Compass Error on attempting yo take off at canal lock

Steve loves to think the compass does nothing. Plenty of the P3 fly aways caused by bad compass readings, Steve.
 
"Just because" doesn't work here. Please answer the question:

How?

The compass has nothing to do with the location - just which way the drone is pointing. The GPS reports the latitude-longitude position to the controller. The compass reports the heading. There is no sensor on the Phantom to determine speed, that is calculated from subsequent positions from the GPS.

Bad compass data will not cause a flyaway. "It happened to me" is not a technical description of how bad compass data can cause a flyaway. It's what we in the engineering world call a coincidence. The controller goes into ATTI mode when there is a significant compass error not to prevent a flyaway, but because without reliable heading information from the compass, your Phantom won't know which way to point when it performs a "return to Home".

The answer is above in very non-rocket science terms. I will, however, copy an paste here for convenience. Not just because, and not because it happened to me, all of my scratch built Naza multirotors and Phantoms are still in my possession.

The compass directional info, calibrated with speed info, creates a location which must match where the gps thinks the P3 is. It is a checks and balances system. (consumer gps is not accurate enough on its own +/-6-15ft) The compass has a whole lot more to do with location than where north is. Thats why compass errors are so critical to pay attention too. That's why taking off on metal and then moving away does not solve your compass error issues, it makes them exponentially worse. Now the checks and balances get checked, but they are not balanced. The FC tries to balance, post haste, and then you get the speed of light fly-a-way. Thats also why the FC now goes automatically goes into atti mode with major compass errors. It was a safety feature not available to the Naza Phantoms or custom builds. Thats also why it is recommended to switch manually into atti with compass errors. You often dont want it switching back to gps and trying to reconcile compass errors on its own.
 
The GPS doesn't "think" anything, it just provides the lat-lon data. The compass doesn't "know" where it is, it only knows which way is north.

Your first statement makes no sense at all because the direction (course) and speed are all from the GPS - NOT the compass. And If you are just flying about without waypoints or RTH engaged, then the compass contributes almost nothing to the flight. If the compass is in error, then the drone will "toilet bowl" -- It won't fly in a straight line, but it will not fly-away. The drone will maintain whatever heading it starts out with when you push the stick forward. It doesn't care which way it is pointed and the GPS doesn't know.
 
I've never figured out why DJI can't print a white paper explaining this without worrying about trademark/copyright/ trade secret information.

Opinions do vary among our local experts, leading to more confusion.
 
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The GPS doesn't "think" anything, it just provides the lat-lon data. The compass doesn't "know" where it is, it only knows which way is north.

Your first statement makes no sense at all because the direction (course) and speed are all from the GPS - NOT the compass. And If you are just flying about without waypoints or RTH engaged, then the compass contributes almost nothing to the flight. If the compass is in error, then the drone will "toilet bowl" -- It won't fly in a straight line, but it will not fly-away. The drone will maintain whatever heading it starts out with when you push the stick forward. It doesn't care which way it is pointed and the GPS doesn't know.

Hi Steve,

Does that possibly mean that if we were to disable RTH on the P3 (and I have not checked how this is done), erratic behavior due to metal interference would be mitigated?
 
Hi Steve,

Does that possibly mean that if we were to disable RTH on the P3 (and I have not checked how this is done), erratic behavior due to metal interference would be mitigated?
You don't disable RTH, you either enter RTH or not. Because RTH is a safety feature and RTH won't work without a compass and GPS, the Phantom won't start if either is giving bad data.

Here's how compass error presents in flight. If you point your drone due East (90° heading), then move your drone left-right-forward or back, up or down, it should always be pointing to a heading of 90°. Because the compass tells the FC if it drifts to 89° or 91° and the FC corrects the heading. If you are flying into a magnetic anomaly, such as approaching a steel ship or building, the interference will change where 90° is, but the FC will keep the drone pointed to 90°, so you will see the drone physically rotate as you pass over the anomaly or other interference. Most operators will never be consciously aware of this as they are not usually flying waypoints but keeping the camera pointed at their target oblivious to what the compass says. You always have two waypoints in your FC -- the current position and home. When you or your drone decides to RTH, the FC calculates the heading needed to proceed to home, turns the drone until the compass reaches that heading and the drone starts to move. Every few milliseconds the FC calculates the course from the GPS . If the course is not good, I.E. "If I continue on this course, I will not get to my next waypoint", usually because of wind, the FC adjusts the heading needed to correct the course. Assuming no wind, if your compass is 30° off due to interference, the RTH will initially head in a direction that won't take it home. A few milliseconds later, the FC makes a heading correction. Repeat- every few milliseconds. This is how a compass error will produce the "toilet bowl" effect. The drone will eventually get to the home waypoint, but it will take the scenic route.

This is how the FC handles a calibration error or interference from a magnetic anomaly. A bad or no compass produces another outcome because the FC never sees a heading change. This could result in a fly-away as the FC tries in vain to turn the drone to a new heading.

I am not privy to the code in the Phantom controllers, but this is how the compass is used in my Arduino based 3DR hex where I do have the source code.
 
You don't disable RTH, you either enter RTH or not. ...

.
Thanks Steve for the detail behind the P3 logic. I am amazed that this small device can number crunch all this positional data at the same time as capturing and saving 4K video and beaming a down convert 720P steam to over 2 (or more) kms.

