/****************************************************************************
 *
 *   Copyright (c) 2015-2020 Estimation and Control Library (ECL). All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 * 3. Neither the name ECL nor the names of its contributors may be
 *    used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
 * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 *
 ****************************************************************************/

/**
 * @file control.cpp
 * Control functions for ekf attitude and position estimator.
 *
 * @author Paul Riseborough <p_riseborough@live.com.au>
 *
 */


#include "ekf.h"
#include <mathlib/mathlib.h>

void Ekf::controlFusionModes()
{
	// Store the status to enable change detection
	_control_status_prev.value = _control_status.value;

	// monitor the tilt alignment
	if (!_control_status.flags.tilt_align) {
		// whilst we are aligning the tilt, monitor the variances
		const Vector3f angle_err_var_vec = calcRotVecVariances();

		// Once the tilt variances have reduced to equivalent of 3deg uncertainty
		// and declare the tilt alignment complete
		if ((angle_err_var_vec(0) + angle_err_var_vec(1)) < sq(math::radians(3.0f))) {
			_control_status.flags.tilt_align = true;

			// send alignment status message to the console
			const char *height_source = nullptr;

			if (_control_status.flags.baro_hgt) {
				height_source = "baro";

			} else if (_control_status.flags.ev_hgt) {
				height_source = "ev";

			} else if (_control_status.flags.gps_hgt) {
				height_source = "gps";

			} else if (_control_status.flags.rng_hgt) {
				height_source = "rng";

			} else {
				height_source = "unknown";

			}

			if (height_source) {
				ECL_INFO("%llu: EKF aligned, (%s hgt, IMU buf: %i, OBS buf: %i)",
					 (unsigned long long)_imu_sample_delayed.time_us, height_source, (int)_imu_buffer_length, (int)_obs_buffer_length);
			}
		}
	}

	// check for intermittent data (before pop_first_older_than)
	const baroSample &baro_init = _baro_buffer.get_newest();
	_baro_hgt_faulty = !isRecent(baro_init.time_us, 2 * BARO_MAX_INTERVAL);

	const gpsSample &gps_init = _gps_buffer.get_newest();
	_gps_hgt_intermittent = !isRecent(gps_init.time_us, 2 * GPS_MAX_INTERVAL);

	// check for arrival of new sensor data at the fusion time horizon
	_time_prev_gps_us = _gps_sample_delayed.time_us;
	_gps_data_ready = _gps_buffer.pop_first_older_than(_imu_sample_delayed.time_us, &_gps_sample_delayed);
	_mag_data_ready = _mag_buffer.pop_first_older_than(_imu_sample_delayed.time_us, &_mag_sample_delayed);

	if (_mag_data_ready) {
		_mag_lpf.update(_mag_sample_delayed.mag);

		// if enabled, use knowledge of theoretical magnetic field vector to calculate a synthetic magnetomter Z component value.
		// this is useful if there is a lot of interference on the sensor measurement.
		if (_params.synthesize_mag_z && (_params.mag_declination_source & MASK_USE_GEO_DECL) && (_NED_origin_initialised
				|| PX4_ISFINITE(_mag_declination_gps))) {

			const Vector3f mag_earth_pred = Dcmf(Eulerf(0, -_mag_inclination_gps, _mag_declination_gps)) * Vector3f(_mag_strength_gps, 0, 0);
			_mag_sample_delayed.mag(2) = calculate_synthetic_mag_z_measurement(_mag_sample_delayed.mag, mag_earth_pred);
			_control_status.flags.synthetic_mag_z = true;

		} else {
			_control_status.flags.synthetic_mag_z = false;
		}
	}

	_delta_time_baro_us = _baro_sample_delayed.time_us;
	_baro_data_ready = _baro_buffer.pop_first_older_than(_imu_sample_delayed.time_us, &_baro_sample_delayed);

	// if we have a new baro sample save the delta time between this sample and the last sample which is
	// used below for baro offset calculations
	if (_baro_data_ready) {
		_delta_time_baro_us = _baro_sample_delayed.time_us - _delta_time_baro_us;
	}

	{
		// Get range data from buffer and check validity
		const bool is_rng_data_ready = _range_buffer.pop_first_older_than(_imu_sample_delayed.time_us, _range_sensor.getSampleAddress());
		_range_sensor.setDataReadiness(is_rng_data_ready);

		// update range sensor angle parameters in case they have changed
		_range_sensor.setPitchOffset(_params.rng_sens_pitch);
		_range_sensor.setCosMaxTilt(_params.range_cos_max_tilt);
		_range_sensor.setQualityHysteresis(_params.range_valid_quality_s);

		_range_sensor.runChecks(_imu_sample_delayed.time_us, _R_to_earth);
	}

	if (_range_sensor.isDataHealthy()) {
		// correct the range data for position offset relative to the IMU
		const Vector3f pos_offset_body = _params.rng_pos_body - _params.imu_pos_body;
		const Vector3f pos_offset_earth = _R_to_earth * pos_offset_body;
		_range_sensor.setRange(_range_sensor.getRange() + pos_offset_earth(2) / _range_sensor.getCosTilt());
	}

	// We don't fuse flow data immediately because we have to wait for the mid integration point to fall behind the fusion time horizon.
	// This means we stop looking for new data until the old data has been fused, unless we are not fusing optical flow,
	// in this case we need to empty the buffer
	if (!_flow_data_ready || !_control_status.flags.opt_flow) {
		_flow_data_ready = _flow_buffer.pop_first_older_than(_imu_sample_delayed.time_us, &_flow_sample_delayed);
	}

	// check if we should fuse flow data for terrain estimation
	if (!_flow_for_terrain_data_ready && _flow_data_ready && _control_status.flags.in_air) {
		// TODO: WARNING, _flow_data_ready can be modified in controlOpticalFlowFusion
		// due to some checks failing
		// only fuse flow for terrain if range data hasn't been fused for 5 seconds
		_flow_for_terrain_data_ready = isTimedOut(_time_last_hagl_fuse, (uint64_t)5E6);
		// only fuse flow for terrain if the main filter is not fusing flow and we are using gps
		_flow_for_terrain_data_ready &= (!_control_status.flags.opt_flow && _control_status.flags.gps);
	}

	_ev_data_ready = _ext_vision_buffer.pop_first_older_than(_imu_sample_delayed.time_us, &_ev_sample_delayed);
	_tas_data_ready = _airspeed_buffer.pop_first_older_than(_imu_sample_delayed.time_us, &_airspeed_sample_delayed);

	// check for height sensor timeouts and reset and change sensor if necessary
	controlHeightSensorTimeouts();

	// control use of observations for aiding
	controlMagFusion();
	controlOpticalFlowFusion();
	controlGpsFusion();
	controlAirDataFusion();
	controlBetaFusion();
	controlDragFusion();
	controlHeightFusion();

	// Additional data odoemtery data from an external estimator can be fused.
	controlExternalVisionFusion();

	// Additional horizontal velocity data from an auxiliary sensor can be fused
	controlAuxVelFusion();

	// Fake position measurement for constraining drift when no other velocity or position measurements
	controlFakePosFusion();

	// check if we are no longer fusing measurements that directly constrain velocity drift
	update_deadreckoning_status();
}

void Ekf::controlExternalVisionFusion()
{
	// Check for new external vision data
	if (_ev_data_ready) {

		if (_inhibit_ev_yaw_use) {
			stopEvYawFusion();
		}

		// if the ev data is not in a NED reference frame, then the transformation between EV and EKF navigation frames
		// needs to be calculated and the observations rotated into the EKF frame of reference
		if ((_params.fusion_mode & MASK_ROTATE_EV) && ((_params.fusion_mode & MASK_USE_EVPOS)
				|| (_params.fusion_mode & MASK_USE_EVVEL)) && !_control_status.flags.ev_yaw) {

