GPS on Sport's Fields - too much error

Hi everyone, im recently starting with Arduino, reading a lot, but i have some question about my project...

I need to collect data (maybe 3 times in 1 second) of Velocity and Position of one player in a soccer field. My doubt is that GPS is not so accurate for the area I need to control, theres to much error.

The error of the position can not be more than 20cm.

what do you suggest i have to use for this? how can a I reduce the error?

thanks

Your doubt is spot on.

Before anyone else jumps on this, I will give the "stock" answer: If you contract a military-grade system costing many grand from a specialist developer, you might get some results.

You need to Google “differential GPS”

The "NavSpark" is a GPS receiver/processor designed for this type of problem. Not a simple project but it can be done reasonably inexpensively, and you can get 10cm level accuracy. Slapping together a rover is still a complex project because accuracy depends so much on getting good GPS signals.

The "Piksi" is another option; a little more expensive but seems to have packaged everything together a bit better.