ardupilot/libraries/AP_NavEKF/EKFGSF_yaw.h

#pragma once

#pragma GCC optimize("O2")

#include <AP_NavEKF/AP_Nav_Common.h>
#include <AP_Math/AP_Math.h>
#include <AP_Math/vectorN.h>
#include <AP_Logger/LogStructure.h>

#define IMU_DT_MIN_SEC 0.001f // Minimum delta time between IMU samples (sec)

class EKFGSF_yaw
{
public:
    // Constructor
    EKFGSF_yaw();

    // Update Filter States - this should be called whenever new IMU data is available
    void update(const Vector3F &delAng,// IMU delta angle rotation vector measured in body frame (rad)
                const Vector3F &delVel,// IMU delta velocity vector measured in body frame (m/s)
                const ftype delAngDT, // time interval that delAng was integrated over (sec) - must be no less than IMU_DT_MIN_SEC
                const ftype delVelDT, // time interval that delVel was integrated over (sec) - must be no less than IMU_DT_MIN_SEC
                bool runEKF,          // set to true when flying or movement suitable for yaw estimation
                ftype TAS);           // true airspeed used for centripetal accel compensation - set to 0 when not required.

    // Fuse NE velocty mesurements and update the EKF's and GSF state and covariance estimates
    // Should be called after update(...) whenever new velocity data is available
    void fuseVelData(const Vector2F &vel,    // NE velocity measurement (m/s)
                     const ftype velAcc);   // 1-sigma accuracy of velocity measurement (m/s)

    // set the gyro bias in rad/sec
    void setGyroBias(Vector3f &gyroBias);

    // get yaw estimated and corresponding variance return false if
    // yaw estimation is inactive.  n_clips will contain the number of
    // models which were *not* used to create the yaw and yawVariance
    // return values.
    bool getYawData(ftype &yaw, ftype &yawVariance, uint8_t *n_clips=nullptr) const;

    // get the length of the weighted average velocity innovation vector
    // return false if not available
    bool getVelInnovLength(ftype &velInnovLength) const;

    // log EKFGSF data on behalf of an EKF caller.  id0 and id1 are the
    // IDs of the messages to log, e.g. LOG_NKY0_MSG, LOG_NKY1_MSG
    void Log_Write(uint64_t time_us, LogMessages id0, LogMessages id1, uint8_t core_index);

private:

#if MATH_CHECK_INDEXES
    typedef VectorN<ftype,2> Vector2;
    typedef VectorN<ftype,3> Vector3;
    typedef VectorN<VectorN<ftype,3>,3> Matrix3;
#else
    typedef ftype Vector2[2];
    typedef ftype Vector3[3];
    typedef ftype Matrix3[3][3];

#endif

    // Parameters
    const ftype EKFGSF_gyroNoise{1.0e-1};  // yaw rate noise used for covariance prediction (rad/sec)
    const ftype EKFGSF_accelNoise{2.0};    // horizontal accel noise used for covariance prediction (m/sec**2)
    const ftype EKFGSF_tiltGain{0.2};      // gain from tilt error to gyro correction for complementary filter (1/sec)
    const ftype EKFGSF_gyroBiasGain{0.04}; // gain applied to integral of gyro correction for complementary filter (1/sec)
    const ftype EKFGSF_accelFiltRatio{10.0}; // ratio  of time constant of AHRS tilt correction to time constant of first order LPF applied to accel data used by ahrs

    // Declarations used by the bank of AHRS complementary filters that use IMU data augmented by true
    // airspeed data when in fixed wing mode to estimate the quaternions that are used to rotate IMU data into a
    // Front, Right, Yaw frame of reference.
    Vector3F delta_angle;
    Vector3F delta_velocity;
    ftype angle_dt;
    ftype velocity_dt;
    struct ahrs_struct {
        Matrix3F R;             // matrix that rotates a vector from body to earth frame
        Vector3F gyro_bias;     // gyro bias learned and used by the quaternion calculation
        bool aligned;           // true when AHRS has been aligned
        ftype accel_FR[2];      // front-right acceleration vector in a horizontal plane (m/s/s)
        ftype vel_NE[2];        // NE velocity vector from last GPS measurement (m/s)
        bool fuse_gps;          // true when GPS should be fused on that frame
        ftype accel_dt;         // time step used when generating _simple_accel_FR data (sec)
    };
    ahrs_struct AHRS[N_MODELS_EKFGSF];
    bool ahrs_tilt_aligned;         // true the initial tilt alignment has been calculated
    ftype accel_gain;               // gain from accel vector tilt error to rate gyro correction used by AHRS calculation
    Vector3F ahrs_accel;            // filtered body frame specific force vector used by AHRS calculation (m/s/s)
    ftype ahrs_accel_norm;          // length of body frame specific force vector used by AHRS calculation (m/s/s)
    ftype true_airspeed;            // true airspeed used to correct for centripetal acceleratoin in coordinated turns (m/s)

    // Runs quaternion prediction for the selected AHRS using IMU (and optionally true airspeed) data
    void predictAHRS(const uint8_t mdl_idx);

    // Applies a body frame delta angle to a body to earth frame rotation matrix using a small angle approximation
    Matrix3F updateRotMat(const Matrix3F &R, const Vector3F &g) const;

    // Initialises the tilt (roll and pitch) for all AHRS using IMU acceleration data
    void alignTilt();

    // Initialises the yaw angle for all AHRS using a uniform distribution of yaw angles between -180 and +180 deg
    void alignYaw();

    // The Following declarations are used by bank of EKF's that estimate yaw angle starting from a different yaw hypothesis for each filter.

    struct EKF_struct {
        ftype X[3];     // Vel North (m/s),  Vel East (m/s), yaw (rad)
        ftype P[3][3];  // covariance matrix
        ftype S[2][2];  // N,E velocity innovation variance (m/s)^2
        ftype innov[2]; // Velocity N,E innovation (m/s)
    };
    EKF_struct EKF[N_MODELS_EKFGSF];
    bool vel_fuse_running;  // true when the bank of EKF's has started fusing GPS velocity data
    bool run_ekf_gsf;       // true when operating condition is suitable for to run the GSF and EKF models and fuse velocity data

    // Resets states and covariances for the EKF's and GSF including GSF weights, but not the AHRS complementary filters
    void resetEKFGSF();

    // Runs the state and covariance prediction for the selected EKF
    void predict(const uint8_t mdl_idx);

    // Runs the state and covariance update for the selected EKF using the GPS NE velocity measurement
    // Returns false if the sttae and covariance correction failed
    bool correct(const uint8_t mdl_idx, const Vector2F &vel, const ftype velObsVar);

    // Forces symmetry on the covariance matrix for the selected EKF
    void forceSymmetry(const uint8_t mdl_idx);

    // The following declarations are used  by the Gaussian Sum Filter that combines the state estimates from the bank of
    // EKF's to form a single state estimate.

    struct GSF_struct {
        ftype yaw;                      // yaw (rad)
        ftype yaw_variance;             // Yaw state variance (rad^2)
        ftype weights[N_MODELS_EKFGSF]; // Weighting applied to each EKF model. Sum of weights is unity.
    };
    GSF_struct GSF;

    // Returns the probability for a selected model assuming a Gaussian error distribution
    // Used by the Guassian Sum Filter to calculate the weightings when combining the outputs from the bank of EKF's
    ftype gaussianDensity(const uint8_t mdl_idx) const;

    // number of models whose weights underflowed due to excessive
    // innovation variances:
    uint8_t n_clips;
};