New York City had the distinction on Tuesday of being the first to carry FEMA's new mobile emergency system and explained how it worked. PLAN (Personal Localized Alerting Network) will reach the city half a year before everywhere else and will give those with cellphones and other mobile devices a location-based warning of imminent threats. The system would help give the city a way of warning residents of a terrorist attack or a disaster by notifying just those most likely to be affected in a given area.