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"

// GPS pre-flight check bit locations
#define MASK_GPS_NSATS  (1<<0)
#define MASK_GPS_GDOP   (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(uint64_t time_usec, struct gps_message *gps)
{
	// run gps checks if we have not yet set the NED origin or have not started using GPS
	if (!_NED_origin_initialised || !_control_status.flags.gps) {
		// if we have good GPS data and the NED origin is not set, set to the last GPS fix
		if (gps_is_good(gps) && !_NED_origin_initialised) {
			printf("gps is good -  setting EKF origin\n");
			// Initialise projection
			double lat = gps->lat / 1.0e7;
			double lon = gps->lon / 1.0e7;
			map_projection_init(&_pos_ref, lat, lon);
			_gps_alt_ref = gps->alt / 1e3f;
			_NED_origin_initialised = true;
			_last_gps_origin_time_us = _time_last_imu;
		}
	}

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

	} else {
		return false;
	}

	return false;
}

/*
 * 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(struct 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 geometric dilution of precision
	_gps_check_fail_status.flags.gdop = (gps->gdop > _params.req_gdop);

	// Check the reported horizontal position accuracy
	_gps_check_fail_status.flags.hacc = (gps->eph > _params.req_hacc);

	// Check the reported vertical position accuracy
	_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);

	// Calculate position movement since last measurement
	float delta_posN = 0.0f;
	float delta_PosE = 0.0f;
	double lat = gps->lat * 1.0e-7;
	double lon = gps->lon * 1.0e-7;

	if (_pos_ref.init_done) {
		map_projection_project(&_pos_ref, lat, lon, &delta_posN, &delta_PosE);

	} else {
		map_projection_init(&_pos_ref, lat, lon);
		_gps_alt_ref = gps->alt * 1e-3f;
	}

	// Calculate time lapsed since last update, limit to prevent numerical errors and calculate the lowpass filter coefficient
	const float filt_time_const = 10.0f;
	float dt = fminf(fmaxf(float(_time_last_imu - _last_gps_origin_time_us) * 1e-6f, 0.001f), filt_time_const);
	float filter_coef = dt / filt_time_const;

	// Calculate the horizontal drift velocity components and limit to 10x the threshold
	float vel_limit = 10.0f * _params.req_hdrift;
	float velN = fminf(fmaxf(delta_posN / dt, -vel_limit), vel_limit);
	float velE = fminf(fmaxf(delta_PosE / dt, -vel_limit), vel_limit);

	// Apply a low pass filter
	_gpsDriftVelN = velN * filter_coef + _gpsDriftVelN * (1.0f - filter_coef);
	_gpsDriftVelE = velE * filter_coef + _gpsDriftVelE * (1.0f - filter_coef);

	// Calculate the horizontal drift speed and fail if too high
	// This check can only be used if the vehicle is stationary during alignment
	if (!_control_status.flags.armed) {
		float drift_speed = sqrtf(_gpsDriftVelN * _gpsDriftVelN + _gpsDriftVelE * _gpsDriftVelE);
		_gps_check_fail_status.flags.hdrift = (drift_speed > _params.req_hdrift);

	} else {
		_gps_check_fail_status.flags.hdrift = false;
	}

	// Save current position as the reference for next time
	map_projection_init(&_pos_ref, lat, lon);
	_last_gps_origin_time_us = _time_last_imu;

	// Calculate the vertical drift velocity and limit to 10x the threshold
	vel_limit = 10.0f * _params.req_vdrift;
	float velD = fminf(fmaxf((_gps_alt_ref - gps->alt * 1e-3f) / dt, -vel_limit), vel_limit);

	// Save the current height as the reference for next time
	_gps_alt_ref = gps->alt * 1e-3f;

	// Apply a low pass filter to the vertical velocity
	_gps_drift_velD = velD * filter_coef + _gps_drift_velD * (1.0f - filter_coef);

	// Fail if the vertical drift speed is too high
	// This check can only be used if the vehicle is stationary during alignment
	if (!_control_status.flags.armed) {
		_gps_check_fail_status.flags.vdrift = (fabsf(_gps_drift_velD) > _params.req_vdrift);

	} else {
		_gps_check_fail_status.flags.vdrift = false;
	}

	// Check the magnitude of the filtered horizontal GPS velocity
	// This check can only be used if the vehicle is stationary during alignment
	if (!_control_status.flags.armed) {
		vel_limit = 10.0f * _params.req_hdrift;
		float velN = fminf(fmaxf(gps->vel_ned[0], -vel_limit), vel_limit);
		float velE = fminf(fmaxf(gps->vel_ned[1], -vel_limit), vel_limit);
		_gps_velN_filt = velN * filter_coef + _gps_velN_filt * (1.0f - filter_coef);
		_gps_velE_filt  = velE * filter_coef + _gps_velE_filt  * (1.0f - filter_coef);
		float horiz_speed = sqrtf(_gps_velN_filt * _gps_velN_filt + _gps_velE_filt * _gps_velE_filt);
		_gps_check_fail_status.flags.hspeed = (horiz_speed > _params.req_hdrift);

	} else {
		_gps_check_fail_status.flags.hspeed = false;
	}

	// Check  the filtered difference between GPS and EKF vertical velocity
	vel_limit = 10.0f * _params.req_vdrift;
	float vertVel = fminf(fmaxf((gps->vel_ned[2] - _state.vel(2)), -vel_limit), vel_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.gdop    && (_params.gps_check_mask & MASK_GPS_GDOP)) ||
		(_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;
	}

	// continuous period without fail of 10 seconds required to return a healthy status
	if (_time_last_imu - _last_gps_fail_us > 1e7) {
		return true;
	}

	return false;
}