gps locked on @ 6 sats

[twitchyzero]

took off hoping to gain more sats...took maybe another minute at a different position to finally gain 10 sats.

upon taking off it was very unstable...significantly drifting. Only light wind. Not sure if it's due to the magnetic interf I was getting around launch point. Wasn't paying attention to HDOP

I was surprised it locked at 6....it still RTH fairly accurately though

I'm not too shocked at the flying behavior but I'm a little taken back Solo can be armed like this.

Is there a definitive pre-requisite to arming with GPS lock? In the past the lowest # ive seen is may be 8 sats. Typically around 10.

Is there a unofficial HDOP or sats number you wait on to take off comfortably?

 

[Anthony Horrigan]

I have experienced the same behaviour with 6 sats, so I wait for at least 8 sats and the drone is very stable, or another option is to move to a better location with better exposure to the sky.

 

[pyrate]

you are over thinking it
For months we have been trying to tell people number of satellites is but one of many factors and not even the most important
It is the quality of the signal and data from the acquired sats that is important
And yes Solo / Ardupilot have an extensive pre-arm check for GPS. It is why you do not hear about fly aways from 3DR versus others

 

[twitchyzero]

I dont know if overthinking is the right description...the Solo took off irratically and that should be a cause for concern.
OK # of sats isn't the end-all be-all...then is there an HDOPs that I should be looking for prior to take-off?
are you sure about that no fly-away claim..? I'm almost certain I've read a few on here...not as much as the competitor but i think there's still the risk.

 

[SPP]

6 Sats is the minimum allowed by latest firmware .... as go and "FLY". I think 6 as minimum been a few firmware ago.
However, if these 6 sats are poor signal or clump together very very 11 to 1 o'clock, accuracy wont be so good.
Need some to be more spread apart.

Download the Tlog and see the HDOP, perhaps it will give some insight.
.

 

[pyrate]

Yes HDOP is a big part of it, and if memory serves solo requires 2 or less on HDOP
I still think there is more to it then just number of sats and HDOP
for example does HDOP have to be maintained for a period.
Not sure
On fly aways, please post your evidence of a true "fly away"
Meaning solo just took off and without control flew away.
There are two reasons it might happen
You had a tower mission loaded and did not know it, and the copter was flying to the first way point
The home position was not properly established and solo was attempting to go home.

Both of which can be countered by switching to manual.

Also some people have flown near to buildings or other obstructions and had GPS glitches that would cause solo to move erratically in fly
were you in an open field?

I have heard that called a fly away, but that is not what DJI is famous for.
Where they just take off and never come back

 

[Jubalr]

It's been discussed on here extensively. #of satellites and HDOP alone are not what determines a lock. sAcc is a parameter that weighs heavily in determining lock with the Solo. This has to do with timing. Here is some great discussion on the issue including one of the lead designers behind the Solo..

 

[ajdelange]

At a very high level the error in the vehicle's estimated position is

K = (1/A)*Kn*(1/A_) + B*(1/A)*Kp*(1/A_)*B

K is in units of ft^2. Kn is also in units of ft^2 and represents the error the receiver makes in estimating the pseudorange to the satellites. HDOP is part of A. Obviously the larger A and the smaller Kn the better the quality of the fix. Kn depends on things like signal to noise ratio, multipath, attenuation... A depends only on the geometry which includes the number of satellites. In general (1/A)*(1/A_) is proportional to 1/N where N is the number of satellites but it also depends on where the satellites are. So, assuming favorable distribution, doubling the number of satellites reduces the standard error (the square root of K in units of feet) by 1/sqrt(2) that is, by 30%.

Some of the linked remarks from the so called experts are correct and some show considerable naivete. The observation that the antenna required to receive Glonass and GPS must be wider band than that required for GPS alone is certainly valid as is the observation that this may effect SNR and thus Kn but good SNR can be restored by proper filtering beyond the antenna. A comment as to what to do if a Glonass satellite is added to the mix which satellite improves DOP (A) but degrades Kn reflects failure to appreciate that a good receiver (which knows A and Kn) simply calculates K to see if adding the satellite improves it or not before adding it into the nav solution.

