px4_autopilot/EKF/gps_checks.cpp

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/**
 * @file gps_checks.cpp
 * Perform pre-flight and in-flight GPS quality checks
 *
 * @author Paul Riseborough <p_riseborough@live.com.au>
 *
 */

#include "ekf.h"

#include <ecl.h>
#include <geo_lookup/geo_mag_declination.h>
#include <mathlib/mathlib.h>

// GPS pre-flight check bit locations
#define MASK_GPS_NSATS  (1<<0)
#define MASK_GPS_PDOP   (1<<1)
#define MASK_GPS_HACC   (1<<2)
#define MASK_GPS_VACC   (1<<3)
#define MASK_GPS_SACC   (1<<4)
#define MASK_GPS_HDRIFT (1<<5)
#define MASK_GPS_VDRIFT (1<<6)
#define MASK_GPS_HSPD   (1<<7)
#define MASK_GPS_VSPD   (1<<8)

bool Ekf::collect_gps(const gps_message &gps)
{
	// Run GPS checks always
	_gps_checks_passed = gps_is_good(gps);

	if (_filter_initialised && !_NED_origin_initialised && _gps_checks_passed) {
		// If we have good GPS data set the origin's WGS-84 position to the last gps fix
		const double lat = gps.lat * 1.0e-7;
		const double lon = gps.lon * 1.0e-7;

		if (!map_projection_initialized(&_pos_ref)) {
			map_projection_init_timestamped(&_pos_ref, lat, lon, _time_last_imu);

			// if we are already doing aiding, correct for the change in position since the EKF started navigating
			if (isHorizontalAidingActive()) {
				double est_lat;
				double est_lon;
				map_projection_reproject(&_pos_ref, -_state.pos(0), -_state.pos(1), &est_lat, &est_lon);
				map_projection_init_timestamped(&_pos_ref, est_lat, est_lon, _time_last_imu);
			}
		}

		// Take the current GPS height and subtract the filter height above origin to estimate the GPS height of the origin
		_gps_alt_ref = 1e-3f * (float)gps.alt + _state.pos(2);
		_NED_origin_initialised = true;
		_earth_rate_NED = calcEarthRateNED((float)_pos_ref.lat_rad);
		_last_gps_origin_time_us = _time_last_imu;

		const bool declination_was_valid = ISFINITE(_mag_declination_gps);

		// set the magnetic field data returned by the geo library using the current GPS position
		_mag_declination_gps = get_mag_declination_radians(lat, lon);
		_mag_inclination_gps = get_mag_inclination_radians(lat, lon);
		_mag_strength_gps = get_mag_strength_gauss(lat, lon);

		// request a reset of the yaw using the new declination
		if (_params.mag_fusion_type == MAG_FUSE_TYPE_NONE) {
			// try to reset the yaw using the EKF-GSF yaw estimator
			_do_ekfgsf_yaw_reset = true;
			_ekfgsf_yaw_reset_time = 0;

		} else {
			if (!declination_was_valid) {
				_mag_yaw_reset_req = true;
			}
		}

		// save the horizontal and vertical position uncertainty of the origin
		_gps_origin_eph = gps.eph;
		_gps_origin_epv = gps.epv;

		// if the user has selected GPS as the primary height source, switch across to using it
		if (_params.vdist_sensor_type == VDIST_SENSOR_GPS) {
			startGpsHgtFusion();
		}

		ECL_INFO_TIMESTAMPED("GPS checks passed");

	} else if (!_NED_origin_initialised) {
		// a rough 2D fix is still sufficient to lookup declination
		if ((gps.fix_type >= 2) && (gps.eph < 1000)) {

			const bool declination_was_valid = ISFINITE(_mag_declination_gps);

			// If we have good GPS data set the origin's WGS-84 position to the last gps fix
			const double lat = gps.lat * 1.0e-7;
			const double lon = gps.lon * 1.0e-7;

