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mavic pro negative altitude on landing

jimmy johnson

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Hi guys,

I got an official refurb mavic to replace my last one (that I lost in the water). Anyway, one thing I am noticing with it is that when it lands, I have a -1 to -2 meter reading when I land and touch down. Never saw this with the old one. Doesn't seem to have any problems with flight or landing, just these negative readings which are obviously in accurate ( should show zero). Doesn't matter if I take off and land in same spot right away. Is this something to be concerned about? I recalibrated IMU and compass several times but I did it in a bumpy field that wasn't perfectly level. The IMU and compass are both showing green readings on the sensor status.
 
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niki

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The showing altitude is interpolated by the values of the 3 separate sensors - barometer, gps and downward vision positioning system. So you should check those.

The barometer itself can show wrong reading if there is a storm comming.

The GPS can have wrong reading with some electromagnetic interference. This is different from the number of sats.

The vision system can have wrong reading if the ultrasound sensors and or the downward camera are obscured by something. Or if flying over difficult to read ground texture like water or grass.

If the problem is repeating everytime and you have the downward vision switched on I guess you should try to switch it off. If that fixes the negative altitude after landing then you have to try to calibrate the downward vision using dji assistant and computer.

If that and as well more precise IMU and Compass calibration doesnt fix the problem then maybe some of the sensors is faulty and then only dji service is the solution.
 
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jimmy johnson

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The showing altitude is interpolated by the values of the 3 separate sensors - barometer, gps and downward vision positioning system. So you should check those.

The barometer itself can show wrong reading if there is a storm comming.

The GPS can have wrong reading with some electromagnetic interference. This is different from the number of sats.

The vision system can have wrong reading if the ultrasound sensors and or the downward camera are obscured by something. Or if flying over difficult to read ground texture like water or grass.

If the problem is repeating everytime and you have the downward vision switched on I guess you should try to switch it off. If that fixes the negative altitude after landing then you have to try to calibrate the downward vision using dji assistant and computer.

If that and as well more precise IMU and Compass calibration doesnt fix the problem then maybe some of the sensors is faulty and then only dji service is the solution.


I recalibrated the IMU on a flat surface in the house and also recalibrated the compass. I tried to do the VPS calibration with DJI Assistant 2 1.1.2-2 and 1.2.4 (latest link on the site). Both will not allow me to recalibrate with the Frame Alignment error and it keeps telling me to reboot the drone, never leaving Tutorial mode. I gave up on trying this. I'm using Windows 10 with a 17inch laptop screen.

Took out for some more quick test flights to investigate this issue and gather some data. I tried disabling the downward sensor like you said and it still shows negative readings. I changed to imperial units and I saw between -0.3 to -2 ft when it landed. It was pretty much the same when I re-enabled it as well except that it hovers up and down much more stable with downward visual sensors enabled (it would hover 1 foot or so up and down with it off at a low altitude).

I checked airdata and look at the readings. It shows that it lands at -0.3 and -2 ft one a few of the takeoff/landing short flights. It shows that it landed at 0ft on others although I thought I remember seeing some small negative readings almost every time (maybe it rounded up to 0). It seems to be better from earlier this week when the worst reading I saw was a landing at -6.2 feet (on airdata).

In the General->Notifications area, I noticed that "Mode changed to confirm landing" happens often around 0.7 ft up to around 1.6 ft. My previous Mavic would initiate this at higher altitudes although not always. I checked the signal readings, and on the Signal Map I have never had even one minor signal error that I could find. It has compass turn rate per 0.1 seconds of around 9.20 during longer flights. In contrast the older Mavic had more signal errors, although usually in the green and the compass turn rate averaged about 4.7.

Anyway, I'm wondering if there is anything to be concerned about here. My main worry is that I do fly it over water in beach environments and I have disabled the downward sensor before on the old drone because I read about unexpected water landings. I don't want to have issues with this one landing in the water obviously. It seems to be function okay in my test flights except for this observation with negative feet on landing. I've flow it out about 600ft out or so and at 400 ft altitude without issues. I am still within the 30 day period to exchange from the Newegg DJI store but that will end soon. I sent an email to DJI support to see what they say.
 

jimmy johnson

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Well DJI replied and said it's fine but I think they though I landed in a spot lower than where my takeoff was. But as I noted before, I have taken off and landed in the exact same spot almost immediately and it shows the negative altitude on landing most times. Anyone ever see this issue or is this something I should be concerned about?
 

niki

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The negative reading is not a big problem but is indicating that there may be other or bigger problem.
I suggest you to check did the positive altitude reading is relatively correct by flying up close to a building or other tall object that you know the height of and compare the reading with what you know for that object. If the positive reading doesnt deviate too much from the real I guess you are fine to fly. But too big difference could limit or extend your max altitude. Also the RTH function relies on correct altitude reading. Imagine if the Mavic is thinking he is at 120 feet but it is actually 90 feet over the ground, this may end with crash in to something on RTH.
 

genesimmons

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i am going to check mine tomorow at landing but i notice when i hover the altitude is wrong by about 4-10ft, some times says 12 ft when its 3 ft off the ground, i didnt pay much attention but now im gonna check haha
 

4wd

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I suspect it's a 'feature' caused by the onboard altimeter heating up during flight.
I've often flown from a hillside and noticed zero altitude is actually about 10 feet too low when it comes back.
This effect probably explains quite a few water crashes - it gradually gets lower than the reading suggests as it heats up.
 

jimmy johnson

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That's an interesting comment about the altimeter heating up. I wonder why some Mavics would have this issue while others do not. That sounds like a hardware problem if that's the case. Someone suggested to me to put try it while it's cold (like put it in the refrigerator for a a bit) and then do a takeoff and landing in the same spot to see if it shows the negative reading to test this theory.

