Acoustic doppler current profiler tilt method based on attitude heading reference system

Abstract

This invention discloses an acoustic Doppler current profiler tilt measurement method based on an attitude and heading reference system, aiming to solve the technical problem of decreased measurement accuracy caused by neglecting the influence of transducer tangential velocity in traditional static correction methods. This invention utilizes the attitude and heading reference system to output high-precision attitude angles and angular velocities in real time. On one hand, it constructs a rotation matrix for static coordinate system transformation; on the other hand, it calculates the radial component of the tangential velocity of each beam in real time based on the angular velocity and a pre-calibrated equivalent radial radius, and subtracts it from the original radial velocity to achieve dynamic compensation. Addressing the problem of non-coplanarity of the same slant range measurement unit among the four beams under dynamic tilt conditions, this invention calculates the instantaneous depth of each unit of each beam based on the instantaneous attitude, performs linear interpolation on the radial velocity and depth sequence of each beam using a predefined standard depth layer, obtains the radial velocity of that beam at a specified depth, extracts the radial velocity, performs compensation, beamforming, and coordinate transformation, and finally subtracts the ship's translational velocity to obtain the absolute water flow velocity in the Earth coordinate system. This invention can effectively suppress tangential velocity interference and improve flow measurement accuracy under ship roll and pitch conditions, and is suitable for underway ADCP flow measurement under ship rolling conditions.

Description

Technical Field

[0001] This invention belongs to the field of underwater acoustic velocity measurement technology, specifically relating to an inclined current measurement method for an acoustic Doppler current profiler based on an attitude and heading reference system, which is suitable for dynamic velocity measurement and error compensation of a traveling ADCP under ship rolling and wave conditions. Background Technology

[0002] The Acoustic Doppler Current Profiler (ADCP) utilizes the acoustic Doppler principle, transmitting acoustic pulses to the bottom of the water through four beams and receiving the echo signals from scattering bodies in the water. It calculates the flow velocity of each water layer using the Doppler frequency shift principle. It has advantages such as high measurement efficiency and low interference with the flow field, and has become the mainstream instrument for measuring the flow velocity of oceans, rivers and lakes.

[0003] During the measurement process of a mobile ADCP, the measuring vessel is subjected to rolling motions due to wind and waves, including roll, pitch, and bow, which causes changes in the attitude of the transducer array. This tilt introduces two types of measurement errors: first, the attitude change causes the measurement coordinate system to rotate, resulting in projection errors of the velocity vector when transforming from the instrument coordinate system to the Earth coordinate system; second, the center of the transducer array is usually offset from the center of the vessel's rolling motion, and the transducers generate additional tangential linear velocity when the vessel rolls. This velocity is superimposed on the acoustic radial velocity measurement, causing an additional deviation in the Doppler frequency shift.

[0004] In existing technologies, static attitude correction methods are typically used to address tilting issues. This involves constructing a rotation matrix using attitude angles output from attitude sensors, such as electronic compasses or inertial measurement units, to transform the measured flow velocity vector from the instrument coordinate system to the Earth coordinate system. However, static attitude correction methods only compensate for projection errors caused by coordinate system rotation, neglecting the influence of tangential velocity resulting from the transducer's deviation from the sway center. When the ship's sway amplitude is large or the sway frequency is high, the tangential velocity can reach the order of centimeters per second. For low-flow-velocity environments or high-precision flow measurement requirements, this error cannot be ignored.

[0005] Furthermore, existing methods typically assume that the measurement units corresponding to the same slant range for the four beams are on the same horizontal plane when dealing with flow velocities at different depths, and directly perform beamforming. However, under dynamic tilting conditions, due to the different directions of the beams and the real-time changes in the ship's attitude, the actual vertical depths corresponding to the same slant range for each beam at the same moment are not equal, and direct beamforming will introduce aliasing errors between different depths.

