ekf2: enable multiple height sources fusion

Instead of having a single height source fused into the EKF and the
other ones "waiting" for a failure or the primary sensor, fuse all
sources in EKF2 at the same time. To prevent the sources from fighting against each
other, the "primary" source is set as reference and the other ones are
running a bias estimator in order to make all the secondary height
sources converge to the primary one.

If the reference isn't available, another one is automatically selected
from a priority list. This secondary reference keeps its current bias
estimate but stops updating it in order to be the new reference as close
as possible to the primary one.

src/modules/ekf2/EKF/gps_fusion.cpp

@@ -102,12 +102,23 @@ void Ekf::updateGpsPos(const gpsSample &gps_sample)
 	auto &gps_pos = _aid_src_gnss_pos;
 	resetEstimatorAidStatus(gps_pos);
 
+	const float height_measurement = gps_sample.hgt - getEkfGlobalOriginAltitude();
+	const float height_measurement_var = getGpsHeightVariance();
+
+	// save current bias and update bias estimator
+	const float bias = _gps_hgt_b_est.getBias();
+	const float bias_var = _gps_hgt_b_est.getBiasVar();
+
+	_gps_hgt_b_est.setMaxStateNoise(height_measurement_var);
+	_gps_hgt_b_est.setProcessNoiseStdDev(height_measurement_var); //TODO: update this
+	_gps_hgt_b_est.fuseBias(height_measurement - (-_state.pos(2)), height_measurement_var + P(9, 9));
+
 	Vector3f position;
 	position(0) = gps_sample.pos(0);
 	position(1) = gps_sample.pos(1);
 
 	// vertical position - gps measurement has opposite sign to earth z axis
-	position(2) = -(gps_sample.hgt - getEkfGlobalOriginAltitude() - _gps_hgt_offset);
+	position(2) = -(height_measurement - bias);
 
 	const float lower_limit = fmaxf(_params.gps_pos_noise, 0.01f);
 
@@ -125,7 +136,7 @@ void Ekf::updateGpsPos(const gpsSample &gps_sample)
 		obs_var(0) = obs_var(1) = sq(math::constrain(gps_sample.hacc, lower_limit, upper_limit));
 	}
 
-	obs_var(2) = getGpsHeightVariance();
+	obs_var(2) = height_measurement_var + bias_var;
 
 	// innovation gate size
 	float innov_gate = fmaxf(_params.gps_pos_innov_gate, 1.f);