delay time: potentially reduce delay time if baro/aux velocity/airspeed is not used

EKF/common.h

@@ -277,6 +277,7 @@ struct parameters {
 	// airspeed fusion
 	float tas_innov_gate{5.0f};		///< True Airspeed innovation consistency gate size (STD)
 	float eas_noise{1.4f};			///< EAS measurement noise standard deviation used for airspeed fusion (m/s)
+	float arsp_thr{0.0f};			///< Airspeed fusion threshold. A value of zero will deactivate airspeed fusion
 
 	// synthetic sideslip fusion
 	float beta_innov_gate{5.0f};		///< synthetic sideslip innovation consistency gate size in standard deviation (STD)

EKF/estimator_interface.cpp

@@ -478,9 +478,14 @@ void EstimatorInterface::setDragData(const imuSample &imu)
 bool EstimatorInterface::initialise_interface(uint64_t timestamp)
 {
 	// find the maximum time delay the buffers are required to handle
-	float max_time_delay_ms = math::max(_params.baro_delay_ms,
-					math::max(_params.auxvel_delay_ms,
-						_params.airspeed_delay_ms));
+	// it's reasonable to assume that barometer is always used, and its delay is low
+	// it's reasonable to assume that aux velocity device has low delay. TODO: check the delay only if the aux device is used
+	float max_time_delay_ms = math::max(_params.baro_delay_ms, _params.auxvel_delay_ms);
+
+	// using airspeed
+	if (_params.arsp_thr > FLT_EPSILON) {
+		max_time_delay_ms = math::max(_params.airspeed_delay_ms, max_time_delay_ms);
+	}
 
 	// mag mode
 	if (_params.mag_fusion_type != MAG_FUSE_TYPE_NONE) {