Commander: add local_position_accuracy_low flag, incl. warning and RTL

Set this flag to true if local position is valid but accuracy low, such that
the operator can be warned before system switches to position-failure failsafe.
Additionally, switch to RTL if currently in Mission or Loiter to try to reach home
or fly out of GNSS-denied area.

Set low accuracy threshold to 50m by default for FW and VTOL.

Signed-off-by: Silvan Fuhrer <silvan@auterion.com>

src/modules/commander/commander_params.c

@@ -1088,3 +1088,20 @@ PARAM_DEFINE_INT32(COM_FLT_TIME_MAX, -1);
  * @unit m/s
  */
 PARAM_DEFINE_FLOAT(COM_WIND_MAX, -1.f);
+
+/**
+ * EPH threshold for RTL
+ *
+ * Specify the threshold for triggering a warning for low local position accuracy. Additionally triggers
+ * a RTL if currently in Mission or Loiter mode.
+ * Local position has to be still declared valid, which is most of all depending on COM_POS_FS_EPH.
+ * Use this feature on systems with dead-reckoning capabilites (e.g. fixed-wing vehicles with airspeed sensor)
+ * to improve the user notification and failure mitigation when flying in GNSS-denied areas.
+ *
+ * Set to -1 to disable.
+ *
+ * @group Commander
+ * @min -1
+ * @unit m
+ */
+PARAM_DEFINE_FLOAT(COM_POS_LOW_EPH, -1.0f);