I am intrigued by the the 'enter RTH' or not. How do you 'not'?
 
Thanks Steve for the detail behind the P3 logic. I am amazed that this small device can number crunch all this positional data at the same time as capturing and saving 4K video and beaming a down convert 720P steam to over 2 (or more) kms.

I am intrigued by the the 'enter RTH' or not. How do you 'not'?
Don't push the button or turn off the remote controller - depending on your drone model.
 
I fly 3D 250 class quads as well using FCs with only gyro even though compass is available on the board. Flies fine. You just have to actually fly a quad. If ATTI mode is just gyro or includes compass should not matter as long as you can fly. Orientation, orientation! At our club I recommend to all to learn to fly first, then buy DJI for vids.
 
Today I flew from the deck of floating dock (steel) near a big ship.

I tried 5 times and last 2 of them were successfull.

First 3 attemps, i took compass error and I had to land again in atti mode and change the position.

Last 2 of them was better. i took of from hand. First couple seconds I had. compass error but it turned back to normal.

So better to takeoff with hand on the steel.

ImageUploadedByPhantomPilots - DJI Phantom Forum1445106770.703766.jpg


ImageUploadedByPhantomPilots - DJI Phantom Forum1445106815.871263.jpg
 
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This afternoon, I thought I could get some really cool footage of my local canal and chose a flat spot near a canal lock to take off. After the usual preflight checks, I turned on the phantom only to be bombarded with a compass error warning and a suggestion to move elsewhere. Unbeknownst to me I moved to a nearly canal bridge but still got the same compass error readout. On further inspection of the lock and the bridge I found their was massive reinforced iron in both structures so I decided to abort the mission. I contemplated whether it would be possible to take of in ATTI mode, if that were possible, but was reluctant to do so due to the enclosed nature of the canal bordered by high trees. I foresaw losing LOS and saying goodbye to my bird.

Now my question which I would like to ask the more experienced on the forum. Could I have started in ATTI mode and flown the mission safely by bypassing the start up procedure that gave me all the compass errors in PGP? Many thanks for informed revise in advance.
The same thing happened to me yesterday in Western Montana with a P3. I was in the Bitterroot National Forest on a narrow gravel road with trees and mountains on both sides and a creek below. I got the green light, but right after take off it went red. I landed and restarted It did the exact same thing 3 or 4 times so I finally flew in ATTI, which I really don't like to do. It doesn't answer your question because I started with green, but the coincidence is worth noting.
 
Today I flew from the deck of floating dock (steel) near a big ship.

I tried 5 times and last 2 of them were successful.

First 3 attempts, i took compass error and I had to land again in atti mode and change the position.

Last 2 of them was better. i took of from hand. First couple seconds I had. compass error but it turned back to normal.

So better to takeoff with hand on the steel.

View attachment 33711

View attachment 33712

Well that is impressive.
Not only steel to the bottom but also steel to both sides and lots of it.
Did you also land at the bottom of the floating dock, and hand catch?
 
This afternoon, I thought I could get some really cool footage of my local canal and chose a flat spot near a canal lock to take off. After the usual preflight checks, I turned on the phantom only to be bombarded with a compass error warning and a suggestion to move elsewhere. Unbeknownst to me I moved to a nearly canal bridge but still got the same compass error readout. On further inspection of the lock and the bridge I found their was massive reinforced iron in both structures so I decided to abort the mission. I contemplated whether it would be possible to take of in ATTI mode, if that were possible, but was reluctant to do so due to the enclosed nature of the canal bordered by high trees. I foresaw losing LOS and saying goodbye to my bird.
unless Iwas
Now my question which I would like to ask the more experienced on the forum. Could I have started in ATTI mode and flown the mission safely by bypassing the start up procedure that gave me all the compass errors in PGP? Many thanks for informed revise in advance.
Think the problem is in the software. Never had a problem calibrating the compass of my P3,unless I was 10 feet from my vehicle.
 
This afternoon, I thought I could get some really cool footage of my local canal and chose a flat spot near a canal lock to take off. After the usual preflight checks, I turned on the phantom only to be bombarded with a compass error warning and a suggestion to move elsewhere. Unbeknownst to me I moved to a nearly canal bridge but still got the same compass error readout. On further inspection of the lock and the bridge I found their was massive reinforced iron in both structures so I decided to abort the mission. I contemplated whether it would be possible to take of in ATTI mode, if that were possible, but was reluctant to do so due to the enclosed nature of the canal bordered by high trees. I foresaw losing LOS and saying goodbye to my bird.be it's some mineral de

Now my question which I would like to ask the more experienced on the forum. Could I have started in ATTI mode and flown the mission safely by bypassing the start up procedure that gave me all the compass errors in PGP? Many thanks for informed revise in advance.
Or maybe it's some mineral deposit underground.
 
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Well that is impressive.
Not only steel to the bottom but also steel to both sides and lots of it.
Did you also land at the bottom of the floating dock, and hand catch?

Yes, I landed to the same place, I got again compass error around 5 meter height. Din't risk, pass to the atti mode and then hand catch.
 
Or maybe it's some mineral deposit underground.
Don't think it was due to geology in the area. I have been working in the mining industry for much of my career. Can only think that it was the Victorian cast iron reinforcement to the canal structures. Incidentally, it took 3 attempts to calibrate the compass in a large open field before the error warning disappeared and the seniors indicted more normal levels.
 

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