While on a training flight yesterday when flying over a block of flats my Mavic Air suddenly lost all GNSS satellites: the number plummeted from 19 to 0 in half a second, then went back to normal in 5-10 seconds, then it repeated all-over again one more time. Here is the log (taken from the DJI GO 4 app): http://www.phantomhelp.com/logviewer/ALU27E2O5XOS64MFRTB7/
The first time it happened at 5m 29.5s, the second time was at 6m 6.1s. Although the site doesn't show this, the drone went into OPTI mode and there were absolutely no problems controlling it, but I decided to head back anyway for the fear of losing the RC connection, which also had been not very stable in that place.
So, I have several questions:
1. Is GPS/GLONASS signal loss a common thing? Personally, I suspect interference but want to be certain;
2. Does the log in the drone's flight recorder have any additional information to help make sure the hardware is functioning as it should?
3. What will the Mavic Air do if it loses both the GNSS satellites and the RC connection (hover, then land where it is if the battery dies)?

Loosing gps is very rare unless flying in an unsuitable envoirment, in your log I can see where there is time missing which couldn’t be recorded because of lack of gps.

Although you said it went into opti mode, this mode is of no use to you unless you are within 30ft from the ground optical vision does not work from 500ft altitude. I’m may have turned on as you were flying over the building , so you may have seen opti coming up on your screen, but more than likely you were in Atti mode.

hallmark007 Posted at 2018-4-16 05:54
Loosing gps is very rare unless flying in an unsuitable envoirment, in your log I can see where there is time missing which couldn’t be recorded because of lack of gps.

Although you said it went into opti mode, this mode is of no use to you unless you are within 30ft from the ground optical vision does not work from 500ft altitude. I’m may have turned on as you were flying over the building , so you may have seen opti coming up on your screen, but more than likely you were in Atti mode.

I was surprised myself, but it was OPTI rather than Atti or anything else. At least that was what the DJI GO 4 was showing. And, by the way, the Mavic was flying much higher than 30ft above the roof.

asaw Posted at 2018-4-16 06:02
I was surprised myself, but it was OPTI rather than Atti or anything else. At least that was what the DJI GO 4 was showing. And, by the way, the Mavic was flying much higher than 30ft above the roof.

There is a second compass in MavAir, I’m not sure but I think it may kick in when you have weak or no gps I think it’s designed to keep its heading, this might be why you seen opti, but it’s only a guess not much has been written about this, maybe a mod can let us know.

I would really like to see an explanation for this. It is weird. I am going to check all my logs as well.

This is disturbing. I have just uploaded some of my new logs to air data and the GPS stats are misleading. This is completely out in the open and should pick up more than 11 satellites.

Kevjones1959 Posted at 2018-4-16 06:55
This is disturbing. I have just uploaded some of my new logs to air data and the GPS stats are misleading. [view_image] This is completely out in the open and should pick up more than 11 satellites.

Remember that the sats pick depends on weather has well, in poor conditions this can affect the sat count, Also keep in mind that minor solar storms and the KP index plays a Hugh role in the detection of sats. Use UAV and Magnetology, 2 great apps that will help keep your drone safe for flying

Mavic Air has only 1 compass not dual compass

rickysuperdog Posted at 2018-4-16 09:06
Mavic Air has only 1 compass not dual compass

Rickysuperdog, when giving out facts you should always check your facts before you post

From DJI's Elektra - Mavic Air has dual IMU and a compass.

Reading DJI's Promotional on Mavic Air, the Vision compass does not use magnetic field, but imaging from cameras.

The time records you don't see is just missing in Phantom Help. All the record are inside the .CSV file.


All the time the drone show GPS mode even if it has 0 GPS sat.
Something is not working as it should. Is the FW up to date?

G_Sig Posted at 2018-4-16 11:04
The time records you don't see is just missing in Phantom Help. All the record are inside the .CSV file.
[view_image]

I suspect the P-GPS doesn't change simply because Phantom Help is unable to correctly decode the log. When I replay it in the GO 4 App itself the mode changes to OPTI where it should. Yes, all the firmware is up to date.