[0006] Therefore, there is an urgent need for an ADCP tilt current measurement method that can simultaneously compensate for coordinate system rotation error and tangential velocity error, and accurately achieve depth layer alignment, so as to improve the current measurement accuracy of underway ADCP under ship rolling conditions. Summary of the Invention

[0007] Purpose of the Invention: To address the technical problems of reduced flow measurement accuracy in existing shipborne acoustic Doppler current profilers (ADCPs) during wave-driven navigation measurements, which fail to account for tangential velocity interference caused by transducer deviation from the ship's center of gravity and fail to accurately correct for beam sampling depth misalignment due to ship rolling, this invention aims to provide a shipborne ADCP dynamic flow velocity measurement compensation method based on an attitude reference system. This method can accurately isolate tangential interference induced by attitude angular velocity from the underlying radial signal and achieve beam depth mapping under dynamic tilt, thereby reconstructing the true three-dimensional velocity of the water flow with high precision.

[0008] Technical solution: This invention provides a shipborne ADCP current velocity compensation and calculation method based on an attitude and bearing reference system, comprising the following steps:

[0009] Step 1: Employ a BW-AH100C MEMS attitude reference system with an attitude angle accuracy of 1°, an angular velocity range of ±400° / s, and an output frequency of 100Hz to output the ship's attitude angle and angular velocity relative to the Earth coordinate system in real time. Simultaneously, a 600kHz ADCP acquires the raw radial velocity sequences of four beams and the bottom-tracking radial velocity. The ADCP sampling trigger is aligned with the PPS second pulse of the attitude reference system to ensure a time synchronization error of less than 1ms.

[0010] Step 2: Real-time estimation and radial compensation of transducer tangential velocity. The arm vector of each transducer center relative to the ship's center of gravity is pre-calibrated. An antisymmetric tensor is constructed using the angular velocity vector to calculate the tangential linear velocity of each transducer in the ship's coordinate system. This velocity is then projected onto the direction of the corresponding beam to obtain the radial tangential velocity component of the beam. This component is subtracted in real-time from the original radial velocity measured by ADCP to obtain the compensated radial velocity. This step effectively eliminates the additional Doppler shift caused by the transducer's deviation from the oscillation center.

[0011] Step 3: Instantaneous Depth Calculation and Standard Water Layer Radial Velocity Extraction. This method uses a geometric analytical approach to determine the instantaneous depth of each range cell. A rotation matrix is ​​constructed using real-time attitude angles to transform the transducer center and the positions of each range cell for each beam to the Earth coordinate system, thereby calculating the instantaneous vertical depth of each range cell. For any predefined standard depth layer, linear interpolation is performed on the radial velocity and depth sequence of each beam to obtain the radial velocity of that beam after compensation at a specified depth.

[0012] Step 4: 3D Flow Velocity Calculation and Coordinate Transformation. The radial velocities of the four beams after compensation at the same depth are grouped into a column vector. The 3D flow velocity in the instrument coordinate system is calculated using a pre-calculated beam transformation matrix. Then, the installation error matrix and real-time attitude rotation matrix are applied sequentially to transform the flow velocity to the Earth coordinate system, obtaining the velocity vector of the water body relative to the transducer.

[0013] Step 5: Ship speed subtraction and absolute current velocity output. Perform the same tangential velocity compensation as in Step 2 on the bottom-tracking radial velocity. Then, through beamforming and coordinate transformation in Step 4, obtain the ship's land-relative velocity in the Earth coordinate system, i.e., the ship's translational velocity. When the bottom-tracking signal quality deteriorates, the velocity provided by an external GNSS receiver can be used as a substitute. Finally, subtract the ship's translational velocity from the relative velocity of the water body to obtain the absolute current velocity of the water body at the target depth. Repeating Steps 3 to 5 yields the full-depth current velocity profile.

[0014] An ADCP tilt flow measurement system for implementing the above method includes:

[0015] Attitude and attitude reference system: BW-AH100C model, with built-in three-axis accelerometer, three-axis gyroscope and three-axis magnetometer. It outputs attitude angle and angular velocity through Kalman filtering and connects to the data processing unit through RS232 / 485 interface. The maximum output frequency is 100Hz.

