I'm afraid I'm not a surveying professional. I'm here as a convergence of a number of other factors. I have no good reason to have a dual frequency GPS receiver, but as one of my hobbies is Amatuer Radio, I have a lot of stuff for reasons that aren't immediately obvious; such as a Trimble Thunderbolt GPS Disciplined Clock. The Thunderbolt led me down the road to a Trimble Lassen IQ, which in turn took me to a Trimble 4000SSi. So far the 4000SSi is the most difficult device to have talk to a computer.