Max Altitude ATTI Mode Compass Calibration Errors Photographing Downtown

[Admiral Cashbar]

Hello all,

This may be my first and last post, not much of a forum user but I felt this story could help others. I am 107 certified and was hired to photograph a 40 story building downtown with my Mavic Pro. Narrow streets meant no GPS, ATTI mode means max altitude of 98 feet. Here were my steps to overcome these issues.

Before flying, set the return to home altitude to something higher than the closest buildings around. In this case that was about 550 feet. If it goes RTH you want it flying over everything.

1: Walked to an open park on one side of the building (still no GPS but a slightly larger area to fly around in, just find the closest open area you can)
2: Take off, go to max altitude (98 feet)
3: Fly around until you find a signal and go into GPS mode. You have a better chance finding GPS at 98 feet than on the ground.
4: Set a home point there. ONCE YOU FIND A GPS SIGNAL YOU CANNOT PASS THE 98 FOOT LIMIT UNTIL YOU SET A HOME POINT.
5: Take it up above the building you are photographing, 50 feet or so, SET A NEW HOME POINT. If it tries RTH you want it going up out of the city.
6: Walk over to the building where you and the VO have the best line of sight.

While photographing the building I ran into magnetic interference. Look at a highrise, you may notice several floors of the building will have no windows. These are mechanical floors that house ventilation, heaters, AC, elevator equipment and whatnot. These systems usually require giant electric motors, powered by huge magnets. Anytime I matched altitude with these mechanical floors I lost compass calibration. I also lost compass calibration when matching altitude of the roof, and the roof of surrounding buildings, as lots of mechanical equipment is found there as well. This is where the steps above are important. With spotty GPS signals, compass calibration errors, unpredictable winds AND you are surrounded by buildings, you are flying with the potential of losing every failsafe built into this bird. The open sky above the city is the safest place for your drone if everything goes wrong.

Just wanted to share my thoughts on how to trick your way through the ATTI mode limits, then reset all these safety systems in case of trouble.

 

[Storyline]

Thanks for your post which could be useful at some point. This is one of the reasons I would like to be able to switch manually into ATTI mode and then be able to fly in a dumb mode just as you would with a RC model aircraft. The tricky bit is that you need some of the systems just to be able to fly and hold a stable hover let alone pilot it around in narrow spaces. Anyone who has flown a model helicopter which just has a heading gyro will know what a handful they are although a quad-copter should be inherently a bit more stable. Maybe there is some old or hacked Mavic firmware which will permit ATTI mode without any geofencing.

 

[Admiral Cashbar]

The Inspire is way better for this purpose. Hovering in narrow spaces isn't a big deal when you maintain visual with the drone, but then you need a camera operator. The moment you look at the controller to frame your shot, the UAV wanders off.

But the Pro's battery life is just so much better...

 

[DanMan32]

In setting home point, your options are AC or RC locations. Since the AC is above the building, don't set homepoint to AC location, lest it land on the building roof.

You can get away without GPS if you are proficient in ATTI, but losing compass is dangerous. Craft may go anywhere, do anything and crash.

 

[nevernamed]

I was under the impression flyaways can happen if you lift off with not enough satellites locked in. am I incorrect?

 

[DanMan32]

If you aren't skilled in ATTI mode, that is true as it will drift with the wind. Without compass though, it may fly away or crash even with ATTI skills.

 

[Storyline]

As I understand it flying in ATTI just means that it will not stay in position and just drift with the wind - that is what used to happen with the old Phantoms anyway. This is why being able to switch into ATTI for a moment or two is so useful as you can immediately feel how strong and which direction the wind is coming from. I am not in any way an expert but I think even in ATTI the Mavic is still flying automatically using I presume the IMU to control it. Even with the same motors on each rotor there will be differences in thrust and it will need some kind of brain to send signals to the ESC's to keep them in balance. In a single rotor helicopter you do not need this as you can control the thrust and just need a single gyro to sense if the tail is wanting to rotate. In the very early days I don't think you even had that so it would take a large amount of skill to even hover.

So to sum up, my feeling is that in ATTI mode and without compass the aircraft should not fly away. In stationary air it should just hover with minor drift. You should be able to fly it with reasonable control except, as the Admiral says above, you could not take your eyes off it even for a second, especially around buildings as you will get all sorts of gusts and eddies in the wind. Once above the building line it should be much easier.

To get round the problem of needing a second person to trigger the photography I suppose it might be possible to just leave the camera running and use frame grabs which I have read are nearly as good when using the Mavic camera with optimum settings.

 

[Admiral Cashbar]

When the bird lost GPS and compass calibration, it didn't start spinning or yawing. I continued to fly with normal control inputs.

I took it back up above the sprawl, waited for GPS lock, then walked with my VO back to the block where we launched from.

Frame grabs aren't a bad idea, more work for me though! Horizontal mapping would be a great feature for this kind of work.

 

[Storyline]

When the bird lost GPS and compass calibration, it didn't start spinning or yawing. I continued to fly with normal control inputs.
.....

Useful to know for sure - thanks
Edit: I have been toying with building my own drone recently and apart from getting a camera of choice into the air, the other motivation is learning how all the latest control systems work and how they interact with one another

 

[dbyantis]

I was under the impression flyaways can happen if you lift off with not enough satellites locked in. am I incorrect?

Not always, however I never launch without GPS and HP set.If you maintain VLOS, then the likelihood of a fly-away is reduced.

 

[Storyline]

Not always, however I never launch without GPS and HP set.If you maintain VLOS, then the likelihood of a fly-away is reduced.

I am no expert but I also don't think that a lack of a GPS fix can cause a flyaway. I have hardly read anything about them but have always thought that the only way a Mavic would flyaway is that it had got some incorrect control signal/s somehow eg a stuck stick or electronic malfunction in the controller. It is not suddenly decide to go off on it's own without being told to.

The main risk of no fix before takeoff is a lack of a reliable Home Point and also no way for the Mavic to find its way back if it was using an old Home Point.

 

[DanMan32]

No GPS in and of itself won't typically cause fly away, but without GPS and not low enough for VPS, it will drift with the wind. If you're not equipped to deal with that, it will then effectively be flying away.

 

[DanMan32]

Back in late Jan 2016 DJI had a serious bug in the Go app where the RC location was mapped 2 miles WNW of actual location. If you managed to set your homepoint to your perceived location or had distance limit set, it could fly away.

 

[Meta4]

The main risk of no fix before takeoff is a lack of a reliable Home Point and also no way for the Mavic to find its way back if it was using an old Home Point.

The risk of launching before recording a home point is exaggerated.
Your drone will record a homepoint as soon as it gets a good GPS location fix.
If you are very impatient and launch before getting GPS, your drone will still record a homepoint when it does get GPS.
In most cases that won't be far from where you launched.

The Mavic cannot use an old homepoint because the homepoint evaporates when the drone is powered down.
Every time you start up it has no home point until it gets GPS again.

 

[Storyline]

.....
Your drone will record a homepoint as soon as it gets a good GPS location fix.
.....
The Mavic cannot use an old homepoint because the homepoint evaporates when the drone is powered down.
Every time you start up it has no home point until it gets GPS again.

Most of the time mine does not record a Home Point if it is offline no matter how many satellites I've got.

Did not know that the HP is scrubbed on power down - thanks