EKF: Use specific position observation noise when flying without GPS

A larger position uncertainty is required when flying without GPS to reduce tilt attitude estimation errors caused by vehicle manoeuvring. This needs to be tuneable to allow optimisation for different use cases (e.g. outdoor vs indoor).

EKF/common.h

@@ -130,6 +130,7 @@ struct parameters {
 
         float gps_vel_noise = 5.0e-1f;      // observation noise for gps velocity fusion
         float gps_pos_noise = 1.0f;         // observation noise for gps position fusion
+        float pos_noaid_noise = 10.0f;      // observation noise for non-aiding position fusion
         float baro_noise = 3.0f;            // observation noise for barometric height fusion
         float baro_innov_gate = 3.0f;       // barometric height innovation consistency gate size in standard deviations
         float posNE_innov_gate = 3.0f;      // GPS horizontal position innovation consistency gate size in standard deviations

EKF/vel_pos_fusion.cpp

@@ -84,7 +84,12 @@ void Ekf::fuseVelPosHeight()
 		_vel_pos_innov[3] = _state.pos(0) - _gps_sample_delayed.pos(0);
 		_vel_pos_innov[4] = _state.pos(1) - _gps_sample_delayed.pos(1);
         // observation variance - user parameter defined
+        // if we are in flight and not using GPS, then use a specific parameter
+        if (!_control_status.flags.gps && _control_status.flags.in_air) {
+            R[3] = fmaxf(_params.pos_noaid_noise, 0.5f);
+        } else {
             R[3] = fmaxf(_params.gps_pos_noise, 0.01f);
+        }
         R[3] = R[3] * R[3];
         R[4] = R[3];
         // innovation gate sizes