microelectromechanical systems inertial measurement unit (MEMS-IMU); capturing, over time, a series of images of the terrain below the aircraft using a camera; identifying a terrain feature in an image captured by the camera; deriving a terrain height estimate of the terrain feature; deriving a second position estimate based on a comparison between the terrain height estimate and terrain elevation data extracted from a terrain map; and deriving a third position estimate based on the first position estimate and the second position estimate.