EKF: Reset yaw gyro bias learning when resuming use of magnetometer

The previous lack of yaw drift constraint will also mean that the yaw gyro bias estimate is likely to be incorrect and needs to be re-learned.

EKF/control.cpp

@@ -476,7 +476,14 @@ void Ekf::controlGpsFusion()
 					_control_status.flags.yaw_align = false;
 					_control_status.flags.ev_yaw = false;
 					_control_status.flags.yaw_align = resetMagHeading(_mag_sample_delayed.mag);
-					_mag_inhibit_yaw_reset_req = false;
+					// Handle the special case where we have not been constraining yaw drift or learning yaw bias due
+					// to assumed invalid mag field associated with indoor operation with a downwards looking flow sensor.
+					if (_mag_inhibit_yaw_reset_req) {
+						_mag_inhibit_yaw_reset_req = false;
+						// Zero the yaw bias covariance and set the variance to the initial alignment uncertainty
+						float dt = 0.001f * (float)FILTER_UPDATE_PERIOD_MS;
+						setDiag(P, 12, 12, sq(_params.switch_on_gyro_bias * dt));
+					}
 				}
 
 				// If the heading is valid start using gps aiding