I know you need a minimum of 6 satellites to get a home point but what I want to know is what do DJI consider the maximum PDOP value for safe flight.

My concern is, it is possible to have 6 satellites and the PDOP (according to some) is not sufficient for accurate position calculation. These periods can be as long as 15 minutes, which is long enough for a crash etc.

I would love to know if DJI use PDOP to determine the Phantom's position, if not what do they use.

It is important to get a satellite forcast for the period you are flying but I am not sure of two factors that are used to get a forcast.

The first being the minimum height to determine if the satellite/s are visible to the Phantoms GPS (I use 20 degrees in most situations and 30 degrees when there are hills or suchlike obstructions). Maybe I am being too cautious?

The second factor is this PDOP value. This concerns me because you can have 6 satellites for example and the PDOP be much greater than 3 which apparently is too large a value for accurate position determination.

Regular forum readers can be forgiven for thinking "oh no he's still on about this" and I don't blame them but why can't DJI just answer the question to shed a light on this. I could be wrong but this could be an explanation for some crashes and fly aways because the Phantom was chasing a large error value?

I don't think this is something DJI should be made accountable for but it is in everybodys interest to be fully informed so when we are getting a satellite forcast, we are all inputting the same data and using the same "safe number" for flight.

I live in hope someone from DJI can put my mind at rest (and shut me up!).

Regards

Guy MacDonald

Yeah, you really are gnawing on that bone.  Sheesh, where are you flying that you need that kind of precision?!?  (if the answer is:  "I could tell you but then I'd have to kill you"  well then, never mind... )  Dang, I had to look up GDOP (Geometric Dilution Of Precision) just to see what the heck you were going on about and I dunno, the first thought that occurred to me was, presuming you actually had a definitive answer to that question, how would that change your flying?  Every model, beginning with the Phantom 1 up through the Phantom 2 Vision+ has had the same Hover Accuracy of Vertical:0.8m / Horizontal:2.5m.  So, as long as you keep those parameters in mind, along with a minimum of 7 satellites what could you do differently with an abstract PDOP value?  

guymacdonald@ip Posted at 2015-1-5 15:24
Hello Gil

I am not looking at it with the point of view of accuracy with the respect of me knowing ...

hi, Guy, I understand your concerns
I've actually sent an email to DJI support on a very similar issue (GPS reception related). I got a polite answer, but I asked for more information (and basically an answer to my technical question). I'm still waiting for an answer.

I assume that some of DJI employees watching this forum either don't know or can't answer (for whatever reasons). So, meanwhile we're just guessing.
My personal experience so far included 3 recovered "flyaways", probably due to GPS issues. After that, I've added extra GPS insulation on my P2V+ v2 and, so far, so good... no more flyaways. The insulation didn't increase the satellite reception as reported by other users. A few days ago, I did fly at 5-6 satellites coverage. I was always ready to switch manually to ATTI mode but there was no need - all worked as expected. Today, I was constantly at 5 satellites coverage (a first) and I just decided to fly in ATTI mode on my choice (S1 switch to middle position).
The problem is that, indeed, we may have some weak/faulty GPS antennas... or just our flight sites are not optimal (close to high trees, hills, etc).
Either way, it would be great to get an "official" answer on GPS reception from DJI. Personally, I'm really interested what Phantom software is doing in terms of GPS error correction... And, yes, maybe my GPS unit is not great...

ciprianboboc@gm Posted at 2015-1-5 16:13
hi, Guy, I understand your concerns
I've actually sent an email to DJI support on a very similar i ...

Maybe at the time you were flying, there were only 6 sats up there.

It is the absolute full value of Pi.

guymacdonald@ip Posted at 2015-1-5 18:54
Gerry it is interesting you should say that.

I had someone reply to me that we have at least 12 s ...

You only need 4 sats to get a position fix, but the more sats you can pick up, the more accurate that fix becomes.  I think DJI set in their flight controller that any less than 6 will trigger atti mode.  I have also had fixes on 12 sats from time to time, and a few times 6 or less.  

Here is the sats over me for today

guymacdonald@ip Posted at 2015-1-5 19:12
I understand you don't need 6 satellites to get a fix but also I understand you can have six or mor ...

Accuracy can be as small as 6 feet from the absolute perfect position with the equipment we have on board the Phantom, and keep the cost down.  If you want better accuracy like the airlines have, pinpoint accuracy, it will cost you.  They can drop a bomb down a smokestack by GPS targeting, but that equipment is unaffordable to us.  DJI could supply that if the equipment was small enough, but they would not sell any Phantoms because of the cost.  So affordability figures into the GPS equipment we have.

guymacdonald@ip Posted at 2015-1-5 19:29
Gerry, I understand what you are saying regarding the accuracy in the best scenario but I am talki ...