I really simplified this. Those familiar will have already picked up that A, K etc are not numbers but arrays of numbers (matrices) and that A_ is the transpose of A. K has diagonal elements that represent, respectively, the variances of the estimates of position in northing, easting, altitude and clock. Their square roots are the standard errors in these position components. Treating them as scalars as we have done here would probably cause heart failure in a mathematician but nevertheless makes it clear what the relationships between the components of error are.

The statement has been made that there is more to it than just number of satellites. This is definitely true but number of satellites and their positions relative to the receiver and received signal quality (which may also depend on the position i.e. pseudorange error is worse close to the horizon than at zenith) are equally important.

There are other factors too. Kp represents the uncertainty in things like emphemeris, clock correction, iononspheric delay etc. B scales these uncertainties into pseudorange error. WAAS works by reducing Kp.

 

[Seaair7]

I have a solo and I get 8 sats I also have a x-star premium in the same location I get 18 sats much quicker I also have a typhoon H and get I get 18 sats also ?

 

[ajdelange]

Is there a unofficial HDOP or sats number you wait on to take off comfortably?

 

[ajdelange]

As I tried to explain in my last post what you ultimately care about is not what the geometry will allow (A; DOP) or what the signal to noise ratio and multipath give you for individual satellite range error (Kn) or how well WAAS has corrected the ephemerides (Kp) but the combined effect of all of these which is commonly represented by the square root of sum of the northing and easting terms in K, or more commonly, twice this which is called the "2DRMS". The GPS radio I use to locate my dogs in the woods draws a circle of that radius around the point of its estimated location so you can see not only where the receiver thinks it most likely is but where you actually could be with 95% probability. Other receivers display just the number but the circle on the map is really useful. The SOLO's GPS quality indication would be a lot more useful if it indicated 2DRMS rather than HDOP and 2DRMS is probably a better criterion for making a decision as to whether the GPS fix is reliable enough for launch.

 

[ajdelange]

I have a solo and I get 8 sats I also have a x-star premium in the same location I get 18 sats much quicker I also have a typhoon H and get I get 18 sats also ?

There is no question as to why you get 8 with SOLO and 18 with the others. SOLO does not receive GLONASS. The question is as to whether K is smaller, taking into account all the factors that go into it, with the addition of the 10 GLONASS satellites or not. If it is then the design of SOLO is inferior to that of the other two (in this regard at least).

 

[Seaair7]

The X-star premium has GLONASS and Typhoon H has U-BLOCKS - GLONASS

 

[Seaair7]

They are all good
Solo
X- Star premium
Typhoon H

 

[Tamj]

As I tried to explain in my last post what you ultimately care about is not what the geometry will allow (A; DOP) or what the signal to noise ratio and multipath give you for individual satellite range error (Kn) or how well WAAS has corrected the ephemerides (Kp) but the combined effect of all of these which is commonly represented by the square root of sum of the northing and easting terms in K, or more commonly, twice this which is called the "2DRMS". The GPS radio I use to locate my dogs in the woods draws a circle of that radius around the point of its estimated location so you can see not only where the receiver thinks it most likely is but where you actually could be with 95% probability. Other receivers display just the number but the circle on the map is really useful. The SOLO's GPS quality indication would be a lot more useful if it indicated 2DRMS rather than HDOP and 2DRMS is probably a better criterion for making a decision as to whether the GPS fix is reliable enough for launch.

I'm guessing the ELI5 version is that the Solo app should show a circle of how accurate the positioning is just like Google Maps does when the circle starts off big then gets smaller and smaller as it gets more precise?

 

[Robert Rose]

If you want to check the current status of the US GPS satellites you can visit Welcome to GPS.gov and see an enormous amount of information, plus sign up for email notices. From what I see online the GPS satellites are not up 100%. In January I was having lots of problems getting a GPS fix, but after the recent upgrades it has gotten better. I don't know if this was a Solo issue, a satellite issue, or a time of year issue. I have found that a clear view of the sky is better, and being close to a mountain or trees blocking the horizon view also can create a problem. What doesn't work in the morning might work in the afternoon.