Positional Tracking for vast open spaces?

I dont know exactly what you are doing but t sounds like what you probably want is QR codes (and ignore my post about infrared mocap). Valve has successfully used QR codes in VR demonstrations for motion tracking. QR codes carry data like a bar code so you have the option of writing the co ordinates into the QR code or just a serial number for use with a database. If a camera can see the corners of the QR code then as the QR code is of known size and dimensions a cellphone can tell its 3d position and orientation relative to the QR code. Its a simple triangulation calculation from the four corners. So if a cell phone can see at least one QR code with a known fixed position it knows exactly where it is. With some clever programming it will be able to tell which square is which/where before it is close enough to read it by its position relative to the other known qr codes.