Failsafe docs: document COM_POS_LOW_EPH, COM_POS_LOW_ACT (#26508)

* Failsafe docs: document COM_POS_LOW_EPH, COM_POS_LOW_ACT

* Failsafe docs: address review

docs/en/config/safety.md

@@ -181,17 +181,22 @@ The following settings also apply, but are not displayed in the QGC UI.
 | <a id="GF_PREDICT"></a>Preemptive geofence triggering              | [GF_PREDICT](../advanced_config/parameter_reference.md#GF_PREDICT)           | (Experimental) Trigger geofence if current motion of the vehicle is predicted to trigger the breach (rather than late triggering after the breach). |
 | <a id="CBRK_FLIGHTTERM"></a>Circuit breaker for flight termination | [CBRK_FLIGHTTERM](../advanced_config/parameter_reference.md#CBRK_FLIGHTTERM) | Enables/Disables flight termination action (disabled by default).                                                                                   |
 
-## Position (GNSS) Loss Failsafe
+## Position Estimation Failsafes
+
+This section describes failsafes related to the quality of the vehicle's position estimate.
+
+### Position Loss Failsafe
 
 The _Position Loss Failsafe_ is triggered if the quality of the PX4 position estimate falls below acceptable levels (this might be caused by GPS loss) while in a mode that requires an acceptable position estimate.
-The sections below cover first the trigger and then the failsafe action taken by the controller.
 
 ### Position Loss Failsafe Trigger
 
-There are basically two mechanisms in PX4 to trigger position failsafes:
+The position loss failsafe triggers if the position estimate becomes _invalid_. There are two mechanisms in PX4 to invalidate the position estimate:
 
-- A timeout since the last sensor data was fused that provides direct speed or horizontal position measurements. Sensors that fall into that category are: GNSS, optical flow, airspeed, VIO, auxiliary global position.
-- The estimated horizontal position accuracy exceeds a certain threshold. This check is only done on hovering systems (rotary wing vehicles or VTOLs in hover phase).
+- A timeout since the last sensor data was fused that provides direct speed or horizontal position measurements.
+   - Sensors that fall into that category are: GNSS, optical flow, airspeed, VIO, auxiliary global position.
+- The estimated horizontal position inaccuracy exceeds the threshold [COM_POS_LOW_EPH](../advanced_config/parameter_reference.md#COM_POS_LOW_EPH)
+   - This check is only done on hovering systems (rotary-wing vehicles or VTOLs in hover phase). For fixed-wing vehicles, refer to the [Position Accuracy Low](#position-accuracy-low-failsafe) section.
 
 The relevant parameters shown below.
 
@@ -207,14 +212,24 @@ Multicopters will switch to [Altitude mode](../flight_modes_mc/altitude.md) if a
 Fixed-wing planes, and VTOLs not configured to land in hover ([NAV_FORCE_VT](../advanced_config/parameter_reference.md#NAV_FORCE_VT)), have a parameter ([FW_GPSF_LT](../advanced_config/parameter_reference.md#FW_GPSF_LT)) that defines how long they will loiter (circle with a constant roll angle ([FW_GPSF_R](../advanced_config/parameter_reference.md#FW_GPSF_R)) at the current altitude) after losing position before attempting to land.
 If VTOLs have are configured to switch to hover for landing ([NAV_FORCE_VT](../advanced_config/parameter_reference.md#NAV_FORCE_VT)) then they will first transition and then descend.
 
-The relevant parameters for all vehicles shown below.
+The relevant parameters are:
 
-Parameters that only affect Fixed-wing vehicles:
+| Parameter                                                                                       | Description                                                                                                                                   |
+| ----------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------- |
+| <a id="FW_GPSF_LT"></a>[FW_GPSF_LT](../advanced_config/parameter_reference.md#FW_GPSF_LT)       | Fixed-wing only: Loiter time (waiting at current altitude for position estimation recovery before starting to descend). Set to 0 to disable.  |
+| <a id="FW_GPSF_R"></a>[FW_GPSF_R](../advanced_config/parameter_reference.md#FW_GPSF_R)          | Fixed roll/bank angle while circling.                                                                                                         |
+| <a id="NAV_FORCE_VT"></a>[NAV_FORCE_VT](../advanced_config/parameter_reference.md#NAV_FORCE_VT) | If true, force VTOL takeoff and landing, even in `Descend` failsafe.                                                                          |
 
-| Parameter                                                                                 | Description                                                                                                 |
-| ----------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------- |
-| <a id="FW_GPSF_LT"></a>[FW_GPSF_LT](../advanced_config/parameter_reference.md#FW_GPSF_LT) | Loiter time (waiting for GPS recovery before it goes into land or flight termination). Set to 0 to disable. |
-| <a id="FW_GPSF_R"></a>[FW_GPSF_R](../advanced_config/parameter_reference.md#FW_GPSF_R)    | Fixed roll/bank angle while circling.                                                                       |
+### Position Accuracy Low Failsafe
+
+In Fixed-wing, the position estimate is never strictly invalidated as long as we have a horizontal aiding source, such as an airspeed sensor. In that case, a separate failsafe can be configured that triggers if the position estimate inacuraccy exceeds the threshold [COM_POS_LOW_EPH](../advanced_config/parameter_reference.md#COM_POS_LOW_EPH). The failsafe action is taken if the vehicle is in mission or hold mode, otherwise it is only a warning. The relevant parameters are:
+
+| Parameter                                                                                                | Description                                                                   |
+| -------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------|
+| <a id="COM_POS_LOW_EPH"></a>[COM_POS_LOW_EPH](../advanced_config/parameter_reference.md#COM_POS_LOW_EPH) | Position inaccuracy threshold above which COM_POS_LOW_ACT is taken.           |
+| <a id="COM_POS_LOW_ACT"></a>[COM_POS_LOW_ACT](../advanced_config/parameter_reference.md#COM_POS_LOW_ACT) | Failsafe action taken when position inaccuracy is above configured threshold. |
+
+Note that if there is no horizontal aiding source anymore, the position estimate is invalidated after `EKF2_NOAID_TOUT`, and the standard position loss failsafe applies.
 
 ## Offboard Loss Failsafe