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MikB
Captain
Flight distance : 13346 ft
France
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Does anyone have specific information about how the Spark fixes its position using satellites?

I have read on here that it uses both GPS and Glonass satellites.

I have also read that Spark needs a minimum of 6 GPS OR 6 Glonass satellites for a satisfactory fix. I presume then, it has separate solutions for both systems and since the minimum number to establish a unique fix is 4 satellites I presume some degree of redundancy is needed.

The Spark only tells you the total number of satellites being received so logically we need to see a minimum of 11 satellites before being certain of a good position from one or the other.

What is not mentioned is the geometry of the satellites. Are they near the horizon at the time you fly, or more to one side of your AC than the other? Also they are obviously moving and changing position all the time.

What is going through my mind is, once you have taken off, is it possible/likely that you can go from a strong position fix to a bad position fix by losing a satellite due to an obstruction e.g. a building or getting a reflected (multipath) signal.

Could this explain any of the “fly aways” we read about?

Personally I check on UAV forecast to see number of satellites. I tick and untick the “show Glonass “ box to see which satellites will be available. I am surprised to find that there are times in the day when flying is not recommended. There are plenty of other sites which show the actual geometry but I am not quite that anal to check them every time I fly.

2018-2-5
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Alberto83
New

Spain
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Hi, with the latest DJI Go 4 version 4.2.5 and latest firmware installed on Mavic Pro, the drone can't connect to my iphone with software 11.2.5, any solutions for that? Write me to movify@movify.es
2018-2-5
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MikB
Captain
Flight distance : 13346 ft
France
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Hi Alberto. This is a thread on a subject that will be of little interest to most users. You would be better off starting a new thread or adding to one relevant to your specific problem
2018-2-5
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A CW
Captain
Flight distance : 13838848 ft
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United Kingdom
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Yep - you got it! I personally always wait for 12 before take off.
2018-2-5
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BeastMaster_101
First Officer
Flight distance : 1082992 ft
Canada
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I take off with 11-12 satellites but when I reach 60 meters and above it always shows 18-19 satellites and I recommend you do the same
2018-2-5
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mtnlandpix
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United States
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I am in the mountains of Western North Carolina.  If I waited to access 11 birds I would never get off the ground.  I usually take off with 7 or 8 birds.  I suppose it is all the 5000 and 6000 ft mountains around us.
2018-2-5
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SOUNDWAVE439
Second Officer
Flight distance : 115597 ft
United States
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I like this discussion for the curious minds
2018-2-5
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hallmark007
Captain
Flight distance : 6342851 ft
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Ireland
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You are quite correct 6 is really the magic number for flying, one thing you must check before flying and during flying is the bar graph , this is your real measurement of how good your gps is and you should be looking at having 4/5 bars lit up to fly with strong gps.
2018-2-5
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DJI Elektra
Administrator
China
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Alberto83 Posted at 2018-2-5 04:55
Hi, with the latest DJI Go 4 version 4.2.5 and latest firmware installed on Mavic Pro, the drone can't connect to my iphone with software 11.2.5, any solutions for that? Write me to

Sir, we are sorry about the trouble it brings. The situation is forwarded and our engineers are working on that. Please downgrade the IOS system to 11.2.2 and check if it works. Please keep us updated, thanks.
2018-2-5
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DJI Elektra
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China
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MikB Posted at 2018-2-5 11:54
Hi Alberto. This is a thread on a subject that will be of little interest to most users. You would be better off starting a new thread or adding to one relevant to your specific problem

Mik, we would recommend users fly in a wide place with good GPS signal. Please note that drone needs over 6 satellites to record the home point. Thanks for your attention.
2018-2-5
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MikB
Captain
Flight distance : 13346 ft
France
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DJI Elektra Posted at 2018-2-5 22:51
Mik, we would recommend users fly in a wide place with good GPS signal. Please note that drone needs over 6 satellites to record the home point. Thanks for your attention.

Hi DJI Elektra. thanks for that. I understand that it is sensible to fly with good GPS coverage - I am just trying to work out what that is and when not to fly. As mtnlandpix points out your horizon make a lot of difference to that decision.


When you say "more than 6 satellites" do you mean 7 satellites. This is not as daft a question as it sounds because some programs give estimates of satellites that are available over a certain horizon and come up with a decimal no like 6.6 even though there can only be an integral number of birds.

Also, do you mean more than 6 GPS OR more than 6 Glonass satellites

or do you mean more than 6 GPS AND Glonass satellites (combined)?

Also, will Gallileo be taken into account in the near future.


2018-2-6
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Martin Lines
Second Officer
Flight distance : 19147 ft
United Kingdom
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Its not how many satellites it can see, it is the spacing of the satellites in the sky.

4 satellites nicely spread across the hemisphere gives a good fix. 12 satellites bunched up in one place gives a bad fix. As they are moving in their different orbits and the earth is rotating the spread is always changing. As has been mentioned it is all about a good horizon. In the shadow of a building or large object you are only seeing satellites to one side so again you are not getting the spacial diversity need for a good fix
2018-2-6
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MikB
Captain
Flight distance : 13346 ft
France
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Martin Lines Posted at 2018-2-6 01:44
Its not how many satellites it can see, it is the spacing of the satellites in the sky.