			// set the magnetic field data returned by the geo library using the current GPS position
			_mag_declination_gps = get_mag_declination_radians(lat, lon);
			_mag_inclination_gps = get_mag_inclination_radians(lat, lon);
			_mag_strength_gps = get_mag_strength_gauss(lat, lon);

			// request mag yaw reset if there's a mag declination for the first time
			if (_params.mag_fusion_type != MAG_FUSE_TYPE_NONE) {
				if (!declination_was_valid && ISFINITE(_mag_declination_gps)) {
					_mag_yaw_reset_req = true;
				}
			}

			_earth_rate_NED = calcEarthRateNED((float)math::radians(lat));
		}
	}

	// start collecting GPS if there is a 3D fix and the NED origin has been set
	return _NED_origin_initialised && (gps.fix_type >= 3);
}

/*
 * Return true if the GPS solution quality is adequate to set an origin for the EKF
 * and start GPS aiding.
 * All activated checks must pass for 10 seconds.
 * Checks are activated using the EKF2_GPS_CHECK bitmask parameter
 * Checks are adjusted using the EKF2_REQ_* parameters
*/
bool Ekf::gps_is_good(const gps_message &gps)
{
	// Check the fix type
	_gps_check_fail_status.flags.fix = (gps.fix_type < 3);

	// Check the number of satellites
	_gps_check_fail_status.flags.nsats = (gps.nsats < _params.req_nsats);

	// Check the position dilution of precision
	_gps_check_fail_status.flags.pdop = (gps.pdop > _params.req_pdop);

	// Check the reported horizontal and vertical position accuracy
	_gps_check_fail_status.flags.hacc = (gps.eph > _params.req_hacc);
	_gps_check_fail_status.flags.vacc = (gps.epv > _params.req_vacc);

	// Check the reported speed accuracy
	_gps_check_fail_status.flags.sacc = (gps.sacc > _params.req_sacc);

	// check if GPS quality is degraded
	_gps_error_norm = fmaxf((gps.eph / _params.req_hacc), (gps.epv / _params.req_vacc));
	_gps_error_norm = fmaxf(_gps_error_norm, (gps.sacc / _params.req_sacc));

	// Calculate time lapsed since last update, limit to prevent numerical errors and calculate a lowpass filter coefficient
	constexpr float filt_time_const = 10.0f;
	const float dt = math::constrain(float(int64_t(_time_last_imu) - int64_t(_gps_pos_prev.timestamp)) * 1e-6f, 0.001f, filt_time_const);
	const float filter_coef = dt / filt_time_const;

	// The following checks are only valid when the vehicle is at rest
	const double lat = gps.lat * 1.0e-7;
	const double lon = gps.lon * 1.0e-7;

	if (!_control_status.flags.in_air && _control_status.flags.vehicle_at_rest) {
		// Calculate position movement since last measurement
		float delta_pos_n = 0.0f;
		float delta_pos_e = 0.0f;

		// calculate position movement since last GPS fix
		if (_gps_pos_prev.timestamp > 0) {
			map_projection_project(&_gps_pos_prev, lat, lon, &delta_pos_n, &delta_pos_e);

		} else {
			// no previous position has been set
			map_projection_init_timestamped(&_gps_pos_prev, lat, lon, _time_last_imu);
			_gps_alt_prev = 1e-3f * (float)gps.alt;
		}

		// Calculate the horizontal and vertical drift velocity components and limit to 10x the threshold
		const Vector3f vel_limit(_params.req_hdrift, _params.req_hdrift, _params.req_vdrift);
		Vector3f pos_derived(delta_pos_n, delta_pos_e, (_gps_alt_prev - 1e-3f * (float)gps.alt));
		pos_derived = matrix::constrain(pos_derived / dt, -10.0f * vel_limit, 10.0f * vel_limit);

		// Apply a low pass filter
		_gps_pos_deriv_filt = pos_derived * filter_coef + _gps_pos_deriv_filt * (1.0f - filter_coef);