How would I go about measuring the height of something that is known while out in the field? It would be nice to know the heights of telephone poles and buildings but I'm not sure how to measure that without specialized tools.
 
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MavicCF

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That's an interesting comment about the altimeter heating up. I wonder why some Mavics would have this issue while others do not. That sounds like a hardware problem if that's the case. Someone suggested to me to put try it while it's cold (like put it in the refrigerator for a a bit) and then do a takeoff and landing in the same spot to see if it shows the negative reading to test this theory.

How would I go about measuring the height of something that is known while out in the field? It would be nice to know the heights of telephone poles and buildings but I'm not sure how to measure that without specialized tools.
Landing at a negative altitude is a common occurrence caused by change in barometric pressure. Really nothing to worry about and not a problem with the hardware.
The barometer is the way the Mavic keeps track of altitude while above the range of the VPS system. A lot of things affect barometric pressure so even ascending or descending can cause a change in it. I would say 1-2 m is nothing to worry about.
 

Mossiback

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Is your home point recorded before you take off?

EDIT: After testing with my MP, changing the home point made no difference in height displayed.
 
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jimmy johnson

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Took out nearby to test again. When it was cold after just turning it on, I took off about 20 feet in the air near a utility pole that I estimated to be about 18 - 19 ft and then landed within less than a minute. First landing was -4.5 ft or so. I kept retesting taking off and landing and noticed the altitude got more accurate or less negative as time went on. -4 ft, -2.3 ft, -1 ft are some values I remember seeing.

During some of the quick takeoff/landing tests, I noticed that the drone indicated about 2 - 3 ft when it was really about 5 or 6 ft off the ground (about my eye level or slightly above my head). It started to get more accurate as where it was about one foot off. I had it do higher altitudes straight up (to 180 ft) and come back down and by the time it was about 50% battery I noticed it was about 1 foot off from the true reading at eye level and landing at -1 ft. A couple landings were accurate at zero. Looking at airdata, if it goes into landing mode at around 2 ft, then it will land at 0 feet. But if it goes into landing mode at a lower value than that (e.g. 0.7 feet or lower) then it will land at a negative value. The drone seems to be about 2 feet high before the downward sensors stop the drone, detect landing mode, and start the auto land with the stick held down.

My experience is that it seems to get more accurate as it warms up in these recent tests. I get -2 to -6 ft in the first flights and then it seems to level off at -1 ft landings and sometimes 0. However, this is not always case because there were logs where the drone was warmed up but showed a higher negative (-4 to -6 feet landing) after a longer flight. Maybe that was the barometric pressure changing like that linked thread described to some extent.

Is this normal? Should I exchange it for a different drone or do the people here think it's acceptable? I have a few more days to exchange but I'm trying figure out if this is a problem or not.
 

niki

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I think that deviation of around 10ft to 15ft is not a problem for having fun but usually should be smaller than that. I also had a few flights showing -2ft -3ft.

I would set the RTH altitude 20-30ft more than usual and will go to fly :)
 

jimmy johnson

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I think that deviation of around 10ft to 15ft is not a problem for having fun but usually should be smaller than that. I also had a few flights showing -2ft -3ft.

I would set the RTH altitude 20-30ft more than usual and will go to fly :)

Deviation is between -1 to -6.8 feet, not meters. Mainly am hoping I would not get unexpected water landings.
 

jimmy johnson

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I guess I will exchange it. DJI support has been ignoring me and would prefer to have a drone without a defect out of the box. Sounds like from the posts here it is not serious but maybe it could be down the road.
 

MavicCF

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I guess I will exchange it. DJI support has been ignoring me and would prefer to have a drone without a defect out of the box. Sounds like from the posts here it is not serious but maybe it could be down the road.
I've had mine for over a year now and have experienced slight deviations like you mentioned numerous times with no related issues. There is nothing wrong with the hardware that would causes this issue, it is merely a result of the flight environment.
 

jimmy johnson

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I've had mine for over a year now and have experienced slight deviations like you mentioned numerous times with no related issues. There is nothing wrong with the hardware that would causes this issue, it is merely a result of the flight environment.

Yeah but how is the flight environment changing within seconds? Flight times do not matter. It lands negative each time.
 

MavicCF

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Yeah but how is the flight environment changing within seconds? Flight times do not matter. It lands negative each time.
Could you post a flight DAT? I'll shut up too, really if you really feel it's defective, it is probably best to return it.
 

jimmy johnson

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Could you post a flight DAT? I'll shut up too, really if you really feel it's defective, it is probably best to return it.

I think if Mavics were landing negative every time, there would probably be hundreds of posts rather than just a handful that I've found. I looked at the airdata on my older mavic and I saw a couple negative landings out of dozens of flights, but it wasn't *almost every time* and that is why I think it is an anomaly. I was trying to gauge the seriousness of the situation by posting here. I'm not happy about trying to do an exchange as this was a refurb and I could get another Mavic that has a more serious problem. It's also a hassle. It's a dice roll, hence the discounted price. It could be the reason it was sent in before and DJI QC didn't catch it or didn't care, who knows.

I wouldn't mind posting a DAT. I'm assuming you grab this from DJI Assistant? If you can point me to a quick guide and place to post it, I can do that tomorrow morning.
 
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