Does RTH work with Compass Error?
- Thread starter Mimoid
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N017RW
Premium Pilot
Try driving backwards and see if it points in the same direction you are moving.
This will prove if you are seeing Course or Heading.
The amount you rotate is irrelevant. Just nudge to the right a little bit and go forward. Compare the new coordinates to the old and rotate & move based on that. Repeat. It won't be pretty or efficient, but the drone will move toward the RTH point.
The drama that I didn't have to deal with is altitude. Couple of different options, none of which are bullet proof. All of which are probably better than a random flyaway.
ianwood
Taco Wrangler
If you think it's easy to determine heading from GPS points because your car does it, there's an important difference you need to consider:
Your car is just a little less likely to drift in the wind. Hopefully, it's doesn't drift at all! And actually, it's more likely that your car does have a compass in it if the navigation system is any good.
The compass is there for a reason. If you want to nix the compass, start looking at RTK which is very precise but equally expensive. In the meantime, the compass is essential. No way home without it.
Your car is just a little less likely to drift in the wind. Hopefully, it's doesn't drift at all! And actually, it's more likely that your car does have a compass in it if the navigation system is any good.
The compass is there for a reason. If you want to nix the compass, start looking at RTK which is very precise but equally expensive. In the meantime, the compass is essential. No way home without it.
Nobody is talking about wanting to get rid of the compass. We're talking about how to RTH when the compass shits itself and only the GPS is available.
In that situation, you RTH. The logic is simple. The execution is kludgy because it's a series of actions based off, essentialy'ish, random information. And there is the altitude and, as you rightfully mention, wind/drift factors. All in all, still better than just letting the drone getting a mind of it's own and abandoning you.
In fact, I'd think DJI could easily implement an option as to what to do if you lose compass but maintain GPS. Make it user selectable.
- If RC connected, force ATTI mode.
- If RC not connected... hover, land straight down, go to GPS-only RTH, raise, lower, blah blah. Needs more thought.
- Retrace last x seconds of flight. This could be the best option as it may come closest to assuring you don't hit anything. It, of course, has some randomness and inaccuracies, but the user may want to have this option.
ianwood
Taco Wrangler
A GPS rewind function, similar to an automatic zip line I guess, assumes the ability to accurately get from point a to point b using GPS. This is something you cannot do easily without a compass.
Okay. So you didn't mean "rotate a certain amount." You meant "rotate it a little bit." So you "nudge the aircraft to the right a little bit". Okay? I'm using your words. I suppose you mean you want to "nudge" it clockwise a little bit. So you do that. Okay? The craft is now rotating clockwise. Okay? Stay with me. You still have no compass. What's next?
Oso
Premium Pilot
My take is that he is effectively saying it could have been designed sort of like the children's game of Hot and Cold. After ascending to the preset RTH height, the Phantom could be designed to move a short distance and then compare it's current location to the home point while GPS keeps it from drifting. Is it getting closer to the home point? Yes, you're getting warmer. No, you're getting colder you went the wrong way try again. If no, the Phantom rotates a number of degrees, moves a short distance and then compares again. Closer to home point this time? Yes, you're getting warmer keep going for a while and then repeat the process. Now you're getting very hot, etc. After a few of these comparisons, the Phantom could determine a reasonably accurate direction back to the home point and then stop to compare less frequently along the way.
I for sure don't want to put words into his mouth, but that's my understanding of what he's trying to say.
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My point is, that strategy might work if it was a car. But in the air, once the aircraft begins to rotate, even a little bit, you're done. GPS won't help. There is no way to stop the rotation without a compass. You're doomed.
alokbhargava
Premium Pilot
Bingo. You put words in my mouth way better than I take them out. Thanks.
It's not clean, but perfectly doable. This seems to be an awfully reasonable RTH when the world (other than GPS, of course) has come to an end. When the execution is controlled by a number of user-selectable variables, you have the best possible fail safe.
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Just had compass error last week 9,000ft away from home point. DJI go showed where the bird was on the map but orientation of the bird was not correct in the app. Switched to ATTI mode and relied on visuals in the FPV feed to navigate back home and used the map as a reference to make sure I was heading in the right direction. I'm not sure if RTH would have worked because DJI Go was reporting that my drone was facing west the whole time while I was flying actually pointed and flying in a northwesterly direction. Check out the video
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Wrong. But to each his own.
Stopping the rotation would be best, but even if it can't the algorithm still works. Just somewhat less efficiently. Which is still more efficient than the drone disappearing on a fly away. I believe the best economy in the algorithm can be made by simply knowing if it's closer to home direction to rotate left or right. As someone earlier mentioned, this can be controlled by varying prop speeds.
It's no different than Hot or Cold. You've taken away one of the child's senses (in this case sight), but they still have the second relevant sense - hearing. Well, in our use case, the drone has lost its sense of direction, but not it's sense of position. The job would get done.
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Same thing for my after-market dash GPS as well as my smart phone. After some movement, it makes a good guess as to what direction you're heading. But the P3 needs the compass.
Note: without compass, if you have the green line on your map, you could fly back along it, even if the compass doesn't orient the arrow icon to the direction you're flying. Also, you can fly FPV, using the video feed to recognize markers to fly by.
I was using your example. You said you'd rotate it a certain amount. Then you said you'd rotate it a little. Rotating it will not work for obvious reasons - you (or your algorithm) will never be able to stop the rotation. Forwards would be backwords. Left would be backwards. Forwards would be left. Forwards would be right. Forwards would be right again.. Left would be forwards. And so on. You'd be helplessly lost.
Out.
ianwood
Taco Wrangler
The hot and cold game won't work in anything but a perfectly stable wind (not common on the surface), perfect GPS signal, and with a LOT of space to get it wrong. More importantly, it will be almost impossible for the FC to figure out when to play this hot and cold game. It is very hard to know when compass data is bad. The compass error in the FC is a guess and it's flawed. The last thing anyone wants is for it to false positive and abandon good compass data. Finally, the hot and cold game would be yet another flight mode that would require its own navigation and AHRS logic. Not likely.
Hopefully your children are smarter than a Phantom!
Hopefully your children are smarter than a Phantom!
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Yeah, that pretty much settles it. RTH with a scrambled compass is a no-go. Basically if you lose both the compass AND the camera image, your precious Phantom is most likely a goner. You may get lucky by pulling the (right) stick back towards you to bring the drone closer until the image returns, but if you have turned away from the original heading, you will just fly deeper into oblivion.
With this in mind, I find it unbelievable risky to fly further our than 5000-6000 feet, as it is usually at that distance the video feed is starting to become weak.
//Tom
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N017RW
Premium Pilot
Sample two positions at a time interval and you got a vector.
Current position to home is another vector.
Trial and error would work. Move forward. Check vector. Rotate a bit, move forward, check again.
I don't doubt the resulting route would be far from straight but it would bring the machine closer to home.
Sent from my iPhone using PhantomPilots mobile app
Current position to home is another vector.
Trial and error would work. Move forward. Check vector. Rotate a bit, move forward, check again.
I don't doubt the resulting route would be far from straight but it would bring the machine closer to home.
Sent from my iPhone using PhantomPilots mobile app
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