			// rotate EV measurements into the EKF Navigation frame
			calcExtVisRotMat();
		}

		// external vision aiding selection logic
		if (_control_status.flags.tilt_align && _control_status.flags.yaw_align) {

			// check for a external vision measurement that has fallen behind the fusion time horizon
			if (isRecent(_time_last_ext_vision, 2 * EV_MAX_INTERVAL)) {
				// turn on use of external vision measurements for position
				if (_params.fusion_mode & MASK_USE_EVPOS && !_control_status.flags.ev_pos) {
					startEvPosFusion();
				}

				// turn on use of external vision measurements for velocity
				if (_params.fusion_mode & MASK_USE_EVVEL && !_control_status.flags.ev_vel) {
					startEvVelFusion();
				}
			}
		}

		// external vision yaw aiding selection logic
		if (!_inhibit_ev_yaw_use && (_params.fusion_mode & MASK_USE_EVYAW) && !_control_status.flags.ev_yaw
		    && _control_status.flags.tilt_align) {

			// don't start using EV data unless data is arriving frequently
			if (isRecent(_time_last_ext_vision, 2 * EV_MAX_INTERVAL)) {
				if (resetYawToEv()) {
					_control_status.flags.yaw_align = true;
					startEvYawFusion();
				}
			}
		}

		// determine if we should use the horizontal position observations
		if (_control_status.flags.ev_pos) {

			Vector3f ev_pos_obs_var;
			Vector2f ev_pos_innov_gates;

			// correct position and height for offset relative to IMU
			const Vector3f pos_offset_body = _params.ev_pos_body - _params.imu_pos_body;
			const Vector3f pos_offset_earth = _R_to_earth * pos_offset_body;
			_ev_sample_delayed.pos -= pos_offset_earth;

			// Use an incremental position fusion method for EV position data if GPS is also used
			if (_params.fusion_mode & MASK_USE_GPS) {
				_fuse_hpos_as_odom = true;

			} else {
				_fuse_hpos_as_odom = false;
			}

			if (_fuse_hpos_as_odom) {
				if (!_hpos_prev_available) {
					// no previous observation available to calculate position change
					_hpos_prev_available = true;

				} else {
					// calculate the change in position since the last measurement
					Vector3f ev_delta_pos = _ev_sample_delayed.pos - _pos_meas_prev;

					// rotate measurement into body frame is required when fusing with GPS
					ev_delta_pos = _R_ev_to_ekf * ev_delta_pos;

					// use the change in position since the last measurement
					_ev_pos_innov(0) = _state.pos(0) - _hpos_pred_prev(0) - ev_delta_pos(0);
					_ev_pos_innov(1) = _state.pos(1) - _hpos_pred_prev(1) - ev_delta_pos(1);

					// observation 1-STD error, incremental pos observation is expected to have more uncertainty
					Matrix3f ev_pos_var = matrix::diag(_ev_sample_delayed.posVar);
					ev_pos_var = _R_ev_to_ekf * ev_pos_var * _R_ev_to_ekf.transpose();
					ev_pos_obs_var(0) = fmaxf(ev_pos_var(0, 0), sq(0.5f));
					ev_pos_obs_var(1) = fmaxf(ev_pos_var(1, 1), sq(0.5f));
				}

				// record observation and estimate for use next time
				_pos_meas_prev = _ev_sample_delayed.pos;
				_hpos_pred_prev = _state.pos.xy();

			} else {
				// use the absolute position
				Vector3f ev_pos_meas = _ev_sample_delayed.pos;
				Matrix3f ev_pos_var = matrix::diag(_ev_sample_delayed.posVar);

				if (_params.fusion_mode & MASK_ROTATE_EV) {
					ev_pos_meas = _R_ev_to_ekf * ev_pos_meas;
					ev_pos_var = _R_ev_to_ekf * ev_pos_var * _R_ev_to_ekf.transpose();
				}

				_ev_pos_innov(0) = _state.pos(0) - ev_pos_meas(0);
				_ev_pos_innov(1) = _state.pos(1) - ev_pos_meas(1);

				ev_pos_obs_var(0) = fmaxf(ev_pos_var(0, 0), sq(0.01f));
				ev_pos_obs_var(1) = fmaxf(ev_pos_var(1, 1), sq(0.01f));

				// check if we have been deadreckoning too long
				if (isTimedOut(_time_last_hor_pos_fuse, _params.reset_timeout_max)) {
					// only reset velocity if we have no another source of aiding constraining it
					if (isTimedOut(_time_last_of_fuse, (uint64_t)1E6) &&
					    isTimedOut(_time_last_hor_vel_fuse, (uint64_t)1E6)) {
						resetVelocity();
					}

					resetHorizontalPosition();
				}
			}

			// innovation gate size
			ev_pos_innov_gates(0) = fmaxf(_params.ev_pos_innov_gate, 1.0f);

			fuseHorizontalPosition(_ev_pos_innov, ev_pos_innov_gates, ev_pos_obs_var, _ev_pos_innov_var, _ev_pos_test_ratio);
		}

		// determine if we should use the velocity observations
		if (_control_status.flags.ev_vel) {

			Vector2f ev_vel_innov_gates;

			_last_vel_obs = getVisionVelocityInEkfFrame();
			_ev_vel_innov = _state.vel - _last_vel_obs;

			// check if we have been deadreckoning too long
			if (isTimedOut(_time_last_hor_vel_fuse, _params.reset_timeout_max)) {
				// only reset velocity if we have no another source of aiding constraining it
				if (isTimedOut(_time_last_of_fuse, (uint64_t)1E6) &&
				    isTimedOut(_time_last_hor_pos_fuse, (uint64_t)1E6)) {
					resetVelocity();
				}
			}

			_last_vel_obs_var = matrix::max(getVisionVelocityVarianceInEkfFrame(), sq(0.05f));

			ev_vel_innov_gates.setAll(fmaxf(_params.ev_vel_innov_gate, 1.0f));

			fuseHorizontalVelocity(_ev_vel_innov, ev_vel_innov_gates, _last_vel_obs_var, _ev_vel_innov_var, _ev_vel_test_ratio);
			fuseVerticalVelocity(_ev_vel_innov, ev_vel_innov_gates, _last_vel_obs_var, _ev_vel_innov_var, _ev_vel_test_ratio);
		}

		// determine if we should use the yaw observation
		if (_control_status.flags.ev_yaw) {
			fuseHeading();
		}

	} else if ((_control_status.flags.ev_pos || _control_status.flags.ev_vel ||  _control_status.flags.ev_yaw)
		   && isTimedOut(_time_last_ext_vision, (uint64_t)_params.reset_timeout_max)) {

		// Turn off EV fusion mode if no data has been received
		stopEvFusion();
		_warning_events.flags.vision_data_stopped = true;
		ECL_WARN("vision data stopped");
	}
}

void Ekf::controlOpticalFlowFusion()
{
	// Check if on ground motion is un-suitable for use of optical flow
	if (!_control_status.flags.in_air) {
		updateOnGroundMotionForOpticalFlowChecks();

