You can get centimeter accuracy with gps. http://swift-nav.com/piksi.html http://swift-nav.com/piksi.html Even with cheaper modules and phones that dont carrie

You can get centimeter accuracy with gps. http://swift-nav.com/piksi.html http://swift-nav.com/piksi.html Even with cheaper modules and phones that dont carrier phase rtk, you can get meter which would be plenty for this application.