[0016] Acoustic Doppler current profiler: operating frequency 600kHz, four-beam Janus configuration, beam tilt angle known, transducer geometry parameters obtained through factory calibration, attitude reference system built into the ADCP internal motherboard.

[0017] Data processing unit: Adopting an embedded industrial computer or FPGA+DSP architecture, it integrates a time synchronization module, tangential compensation module, depth mapping module, beamforming module, coordinate transformation module, and ship speed deduction module. It receives attitude reference system and ADCP data in real time and outputs the corrected flow velocity profile.

[0018] Power and communication interface: Provides DC power and transmits data to the host computer via Ethernet or serial port.

[0019] Preferably, the lever arm vector, installation error angle, and transducer center position are obtained through factory calibration or precise on-site measurement. Alternatively, a dock calibration method can be used to perform standard swaying motion of the ship under still water conditions, while recording the ADCP radial velocity and attitude reference system angular velocity, and then obtaining the equivalent lever arm vector through least squares fitting inversion.

[0020] Preferably, the system can also be connected to an external GNSS receiver to provide the ship's translational speed when bottom tracking fails, ensuring the system continues to operate in deep water environments.

[0021] Beneficial effects:

[0022] 1. This invention eliminates the random error in tangential velocity caused by eccentric installation, breaking through the limitation of traditional equipment that only performs simple static rotation of the coordinate system. It introduces a high-frequency attitude matrix and angular velocity tensor operator to accurately calculate the high-frequency parasitic tangential motion caused by the lever effect, and directly performs subtraction and stripping within the beam coordinate system, fundamentally eliminating the speed measurement distortion caused by the ship's pitch and roll.

[0023] 2. It solves the problem of beam depth sounding misalignment under dynamic attitude changes, breaks the previous rough assumption that the same slant range equals the same depth, and ensures that the data of the four beams participating in the three-dimensional orthogonal solution are strictly derived from the same horizontal water body in reality, thus eliminating the calculation error of profile velocity stratification. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the flow measurement system of the present invention;

[0025] Figure 2 This is a flowchart of the ADCP tilt flow measurement method based on the attitude heading reference system. Detailed Implementation

[0026] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.

[0027] This invention provides a method for ADCP flow velocity compensation and calculation based on an attitude reference system, such as... Figure 1 As shown, the equipment used mainly includes a current-measuring vessel, a mobile acoustic Doppler current profiler (AHRS) mounted on the bottom of the hull, and a BW-AH100C attitude reference system (AHRS) mounted on the mainboard inside the ADCP. The current-measuring vessel carries the ADCP and related equipment, providing a measurement platform. During the measurement process, it experiences roll, pitch, and yaw motions due to wind and waves. A shipborne ADCP with a working frequency of 600kHz is used, featuring a built-in four-beam Janus transducer array to emit acoustic pulses and receive echo signals from scatterers in the water. The original radial velocity is calculated using a complex autocorrelation algorithm. The BW-AH100C MEMS attitude reference system is used, its core comprising a three-axis MEMS gyroscope, a three-axis MEMS accelerometer, and a three-axis magnetometer. The AHRS fuses sensor data through a six-state Kalman filter, outputting a 100Hz frequency and providing high-precision attitude angles and angular velocities in real time. The AHRS is directly connected to the ADCP's main processor via an internal bus and is rigidly mounted inside the ADCP housing.

[0028] The shipborne ADCP tilt current measurement method based on the attitude and bearing reference system specifically includes the following steps:

[0029] Step 1: Define the coordinate system

[0030] To fully describe the spatial geometric relationships during the measurement process, three rectangular coordinate systems are defined:

[0031] Let the Earth coordinate system be It was established by shifting and interchanging the northeast coordinate system. The axis points due east. The axis points due north. The axis is perpendicular to the horizontal plane and points directly downwards, towards the seabed. The depth of the origin, 𝑂, from the water surface is . The total water depth of the measured sea area is The observed depth of a specific water layer is denoted as This coordinate system is used to ultimately express the absolute velocity vector.