	} else {
		resetOnGroundMotionForOpticalFlowChecks();
	}

	// Accumulate autopilot gyro data across the same time interval as the flow sensor
	_imu_del_ang_of += _imu_sample_delayed.delta_ang - _state.delta_ang_bias;
	_delta_time_of += _imu_sample_delayed.delta_ang_dt;

	if (_flow_data_ready) {
		const bool is_quality_good = (_flow_sample_delayed.quality >= _params.flow_qual_min);
		const bool is_magnitude_good = !_flow_sample_delayed.flow_xy_rad.longerThan(_flow_sample_delayed.dt * _flow_max_rate);
		const bool is_tilt_good = (_R_to_earth(2, 2) > _params.range_cos_max_tilt);

		const float delta_time_min = fmaxf(0.7f * _delta_time_of, 0.001f);
		const float delta_time_max = fminf(1.3f * _delta_time_of, 0.2f);
		const bool is_delta_time_good = _flow_sample_delayed.dt >= delta_time_min && _flow_sample_delayed.dt <= delta_time_max;
		const bool is_body_rate_comp_available = calcOptFlowBodyRateComp();

		if (is_quality_good
		    && is_magnitude_good
		    && is_tilt_good
		    && is_body_rate_comp_available
		    && is_delta_time_good) {
			// compensate for body motion to give a LOS rate
			_flow_compensated_XY_rad = _flow_sample_delayed.flow_xy_rad - _flow_sample_delayed.gyro_xyz.xy();

		} else if (!_control_status.flags.in_air && is_body_rate_comp_available) {

			if (!is_delta_time_good) {
				// handle special case of SITL and PX4Flow where dt is forced to
				// zero when the quaity is 0
				_flow_sample_delayed.dt = delta_time_min;
			}

			// when on the ground with poor flow quality,
			// assume zero ground relative velocity and LOS rate
			_flow_compensated_XY_rad.setZero();

		} else {
			// don't use this flow data and wait for the next data to arrive
			_flow_data_ready = false;
			_flow_for_terrain_data_ready = false; // TODO: find a better place
		}
	}

	// New optical flow data is available and is ready to be fused when the midpoint of the sample falls behind the fusion time horizon
	if (_flow_data_ready) {
		// Inhibit flow use if motion is un-suitable or we have good quality GPS
		// Apply hysteresis to prevent rapid mode switching
		const float gps_err_norm_lim = _control_status.flags.opt_flow ? 0.7f : 1.0f;

		// Check if we are in-air and require optical flow to control position drift
		const bool is_flow_required = _control_status.flags.in_air
					      && (_is_dead_reckoning // is doing inertial dead-reckoning so must constrain drift urgently
						  || isOnlyActiveSourceOfHorizontalAiding(_control_status.flags.opt_flow)
						  || (_control_status.flags.gps && (_gps_error_norm > gps_err_norm_lim))); // is using GPS, but GPS is bad


		// inhibit use of optical flow if motion is unsuitable and we are not reliant on it for flight navigation
		const bool preflight_motion_not_ok = !_control_status.flags.in_air
						     && ((_imu_sample_delayed.time_us > (_time_good_motion_us + (uint64_t)1E5))
								     || (_imu_sample_delayed.time_us < (_time_bad_motion_us + (uint64_t)5E6)));
		const bool flight_condition_not_ok = _control_status.flags.in_air && !isTerrainEstimateValid();

		_inhibit_flow_use = ((preflight_motion_not_ok || flight_condition_not_ok) && !is_flow_required)
				    || !_control_status.flags.tilt_align;

		// Handle cases where we are using optical flow but we should not use it anymore
		if (_control_status.flags.opt_flow) {
			if (!(_params.fusion_mode & MASK_USE_OF)
			    || _inhibit_flow_use) {

				stopFlowFusion();
				return;
			}
		}

		// optical flow fusion mode selection logic
		if ((_params.fusion_mode & MASK_USE_OF) // optical flow has been selected by the user
		    && !_control_status.flags.opt_flow // we are not yet using flow data
		    && !_inhibit_flow_use) {
			// If the heading is valid and use is not inhibited , start using optical flow aiding
			if (_control_status.flags.yaw_align) {
				// set the flag and reset the fusion timeout
				_control_status.flags.opt_flow = true;
				_time_last_of_fuse = _time_last_imu;

				// if we are not using GPS or external vision aiding, then the velocity and position states and covariances need to be set
				const bool flow_aid_only = !isOtherSourceOfHorizontalAidingThan(_control_status.flags.opt_flow);

				if (flow_aid_only) {
					resetVelocity();
					resetHorizontalPosition();
				}
			}
		}

		if (_control_status.flags.opt_flow) {
			// Wait until the midpoint of the flow sample has fallen behind the fusion time horizon
			if (_imu_sample_delayed.time_us > (_flow_sample_delayed.time_us - uint32_t(1e6f * _flow_sample_delayed.dt) / 2)) {
				// Fuse optical flow LOS rate observations into the main filter only if height above ground has been updated recently
				// but use a relaxed time criteria to enable it to coast through bad range finder data
				if (isRecent(_time_last_hagl_fuse, (uint64_t)10e6)) {
					fuseOptFlow();
					_last_known_posNE = _state.pos.xy();
				}

				_flow_data_ready = false;
			}

			// handle the case when we have optical flow, are reliant on it, but have not been using it for an extended period
			if (isTimedOut(_time_last_of_fuse, _params.reset_timeout_max)
			    && !isOtherSourceOfHorizontalAidingThan(_control_status.flags.opt_flow)) {

				resetVelocity();
				resetHorizontalPosition();
			}
		}

	} else if (_control_status.flags.opt_flow
		   && (_imu_sample_delayed.time_us >  _flow_sample_delayed.time_us + (uint64_t)10e6)) {

		stopFlowFusion();
	}
}

void Ekf::updateOnGroundMotionForOpticalFlowChecks()
{
	// When on ground check if the vehicle is being shaken or moved in a way that could cause a loss of navigation
	const float accel_norm = _accel_vec_filt.norm();

	const bool motion_is_excessive = ((accel_norm > (CONSTANTS_ONE_G * 1.5f)) // upper g limit
					  || (accel_norm < (CONSTANTS_ONE_G * 0.5f)) // lower g limit
					  || (_ang_rate_magnitude_filt > _flow_max_rate) // angular rate exceeds flow sensor limit
					  || (_R_to_earth(2, 2) < cosf(math::radians(30.0f)))); // tilted excessively

	if (motion_is_excessive) {
		_time_bad_motion_us = _imu_sample_delayed.time_us;

	} else {
		_time_good_motion_us = _imu_sample_delayed.time_us;
	}
}

void Ekf::resetOnGroundMotionForOpticalFlowChecks()
{
	_time_bad_motion_us = 0;
	_time_good_motion_us = _imu_sample_delayed.time_us;
}

void Ekf::controlGpsFusion()
{
	// Check for new GPS data that has fallen behind the fusion time horizon
	if (_gps_data_ready) {

		const bool gps_checks_passing = isTimedOut(_last_gps_fail_us, (uint64_t)5e6);
		const bool gps_checks_failing = isTimedOut(_last_gps_pass_us, (uint64_t)5e6);

		controlGpsYawFusion(gps_checks_passing, gps_checks_failing);

		// Determine if we should use GPS aiding for velocity and horizontal position
		// To start using GPS we need angular alignment completed, the local NED origin set and GPS data that has not failed checks recently
		if ((_params.fusion_mode & MASK_USE_GPS) && !_control_status.flags.gps) {
			if (_control_status.flags.tilt_align && _NED_origin_initialised && gps_checks_passing) {
				// If the heading is not aligned, reset the yaw and magnetic field states
				// Do not use external vision for yaw if using GPS because yaw needs to be
				// defined relative to an NED reference frame
				if (_control_status.flags.ev_yaw
				    || _mag_inhibit_yaw_reset_req
				    || _mag_yaw_reset_req) {

					_mag_yaw_reset_req = true;

					// Stop the vision for yaw fusion and do not allow it to start again
					stopEvYawFusion();
					_inhibit_ev_yaw_use = true;

				} else if (_control_status.flags.yaw_align) {
					// If the heading is valid start using gps aiding
					startGpsFusion();
				}
			}

