trilateration in star topology

HI ,
please , i want an explanation.

I'm working on implementing a platform for localisation in wireless sensor networks.
I have only xbee 802.15.4 serie 1 , can't upgrade them (cause of the arduino xbee shield i have) so can't support mesh toplogy.

So i 'm wondering for a very long time how can i implement the trilateration with star topology, any ideas please??