Rebar Question

[MapMaker53]

I'll be flying at a small concrete materials site next week in a congested urban area. Every inch of the site is concrete, which I am sure is rebar reinforced to handle their heavy equipment traffic (dump trucks, cement mixers, etc.). I'm considering bringing a short 2 ft wooden step ladder to launch the drone from, but I am unsure as to how far above the rebar the drone needs to be in order for the compass to not be affected by it. Any opinions or experience with this?

 

[BigAl07]

I've ran into a similar situation flying for the city from their Parking Garage. I was able to launch with it on a launch pad on top of a cardboard box that was at the most 18" above the surface. I don't know how much "protection" the closed cell foam launch pad" added so that has to be factored in as well.

 

[MapMaker53]

Thanks for the suggestion, Al. I think I will add the original small foam carrying case the P4P came in on top of the step ladder as a launch pad. That will give me an extra 9 inches of dense foam buffer.

 

[Dave Pitman]

I'll be flying at a small concrete materials site next week in a congested urban area. Every inch of the site is concrete, which I am sure is rebar reinforced to handle their heavy equipment traffic (dump trucks, cement mixers, etc.). I'm considering bringing a short 2 ft wooden step ladder to launch the drone from, but I am unsure as to how far above the rebar the drone needs to be in order for the compass to not be affected by it. Any opinions or experience with this?

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You should be good with a couple of feet separation. Obviously don't set up too close to metal buildings common with those operations. Do a manual take off and hover 20' off the deck and observe the stability. If you don't get any errors and the craft is stable, you should be good for manual and automated ops from that spot. The wicked compass problems generally occur as you move the craft away from the metallic source away from the start-up point.

 

[MapMaker53]

Thanks Dave. Good advice. This job will be all manual flying. The purpose is to take some video and photos of the small marsh areas along the bottom of that cement bulkhead. Will also turn off the downward sensors as I will be flying fairly low over the water.

 

[MS Coast]

You should be good with a couple of feet separation. Obviously don't set up too close to metal buildings common with those operations. Do a manual take off and hover 20' off the deck and observe the stability. If you don't get any errors and the craft is stable, you should be good for manual and automated ops from that spot. The wicked compass problems generally occur as you move the craft away from the metallic source away from the start-up point.

Wouldn't verifying the drone's indicated heading while hovering let you know whether there had been any effects from the rebar during startup?

 

[Ajkm]

Perhaps using a magnetometer app (such as Tesla Recorder) would help identify the best location with the least EMR.

 

[Dave Pitman]

Wouldn't verifying the drone's indicated heading while hovering let you know whether there had been any effects from the rebar during startup?

Perhaps. In general, I tend not to put a lot of confidence in the that bit of telemetry.

There are basically 2 types of compass error that you will run into.

1. The easy one. You start up and there is a compass error and you can't fly until you address it.

2. The more serious one. The craft thinks it is fine. But in reality, the compass has done it's start up check based upon false information, i.e. a significant iron object's influence. The craft will start up and take off just fine. But as it moves away from that influencing object, the craft gets confused and begins to "toilet bowl" while trying to reconcile the compass to the gps. If this happens and you're looking for it or familiar with what's happening, it can be controlled while terminating the flight. If unaddressed this condition will steadily become more aggressive and result in a crash or "flyaway". User's sometimes never know it was caused by starting up with magnetic interference and simply report that the craft was unresponsive. You don't get a compass error notification because the compass thinks that it is fine.

 

[MS Coast]

User's sometimes never know it was caused by starting up with magnetic interference and simply report that the craft was unresponsive. You don't get a compass error notification because the compass thinks that it is fine.

No error message. But wouldn't the orientation indicated by the compass be in error? That is, can't the pilot see that there's an issue if they aim the drone in a known direction and don't see the same orientation indicated by the compass?

 

[Dave Pitman]

No error message. But wouldn't the orientation indicated by the compass be in error? That is, can't the pilot see that there's an issue if they aim the drone in a known direction and don't see the same orientation indicated by the compass?