		}  else if (_control_status.flags.gps
			    && (!(_params.fusion_mode & MASK_USE_GPS) || !_control_status.flags.yaw_align)) {

			stopGpsFusion();
		}

		// Handle the case where we are using GPS and another source of aiding and GPS is failing checks
		if (_control_status.flags.gps && gps_checks_failing && isOtherSourceOfHorizontalAidingThan(_control_status.flags.gps)) {
			stopGpsFusion();

			// Reset position state to external vision if we are going to use absolute values
			if (_control_status.flags.ev_pos && !(_params.fusion_mode & MASK_ROTATE_EV)) {
				resetHorizontalPosition();
			}

			_warning_events.flags.gps_quality_poor = true;
			ECL_WARN("GPS quality poor - stopping use");
		}

		// handle case where we are not currently using GPS, but need to align yaw angle using EKF-GSF before
		// we can start using GPS
		const bool align_yaw_using_gsf = !_control_status.flags.gps && _do_ekfgsf_yaw_reset
						 && isTimedOut(_ekfgsf_yaw_reset_time, 5000000);

		if (align_yaw_using_gsf) {
			if (resetYawToEKFGSF()) {
				_ekfgsf_yaw_reset_time = _time_last_imu;
				_do_ekfgsf_yaw_reset = false;
			}
		}

		// handle the case when we now have GPS, but have not been fusing it for an extended period
		if (_control_status.flags.gps) {
			// We are relying on aiding to constrain drift so after a specified time
			// with no aiding we need to do something
			bool do_vel_pos_reset = isTimedOut(_time_last_hor_pos_fuse, _params.reset_timeout_max)
						&& isTimedOut(_time_last_delpos_fuse, _params.reset_timeout_max)
						&& isTimedOut(_time_last_hor_vel_fuse, _params.reset_timeout_max)
						&& isTimedOut(_time_last_of_fuse, _params.reset_timeout_max);

			// We haven't had an absolute position fix for a longer time so need to do something
			do_vel_pos_reset = do_vel_pos_reset || isTimedOut(_time_last_hor_pos_fuse, 2 * _params.reset_timeout_max);

			/* Logic controlling the reset of navigation filter yaw to the EKF-GSF estimate to recover from loss of
			   navigation casued by a bad yaw estimate.

			   A rapid reset to the EKF-GSF estimate is performed after a recent takeoff if horizontal velocity
			   innovation checks fail. This enables recovery from a bad yaw estimate. After 30 seconds from takeoff,
			   different test criteria are used that take longer to trigger and reduce false positives. A reset is
			   not performed if the fault condition was present before flight to prevent triggering due to GPS glitches
			   or other sensor errors.

			   The yaw reset to the EKF-GSF estimate can be requested externally at any time during flight.

			   The total number of resets allowed per boot cycle is limited.

			   The minimum time interval between resets to the EKF-GSF estimate is limited to allow the EKF-GSF time
			   to improve its estimate if the previous reset was not successful.

			   A reset is not performed when getting GPS back after a significant period of no data because the timeout
			   could have been caused by bad GPS.
			*/

			const bool recent_takeoff_nav_failure = _control_status.flags.in_air &&
								!isTimedOut(_time_last_on_ground_us, 30000000) &&
								isTimedOut(_time_last_hor_vel_fuse, _params.EKFGSF_reset_delay) &&
								(_time_last_hor_vel_fuse > _time_last_on_ground_us);

			const bool inflight_nav_failure = _control_status.flags.in_air &&
							  do_vel_pos_reset &&
							  (_time_last_hor_vel_fuse > _time_last_on_ground_us) &&
							  (_time_last_hor_pos_fuse > _time_last_on_ground_us);

			bool is_yaw_failure = false;

			if ((recent_takeoff_nav_failure || inflight_nav_failure) && _time_last_hor_vel_fuse > 0) {
				// Do sanity check to see if the innovation failures is likely caused by a yaw angle error
				// by measuring the angle between the velocity estimate and the last velocity observation
				// Only use those vectors if their norm if they are larger than 4 times their noise standard deviation
				const float vel_obs_xy_norm_sq = _last_vel_obs.xy().norm_squared();
				const float vel_state_xy_norm_sq = _state.vel.xy().norm_squared();

				const float vel_obs_threshold_sq = fmaxf(sq(4.f) * (_last_vel_obs_var(0) + _last_vel_obs_var(1)), sq(0.4f));
				const float vel_state_threshold_sq = fmaxf(sq(4.f) * (P(4, 4) + P(5, 5)), sq(0.4f));

				if (vel_obs_xy_norm_sq > vel_obs_threshold_sq && vel_state_xy_norm_sq > vel_state_threshold_sq) {
					const float obs_dot_vel = Vector2f(_last_vel_obs).dot(_state.vel.xy());
					const float cos_sq = sq(obs_dot_vel) / (vel_state_xy_norm_sq * vel_obs_xy_norm_sq);

					if (cos_sq < sq(cosf(math::radians(25.f))) || obs_dot_vel < 0.f) {
						// The angle between the observation and the velocity estimate is greater than 25 degrees
						is_yaw_failure = true;
					}
				}
			}

			// Detect if coming back after significant time without GPS data
			const bool gps_signal_was_lost = isTimedOut(_time_prev_gps_us, 1000000);
			const bool do_yaw_vel_pos_reset = (_do_ekfgsf_yaw_reset || is_yaw_failure) &&
							  _ekfgsf_yaw_reset_count < _params.EKFGSF_reset_count_limit &&
							  isTimedOut(_ekfgsf_yaw_reset_time, 5000000) &&
							  !gps_signal_was_lost;

			if (do_yaw_vel_pos_reset) {
				if (resetYawToEKFGSF()) {
					_ekfgsf_yaw_reset_time = _time_last_imu;
					_do_ekfgsf_yaw_reset = false;
					_ekfgsf_yaw_reset_count++;

					// Reset the timeout counters
					_time_last_hor_pos_fuse = _time_last_imu;
					_time_last_delpos_fuse = _time_last_imu;
					_time_last_hor_vel_fuse = _time_last_imu;
					_time_last_of_fuse = _time_last_imu;
				}

			} else if (do_vel_pos_reset) {
				// use GPS velocity data to check and correct yaw angle if a FW vehicle
				if (_control_status.flags.fixed_wing && _control_status.flags.in_air) {
					// if flying a fixed wing aircraft, do a complete reset that includes yaw
					_control_status.flags.mag_aligned_in_flight = realignYawGPS();
				}

				resetVelocity();
				resetHorizontalPosition();
				_velpos_reset_request = false;
				_warning_events.flags.gps_fusion_timout = true;
				ECL_WARN("GPS fusion timeout - reset to GPS");

				// Reset the timeout counters
				_time_last_hor_pos_fuse = _time_last_imu;
				_time_last_hor_vel_fuse = _time_last_imu;
			}
		}

		// Only use GPS data for position and velocity aiding if enabled
		if (_control_status.flags.gps) {

			Vector2f gps_vel_innov_gates; // [horizontal vertical]
			Vector2f gps_pos_innov_gates; // [horizontal vertical]
			Vector3f gps_pos_obs_var;

			// correct velocity for offset relative to IMU
			const Vector3f pos_offset_body = _params.gps_pos_body - _params.imu_pos_body;
			const Vector3f vel_offset_body = _ang_rate_delayed_raw % pos_offset_body;
			const Vector3f vel_offset_earth = _R_to_earth * vel_offset_body;
			_gps_sample_delayed.vel -= vel_offset_earth;

