Bof - It`s so that their new Surface Movement Radar system is able to maintain the identity of each aircraft as it moves around the airport.
From what I understand the Mode S "squitter" transmits the aircraft`s flight plan call-sign once per second and it is this which the system uses.
I`m sure one of the tower experts will give the Full Monty about this in time but I hope this is enough to be going on with.