[0032] Let the ship's coordinate system be... With the ship's center of mass R as the origin, establish axial directions along the geometric center lines of the hull. The axis serves as a transverse axis, pointing to the starboard side; The axis points directly forward of the bow; The axis is perpendicular to the deck and points towards the seabed. This coordinate system is used to describe the installation position of the ADCP on the ship and the ship's own rolling motion.

[0033] Set up the instrument coordinate system The origin is P, which is the geometric center of the four transducer arrays. The axis serves as the transverse axis of the transducer array, passing through the center of transducers 1 and 2, with the positive direction pointing from transducer 1 to transducer 2; The axis serves as the longitudinal axis, passing through the center of transducers 4 and 3, with the positive direction pointing from transducer 4 to transducer 3; The axis is perpendicular to the transducer surface and points downwards.

[0034] Step 2: Derivation of the rotation matrix

[0035] To achieve coordinate system transformations, three basic rotation matrices need to be constructed. The transformation matrix for clockwise rotation around the coordinate axes is defined as follows:

[0036] Let the roll angle be... , around Rotation matrix for clockwise rotation of the axis for:

[0037]

[0038] Let the pitch angle be... , around Rotation matrix for clockwise rotation of the axis for:

[0039]

[0040] Let the yaw angle be... , around Rotation matrix for clockwise rotation of the axis for:

[0041]

[0042] Combining attitude angles and rotation sequence, rotating in the order of roll, pitch, and yaw, the transformation matrix from the ship coordinate system to the earth coordinate system. for:

[0043]

[0044] Expanding, we get:

[0045]

[0046] Because the Attitude and Heading Reference System (AHRS) is rigidly integrated into the ADCP and aligned with the instrument coordinate system, the attitude angles output by the AHRS directly represent the attitude of the instrument coordinate system relative to the Earth coordinate system. If the transducer has a fixed installation offset angle relative to the ship, i.e., the instrument coordinate system and the ship coordinate system are not perfectly parallel, then the installation error angle needs to be pre-calibrated. , , And construct a rotation matrix from the instrument coordinate system to the ship coordinate system. We can obtain:

[0047]

[0048] Expanded to:

[0049]

[0050] The velocity measured for each sound wave is a radial scalar velocity along the acoustic axis. To reconstruct the radial velocities of the four beams into three-dimensional orthogonal velocities in a Cartesian coordinate system, the following steps are taken. Therefore, a transformation matrix between the two needs to be established.

[0051] In the instrument coordinate system, let the unit direction vectors of the four beams form the forward projection matrix 𝐶. The set beam tilt angle is . azimuth angle is Fixed four-beam ADCP azimuth angle is , +90° +180° and The direction vector matrix 𝐶 of the +270° beams 1 to 4 can be represented as:

[0052]

[0053] Let the radial velocity components actually measured by the four beams be... Then the forward projection mapping of the radial velocity satisfies:

[0054]

[0055] The analytical solution for the three-dimensional velocity is obtained using the least squares method, which involves solving for the left pseudo-inverse of matrix 𝐶. Let the beam conversion matrix be... ,have to:

[0056]

[0057] Step 3: Calculation of the radial component of the tangential velocity

[0058] 1. Derivation of lever arm vector

[0059] Let the lever arm vector be Let represent the fixed position vector of the center of the i-th transducer in the ship's coordinate system. Assume that the four transducers of the ADCP are uniformly distributed in a radius of . On the circle, the fixed position of the geometric center P of the transducer array in the ship coordinate system is: Then the fixed position vector of the transducer in the instrument coordinate system is:

[0060]

[0061]

[0062]

[0063]

[0064] If there is an installation error angle between the instrument coordinate system and the ship coordinate system, it is necessary to use a rotation matrix. Transform the coordinates from the instrument coordinate system to the ship coordinate system. Therefore, the position vector of the i-th transducer in the ship coordinate system is:

[0065]

[0066] Since the origin G of the ship's coordinate system is located at the ship's center of gravity, which is also the center of mass, i.e., the geometric rotation center during the rolling motion, the lever arm vector of the transducer relative to the ship's center of gravity is:

[0067]

[0068] 2. Beam direction vector

[0069] The unit direction vector of each beam in the instrument coordinate system is determined by the beam tilt angle and azimuth angle. For the Janus configuration, the azimuth angle is taken as... , usually include:

[0070] Beam 1:

[0071] Beam 2:

[0072] Beam 3:

[0073] Beam 4:

[0074] Since the direction of the beam in the ship's coordinate system changes with installation errors, the unit direction vector of the i-th beam in the ship's coordinate system is:

[0075]

[0076] 3. Coordinate system transformation for angular velocity

[0077] Since the AHRS is fixed inside the ADCP, its output triaxial angular velocity vector is a vector defined in the instrument coordinate system, denoted as . ,have to:

[0078]

[0079] Transform the angular velocity to the ship's coordinate system and use the installation deflection rotation matrix from step two. The true angular velocity vector in the ship's coordinate system can be obtained as follows:

[0080]

[0081] in, To surround the horizontal axis The roll rate is positive when the starboard side sinks. To surround the vertical axis The pitching angular velocity is positive when the bow pitches up; For around the vertical axis The yaw rate is positive when the bow is to the left.

[0082] Based on rigid body kinematics, the antisymmetric tensor matrix of the transformed angular velocity components in the ship coordinate system is obtained. for:

[0083]

[0084] 4. Tangential linear velocity

[0085] The tangential linear velocity of the i-th transducer due to the ship's rolling motion In the ship coordinate system, it is represented as:

[0086]

[0087] Projecting the calculated tangential linear velocity onto the radial direction of the beam yields the swaying tangential component of the contamination radial velocity. for:

[0088]

[0089] 5. Radial velocity compensation

[0090] Subtracting the contamination component entirely from the raw radial velocity measured by ADCP yields the pure radial velocity resulting from the relative motion of the water flow:

[0091]

[0092] in, The raw radial velocity measured by ADCP This refers to the distance unit number; The radial velocity, which does not contain a tangential component, is generated by the relative motion of the water flow.

[0093] Step 4: Dynamic Depth Mapping and Radial Velocity Extraction from Standard Water Layer

[0094] When the hull is tilted, the same tilt distance The actual vertical depth varies for each beam. To obtain a standard depth layer... To determine the flow velocity, the instantaneous depth of each distance cell must be calculated first, and then interpolation must be performed.

[0095] The unit direction vector of the i-th beam in the Earth coordinate system is:

[0096]

[0097] Let the vertical component of this direction vector be . Then the instantaneous depth of the k-th range cell of the i-th beam is:

[0098]

[0099] A predefined set of standard depth layers For each standard depth Perform the following operations for each beam i:

[0100] Depth-radial velocity sequence of the beam Find two adjacent points that satisfy If it exists, linear interpolation is used to obtain the depth of the beam. Radial velocity after compensation:

[0101]

[0102] like If the depth exceeds the effective depth range of the beam, then the beam is marked as invalid.

[0103] Step 5: 3D Flow Velocity Calculation

[0104] For each standard depth layer The interpolated radial velocities of the four beams are arranged into a column vector:

[0105]

[0106] The beam conversion matrix converts the radial velocity into a three-dimensional flow velocity in the instrument coordinate system, and then the rotation matrix converts it into a three-dimensional flow velocity in the Earth coordinate system. for:

[0107]

[0108] Step Six: Obtaining the ship's translational speed and calculating the absolute current velocity

