If you're doing just a one -off, buying a consumer GPS may be a good idea as you get a user interface. Most have a serial (rs232) interface that talks NMEA which is a well documented ascii style protocol. Also the choice depends on the accuracy you want. If you're prepared to pay the money, 2cm is achievable but you probably looking at >USD$10,000. Companies like Novatel and Trimble can help you with the whole range of solutions. Note that using an 8051 to do navigation may not be a good idea if you require a lot of floating point calcs - you may require something with a little more grunt and leave the 8051 to handle the lower level stuff. Using a Windows CE, Palm or embedded pc may be suitable choices here.