4 satellites nicely spread across the hemisphere gives a good fix. 12 satellites bunched up in one place gives a bad fix. As they are moving in their different orbits and the earth is rotating the spread is always changing. As has been mentioned it is all about a good horizon. In the shadow of a building or large object you are only seeing satellites to one side so again you are not getting the spacial diversity need for a good fix

You are correct Martin the number of satellites required for a unique fix is four. The geometry of them affects the accuracy of the fix (another whole can of worms). It is not likely you will get 12 satellites bunched together because their flights are carefully planned but it IS likely that a satellite can become obscured by a building, by a satellite dipping below the horizon or by picking up a reflected path.
What I am trying to understand is how the DJI software deals with the info. Does it not accept a fix until it has a fix of 6 (more according to DJI Electra) satellites? It makes sense because that helps deal with the above problems I mentioned. Does it mix GPS and Glonass, in which case it will need a minimum fix of 3 from one system and 2 from the other OR does it have separate solutions for each and accept the one that gives the best RMS accuracy?
Although it might seem a bit esoteric for some people, I believe it is important to understand what could cause a problem with GPS before it occurs and simply blaming it on wifi or magnetic interference.
2018-2-6
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hallmark007
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MikB Posted at 2018-2-6 03:34
You are correct Martin the number of satellites required for a unique fix is four. The geometry of them affects the accuracy of the fix (another whole can of worms). It is not likely you will get 12 satellites bunched together because their flights are carefully planned but it IS likely that a satellite can become obscured by a building, by a satellite dipping below the horizon or by picking up a reflected path.
What I am trying to understand is how the DJI software deals with the info. Does it not accept a fix until it has a fix of 6 (more according to DJI Electra) satellites? It makes sense because that helps deal with the above problems I mentioned. Does it mix GPS and Glonass, in which case it will need a minimum fix of 3 from one system and 2 from the other OR does it have separate solutions for each and accept the one that gives the best RMS accuracy?
Although it might seem a bit esoteric for some people, I believe it is important to understand what could cause a problem with GPS before it occurs and simply blaming it on wifi or magnetic interference.

Your receiver operates from one source of satellites and can choose which is the strongest, we often see good to fly when registering only 7 satellites, this will mean that you have enough of one type to fly safely, in turn we see 7 satellites and reported not good to fly or weak gps signal, you numbers total are shown in your telemetry but you signal to fly is represented in the graph bar, some time you will see only 8 satellites and 5 bars lit up, other times you will see 8 satellites and only 3 bars lit up, before I always try to insure 4 bars are lit up.
One thing I will say loosing gps in dji drones is a very rare thing, I haven’t seen two cases on the spark forum here where gps was lost flying in open areas. Yes gps dropped because of conflict in data and weak gps at ground level and weak gps when flying undercover of trees and buildings, so yes envoirment can be a cause of weak and loss of gps.
2018-2-6
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rimza
Captain
Flight distance : 3307 ft
Brunei
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Yes look for both number of satellites and signal bars.  Sometimes you do see high number of satellite counts but only 2 bars of signal.
2018-2-6
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Zbig
Second Officer
Flight distance : 7349 ft
Poland
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MikB Posted at 2018-2-6 03:34
You are correct Martin the number of satellites required for a unique fix is four. The geometry of them affects the accuracy of the fix (another whole can of worms). It is not likely you will get 12 satellites bunched together because their flights are carefully planned but it IS likely that a satellite can become obscured by a building, by a satellite dipping below the horizon or by picking up a reflected path.
What I am trying to understand is how the DJI software deals with the info. Does it not accept a fix until it has a fix of 6 (more according to DJI Electra) satellites? It makes sense because that helps deal with the above problems I mentioned. Does it mix GPS and Glonass, in which case it will need a minimum fix of 3 from one system and 2 from the other OR does it have separate solutions for each and accept the one that gives the best RMS accuracy?
Although it might seem a bit esoteric for some people, I believe it is important to understand what could cause a problem with GPS before it occurs and simply blaming it on wifi or magnetic interference.

I'd be surprised if aircraft's firmware had this kind of low level access and such tight and detailed control over the intricacies of GPS receiver's operation. The way you, as a system designer, usually go about integrating a GPS receiver in your system is to rely on the receiver module's own internal firmware for all the gory details and just to listen to the NMEA data incoming over the serial link that basically says "you are here". It's the GPS module's controller that does all the heavy lifting of actually listening to the satellites' signals, receiving and analyzing the almanac and ephemeris data and calculating your position based on all that. I'm guessing that DJI's firmware doesn't have much say as of what particular constellation use at the moment, etc. I'd suspect the receiver is "fusing" the data from whatever is available at the moment but that's pure speculation on my side now.
2018-2-6
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