[0109] Considering that the velocity measured by ADCP still includes the effect of the ship's own translational motion, it is necessary to subtract the ship speed, which represents the translational velocity of the ship's center of gravity in the geodetic coordinate system, given by the bottom tracking or external navigation system. If bottom tracking estimation is used, the bottom tracking radial velocity (subtracting the tangential velocity) is transformed to the Earth coordinate system to obtain the ship's velocity relative to the seabed. The bottom tracking radial velocity after compensating for the tangential component is then calculated. for:

[0110]

[0111] in, The bottom-tracking radial velocity is measured by ADCP. Since bottom-tracking measures the velocity of the seabed relative to ADCP and there is no moving bottom, the ship's translational velocity can be obtained from the bottom-tracking velocity:

[0112]

[0113] When bottom tracking fails, a GNSS receiver or integrated navigation system can be used, and the ship speed can be directly obtained by differentiating the ship's center of gravity position vector.

[0114] Target depth The actual water flow velocity in the geodetic coordinate system at a given location can be expressed as: ,have to:

[0115]

[0116] Repeating steps four through six yields the absolute current velocities for all standard depth layers, forming a complete velocity profile. This invention incorporates the attitude angles and angular velocities measured in real-time by the attitude reference system into the entire ADCP current measurement process. First, it subtracts the radial component of the tangential velocity caused by transducer eccentricity in the beam domain, eliminating the lever arm effect error ignored by traditional methods. Second, it uses instantaneous attitude calculation to determine the actual propagation time of each beam to the specified depth layer, achieving isodepth sampling under dynamic tilt, avoiding depth misalignment caused by the fixed slant range assumption. Finally, through beamforming, coordinate transformation, and ship speed subtraction, it outputs a high-precision absolute velocity profile in the Earth coordinate system. This invention provides a current measurement method for ADCP when the measuring vessel is tilted, effectively eliminating transient projection distortion and Doppler frequency shift contamination in harsh sea conditions, significantly improving the data fidelity and absolute velocity accuracy of underway ADCP current measurement under complex attitude conditions.

Claims

1. A tilting current measurement method using an acoustic Doppler current profiler based on an attitude and heading reference system, characterized in that, Includes the following steps: Step 1: Utilize an attitude reference system to measure the ship's attitude angles and angular velocities relative to the Earth coordinate system in real time; the attitude angles include yaw angle, pitch angle, and roll angle, and the angular velocities include roll angular velocity, pitch angular velocity, and yaw angular velocity; the attitude reference system has a pitch angle measurement range of ±90°, a roll angle measurement range of ±180°, a heading angle measurement accuracy of 1°, and an angular velocity range of ±400° / s; Step 2: Construct a rotation matrix from the ship coordinate system to the Earth coordinate system based on the attitude angle, which will be used for subsequent static coordinate transformation; Step 3: Based on the angular velocity and the pre-calibrated lever vector of each transducer center relative to the ship's center of gravity, calculate the projection component of the tangential velocity of each transducer caused by the ship's sway in the corresponding beam direction, and subtract the projection component from the original radial velocity of each beam to obtain the compensated radial velocity. Step 4: The radial velocities after four beam compensations are calculated into three-dimensional flow velocities in the instrument coordinate system through the beam transformation matrix, and then transformed into the Earth coordinate system through the installation error matrix and the rotation matrix to obtain the velocity vector of the water body relative to the transducer in the Earth coordinate system. Step 5: After deducting the ship's translational velocity, output the absolute water flow velocity in the Earth coordinate system.

2. The ADCP tilt flow measurement method based on an attitude and heading reference system according to claim 1, characterized in that, The calculation method for the tangential velocity projection component in step three is as follows: Let the lever arm vector of the i-th transducer in the ship coordinate system be... The ship's angular velocity vector is Then the tangential velocity in the ship coordinate system is ,in for The antisymmetric matrix; projecting the tangential velocity onto the unit direction vector of the i-th beam in the ship coordinate system. The tangential velocity components in the beam coordinate system are obtained. The compensated radial velocity is .

