terrain_estimator: introduce quality metric in rng

This commit introduces a quality measure in the range data. It is
used to properly decide whether to initialize the HAGL estimate on
 sensor data or MIN_HGT parameter, as well as in the decision of
whether a 'fake' measurement should be published on the ground to
allow for optical flow take-offs.

EKF/common.h

@@ -133,6 +133,7 @@ struct baroSample {
 struct rangeSample {
 	float       rng;	///< range (distance to ground) measurement (m)
 	uint64_t    time_us;	///< timestamp of the measurement (uSec)
+	uint8_t	    quality; ///< quality indicator between 0 and 255
 };
 
 struct airspeedSample {

EKF/control.cpp

@@ -1198,7 +1198,7 @@ void Ekf::checkRangeDataValidity()
 		return;
 	} else {
 		// reset fault status when we get new data
-		_rng_hgt_faulty = false;
+		_rng_hgt_faulty = (_range_sample_delayed.quality == 0);
 	}
 
 	// Check if excessively tilted
@@ -1244,7 +1244,7 @@ void Ekf::checkRangeDataValidity()
 			}
 
 			_control_status.flags.rng_stuck = true;
-			_rng_hgt_faulty = true;
+			_rng_hgt_faulty = (_range_sample_delayed.quality == 0);
 		}
 
 	} else {

EKF/estimator_interface.cpp

@@ -314,7 +314,7 @@ void EstimatorInterface::setAirspeedData(uint64_t time_usec, float true_airspeed
 	}
 }
 
-void EstimatorInterface::setRangeData(uint64_t time_usec, float data)
+void EstimatorInterface::setRangeData(uint64_t time_usec, float data, uint8_t quality)
 {
 	if (!_initialised || _range_buffer_fail) {
 		return;
@@ -336,6 +336,7 @@ void EstimatorInterface::setRangeData(uint64_t time_usec, float data)
 		rangeSample range_sample_new;
 		range_sample_new.rng = data;
 		range_sample_new.time_us = time_usec - _params.range_delay_ms * 1000;
+		range_sample_new.quality = quality;
 		_time_last_range = time_usec;
 
 		_range_buffer.push(range_sample_new);

EKF/estimator_interface.h

@@ -191,7 +191,7 @@ public:
 	void setAirspeedData(uint64_t time_usec, float true_airspeed, float eas2tas);
 
 	// set range data
-	void setRangeData(uint64_t time_usec, float data);
+	void setRangeData(uint64_t time_usec, float data, uint8_t quality);
 
 	// set optical flow data
 	// if optical flow sensor gyro delta angles are not available, set gyroXYZ vector fields to NaN and the EKF will use its internal delta angle data instead

EKF/terrain_estimator.cpp

@@ -48,7 +48,7 @@ bool Ekf::initHagl()
 	// get most recent range measurement from buffer
 	const rangeSample &latest_measurement = _range_buffer.get_newest();
 
-	if ((_time_last_imu - latest_measurement.time_us) < (uint64_t)2e5 && _R_rng_to_earth_2_2 > _params.range_cos_max_tilt) {
+	if (!_rng_hgt_faulty && (_time_last_imu - latest_measurement.time_us) < (uint64_t)2e5 && _R_rng_to_earth_2_2 > _params.range_cos_max_tilt) {
 		// if we have a fresh measurement, use it to initialise the terrain estimator
 		_terrain_vpos = _state.pos(2) + latest_measurement.rng * _R_rng_to_earth_2_2;
 		// initialise state variance to variance of measurement
@@ -66,7 +66,7 @@ bool Ekf::initHagl()
 		// Use the ground clearance value as our uncertainty
 		_terrain_var = sq(_params.rng_gnd_clearance);
 		// this is a guess
-		return false;
+		return true;
 
 	} else {
 		// no information - cannot initialise
@@ -99,8 +99,9 @@ void Ekf::runTerrainEstimator()
 
 		// if stationary on the ground and no range data for over a second, fake a measurement
 		// to handle bad range finder data when on ground
-		if (!_range_data_ready && !_control_status.flags.in_air && _vehicle_at_rest && (_time_last_imu - _time_last_hagl_fuse) > (uint64_t)1E6) {
+		if (_rng_hgt_faulty && !_range_data_ready && !_control_status.flags.in_air && _vehicle_at_rest && (_time_last_imu - _time_last_hagl_fuse) > (uint64_t)1E6) {
 			_range_data_ready = true;
+			_rng_hgt_faulty = false;
 			_range_sample_delayed.rng = _params.rng_gnd_clearance;
 		}