Like I said, perhaps. Over the years that heading telemetry has not been 100% reliable and has changed with various firmwares. In general, sure if in the last 10 flights, it has been spot on but now is 90 degrees off, that would be a clue. But if the craft is stable, and acts appropriately to control input, I would take that over what that heading indicator says. It's not a wet compass by any means is what I'm trying to impart.

 

[MS Coast]

In general, sure if in the last 10 flights, it has been spot on but now is 90 degrees off, that would be a clue.

Just what I'm saying. Only I'd put the threshold at 15 or so degrees of difference between the indicated heading and what I know it to actually be. 90 degrees of difference I'd call a major issue to warrant immediately landing the drone. I pause after takeoff on every flight to check the compass. So far, no issues. But I'm cautious about launching near ferrous materials or hardware that might produce a strong magnetic field.

I've not had any heading/compass issues with any of the four DJI drones I've flown, Mini and later. Perhaps I've been lucky.

But if the craft is stable, and acts appropriately to control input, I would take that over what that heading indicator says. It's not a wet compass by any means is what I'm trying to impart.

I wouldn't feel at all comfortable flying with an indicated heading significantly at odds with reality, even if flight was stable and controls properly responsive.

 

[Dave Pitman]

I wouldn't feel at all comfortable flying with an indicated heading significantly at odds with reality, even if flight was stable and controls properly responsive.

If it works for you, then that is all that matters. I very often use apps that do not hook the heading and it is never displayed.

I am very comfortable operating without it. I have experienced the compass error referred to as #2 above.... once. It is recognizable if you are aware. The old Phantom 1 would sometimes exhibit similar behavior probably just do to the primitive nature of it's compass/gps integration.

Some Cendence controllers specifically (I believe) have had problems with the heading indicator not working at all. Users of it (many) would be out of luck if they depended solely on that telemetry.

The craft won't be stable and predictable after moving away from an environment that has negatively interfered with the compass at take off. You don't even need an app to make that determination.
But, like I said. If it is part of your routine, nothing wrong with that. I just would advise that you consider flight characteristics as the primary determination of IMU and compass health.

 

[R.Perry]

Whenever I begin a new flight, in a new location I always do a compass calibration for the exact reasons Dave mentioned.

To me it only takes a few seconds to do, and it is good insurance.
One other thing, I never start a flight until I hear that the home point has been recorded, and I verify the home point on my tablet.

 

[MapMaker53]

All turned out well that day. It was a beautiful day to fly, even though flights were below 100ft AGL as promised to FAA/ATC. I wound up launching from the surface of my case to avoid any rebar effect. I mostly used Tripod Mode as I was flying low to the water and very close to the concrete bulkhead and other obstacles along it's entire length. In order for the client to understand exactly where the drone was as it filmed, I recorded the Litchi screen and then cropped/superimposed both the small Litchi map and Lat/Lon coordinates onto the 4K video using DaVinci Resolve. It worked out really well.

 

[Snowghost]

Whenever I begin a new flight, in a new location I always do a compass calibration for the exact reasons Dave mentioned.

To me it only takes a few seconds to do, and it is good insurance.
One other thing, I never start a flight until I hear that the home point has been recorded, and I verify the home point on my tablet.

You might want to rethink compass calibrations. This is a good read. I've not calibrated my P4P v2 even out of the box. She has 90 flights various locations. I go by the app and of course checking the map.

A short explanation of compass function, calibration and errors.

BACKGROUND The aircraft compass is often misunderstood, both in terms of function and purpose, but it is important to understand what the compass measures, why, and what happens to that information. At the most basic level the purpose of the compass is to measure the earth’s magnetic field in...

mavicpilots.com

 

[R.Perry]

I have a habit of rethinking a lot of things, and that was a good read. Here is what I have noticed, failing to do a compass calibration has caused the Inspire 2 to RTH and miss the launch point by several feet. I know, that isn't a big deal, I can fly it to the pad.
As we know the magnetic north and true north has changed over the years, and continues to, and yes it is very slowly but I like accuracy and erroring on the side of caution.