I've never lost control of either of my Phantoms when I was only receiving 6 sats.

guymacdonald@ip Posted at 2015-1-5 19:40
I have had a brain-wave! (maybe)

Tell me if this would work or if I could do it better.

Are you trying to make predictions on what would happen IF?

guymacdonald@ip Posted at 2015-1-5 19:47
To put it another way, maybe a maximum PDOP is a safer benchmark than a minimum satellite number.
...

6 sats or more, you have a GPX fix, 5 sats or less, you do not have a fix,  that the way it is programmed into the flight controller.  It's that simple.  It is what it is.

guymacdonald@ip Posted at 2015-1-5 20:06
Got it, (=>6 ok but

If there are 6 sats up there and you can only connect to 5 of them, how is it a lie?  If you cannot connect to it, you cannot connect to it,  It's that simple.  The 6th. sat might be to low on the horizon, it might be partially blocked by something.  
You know there are 6 people in your yard and you take a picture, and you only see 5 people.  Is it a lie to say there are 6 people there, or is the one in the back blocked by the other 5 people?  These sats are scattered over 180 degrees in the sky.

guymacdonald@ip Posted at 2015-1-5 20:18
I am not talking about not connecting or being blocked.

In fact I am talking about getting data f ...

If you are connected to 6 you are getting data from 6.

guymacdonald@ip Posted at 2015-1-5 20:23
The greater the distance between the satellites the greater the accuracy.

Something to do with the ...

They are scattered all over the sky.  Try this,, go to
http://satpredictor.navcomtech.com/
enter in 0 for elevation, then 10, then 20 up to 90 degrees and you will see how many are at altitudes of 10 degrees, 20 degrees, 30 degrees and so on.  That is a BIG sky up there to have 6 to 12 sats in it.  They are spread apart quite a bit.  Each time you change the number pick a time as reference and use that on all of them.

I just did that for 12:30 pm  today at my location
o degrees = 15 sats
10 degrees = 9 sats
20 degrees = 5 sats
30 degrees = 4 sats
40 degrees = 3 sats
50 degrees = 2 sats
60 to 80 degrees = 1 sat
as the cone gets smaller the sats numbers go down, more on the horizon than above me.  They are scattered widely apart

guymacdonald@ip Posted at 2015-1-5 20:43
Yes got all that, but sometimes (can be as litttle as 5 minutes or as long as 15 minutes) they shar ...

That is your choice not to believe in their accuracy.  If you have connection to a sat, you have accuracy.

guymacdonald@ip Posted at 2015-1-5 20:51
I hope you are right!

Regards

Math doesn't lie. 2 + 2 always equals 4, Algebra, trigonometry and calculus are absolutes.

guymacdonald@ip Posted at 2015-1-5 20:56
its interesting the chart classifies 6 satellites as marginal.

Would you fly in marginal periods? ...

That is probably the safety feature, 4 sats you can get a 3 dimensional position, but not that accurate to requirements of a few feet, but 6 minimum will put you in an acceptable range to work with.
I also fly with less than 6

guymacdonald@ip Posted at 2015-1-5 21:04
Then why do we have PDOP, TDOP & HDOP?

Regards

Just to keep you asking questions.

guymacdonald@ip Posted at 2015-1-5 21:07
Good answer!

The things you have mentioned are probably more for airliners and military flights where accuracy are more demanding than what we would require flying a quadcopter.

ok I am new to all this, but does the compass play a part? been puzzling why it has a compass and GPS.

Peter.

Naffusername, the compass is not needed for positioning.

Naffusername Posted at 2015-1-5 21:16
ok I am new to all this, but does the compass play a part? been puzzling why it has a compass and GP ...

Peter, the compass is important for orientation. "Keep the current position" logic takes into account the quad orientation for corrections - so it knows which way to go.
Compass readings are slso used for course lock.

Gerry1124 Posted at 2015-1-5 17:06
Maybe at the time you were flying, there were only 6 sats up there.

I had 8 satellites at that time at over 20 degrees altitude. There were 11 at over 5 degrees.

Oh my... The debris field stretches across more than seven hours!  What have I done?!?  I had no idea of the devastation I was going to wreak by opening that can...  

I will involk the Epiphany of the Phantom to get me through these times of worry:
     Stay calm and wait for 7 satellites

guymacdonald@ip Posted at 2015-1-6 09:59
Hello Gerry,

I slept on our lengthy conversation and now have a better understanding of what you  ...

Do you pay for your maps to upgrade your tomtom equipment?

jajahuggins Posted at 2015-1-5 22:47
Naffusername, the compass is not needed for positioning.

Then warch this:

My phantom freaked out with a bad compass position.

guymacdonald@ip Posted at 2015-1-6 09:59
Hello Gerry,

I slept on our lengthy conversation and now have a better understanding of what you  ...

I can tell you how to get free map downloads for your tomtom.