You have to remember just because the App shows satellites (regardless of how many) just means that they are there. They could very well be low on the horizon making it difficult to lock on, Always look at the signal strength (How many bars) are showing to determine how well of a GPS lock you have.

QuadKid Posted at 2018-4-16 11:53
You have to remember just because the App shows satellites (regardless of how many) just means that they are there. They could very well be low on the horizon making it difficult to lock on, Always look at the signal strength (How many bars) are showing to determine how well of a GPS lock you have.
https://www.youtube.com/watch?v=9pluWDoZhTw

In my case, I had all 5 bars with 19+ satellites. Phantom Help doesn't show it, but it is in the record as DJI GO 4 also shows the bars.

I dont know if you guys recall the first car navigation GPS devices TomTom and Garmin etc. How many of you can remember how long they took to find the satellites. Sometimes you would sit in your driveway for a few minutes waiting for the device to get a location. Today when you power these things on they seem to almost find the satellites without delay (give or take a few seconds). Why is this ???

Could this aircraft be suffering from poorly placed antena, or some sort of software logic that is incorporated into most GPS nav systems. It just seems like my Air is finding satellites like the old GPS nav systems (very slowly and weakly).

asaw Posted at 2018-4-16 12:16
In my case, I had all 5 bars with 19+ satellites. Phantom Help doesn't show it, but it is in the record as DJI GO 4 also shows the bars.

Scratch'in my head on this one ??

Kevjones1959 Posted at 2018-4-16 12:17
I dont know if you guys recall the first car navigation GPS devices TomTom and Garmin etc. How many of you can remember how long they took to find the satellites. Sometimes you would sit in your driveway for a few minutes waiting for the device to get a location. Today when you power these things on they seem to almost find the satellites without delay (give or take a few seconds). Why is this ???

Could this aircraft be suffering from poorly placed antena, or some sort of software logic that is incorporated into most GPS nav systems. It just seems like my Air is finding satellites like the old GPS nav systems (very slowly and weakly).

All dji aircraft take time to locate satellites, MavAir is just as quick as any of the others.

QuadKid Posted at 2018-4-16 12:20
Scratch'in my head on this one ??

I am almost sure the reason why my MA lost satellites was some kind of EM interference. I just had never seen anything like that before and didn't suspect to see it in that place.

hallmark007 Posted at 2018-4-16 12:21
All dji aircraft take time to locate satellites, MavAir is just as quick as any of the others.

You mean just as slow ????

asaw Posted at 2018-4-16 12:26
I am almost sure the reason why my MA lost satellites was some kind of EM interference. I just had never seen anything like that before and didn't suspect to see it in that place.

I tend to think you are right, just looked at the CSV file, definitely dropped to 0 quickly then regained a few then dropped to 0 again, almost like a radar wave was passing through. Just a guess, hopefully it's not persistent and you can fly with some confidence.

If you lose RC connection for more than 3 seconds it should initiate RTH.

Kevjones1959 Posted at 2018-4-16 12:31
You mean just as slow ????

No I don’t mean just as slow, but if you looking for instant gps then buying an $800 drone is not going to give you that.
Your sat nav and phone also get satellite signal being reflected by masts cell towers etc, your aircraft receiver can only get its signal direct form gps satellites.
If your having to wait a long time to get gps signal I suggest you move location or return your MavAir to dji to check out your receiver, mine takes around 30 seconds to receive upwards of 12 satellites.

Kevjones1959 Posted at 2018-4-16 12:17
I dont know if you guys recall the first car navigation GPS devices TomTom and Garmin etc. How many of you can remember how long they took to find the satellites. Sometimes you would sit in your driveway for a few minutes waiting for the device to get a location. Today when you power these things on they seem to almost find the satellites without delay (give or take a few seconds). Why is this ???

Could this aircraft be suffering from poorly placed antena, or some sort of software logic that is incorporated into most GPS nav systems. It just seems like my Air is finding satellites like the old GPS nav systems (very slowly and weakly).

