docs: add GNSS check failsafe documentation

Update safety.md and releases/main.md to document the new GNSS check
failsafe (SYS_HAS_NUM_GNSS, COM_GPS_LOSS_ACT) introduced in PX4.

docs/en/config/safety.md

@@ -229,6 +229,22 @@ In Fixed-wing, the position estimate is never strictly invalidated as long as we
 
 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.
 
+### GNSS Check Failsafe
+
+<Badge type="tip" text="PX4 v1.18" />
+
+The GNSS check failsafe has two triggers, both configured by [COM_GPS_LOSS_ACT](#COM_GPS_LOSS_ACT):
+
+- **Count drop**: the number of receivers with a 3D fix drops below [SYS_HAS_NUM_GNSS](#SYS_HAS_NUM_GNSS). No action when `SYS_HAS_NUM_GNSS=0`, only a warning.
+- **Position divergence**: two receivers report positions that disagree by more than their expected separation on the vehicle (configured via [SENS_GPS0_OFFX](../advanced_config/parameter_reference.md#SENS_GPS0_OFFX), [SENS_GPS0_OFFY](../advanced_config/parameter_reference.md#SENS_GPS0_OFFY), [SENS_GPS1_OFFX](../advanced_config/parameter_reference.md#SENS_GPS1_OFFX), [SENS_GPS1_OFFY](../advanced_config/parameter_reference.md#SENS_GPS1_OFFY)) plus their reported accuracy margin. Always produces a warning regardless of `SYS_HAS_NUM_GNSS`; the `COM_GPS_LOSS_ACT` failsafe action is only triggered when `SYS_HAS_NUM_GNSS=2`. Make sure to configure the lever arm offsets of both receivers for accurate detection.
+
+When `COM_GPS_LOSS_ACT` is set to Warning, both triggers produce a warning only and never block arming. When set to Return, Land, or Terminate, both triggers block arming and fire the configured in-flight failsafe action (subject to the `SYS_HAS_NUM_GNSS` condition above).
+
+| Parameter                                                                                                      | Description                                                                                                                                                    |
+| -------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| <a id="SYS_HAS_NUM_GNSS"></a>[SYS_HAS_NUM_GNSS](../advanced_config/parameter_reference.md#SYS_HAS_NUM_GNSS) | Number of GNSS receivers required for arming and flight. `0`: No minimum (default), `1`: Require one, `2`: Require two.                                       |
+| <a id="COM_GPS_LOSS_ACT"></a>[COM_GPS_LOSS_ACT](../advanced_config/parameter_reference.md#COM_GPS_LOSS_ACT)   | Failsafe action when a GNSS failure is detected. `0`: Warning only (default), `1`: Return, `2`: Land, `3`: Terminate. Values above Warning also block arming. |
+
 ## Offboard Loss Failsafe
 
 The _Offboard Loss Failsafe_ is triggered if the offboard link is lost while under [Offboard control](../flight_modes/offboard.md).

docs/en/releases/main.md

@@ -44,6 +44,7 @@ Please continue reading for [upgrade instructions](#upgrade-guide).
 - [Remote ID (Open Drone ID) in-flight failsafe](../peripherals/remote_id.md): extended [COM_ARM_ODID](../advanced_config/parameter_reference.md#COM_ARM_ODID) to also trigger a configurable failsafe action (Return, Land, or Terminate) if the Remote ID heartbeat is lost while airborne. Users previously on `COM_ARM_ODID=2` retain the same arming behaviour; set to `3` or higher to enable the in-flight action. ([PX4-Autopilot#27029](https://github.com/PX4/PX4-Autopilot/pull/27029))
 - [QGroundControl Bootloader Update](../advanced_config/bootloader_update.md#qgc-bootloader-update-sys-bl-update) via the [SYS_BL_UPDATE](../advanced_config/parameter_reference.md#SYS_BL_UPDATE) parameter has been re-enabled after being broken for a number of releases. ([PX4-Autopilot#25032: build: romf: fix generation of rc.board_bootloader_upgrade](https://github.com/PX4/PX4-Autopilot/pull/25032)).
 - [Feature: Allow prioritization of manual control inputs based on their instance number in ascending or descending order](../config/manual_control.md#px4-configuration). ([PX4-Autopilot#25602: Ascending and descending manual control input priorities](https://github.com/PX4/PX4-Autopilot/pull/25602)).
+- [GNSS check failsafe](../config/safety.md#gnss-check-failsafe): new configurable failsafe triggered by two conditions: active receiver count drops below [SYS_HAS_NUM_GNSS](../advanced_config/parameter_reference.md#SYS_HAS_NUM_GNSS) (warning only when `0`), or two receivers report inconsistent positions (warning always, failsafe action only when `SYS_HAS_NUM_GNSS=2`). Action configured via [COM_GPS_LOSS_ACT](../advanced_config/parameter_reference.md#COM_GPS_LOSS_ACT) (Warning, Return, Land, or Terminate). ([PX4-Autopilot#26863](https://github.com/PX4/PX4-Autopilot/pull/26863))
 
 ### Control