			// correct position and height for offset relative to IMU
			const Vector3f pos_offset_earth = _R_to_earth * pos_offset_body;
			_gps_sample_delayed.pos -= pos_offset_earth.xy();
			_gps_sample_delayed.hgt += pos_offset_earth(2);

			const float lower_limit = fmaxf(_params.gps_pos_noise, 0.01f);

			if (isOtherSourceOfHorizontalAidingThan(_control_status.flags.gps)) {
				// if we are using other sources of aiding, then relax the upper observation
				// noise limit which prevents bad GPS perturbing the position estimate
				gps_pos_obs_var(0) = gps_pos_obs_var(1) = sq(fmaxf(_gps_sample_delayed.hacc, lower_limit));

			} else {
				// if we are not using another source of aiding, then we are reliant on the GPS
				// observations to constrain attitude errors and must limit the observation noise value.
				float upper_limit = fmaxf(_params.pos_noaid_noise, lower_limit);
				gps_pos_obs_var(0) = gps_pos_obs_var(1) = sq(math::constrain(_gps_sample_delayed.hacc, lower_limit, upper_limit));
			}

			_gps_sample_delayed.sacc = fmaxf(_gps_sample_delayed.sacc, _params.gps_vel_noise);

			_last_vel_obs_var.setAll(sq(_gps_sample_delayed.sacc));
			_last_vel_obs_var(2) *= sq(1.5f);

			// calculate innovations
			_last_vel_obs = _gps_sample_delayed.vel;
			_gps_vel_innov = _state.vel - _last_vel_obs;
			_gps_pos_innov.xy() = Vector2f(_state.pos) - _gps_sample_delayed.pos;

			// set innovation gate size
			gps_pos_innov_gates(0) = fmaxf(_params.gps_pos_innov_gate, 1.0f);
			gps_vel_innov_gates(0) = gps_vel_innov_gates(1) = fmaxf(_params.gps_vel_innov_gate, 1.0f);

			// fuse GPS measurement
			fuseHorizontalVelocity(_gps_vel_innov, gps_vel_innov_gates, _last_vel_obs_var, _gps_vel_innov_var, _gps_vel_test_ratio);
			fuseVerticalVelocity(_gps_vel_innov, gps_vel_innov_gates, _last_vel_obs_var, _gps_vel_innov_var, _gps_vel_test_ratio);
			fuseHorizontalPosition(_gps_pos_innov, gps_pos_innov_gates, gps_pos_obs_var, _gps_pos_innov_var, _gps_pos_test_ratio);
		}

	} else if (_control_status.flags.gps && (_imu_sample_delayed.time_us - _gps_sample_delayed.time_us > (uint64_t)10e6)) {
		stopGpsFusion();
		_warning_events.flags.gps_data_stopped = true;
		ECL_WARN("GPS data stopped");

	}  else if (_control_status.flags.gps && (_imu_sample_delayed.time_us - _gps_sample_delayed.time_us > (uint64_t)1e6)
		    && isOtherSourceOfHorizontalAidingThan(_control_status.flags.gps)) {
		// Handle the case where we are fusing another position source along GPS,
		// stop waiting for GPS after 1 s of lost signal
		stopGpsFusion();
		_warning_events.flags.gps_data_stopped_using_alternate = true;
		ECL_WARN("GPS data stopped, using only EV, OF or air data");
	}
}

void Ekf::controlGpsYawFusion(bool gps_checks_passing, bool gps_checks_failing)
{
	if (!(_params.fusion_mode & MASK_USE_GPSYAW)
	    || _is_gps_yaw_faulty) {

		stopGpsYawFusion();
		return;
	}

	const bool is_new_data_available = PX4_ISFINITE(_gps_sample_delayed.yaw);

	if (is_new_data_available) {

		const bool continuing_conditions_passing = !gps_checks_failing;

		const bool is_gps_yaw_data_intermittent = !isRecent(_time_last_gps_yaw_data, 2 * GPS_MAX_INTERVAL);

		const bool starting_conditions_passing = continuing_conditions_passing
				&& _control_status.flags.tilt_align
				&& gps_checks_passing
				&& !is_gps_yaw_data_intermittent
				&& !_gps_hgt_intermittent;

		_time_last_gps_yaw_data = _time_last_imu;

		if (_control_status.flags.gps_yaw) {

			if (continuing_conditions_passing) {

				fuseGpsYaw();

				const bool is_fusion_failing = isTimedOut(_time_last_gps_yaw_fuse, _params.reset_timeout_max);

				if (is_fusion_failing) {
					if (_nb_gps_yaw_reset_available > 0) {
						// Data seems good, attempt a reset
						resetYawToGps();
						_nb_gps_yaw_reset_available--;

					} else if (starting_conditions_passing) {
						// Data seems good, but previous reset did not fix the issue
						// something else must be wrong, declare the sensor faulty and stop the fusion
						_is_gps_yaw_faulty = true;
						stopGpsYawFusion();

					} else {
						// A reset did not fix the issue but all the starting checks are not passing
						// This could be a temporary issue, stop the fusion without declaring the sensor faulty
						stopGpsYawFusion();
					}

					// TODO: should we give a new reset credit when the fusion does not fail for some time?
				}

			} else {
				// Stop GPS yaw fusion but do not declare it faulty
				stopGpsYawFusion();
			}

		} else {
			if (starting_conditions_passing) {
				// Try to activate GPS yaw fusion
				if (resetYawToGps()) {
					_control_status.flags.yaw_align = true;
					_nb_gps_yaw_reset_available = 1;
					startGpsYawFusion();
				}
			}
		}

	} else if (_control_status.flags.gps_yaw && isTimedOut(_time_last_gps_yaw_data, _params.reset_timeout_max)) {
		// No yaw data in the message anymore. Stop until it comes back.
		stopGpsYawFusion();
	}

	// Before takeoff, we do not want to continue to rely on the current heading
	// if we had to stop the fusion
	if (!_control_status.flags.in_air
	    && !_control_status.flags.gps_yaw
	    && _control_status_prev.flags.gps_yaw) {
		_control_status.flags.yaw_align = false;
	}
}

void Ekf::controlHeightSensorTimeouts()
{
	/*
	 * Handle the case where we have not fused height measurements recently and
	 * uncertainty exceeds the max allowable. Reset using the best available height
	 * measurement source, continue using it after the reset and declare the current
	 * source failed if we have switched.
	*/

	checkVerticalAccelerationHealth();

	// check if height is continuously failing because of accel errors
	const bool continuous_bad_accel_hgt = isTimedOut(_time_good_vert_accel, (uint64_t)_params.bad_acc_reset_delay_us);

	// check if height has been inertial deadreckoning for too long
	// in vision hgt mode check for vision data
	const bool hgt_fusion_timeout = isTimedOut(_time_last_hgt_fuse, (uint64_t)5e6);

	if (hgt_fusion_timeout || continuous_bad_accel_hgt) {

		bool request_height_reset = false;
		const char *failing_height_source = nullptr;
		const char *new_height_source = nullptr;

		if (_control_status.flags.baro_hgt) {
			// check if GPS height is available
			const gpsSample &gps_init = _gps_buffer.get_newest();
			const bool gps_hgt_accurate = (gps_init.vacc < _params.req_vacc);

			// check for inertial sensing errors in the last BADACC_PROBATION seconds
			const bool prev_bad_vert_accel = isRecent(_time_bad_vert_accel, BADACC_PROBATION);

			// reset to GPS if adequate GPS data is available and the timeout cannot be blamed on IMU data
			const bool reset_to_gps = !_gps_hgt_intermittent &&
						  ((gps_hgt_accurate && !prev_bad_vert_accel) || _baro_hgt_faulty);

			if (reset_to_gps) {
				// set height sensor health
				_baro_hgt_faulty = true;

				startGpsHgtFusion();

				request_height_reset = true;
				failing_height_source = "baro";
				new_height_source = "gps";

