fw_tecs: Support tighter altitude tracking during low-height flight

msg/TecsStatus.msg

@@ -2,6 +2,7 @@ uint64 timestamp		# time since system start (microseconds)
 
 float32 altitude_sp			# Altitude setpoint AMSL [m]
 float32 altitude_reference		# Altitude setpoint reference AMSL [m]
+float32 altitude_time_constant		# Time constant of the altitude tracker [s]
 float32 height_rate_reference		# Height rate setpoint reference [m/s]
 float32 height_rate_direct		# Direct height rate setpoint from velocity reference generator [m/s]
 float32 height_rate_setpoint		# Height rate setpoint [m/s]

src/lib/tecs/TECS.hpp

@@ -647,6 +647,14 @@ class TECS
 	float get_pitch_setpoint() {return _control.getPitchSetpoint();}
 	float get_throttle_setpoint() {return _control.getThrottleSetpoint();}
 
+	/**
+	 * Returns the altitude tracking time constant
+	 */
+	float get_altitude_error_time_constant() const
+	{
+		return 1.0f / math::max(_control_param.altitude_error_gain, 0.01f);
+	}
+
 	uint64_t timestamp() { return _update_timestamp; }
 	float get_underspeed_ratio() { return _control.getRatioUndersped(); }