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GPS GNSS Accuracy
Please read this page, before.
Who more accuracy U-blox 6 generation (GPS) or U-blox 8 generation (GNSS)?
NAVSTAR GPS accuracy - about 2-4* m
Glonass accuracy - 3-6** m, (outside of Russia accuracy fall down to 9 m)
Galileo - first services should be launched at the turn of 2014 and 2015. Full completion of the Galileo system is expected by 2019
BeiDou - accuracy 10 m in Asia-Pacific (by 2020 global BeiDou-2 also known as COMPASS)
Indian Regional Navigation Satellite System - accuracy unknown (for India)
Quasi-Zenith Satellite System - accuracy unknown (for Japan)
How we see Global Position system is only Navstar GPS and Glonass, if use both of this system we will have GNSS that totally more accuracy than GPS or Glonas separate. To get more accuracy we must use our Satellite-based augmentation system (SBAS) or more expansive element's (it's wright bellow).
EU - EGNOS
USA - WAAS
Russia - SDCM
Japan - MSAS and QZSS
For example EGNOS in EU with GPS give the horizontal position accuracy is at the meter level.
Now we come clouser to our U-Blox modules and other GPS/GNSS modules.
In all series of U-Blox modules you can find sub line of products marked with letter P and you can say that this sub line is more precision than M. You are right and wrong :). Yes it's can give sub meter precision but with great "BUT".
First of all you must change that cheap ceramic antenna to the normal (still not not precision antenna), for example. Second you must use one of your domestic DGPS services (they are not free), and GPS element or flight controller (FC), or GPS element and FC together can work with RTKLIB live/online and off course it must be connected to DGPS service (internet, radio, etc). If all this work together maximum you have precision of 0.30 m - not more.
But, if I'm right "P" sub line of U-blox series only can give offline raw data from GPS/GNSS, if you want it live/online? you must buy another GPS/GNSS module - for example NovAtel, they are cheapest of all professional modules and it will be about 1000$.
Ok it's great you buy OEM GPS model from NovAtel or else manufacture greater than 1000$, now you can have sub 0.30 m precision, - but you must change your antenna to the precision one. Their prices and size you can look here.
Are you really need sub meter precision :)
You can say, that you buy 2 Neo-6M GPS element's and make 1.0-1.5 precision.
No - it wouldn't be so. By the theory of the smallest equals - n times of measurement must be straight to the infinity. How you see infinity is not equals 2 measurement from 2 different GPS element's.
And one more thing - cheap antennas catch reflection signal (for precision GPS/GNSS measurement it's one of most biggest errors), summary in one moment it can give a big coordinate jump out in any side and who knows where your's UAV's go :)
For me 1 GPS/GNSS element enough :)