			} else if (!_baro_hgt_faulty) {
				request_height_reset = true;
				failing_height_source = "baro";
				new_height_source = "baro";
			}

		} else if (_control_status.flags.gps_hgt) {
			// check if GPS height is available
			const gpsSample &gps_init = _gps_buffer.get_newest();
			const bool gps_hgt_accurate = (gps_init.vacc < _params.req_vacc);

			// check the baro height source for consistency and freshness
			const baroSample &baro_init = _baro_buffer.get_newest();
			const float baro_innov = _state.pos(2) - (_hgt_sensor_offset - baro_init.hgt + _baro_hgt_offset);
			const bool baro_data_consistent = fabsf(baro_innov) < (sq(_params.baro_noise) + P(9, 9)) * sq(_params.baro_innov_gate);

			// if baro data is acceptable and GPS data is inaccurate, reset height to baro
			const bool reset_to_baro = !_baro_hgt_faulty &&
						   ((baro_data_consistent && !gps_hgt_accurate) || _gps_hgt_intermittent);

			if (reset_to_baro) {
				startBaroHgtFusion();

				request_height_reset = true;
				failing_height_source = "gps";
				new_height_source = "baro";

			} else if (!_gps_hgt_intermittent) {
				request_height_reset = true;
				failing_height_source = "gps";
				new_height_source = "gps";
			}

		} else if (_control_status.flags.rng_hgt) {

			if (_range_sensor.isHealthy()) {
				request_height_reset = true;
				failing_height_source = "rng";
				new_height_source = "rng";

			} else if (!_baro_hgt_faulty) {
				startBaroHgtFusion();

				request_height_reset = true;
				failing_height_source = "rng";
				new_height_source = "baro";
			}

		} else if (_control_status.flags.ev_hgt) {
			// check if vision data is available
			const extVisionSample &ev_init = _ext_vision_buffer.get_newest();
			const bool ev_data_available = isRecent(ev_init.time_us, 2 * EV_MAX_INTERVAL);

			if (ev_data_available) {
				request_height_reset = true;
				failing_height_source = "ev";
				new_height_source = "ev";

			} else if (_range_sensor.isHealthy()) {
				// Fallback to rangefinder data if available
				setControlRangeHeight();
				request_height_reset = true;
				failing_height_source = "ev";
				new_height_source = "rng";

			} else if (!_baro_hgt_faulty) {
				startBaroHgtFusion();

				request_height_reset = true;
				failing_height_source = "ev";
				new_height_source = "baro";
			}
		}

		if (failing_height_source && new_height_source) {
			_warning_events.flags.height_sensor_timeout = true;
			ECL_WARN("%s hgt timeout - reset to %s", failing_height_source, new_height_source);
		}

		// Reset vertical position and velocity states to the last measurement
		if (request_height_reset) {
			resetHeight();
			// Reset the timout timer
			_time_last_hgt_fuse = _time_last_imu;
		}
	}
}

void Ekf::checkVerticalAccelerationHealth()
{
	// Check for IMU accelerometer vibration induced clipping as evidenced by the vertical
	// innovations being positive and not stale.
	// Clipping usually causes the average accel reading to move towards zero which makes the INS
	// think it is falling and produces positive vertical innovations.
	// Don't use stale innovation data.
	bool is_inertial_nav_falling = false;
	bool are_vertical_pos_and_vel_independant = false;

	if (isRecent(_vert_pos_fuse_attempt_time_us, 1000000)) {
		if (isRecent(_vert_vel_fuse_time_us, 1000000)) {
			// If vertical position and velocity come from independent sensors then we can
			// trust them more if they disagree with the IMU, but need to check that they agree
			const bool using_gps_for_both = _control_status.flags.gps_hgt && _control_status.flags.gps;
			const bool using_ev_for_both = _control_status.flags.ev_hgt && _control_status.flags.ev_vel;
			are_vertical_pos_and_vel_independant = !(using_gps_for_both || using_ev_for_both);
			is_inertial_nav_falling |= _vert_vel_innov_ratio > _params.vert_innov_test_lim && _vert_pos_innov_ratio > 0.0f;
			is_inertial_nav_falling |= _vert_pos_innov_ratio > _params.vert_innov_test_lim && _vert_vel_innov_ratio > 0.0f;

		} else {
			// only height sensing available
			is_inertial_nav_falling = _vert_pos_innov_ratio > _params.vert_innov_test_lim;
		}
	}

	// Check for more than 50% clipping affected IMU samples within the past 1 second
	const uint16_t clip_count_limit = 1000 / FILTER_UPDATE_PERIOD_MS;
	const bool is_clipping = _imu_sample_delayed.delta_vel_clipping[0] ||
				 _imu_sample_delayed.delta_vel_clipping[1] ||
				 _imu_sample_delayed.delta_vel_clipping[2];

	if (is_clipping && _clip_counter < clip_count_limit) {
		_clip_counter++;

	} else if (_clip_counter > 0) {
		_clip_counter--;
	}

	const bool is_clipping_frequently = _clip_counter > 0;

	// if vertical velocity and position are independent and agree, then do not require evidence of clipping if
	// innovations are large
	const bool bad_vert_accel = (are_vertical_pos_and_vel_independant || is_clipping_frequently) && is_inertial_nav_falling;

	if (bad_vert_accel) {
		_time_bad_vert_accel =  _time_last_imu;

	} else {
		_time_good_vert_accel = _time_last_imu;
	}

	// declare a bad vertical acceleration measurement and make the declaration persist
	// for a minimum of BADACC_PROBATION seconds
	if (_fault_status.flags.bad_acc_vertical) {
		_fault_status.flags.bad_acc_vertical = isRecent(_time_bad_vert_accel, BADACC_PROBATION);

	} else {
		_fault_status.flags.bad_acc_vertical = bad_vert_accel;
	}
}

void Ekf::controlHeightFusion()
{
	checkRangeAidSuitability();
	const bool do_range_aid = (_params.range_aid == 1) && isRangeAidSuitable();

	bool fuse_height = false;

	switch (_params.vdist_sensor_type) {
	default:
		ECL_ERR("Invalid hgt mode: %d", _params.vdist_sensor_type);

	// FALLTHROUGH
	case VDIST_SENSOR_BARO:
		if (do_range_aid && _range_sensor.isDataHealthy()) {
			setControlRangeHeight();
			fuse_height = true;

			// we have just switched to using range finder, calculate height sensor offset such that current
			// measurement matches our current height estimate
			if (_control_status_prev.flags.rng_hgt != _control_status.flags.rng_hgt) {
				_hgt_sensor_offset = _terrain_vpos;
			}

		} else if (!do_range_aid && _baro_data_ready && !_baro_hgt_faulty) {
			startBaroHgtFusion();
			fuse_height = true;

		} else if (_control_status.flags.gps_hgt && _gps_data_ready && !_gps_hgt_intermittent) {
			// switch to gps if there was a reset to gps
			fuse_height = true;

			// we have just switched to using gps height, calculate height sensor offset such that current
			// measurement matches our current height estimate
			if (_control_status_prev.flags.gps_hgt != _control_status.flags.gps_hgt) {
				_hgt_sensor_offset = _gps_sample_delayed.hgt - _gps_alt_ref + _state.pos(2);
			}
		}

		break;

	case VDIST_SENSOR_RANGE:
		if (_range_sensor.isDataHealthy()) {
			setControlRangeHeight();
			fuse_height = true;

			if (_control_status_prev.flags.rng_hgt != _control_status.flags.rng_hgt) {
				// we have just switched to using range finder, calculate height sensor offset such that current
				// measurement matches our current height estimate
				// use the parameter rng_gnd_clearance if on ground to avoid a noisy offset initialization (e.g. sonar)
				if (_control_status.flags.in_air && isTerrainEstimateValid()) {
					_hgt_sensor_offset = _terrain_vpos;