The reason is that now they cache the ephemeris data so they know exactly where the location of the GPS satellites are the moment they lock on the first one and get the time and date. And correlation algorithms now run much faster on improved hardware. And btw mast towers do not reflect any GPS signals. The smartphone uses them to get approximate position because their location and coverage area is known and signal strength drops with inverse squared law. Known Wifi SSID's are also used to augment positioning until enough Satellites are acquired.

Kevjones1959 Posted at 2018-4-16 12:17
I dont know if you guys recall the first car navigation GPS devices TomTom and Garmin etc. How many of you can remember how long they took to find the satellites. Sometimes you would sit in your driveway for a few minutes waiting for the device to get a location. Today when you power these things on they seem to almost find the satellites without delay (give or take a few seconds). Why is this ???

Could this aircraft be suffering from poorly placed antena, or some sort of software logic that is incorporated into most GPS nav systems. It just seems like my Air is finding satellites like the old GPS nav systems (very slowly and weakly).

GPS acquisition time is a function of time and distance from the last point it was used.  This is specific to any GPS device, not just DJI drones.

The longer ago it has been or the further away it is since the last time a GPS device is used, the longer it will take to locate the satellites and lock onto them.  So if it has been quite some time since you last used your drone, then it doesn't have the data to be able to predict which satellites would be available in the sky at the current time.  So it has to find them each one by one.  However, if you just recently used it, then it has the ephemeris data stored and can predict what satellites are available in the sky and can grab onto them quickly.  You quite often will see this when you use more than one battery in a session.  

The first flight it will take some time to lock onto the GPS, but the subsequent flights will lock onto the GPS almost instantly.

As ggeorgis mentioned, lots of cellular connected devices these days will use the known locations of the cell towers to help them lock in much quicker.

It may be that when flying over the flats your craft may have gotten confused by reflected GPS signals which is why your lock returned so quickly (as you passed that particular spot). This is a common issue when using a car gps in cities where the unit can lose lock. Only way to confirm is to fly in that exact same spot though.

HedgeTrimmer Posted at 2018-4-16 09:45
From DJI's Elektra - Mavic Air has dual IMU and a compass.

Reading DJI's Promotional on Mavic Air, the Vision compass does not use magnetic field, but imaging from cameras.

Basically it was not a real hardware compass:  
  
“Besides dual IMUs, FlightAutonomy 2.0 has more redundant modules to increase the Mavic Air’s safety level. The Vision Compass will assist the navigation system to estimate flight direction when the compass experiences interference. When only one direction of the front, bottom, or rear vision systems is functioning normally, the aircraft will still position itself successfully. There are two sets of sensor fusion algorithms based on two independent hardware platforms. When an error occurs to one set of algorithms, the aircraft will switch to the other pair seamlessly to resume normal functionality.”

Recently we got no report or cases proving that it was working when the compass 1 was malfunctioned.

KlooGee Posted at 2018-4-16 14:04
GPS acquisition time is a function of time and distance from the last point it was used.  This is specific to any GPS device, not just DJI drones.

The longer ago it has been or the further away it is since the last time a GPS device is used, the longer it will take to locate the satellites and lock onto them.  So if it has been quite some time since you last used your drone, then it doesn't have the data to be able to predict which satellites would be available in the sky at the current time.  So it has to find them each one by one.  However, if you just recently used it, then it has the ephemeris data stored and can predict what satellites are available in the sky and can grab onto them quickly.  You quite often will see this when you use more than one battery in a session.  

Exactly KlooGee, thats my point so why cant the controller get the "initial" position from the tablet or phone and send it to the aircraft. That will narrow down the search and allow the aircraft to connect quicker. I know TomTom has a little last location app inside now.

hallmark007 Posted at 2018-4-16 12:50
No I don’t mean just as slow, but if you looking for instant gps then buying an $800 drone is not going to give you that.
Your sat nav and phone also get satellite signal being reflected by masts cell towers etc, your aircraft receiver can only get its signal direct form gps satellites.
If your having to wait a long time to get gps signal I suggest you move location or return your MavAir to dji to check out your receiver, mine takes around 30 seconds to receive upwards of 12 satellites.

