Adding write-up for midair activation

Author: parakhm95

docs/65-howto-usecases/midair_activation.md

@@ -6,4 +6,15 @@ description: Midair activation of the MRS system
 
 # Midair activation
 
-TODO
+For the cases of flight where you need the aircraft to either activate in the air or be flight-ready on the ground without having to perform a take-off procedure, you can use the midair activation feature. To use this, follow this procedure:
+
+1. In your custom configuration file, set the controller and tracker you desire to be used when midair activation is called:
+```yaml
+    midair_activation:
+      after_activation:
+        controller: "MpcController"
+        tracker: "NullTracker"
+```
+2. Arm your UAV.
+3. Call midair activation using the service call `rosservice call /<vehicle_name>/uav_manager/midair_activation`. This service will request the underlying `hw_api` to switch to offboard and when the status is confirmed, it will switch to the controller and tracker specified in the configuration file.
+4. The UAV will now be engaged with the controller and tracker specified in the configuration file.