				} else if (_control_status.flags.in_air) {
					_hgt_sensor_offset = _range_sensor.getDistBottom() + _state.pos(2);

				} else {
					_hgt_sensor_offset = _params.rng_gnd_clearance;
				}
			}

		} else if (_control_status.flags.baro_hgt && _baro_data_ready && !_baro_hgt_faulty) {
			// fuse baro data if there was a reset to baro
			fuse_height = true;
		}

		break;

	case VDIST_SENSOR_GPS:

		// NOTE: emergency fallback due to extended loss of currently selected sensor data or failure
		// to pass innovation cinsistency checks is handled elsewhere in Ekf::controlHeightSensorTimeouts.
		// Do switching between GPS and rangefinder if using range finder as a height source when close
		// to ground and moving slowly. Also handle switch back from emergency Baro sensor when GPS recovers.
		if (!_control_status_prev.flags.rng_hgt && do_range_aid && _range_sensor.isDataHealthy()) {
			setControlRangeHeight();

			// we have just switched to using range finder, calculate height sensor offset such that current
			// measurement matches our current height estimate
			_hgt_sensor_offset = _terrain_vpos;

		} else if (_control_status_prev.flags.rng_hgt && !do_range_aid) {
			// must stop using range finder so find another sensor now
			if (!_gps_hgt_intermittent && _gps_checks_passed) {
				// GPS quality OK
				startGpsHgtFusion();

			} else if (!_baro_hgt_faulty) {
				// Use baro as a fallback
				startBaroHgtFusion();
			}

		} else if (_control_status.flags.baro_hgt && !do_range_aid && !_gps_hgt_intermittent && _gps_checks_passed) {
			// In baro fallback mode and GPS has recovered so start using it
			startGpsHgtFusion();
		}

		if (_control_status.flags.gps_hgt && _gps_data_ready) {
			fuse_height = true;

		} else if (_control_status.flags.rng_hgt && _range_sensor.isDataHealthy()) {
			fuse_height = true;

		} else if (_control_status.flags.baro_hgt && _baro_data_ready && !_baro_hgt_faulty) {
			fuse_height = true;
		}

		break;

	case VDIST_SENSOR_EV:

		// don't start using EV data unless data is arriving frequently, do not reset if pref mode was height
		if (!_control_status.flags.ev_hgt && isRecent(_time_last_ext_vision, 2 * EV_MAX_INTERVAL)) {
			fuse_height = true;
			setControlEVHeight();

			if (!_control_status_prev.flags.rng_hgt) {
				resetHeight();
			}
		}

		if (_control_status.flags.ev_hgt && _ev_data_ready) {
			fuse_height = true;

		} else if (_control_status.flags.rng_hgt && _range_sensor.isDataHealthy()) {
			fuse_height = true;

		} else if (_control_status.flags.baro_hgt && _baro_data_ready && !_baro_hgt_faulty) {
			fuse_height = true;
		}

		break;
	}

	updateBaroHgtBias();
	updateBaroHgtOffset();

	if (_control_status.flags.rng_hgt
	    && isTimedOut(_time_last_hgt_fuse, 2 * RNG_MAX_INTERVAL)
	    && !_range_sensor.isDataHealthy()
	    && _range_sensor.isRegularlySendingData()
	    && !_control_status.flags.in_air) {

		// If we are supposed to be using range finder data as the primary height sensor, have missed or rejected measurements
		// and are on the ground, then synthesise a measurement at the expected on ground value
		_range_sensor.setRange(_params.rng_gnd_clearance);
		_range_sensor.setDataReadiness(true);
		_range_sensor.setValidity(true); // bypass the checks

		fuse_height = true;
	}

	if (fuse_height) {
		if (_control_status.flags.baro_hgt) {
			Vector2f baro_hgt_innov_gate;
			Vector3f baro_hgt_obs_var;

			// vertical position innovation - baro measurement has opposite sign to earth z axis
			const float unbiased_baro = _baro_sample_delayed.hgt - _baro_b_est.getBias();
			_baro_hgt_innov(2) = _state.pos(2) + unbiased_baro - _baro_hgt_offset;
			// observation variance - user parameter defined
			baro_hgt_obs_var(2) = sq(fmaxf(_params.baro_noise, 0.01f));
			// innovation gate size
			baro_hgt_innov_gate(1) = fmaxf(_params.baro_innov_gate, 1.0f);

			// Compensate for positive static pressure transients (negative vertical position innovations)
			// caused by rotor wash ground interaction by applying a temporary deadzone to baro innovations.
			const float deadzone_start = 0.0f;
			const float deadzone_end = deadzone_start + _params.gnd_effect_deadzone;

			if (_control_status.flags.gnd_effect) {
				if (_baro_hgt_innov(2) < -deadzone_start) {
					if (_baro_hgt_innov(2) <= -deadzone_end) {
						_baro_hgt_innov(2) += deadzone_end;

					} else {
						_baro_hgt_innov(2) = -deadzone_start;
					}
				}
			}

			// fuse height information
			fuseVerticalPosition(_baro_hgt_innov, baro_hgt_innov_gate,
					     baro_hgt_obs_var, _baro_hgt_innov_var, _baro_hgt_test_ratio);

		} else if (_control_status.flags.gps_hgt) {
			Vector2f gps_hgt_innov_gate;
			Vector3f gps_hgt_obs_var;
			// vertical position innovation - gps measurement has opposite sign to earth z axis
			_gps_pos_innov(2) = _state.pos(2) + _gps_sample_delayed.hgt - _gps_alt_ref - _hgt_sensor_offset;
			gps_hgt_obs_var(2) = getGpsHeightVariance();
			// innovation gate size
			gps_hgt_innov_gate(1) = fmaxf(_params.baro_innov_gate, 1.0f);
			// fuse height information
			fuseVerticalPosition(_gps_pos_innov, gps_hgt_innov_gate,
					     gps_hgt_obs_var, _gps_pos_innov_var, _gps_pos_test_ratio);

		} else if (_control_status.flags.rng_hgt) {
			Vector2f rng_hgt_innov_gate;
			Vector3f rng_hgt_obs_var;
			// use range finder with tilt correction
			_rng_hgt_innov(2) = _state.pos(2) - (-math::max(_range_sensor.getDistBottom(),
							     _params.rng_gnd_clearance)) - _hgt_sensor_offset;
			// observation variance - user parameter defined
			rng_hgt_obs_var(2) = fmaxf(sq(_params.range_noise)
						   + sq(_params.range_noise_scaler * _range_sensor.getDistBottom()), 0.01f);
			// innovation gate size
			rng_hgt_innov_gate(1) = fmaxf(_params.range_innov_gate, 1.0f);
			// fuse height information
			fuseVerticalPosition(_rng_hgt_innov, rng_hgt_innov_gate,
					     rng_hgt_obs_var, _rng_hgt_innov_var, _rng_hgt_test_ratio);