		// Calculate the horizontal drift speed and fail if too high
		_gps_drift_metrics[0] = Vector2f(_gps_pos_deriv_filt.xy()).norm();
		_gps_check_fail_status.flags.hdrift = (_gps_drift_metrics[0] > _params.req_hdrift);

		// Fail if the vertical drift speed is too high
		_gps_drift_metrics[1] = fabsf(_gps_pos_deriv_filt(2));
		_gps_check_fail_status.flags.vdrift = (_gps_drift_metrics[1] > _params.req_vdrift);

		// Check the magnitude of the filtered horizontal GPS velocity
		const Vector2f gps_velNE = matrix::constrain(Vector2f(gps.vel_ned.xy()),
							     -10.0f * _params.req_hdrift,
							      10.0f * _params.req_hdrift);
		_gps_velNE_filt = gps_velNE * filter_coef + _gps_velNE_filt * (1.0f - filter_coef);
		_gps_drift_metrics[2] = _gps_velNE_filt.norm();
		_gps_check_fail_status.flags.hspeed = (_gps_drift_metrics[2] > _params.req_hdrift);

		_gps_drift_updated = true;

	} else if (_control_status.flags.in_air) {
		// These checks are always declared as passed when flying
		// If on ground and moving, the last result before movement commenced is kept
		_gps_check_fail_status.flags.hdrift = false;
		_gps_check_fail_status.flags.vdrift = false;
		_gps_check_fail_status.flags.hspeed = false;
		_gps_drift_updated = false;

		resetGpsDriftCheckFilters();

	} else {
		// This is the case where the vehicle is on ground and IMU movement is blocking the drift calculation
		_gps_drift_updated = true;

		resetGpsDriftCheckFilters();
	}

	// save GPS fix for next time
	map_projection_init_timestamped(&_gps_pos_prev, lat, lon, _time_last_imu);
	_gps_alt_prev = 1e-3f * (float)gps.alt;

	// Check  the filtered difference between GPS and EKF vertical velocity
	const float vz_diff_limit = 10.0f * _params.req_vdrift;
	const float vertVel = math::constrain(gps.vel_ned(2) - _state.vel(2), -vz_diff_limit, vz_diff_limit);
	_gps_velD_diff_filt = vertVel * filter_coef + _gps_velD_diff_filt * (1.0f - filter_coef);
	_gps_check_fail_status.flags.vspeed = (fabsf(_gps_velD_diff_filt) > _params.req_vdrift);

	// assume failed first time through
	if (_last_gps_fail_us == 0) {
		_last_gps_fail_us = _time_last_imu;
	}

	// if any user selected checks have failed, record the fail time
	if (
		_gps_check_fail_status.flags.fix ||
		(_gps_check_fail_status.flags.nsats   && (_params.gps_check_mask & MASK_GPS_NSATS)) ||
		(_gps_check_fail_status.flags.pdop    && (_params.gps_check_mask & MASK_GPS_PDOP)) ||
		(_gps_check_fail_status.flags.hacc    && (_params.gps_check_mask & MASK_GPS_HACC)) ||
		(_gps_check_fail_status.flags.vacc    && (_params.gps_check_mask & MASK_GPS_VACC)) ||
		(_gps_check_fail_status.flags.sacc    && (_params.gps_check_mask & MASK_GPS_SACC)) ||
		(_gps_check_fail_status.flags.hdrift  && (_params.gps_check_mask & MASK_GPS_HDRIFT)) ||
		(_gps_check_fail_status.flags.vdrift  && (_params.gps_check_mask & MASK_GPS_VDRIFT)) ||
		(_gps_check_fail_status.flags.hspeed  && (_params.gps_check_mask & MASK_GPS_HSPD)) ||
		(_gps_check_fail_status.flags.vspeed  && (_params.gps_check_mask & MASK_GPS_VSPD))
	) {
		_last_gps_fail_us = _time_last_imu;

	} else {
		_last_gps_pass_us = _time_last_imu;
	}

	// continuous period without fail of x seconds required to return a healthy status
	return isTimedOut(_last_gps_fail_us, (uint64_t)_min_gps_health_time_us);
}