Interesting Litchi Flight. Why was RTH initiated?

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I had an interesting flight with my P4 under Litchi on Wednesday, from the top of a hill, out over a lake to a point on the other side, and then return.

Most distance point was 4.049 kilometres away, and the total flight was 8,175 metres.

The thing I don’t understand was why, at 7:49 mins and on the way back at 3,109 metres out, and with 62% battery, the P4 initiated RTH.

On the way back, cruising at 45 km/h , with only about 3/8th of the mission to complete and heading for home with 62% battery left, it seemed in no danger of not making it back, so why would it initiate RTH?

After that happened it had to climb from -148 metres, to + 108 metres to reach the RTH height, and stopped to do this, right over the middle of the lake. I eventually kicked it up to 35 km/h to get it moving again, and it eventually landed with 35% battery left, a very comfortable position.

The full data log is here http://tinyurl.com/jcze5cn
 
I'm no expert but my guess is that you had Smart RTH turned on. The Phantom determined that it didn't have enough battery left to complete the entire mission so RTH kicked in. Since you said you increased speed on the return trip and made it back with 35% battery left, it would have been less than 35% had you NOT increased speed. The bird seems to be a bit conservative on its estimates of this.

I recently did a Litchi flight over a field with 76 waypoints, each one programmed to stop and take a photo. Somewhere around halfway through, the mission stopped and RTH kicked in even though the bird was almost over my head. I landed, put in a new battery, turned OFF Smart RTH and flew it again. It flew the entire mission and came home. I don't remember the battery level at landing. The only reason I thought of it at the time was that I had just recently watched a video of a 5 mile, one way Litchi mission. In his original try, his Smart RTH kicked in shortly before the end of the flight, turned toward home (which was blocked by a hill) and crashed into the trees. He repeated the mission with Smart RTH off and had no problems. (Side note: If trying a one way mission of that length, it has to be planned as a round trip mission and the pilot needs to get to the end point to manually take control. The reason for this is that no two waypoints can be more than 2000 m apart, INCLUDING the distance between the first and last waypoint. There have to be some waypoints less than 2000 m apart for the return trip even though you have no intention of letting it try to return home).

Here's the video. Be sure to read his description of the flight (below the video):

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The thing that I'm confused about is that it would have had plenty of battery to complete the mission, and probably would have finished with over 40% left. The remaining part of the mission was basically to return to home. I suppose some of these decisions are not that smart as the programming would be pretty complex.
First of all the drone's firmware would need to know about the mission in advance, and this probably doesn't happen. The mission steps probably just get executed serially and the firmware doesn't know what's coming next.
 
Oh, and I didn't know about the 2 km inter waypoint limit. It's a little bit annoying that mission hub doesn't alert to this and you end up having to insert extra waypoints in the field when the pressure's on, rather than in the planning stage.
 

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