Nope I dont expect instant location at all. 30 seconds is fantastic. Mine takes over 2 minutes and I am in the wide open. No houses no electrical interference and no clouds.  I would think anywhere from 20 to 1 minutes would be acceptable. Dont forget while it's sitting there waiting to lock on it is chomping up my battery as well.

joe_gadget Posted at 2018-4-16 14:53
It may be that when flying over the flats your craft may have gotten confused by reflected GPS signals which is why your lock returned so quickly (as you passed that particular spot). This is a common issue when using a car gps in cities where the unit can lose lock. Only way to confirm is to fly in that exact same spot though.

Shot will try it on site again today and actually time it.

Guys thanks for the comments. Just understand I have no one in the vicinity who has a drone that I know, hence i have no reference point other than from your experiences. I really appreciate the comments and advice.  For example one of the guys above says his machine connects in 30 seconds to 12 satellites. So lets just say mine was slower for whatever reason. I have a disconnect in flight I take 2 minutes to reconnect, he takes 30 seconds. See where I am going. I would think mine is normal not having you guys around.  Thanks guys.

joe_gadget Posted at 2018-4-16 14:53
It may be that when flying over the flats your craft may have gotten confused by reflected GPS signals which is why your lock returned so quickly (as you passed that particular spot). This is a common issue when using a car gps in cities where the unit can lose lock. Only way to confirm is to fly in that exact same spot though.

In that case, it would have been a common issue when flying over blocks of flats... But I had already flown over flats and large industrial buildings elsewhere and hadn't experienced anything like that. Even on that day in a spot some 500 metres away.

Hi,

Never seen this before in any flightrecord file. For about 9 seconds max the GPS dropped to zero, GPS data not used by the drone. Normally it would change to ATTI mode, not in this case.
Even weirder is that the Vision became active when the GPS data wasn`t used! (mayby thats why the OPTI was seen on the screen of the device, recorded data says GPS wich was seen).

Tip; i always fly with a screen recorder ON, so i can use that to see wich messages etc were to be seen during the flight (and ofcourse evidence proof when it goes really wrong)

cheers
JJB

JJBspark Posted at 2018-4-16 23:58
Hi,

Never seen this before in any flightrecord file. For about 9 seconds max the GPS dropped to zero, GPS data not used by the drone. Normally it would change to ATTI mode, not in this case.

JJB, thank you very much for your analysis and advice! Is that the viewer from DJI Assistant that you use for viewing the flight records?

rickysuperdog Posted at 2018-4-16 19:19
Basically it was not a real hardware compass:  
  
“Besides dual IMUs, FlightAutonomy 2.0 has more redundant modules to increase the Mavic Air’s safety level. The Vision Compass will assist the navigation system to estimate flight direction when the compass experiences interference. When only one direction of the front, bottom, or rear vision systems is functioning normally, the aircraft will still position itself successfully. There are two sets of sensor fusion algorithms based on two independent hardware platforms. When an error occurs to one set of algorithms, the aircraft will switch to the other pair seamlessly to resume normal functionality.”

Well maybe you might point us to a case where it hasn’t worked, I haven’t seen many compass failures around here with MavAir. You seem to be saying that second compass is not working, how do you know this?

Kevjones1959 Posted at 2018-4-16 21:13
Nope I dont expect instant location at all. 30 seconds is fantastic. Mine takes over 2 minutes and I am in the wide open. No houses no electrical interference and no clouds.  I would think anywhere from 20 to 1 minutes would be acceptable. Dont forget while it's sitting there waiting to lock on it is chomping up my battery as well.

I think all you need to do is fire up your aircraft and take a little video of the length of time it is taking, because IMO 2 minutes is to long, then you will have the proof for dji that you may have a faulty unit, mine certainly is not like this from open air.

JJBspark Posted at 2018-4-16 23:58
Hi,

Never seen this before in any flightrecord file. For about 9 seconds max the GPS dropped to zero, GPS data not used by the drone. Normally it would change to ATTI mode, not in this case.

