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 "../ecl.h"
#include "ekf.h"
#include "mathlib.h"
#include "geo.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 whenever the WGS-84 origin is not set or the vehicle is not using GPS
	// Also run checks if the vehicle is on-ground as the check data can be used by vehicle pre-flight checks
	if (!_control_status.flags.in_air || !_NED_origin_initialised || !_control_status.flags.gps) {
		bool gps_checks_pass = gps_is_good(gps);
		if (!_NED_origin_initialised && gps_checks_pass) {
			// If we have good GPS data set the origin's WGS-84 position to the last gps fix
			double lat = gps->lat / 1.0e7;
			double lon = gps->lon / 1.0e7;
			map_projection_init_timestamped(&_pos_ref, lat, lon, _time_last_imu);

			// if we are already doing aiding, corect for the change in posiiton since the EKF started navigating
			if (_control_status.flags.opt_flow || _control_status.flags.gps || _control_status.flags.ev_pos) {
				double est_lat, 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;
			_last_gps_origin_time_us = _time_last_imu;
			// set the magnetic declination returned by the geo library using the current GPS position
			_mag_declination_gps = math::radians(get_mag_declination(lat, lon));
			// 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 (_primary_hgt_source == VDIST_SENSOR_GPS) {
				ECL_INFO("EKF GPS checks passed (WGS-84 origin set, using GPS height)");
				_control_status.flags.baro_hgt = false;
				_control_status.flags.gps_hgt = true;
				_control_status.flags.rng_hgt = false;
				// zero the sensor offset
				_hgt_sensor_offset = 0.0f;
			} else {
				ECL_INFO("EKF GPS checks passed (WGS-84 origin set)");
			}
		}
	}

	// 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(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;

	// calculate position movement since last GPS fix
	if (_gps_pos_prev.timestamp > 0) {
		map_projection_project(&_gps_pos_prev, lat, lon, &delta_posN, &delta_PosE);
	} 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 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 - _gps_pos_prev.timestamp) * 1e-6f, 0.001f), filt_time_const);
	float filter_coef = dt / filt_time_const;

	// save GPS fix for next time
	map_projection_init_timestamped(&_gps_pos_prev, lat, lon, _time_last_imu);

	// 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.in_air) {
		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;
	}

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

	// 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.in_air) {
		_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.in_air) {
		vel_limit = 10.0f * _params.req_hdrift;
		float gps_velN = fminf(fmaxf(gps->vel_ned[0], -vel_limit), vel_limit);
		float gps_velE = fminf(fmaxf(gps->vel_ned[1], -vel_limit), vel_limit);
		_gps_velN_filt = gps_velN * filter_coef + _gps_velN_filt * (1.0f - filter_coef);
		_gps_velE_filt  = gps_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
	return (_time_last_imu - _last_gps_fail_us > 1e7);
}