GPS Accuracy - uBlox Neo-6M

[Hands0n]

I was setting up a Crius GPS unit for my APM 2.5 (new build) Firefly 450 quad and was examining the functionality and accuracy of it using the uBlox uCentre utility.  Interesting findings.

The Crius GPS is a uBlox NEO-6M (not the V2) and sells quite cheaply these days, mostly from Chinese suppliers and for less than £20 delivered, typically.  There is not a great deal of difference between this and the NEO-6M-V2 in terms of performance as far as I can tell. Although greater persistent memory and faster start-up seem the general improvement.

I am particularly interested in the positional accuracy of these units. They are often cited to be accurate to within 2.5M - I take that to be radial - which is not too bad, but could be the difference between landing on terra firma or somewhere less convenient.

Upon performing a normal start, that is to power it up and wait roughly a minute I got an accuracy with a 2D and 3D lock of 1.74M
[attach=1]

But leaving it for the full three minutes recommended by Peter King the accuracy improved even more, this time down to 0.83M.  A significant improvement still.  And it does rather suggest that the 2.5M quoted is quite conservative.
[attach=2]

Taking things a stage further - I grabbed a screen shot of a Google Earth view in uCentre showing where the GPS unit homed in on its accuracy from power up (warm start).  I am sat alongside the red car on the garden side of the fence.
[attach=3]

And so finally, I performed a Cold Start of the GPS unit using uCentre and the result is shown here - in this case, from cold start to full 2D/3D lock took just over one minute, about 1m 15s.
[attach=4]

So, it looks like these cheap GPS units are more than reasonably accurate, given the time to lock on and settle down.  The short Cold Start time is encouraging. And the recommended three minute wait before arming and taking off is quite conservative, although I am happy to do that to maintain a good chance of the GPS being as accurate as it possibly can.

[guest325]

I';ve got that gps and found exactly the same - it';s really accurate if you leave it to pick up the satellites for long enough, patience is rewarded with better results!

[rickp]

Is it not battery backed? That was one change that happened on the 3DR GPS that made the startup time much better...

[Hands0n]

@rickp, yes it is mate. But from what I read the battery life is short lived, perhaps a day or so. The best proof of how well it does is the Cold Start where it';s memory is purged. One minute and a bit to full 2D/3D lock. After that it only took a few seconds following a LiPo change to regain lock in the FC.

I';d say that the officially quoted 2.5M accuracy is rather conservative.

Interesting other thing I observed. Whilst the GPS locked on quick enough the APM sometimes did not, and required a reboot to get the Blue light that indicates a 3D lock. Every few battery changes this happened. I have applied the GPS configuration file for APM. So that';s a bit odd really.

Sent from my C6603 using Tapatalk 4

[rickp]

Yup, thats my understanding. The battery is good for around 2 days in my experience. I always wait until the HDOP has dropped and I';m currently hacking the minimosd code to show HDOP rather than number of satellites as thats more important to me...

[Tony Campbell]

I';m sure I read somewhere that the APM board actually reconfigures the GPS everytime you switch on. This means that even though you have reconfigured the gps, APM changes it again. Apparently, if you disconnect the TX wire from the GPS after you have done the reconfigure, the APM can';t then change it back. Maybe someone can she some light on that??

[Hands0n]

@Tony - yes the APM does indeed write configs to the GPS. There is a massive discussion between various devs on the subject on this thread (midway through reading it) http://www.diydrones.com/forum/topics/apm-2-5-with-neo-6m-ublox-gps-not-3dr-lea-6?xg_source=activity&id=705844%3ATopic%3A1004467&page=13#comments

[HowDoIfly]

I followed the advice and loaded the custom ublox config file and asked about the APM re-write and was told I can disconnect the rx wire to prevent it but haven';t done so yet. As far as I can tell, my GPS is working just fine, gets a 3D fix by the time APM has finished booting and usually aquires 9 or more satellites in a minute or two. I';m not sure how else to analyse it';s performance. All GPS-related flight modes seem to work with no scarey surprises to date.

[Hands0n]

That sounds like you';ve got it exactly where you need it to be 

[Tony Campbell]

Theres a lot to be said, for not fixing something that aint broke!!!!

[Marty McFly]

Hi as Rick says the HDOP numbers are important,and also the lead developer says wait at least 3 mins after you get a solid blue light on the APM,better as I do 5 mins,Marty. :scotland:

[rickp]

Marty - I wait until the HDOP numbers drop - which on the first boot after a while can take a few minutes. But after a battery change, HDOP is down to below 2 (my limit) before the APM has finished booting...