You analysis is excellent again thanks for that, my thinking was because opti was shown on OP’s telemetry could this have been vision compass?

hallmark007 Posted at 2018-4-17 01:27
You analysis is excellent again thanks for that, my thinking was because opti was shown on OP’s telemetry could this have been vision compass?

Good question, need some test to see when the vison 'compass' is active and when not. Hope to find that answer.  A visual compass ? my brains works like that, when i look at the sun at 12 o`clock i know where South is, guess DJI vision compass is not that clever...

article: https://www.heliguy.com/blog/201 ... -2-aircraft-safety/

rickysuperdog Posted at 2018-4-16 19:19
Basically it was not a real hardware compass:  
  
“Besides dual IMUs, FlightAutonomy 2.0 has more redundant modules to increase the Mavic Air’s safety level. The Vision Compass will assist the navigation system to estimate flight direction when the compass experiences interference. When only one direction of the front, bottom, or rear vision systems is functioning normally, the aircraft will still position itself successfully. There are two sets of sensor fusion algorithms based on two independent hardware platforms. When an error occurs to one set of algorithms, the aircraft will switch to the other pair seamlessly to resume normal functionality.”

I don't think we would get any sort of reports of when it works correctly.  It would just be a normal flight.  Do you have any examples of when it didn't work?  I'm always interested in learning more about how technology works.  Seeing examples of when it isn't working is a great way to learn.  If you've got examples, I'd love to have a look at them.

Kevjones1959 Posted at 2018-4-16 21:09
Exactly KlooGee, thats my point so why cant the controller get the "initial" position from the tablet or phone and send it to the aircraft. That will narrow down the search and allow the aircraft to connect quicker. I know TomTom has a little last location app inside now.

My guess is that this is a function of money.  I believe (but don't know for sure) that DJI uses an off-the-shelf GPS module from another manufacturer in their drones.  I'm not sure the functionality you are talking about exists in an off-the-shelf form already that they could buy at a reasonable price.  So they would have to spend the time (i.e. money and opportunity costs) to engineer the solution.  From my perspective, I don't think they would get any financial rewards in the way of additional sales from undertaking this.  They would also have opportunity costs in assigning engineers to this that could be working on something else that might influence additional sales.

Kevjones1959 Posted at 2018-4-16 21:19
Guys thanks for the comments. Just understand I have no one in the vicinity who has a drone that I know, hence i have no reference point other than from your experiences. I really appreciate the comments and advice.  For example one of the guys above says his machine connects in 30 seconds to 12 satellites. So lets just say mine was slower for whatever reason. I have a disconnect in flight I take 2 minutes to reconnect, he takes 30 seconds. See where I am going. I would think mine is normal not having you guys around.  Thanks guys.

Do you only see this 2 minute acquisition time on the first battery or do you also see it on 2nd and subsequent flights on the same day at the same location?  As others mentioned, it might be useful to see a screen recording of your experience to be able to see if it is different than what everybody else sees.

JJBspark Posted at 2018-4-16 23:58
Hi,

Never seen this before in any flightrecord file. For about 9 seconds max the GPS dropped to zero, GPS data not used by the drone. Normally it would change to ATTI mode, not in this case.

I also have not seen this same scenario where there are a large number of satellites visible with a full signal strength and it drop down to no satellites with no signal strength while in flight.  This seems to be very unique.  I'm also very surprised to see that it didn't drop into ATTI mode.  There don't appear to be any compass errors.  Very interesting case!

I also recommend doing a screen recording during of the flights for the same reason.  Having the visual record of the flight along with the logs can be very useful in trying to figure out what happened.  

Also, its not just the PhantomHelp viewer that shows it in GPS mode during that whole time.  Using the TXTLogToCSVtool also shows it in GPS mode during this period.  

I'm sure its impossible to replicate the flight again, but it would be interesting to see if flying in the same area again results in similar behavior.

Also, kudos to ASAW for immediately bringing the aircraft back home when he saw anomalies in the flight.  I've seen many users continue with their flights after getting errors and then ending up losing their aircraft later in the flight.