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Bit confused about how GPS accuracy is stated for products...

Hionimi

Sky Pirate
#1
Bit of a short question, but I'm confused about GPS accuracy that companies state on their websites or manuals. Like the DJI Naza Lite with GPS that I got in today, they state 2.5 meters accuracy horizontally, and 800 centimeters vertically. (Metric system! :p ) But... Do they mean total or around the objects, so that total would be like... *Gulps.* 5 Meters?

Edit: I meant 80 centimeters, one '0' too many... :confused:
 
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xuzme720

Dedicated foam bender
Mentor
#2
800 cm?!? That's 8 meters! Wow!

I could be wrong, but generally tolerances are stated in a +/- format, which would mean up to 5 meters of error. 5 meters to me sounds like a lot though so maybe they aren't using the normal format and it's only 2.5m.
 
#5
Due to the way GPS works, you can never achieve greater altitude accuracy than lat-long accuracy. More than likely that 80 cm figure comes from a barometric altimeter and not GPS.
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#6
The accuracy quoted is known as CEP -- circular error probable. A 2.5m CEP means, if you drew an 2.5m circle around where the GPS said it was, The GPS is inside that circle.

So why not a perfect location? why are some better than others? It comes from the error in the system.
GPS is measured via time-of-flight. Each satellite has a super accurate, synchronized clock, and the difference between the real time and the measured time tells the GPS how far away it is from each satellite. since it knows where the satellites are in orbit, it can triangulate where it is on earth using those distances.

So where's the error? Couple of sources:

- round-off error can make a *HEUGE* difference. those satellites are a *LONG* way away.
- where the satellites are in the sky -- are all the visible satellites over on one side or are they evenly spaced around the horizon? The closer they are together, the harder it is to measure the position -- also why it's not the same vertically as horizontally, but rocket's right -- it's worse vertically -- they're quoting the baro accuracy in the vertical
- interference (lets assume that's not a problem -- most OEMs do :p )

Why should you care? first a neat trick -- if you've got an 2.5m CEP, and you measure the position over time does the location move quickly or slowly? the answer -- it slowly drifts around that circle --VERY slowly. why is that neat? the difference between one measurement and the next *even while you're moving* will be *EXTREMELY* precise. you might have a huge CEP, but can measure speed very accurately, because you'll keep landing in the same offset in that circle!

OK, DAN, WHY SHOULD I CARE? because you're multirotor will hold it's position my measuring an initial point when you let go of the controls, and it will keep controlling the copter back to that point. Is it really in the lat/long it says it's in? NO! but who cares?!? All it needs to know is it's drifted 3cm left since the last measurement, and that's how far it needs to move back! that 2.5m GPS can *still* measure being 3cm off!

So why do some drift worse than others? processing delay and that nasty round off error. If the processor is having trouble keeping up with the corrections, the GPS doesn't update fast enough, or the control loops aren't tuned well, it'll hunt for that position more.
 

xuzme720

Dedicated foam bender
Mentor
#7
Is the CEP stated as radius or diameter, Dan? I think that's what he was wondering and I was unsure of which one they used...
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#8
formally, it's the potential error between the two points -- real and measured . . . which would make it a radial distance, not a diameter.

Now OEMs are always free to be sloppy with that, but radius is always a small number and marketeers like quoting nicer looking numbers, so it's pretty consistent where I've seen it.
 
#10
just for my information....because knowing is half the battle...blah blah blah. how does a gps system for a fast moving object such as an rc craft differ from say a more conventional gps device? allow me to expound, we just purchased some gps devices for my everyday job that take 100(s) of samples within 2-3 secs and average the points for an EXTREMELY accurate pinpoint location. Do our hobby grade gps systems do the same, multiple samples? or are they vastly different in function? Sorry about the tangent but one would be amazed by the overlap in tech when it comes to this wonderful hobby.
 

Craftydan

Hostage Taker of Quads
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#11
actually in CEP, 100s of samples averaged in 2-3s won't be much smaller -- again the measured point doesn't move very fast in that circle. over several minutes, that's a different story. Early sub-centemeter GPS units used this method, but it could take them up to 30min to get an accurate fix if the satellites were in the wrong place in the sky.

now there are some nifty tricks that signal processors can play to eat away at that error, like precisely measuring pulse edges from one packet to the next to shrink that distance error below the traditional time-stamp calculation . . . and likely other processing tricks I've not heard of ;)

now can the hobby GPS pull the same stunts? depends.

Some, like long-time averaging, can be run on any data stream. You can always crack open that multiwii, dink around with the GPS data stream and process as you like (just leave time for the control loops ;) ).

Some methods rely on features of the hardware . . . and tend to be the pricier solutions -- software can be surprisingly cheap ;)

I do expect that modern hobby grade GPSs are running some processing -- how much/how good is likely connected to that spec'ed error.
 

xuzme720

Dedicated foam bender
Mentor
#12
I think all systems do the same, but in differing degrees. For instance, the cheaper units might not be able to upgrade as many times a second but all will project the route a bit to stay ahead of the fast mover. I think the main difference will be in the update speed and processor power.
 
#13
Dan you da man! Again off topic but seems to be my thing this evening. I have a FY-DOS system and accompanying Hornet osd that has me grounded. i have done a ton of reading up on the gps woes with these systems but have yet to find a clear answer to the following. MY FY-GPS is shot and my limited problem solving has lead nowhere. Would it be possible to purchase the gps below see link as a direct solder up, plug and play? reason for asking is, the fy-gps cannot be purchased separately and ReadyMadeRC despite how much i like them were unable to make an exception in my case. soooo my fpv career is dead in the water seeing as i'm really not interested in purchasing yet another entire system just because of a flipping gps unit.

https://www.sparkfun.com/products/8975
 

xuzme720

Dedicated foam bender
Mentor
#14
I don't know what the specs are on the old unit. Are the serial rates the same? If so, I don't see any reason why they wouldn't play nice.
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#15
Maybe.

kinda of a cop-out answer, but it mostly depends on whether the GPS and you're processing boards are talking the same language. there are a few *very* standard protocols . . . and a few proprietary ones too.

Really hard to tell, unfortunately . . .
 
#16
I don't know what the specs are on the old unit. Are the serial rates the same? If so, I don't see any reason why they wouldn't play nice.
original equipment is a ls23060 v1.0 set up for 3.3v. i cant find that exact version anywhere online, the sparkfun offering is as close as i can get.
 
#17
Maybe.

kinda of a cop-out answer, but it mostly depends on whether the GPS and you're processing boards are talking the same language. there are a few *very* standard protocols . . . and a few proprietary ones too.

Really hard to tell, unfortunately . . .
well if there is anything FYTECH does well...its proprietary....which is unfortunate
 

xuzme720

Dedicated foam bender
Mentor
#18
Like Dan says, if you don't have the communication specs, which is the important bit, you are taking a shot in the dark. Do you know what the baud/refresh rate and protocol is on the original?
 
#20
my third osd/rtl system in less than two years. first the remzibi, then the cyclops storm (what a p.o.s) and now the fy system which only had about 3 months of useful life. this osd game is getting pricey for me....guess its time to go shopping again. SIGH thanks guys.