		} else if (_control_status.flags.ev_hgt) {
			Vector2f ev_hgt_innov_gate;
			Vector3f ev_hgt_obs_var;
			// calculate the innovation assuming the external vision observation is in local NED frame
			_ev_pos_innov(2) = _state.pos(2) - _ev_sample_delayed.pos(2);
			// observation variance - defined externally
			ev_hgt_obs_var(2) = fmaxf(_ev_sample_delayed.posVar(2), sq(0.01f));
			// innovation gate size
			ev_hgt_innov_gate(1) = fmaxf(_params.ev_pos_innov_gate, 1.0f);
			// fuse height information
			fuseVerticalPosition(_ev_pos_innov, ev_hgt_innov_gate,
					     ev_hgt_obs_var, _ev_pos_innov_var, _ev_pos_test_ratio);
		}
	}
}

void Ekf::checkRangeAidSuitability()
{
	if (_control_status.flags.in_air
	    && _range_sensor.isHealthy()
	    && isTerrainEstimateValid()) {
		// check if we can use range finder measurements to estimate height, use hysteresis to avoid rapid switching
		// Note that the 0.7 coefficients and the innovation check are arbitrary values but work well in practice
		const float range_hagl = _terrain_vpos - _state.pos(2);
		const float range_hagl_max = _is_range_aid_suitable ? _params.max_hagl_for_range_aid : (_params.max_hagl_for_range_aid * 0.7f);
		const bool is_in_range = range_hagl < range_hagl_max;

		const float hagl_test_ratio = (_hagl_innov * _hagl_innov / (sq(_params.range_aid_innov_gate) * _hagl_innov_var));
		const bool is_hagl_stable = _is_range_aid_suitable ? (hagl_test_ratio < 1.f) : (hagl_test_ratio < 0.01f);

		if (isHorizontalAidingActive()) {
			const float max_vel = _is_range_aid_suitable ? _params.max_vel_for_range_aid : (_params.max_vel_for_range_aid * 0.7f);
			const bool is_below_max_speed = !_state.vel.xy().longerThan(max_vel);

			_is_range_aid_suitable = is_in_range && is_hagl_stable && is_below_max_speed;

		} else {
			_is_range_aid_suitable = is_in_range && is_hagl_stable;
		}

	} else {
		_is_range_aid_suitable = false;
	}
}

void Ekf::controlAirDataFusion()
{
	// control activation and initialisation/reset of wind states required for airspeed fusion

	// If both airspeed and sideslip fusion have timed out and we are not using a drag observation model then we no longer have valid wind estimates
	const bool airspeed_timed_out = isTimedOut(_time_last_arsp_fuse, (uint64_t)10e6);
	const bool sideslip_timed_out = isTimedOut(_time_last_beta_fuse, (uint64_t)10e6);

	if (_control_status.flags.wind &&
	    (_using_synthetic_position || (airspeed_timed_out && sideslip_timed_out && !(_params.fusion_mode & MASK_USE_DRAG)))) {
		_control_status.flags.wind = false;
	}

	if (_control_status.flags.fuse_aspd && airspeed_timed_out) {
		_control_status.flags.fuse_aspd = false;
	}

	// Always try to fuse airspeed data if available and we are in flight
	if (!_using_synthetic_position && _tas_data_ready && _control_status.flags.in_air) {
		// If starting wind state estimation, reset the wind states and covariances before fusing any data
		if (!_control_status.flags.wind) {
			// activate the wind states
			_control_status.flags.wind = true;
			// reset the timout timer to prevent repeated resets
			_time_last_arsp_fuse = _time_last_imu;
			// reset the wind speed states and corresponding covariances
			resetWindStates();
			resetWindCovariance();
		}

		fuseAirspeed();
	}
}

void Ekf::controlBetaFusion()
{
	if (_using_synthetic_position) {
		return;
	}

	// Perform synthetic sideslip fusion at regular intervals when in-air and sideslip fuson had been enabled externally:
	const bool beta_fusion_time_triggered = isTimedOut(_time_last_beta_fuse, _params.beta_avg_ft_us);

	if (beta_fusion_time_triggered &&
	    _control_status.flags.fuse_beta &&
	    _control_status.flags.in_air) {
		// If starting wind state estimation, reset the wind states and covariances before fusing any data
		if (!_control_status.flags.wind) {
			// activate the wind states
			_control_status.flags.wind = true;
			// reset the timeout timers to prevent repeated resets
			_time_last_beta_fuse = _time_last_imu;
			// reset the wind speed states and corresponding covariances
			resetWindStates();
			resetWindCovariance();
		}

		fuseSideslip();
	}
}

void Ekf::controlDragFusion()
{
	if ((_params.fusion_mode & MASK_USE_DRAG) &&
	    !_using_synthetic_position &&
	    _control_status.flags.in_air &&
	    !_mag_inhibit_yaw_reset_req) {

		if (!_control_status.flags.wind) {
			// reset the wind states and covariances when starting drag accel fusion
			_control_status.flags.wind = true;
			resetWindStates();
			resetWindCovariance();

		} else if (_drag_buffer.pop_first_older_than(_imu_sample_delayed.time_us, &_drag_sample_delayed)) {
			fuseDrag();
		}

	}
}

void Ekf::controlFakePosFusion()
{
	// if we aren't doing any aiding, fake position measurements at the last known position to constrain drift
	// Coincide fake measurements with baro data for efficiency with a minimum fusion rate of 5Hz

	if (!isHorizontalAidingActive()
	    && !(_control_status.flags.fuse_aspd && _control_status.flags.fuse_beta)) {

		// We now need to use a synthetic position observation to prevent unconstrained drift of the INS states.
		_using_synthetic_position = true;

		// Fuse synthetic position observations every 200msec
		if (isTimedOut(_time_last_fake_pos, (uint64_t)2e5)) {

			// Reset position and velocity states if we re-commence this aiding method
			if (isTimedOut(_time_last_fake_pos, (uint64_t)4e5)) {
				_last_known_posNE = _state.pos.xy();
				resetHorizontalPosition();
				resetVelocity();
				_fuse_hpos_as_odom = false;

				if (_time_last_fake_pos != 0) {
					_warning_events.flags.stopping_navigation = true;
					ECL_WARN("stopping navigation");
				}

			}

			_time_last_fake_pos = _time_last_imu;

			Vector3f fake_pos_obs_var;

			if (_control_status.flags.in_air && _control_status.flags.tilt_align) {
				fake_pos_obs_var(0) = fake_pos_obs_var(1) = sq(fmaxf(_params.pos_noaid_noise, _params.gps_pos_noise));

			} else if (_control_status.flags.vehicle_at_rest) {
				// Accelerate tilt fine alignment by fusing more
				// aggressively when the vehicle is at rest
				fake_pos_obs_var(0) = fake_pos_obs_var(1) = sq(0.1f);

			} else {
				fake_pos_obs_var(0) = fake_pos_obs_var(1) = sq(0.5f);
			}

			_gps_pos_innov.xy() = Vector2f(_state.pos) - _last_known_posNE;

			const Vector2f fake_pos_innov_gate(3.0f, 3.0f);

			fuseHorizontalPosition(_gps_pos_innov, fake_pos_innov_gate, fake_pos_obs_var,
					       _gps_pos_innov_var, _gps_pos_test_ratio, true);
		}

	} else {
		_using_synthetic_position = false;
	}

}

void Ekf::controlAuxVelFusion()
{
	const bool data_ready = _auxvel_buffer.pop_first_older_than(_imu_sample_delayed.time_us, &_auxvel_sample_delayed);

	if (data_ready && isHorizontalAidingActive()) {

		const Vector2f aux_vel_innov_gate(_params.auxvel_gate, _params.auxvel_gate);

		_last_vel_obs = _auxvel_sample_delayed.vel;
		_aux_vel_innov = _state.vel - _last_vel_obs;
		_last_vel_obs_var = _aux_vel_innov_var;

		fuseHorizontalVelocity(_aux_vel_innov, aux_vel_innov_gate, _auxvel_sample_delayed.velVar,
				       _aux_vel_innov_var, _aux_vel_test_ratio);

		// Can be enabled after bit for this is added to EKF_AID_MASK
		// fuseVerticalVelocity(_aux_vel_innov, aux_vel_innov_gate, _auxvel_sample_delayed.velVar,
		//		_aux_vel_innov_var, _aux_vel_test_ratio);

	}
}
