n00b Question - Mixed Wifi & GPS positioning - need advice on new project

Hello! I've always been attracted from Arduino & robotics but never get involved in doing anything with it. Now an idea (maybe) come on my mind while trying to solve "a problem". Let's say that I need to track people inside (mainly) & outside buildings: - Lilypad XBEE + XBEE wifi module for quite everyone - Lilypad XBEE + GPS XBEE IF it is needed outside on some people to get more accurate positioning data from sat - I could use 2 "PC stations" outside and/or GPS modules - Point to multipoint connection between people & "PC stations/GPS"

Every wifi Lilypad send ID number, time mark, sync signal and at least 3 RF measurement (2 PC/GPS + 1 from other Lilypad); using more GPS Lilypad will improve accuracy.

Are GPS real time data a must or could PC stations (with informations on theuir location) do the same? Do you think that this is the right way to start a project like this or should I consider other things?