Atti mode is basically manual mode yes? No GPS so you control everything?
No, with losing GPS the system only loses the ability to hold a position steadily and to navigate to a given position (home point) on its own. All other sensors like IMU (3-axis accelerators, 3-axis gyros), Baro (barometric altitude) are still working and are used to assist you in your piloting task.
Atti is still a heavily assisted flight mode using IMU and baro data to support you with self leveling the drone and maintaining a steady altitude. In a pure manual mode you'd loose the self leveling and alt hold ability degrading to a flight mode race quad flyers love (Manual or Acro). So no panic if it reverts to Atti mode. One side effect of not being able to hold a steady position though is drift. Your drone will drift with the wind if you don't counteract the motion with pitch and roll inputs. Apart from this you won't notice a huge difference in flying the drone. Push the stick forward and the drone will pickup speed and fly to where the nose is pointing. Release the stick it will stop, self level and hover happily drifting with the wind. Think you get the picture. One other important aspect: losing GPS does not affect your RC link and video link.
So what do you if your drone reverts to Atti and you still have a good visual:
- don't panic, it won't fall out of the sky immediately
- don't think RTH will save you. Man up.
- Turn it by using the yaw stick until you see the back of the drone
- Pull the the pitch stick moderatly back to fly it towards you
- If it drifts off course due to wind apply some roll inputs
If you lost visual there is still hope:
- don't panic, it won't fall out of the sky immediately
- don't think RTH will save you. Man up even more and fight.
- Use your video link. Turn the drone by using the yaw stick until you identify some known ground reference points.
- Fly by your screen. Don't give up!
If you lose GPS and your video link with the drone out of sight at the same time you are in a double failure mode which should statistically be rare. And you are doomed. Think about staying in LOS or at least video range with lots of margin for errors for the next flight after buying a new drone. Remember, even bad experiences make you a better pilot.
Too bad DJI doesn't allow Spark customers to practice Atti mode in order to demystify it.