3. The ADCP tilt flow measurement method based on an attitude and heading reference system according to claim 1, characterized in that, The beam conversion matrix in step four ,in Beam tilt angle, This is the azimuth angle. for:

4. The ADCP tilt flow measurement method based on an attitude and heading reference system according to claim 1, characterized in that, In step five, the ship's translational velocity is obtained from the bottom-tracking radial velocity after undergoing the same tangential compensation and coordinate transformation, specifically as follows: The bottom tracking radial velocity of the four beams is subjected to tangential compensation as described in claim 2, and then transformed to the Earth coordinate system using the beam transformation matrix, installation error matrix, and rotation matrix to obtain the ship's ground speed. The ship's translational speed is taken as When an external navigation system is used, the ship's translational speed is obtained by differentiating the ship's center of gravity position vector with respect to time.

5. The ADCP tilt flow measurement method based on an attitude and heading reference system according to claim 1, characterized in that, For a specified depth layer Flow velocity measurement also includes a dynamic depth mapping step: The instantaneous direction vectors of each beam in the Earth coordinate system The vertical component is denoted as The initial draft of the transducer is Then the instantaneous depth of the i-th beam and the k-th range cell is A predefined set of standard depth layers Find two adjacent points in the depth-radial velocity sequence of each beam that satisfy... If it exists, linear interpolation is used to obtain the depth of the beam. The compensated radial velocity. Alternatively, the two-way propagation time corresponding to this layer depth can be used to extract the beam's position at depth from the original echo sequence by time. The original radial velocity at the location is then used to perform the tangential compensation as described in claim 2.

6. A tilting current measurement method using an acoustic Doppler current profiler based on an attitude and heading reference system, used to implement the method described in any one of claims 1 to 5, characterized in that, include: The attitude reference system adopts the BW-AH100C type attitude reference system, which has built-in accelerometer, gyroscope and magnetometer. It outputs the attitude angle and angular velocity of the ship coordinate system relative to the earth coordinate system through Kalman filtering. The maximum output frequency is 100Hz. An acoustic Doppler current profiler, operating at a frequency of 600 kHz, is installed on the bottom of a ship to transmit and receive acoustic signals and measure the radial velocity of four beams.

7. The ADCP tilt flow measurement system based on an attitude and heading reference system according to claim 6, characterized in that, The data processing unit includes: The attitude calculation module is used to construct a rotation matrix based on the attitude angles output by the attitude reference system. The tangential compensation module is used to calculate and subtract the tangential velocity projection component based on the angular velocity output by the attitude reference system and the pre-calibrated lever vector; The beamforming module is used to convert the compensated four-beam radial velocities into three-dimensional flow velocities in the instrument coordinate system using a beam transformation matrix. The coordinate transformation module is used to transform the three-dimensional flow velocity in the instrument coordinate system to the Earth coordinate system sequentially through the installation error matrix and the rotation matrix. The ship speed deduction module is used to deduct the ship's translational speed and output the absolute water flow speed.

8. The ADCP tilt flow measurement system based on an attitude and heading reference system according to claim 6, characterized in that, The data processing unit also includes a depth mapping module, which calculates the two-way propagation time of each beam to the specified depth layer based on the instantaneous attitude, and extracts the radial velocity at the corresponding depth from the raw echo signal of the acoustic Doppler current profiler accordingly.

9. The ADCP tilt flow measurement system based on an attitude and heading reference system according to claim 6, characterized in that, The BW-AH100C attitude reference system has a gyroscope zero-bias stability of 30° / h, an angle random walk coefficient of less than 0.1° / √h, a scaling factor nonlinearity of no more than 100ppm, an accelerometer range of ±3.6g, and a zero-bias stability of 0.001mg. The attitude reference system outputs data to the data processing unit via RS232, RS485, or TTL interfaces.

10. The ADCP tilt flow measurement system based on an attitude and heading reference system according to claim 6, characterized in that, The acoustic Doppler current profiler is a shipborne ADCP with a working frequency of 600kHz and a fixed beam tilt angle. The lever vector of the transducer center relative to the ship's center of gravity is obtained through factory calibration or on-site measurement.

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