expand info on using GSC only for control

common/source/docs/common-gcs-only-operation.rst

@@ -19,6 +19,8 @@ Failsafes
 
 It is highly recommended that the GCS failsafe be setup for the vehicle, since GCS connectivity is being relied upon exclusively for vehicle control. See :ref:`apms-failsafe-function`, :ref:`Copter Failsafe <failsafe-landing-page>`, or :ref:`Rover Failsafes <rover-failsafes>`, as appropriate.
 
+RC failsafe can also be used if a joystick is being used to send RC information (RC_OVERRIDES) via MAVLink. If the joystick fails or is disconnected, and normal :ref:`radio-failsafe` will occur.
+
 Of course, other failsafes can be setup and utilized as desired in addition to the GCS failsafe.
 
 Configuration
@@ -29,7 +31,7 @@ Plane and Rover
 
 Both Plane and Rover only require two parameter changes to allow the arming of the vehicle when using only a GCS.
 
--  In Plane, :ref:`THR_FAILSAFE<THR_FAILSAFE>` must be set to zero, while in Rover, :ref:`FS_THR_ENABLE<FS_THR_ENABLE>` must be set to zero.
+-  If you are NOT using a joystick to provide RC overrides via MAVLink, or you do not want a failsafe if the joystick fails, then in Plane, :ref:`THR_FAILSAFE<THR_FAILSAFE>` must be set to zero, while in Rover, :ref:`FS_THR_ENABLE<FS_THR_ENABLE>` must be set to zero.
 -  In both Plane and Rover, :ref:`RC_PROTOCOLs<RC_PROTOCOLs>` should be set to "0" to prevent accidental detection of noise as an RC source. 
 
 The vehicle can then be armed after the GPS has obtained lock and internal initializations have been completed, either by joystick rudder arming (if enabled, see :ref:`ARMING_RUDDER<ARMING_RUDDER>` ) or arming command from the GCS. 
@@ -40,7 +42,7 @@ Copter
 
 Copter has a few additional safety checks that must be accommodated in order to arm with no RC input.
 
--  :ref:`FS_THR_ENABLE<FS_THR_ENABLE>` must be set to zero
+-  :ref:`FS_THR_ENABLE<FS_THR_ENABLE>` must be set to zero if no joystick is being used or you do not want a failsafe if the joystick fails or is disabled.
 -  :ref:`ARMING_CHECK<ARMING_CHECK>` should not include bit 6 (RC check). A value of 65470 will include all other checks. It is not recommended that :ref:`ARMING_CHECK<ARMING_CHECK>` set for 0 (ARM immediately), since important system checks would be bypassed.
 -  assuming that the :ref:`RCMAP_ROLL <RCMAP_ROLL>`, :ref:`RCMAP_PITCH <RCMAP_PITCH>`, :ref:`RCMAP_THROTTLE <RCMAP_THROTTLE>` and :ref:`RCMAP_YAW <RCMAP_YAW>` parameters are default, then the RC 1 thru 4 channels' maximum and minimum parameters must be changed slightly. This is because Copter checks to see that the RC calibration has been done. But since no RC is used in the system, this step will not have occurred. Therefore, to emulate that this has happened, the min and max values need to be changed from their defaults of 1100 and 1900, respectively. Values of 1101 and 1901, will do for the ``RCx_MIN`` and ``RCx_MAX`` for the roll,pitch, throttle, and yaw RC channels.