After more than five years of planning, a first-of-its-kind national cell phone alert system is coming to the city.

Mayor Michael Bloomberg was joined this morning by city and federal officials by the World Trade Center site to announce the details of the new Personal Localized Alerting Network, or PLAN system.

The technology effort will send emergency text messages of 90 characters or less to cell phones in case of a terrorist attack, natural disaster or other serious threat. The messages will have a unique ring tone.

The PLAN system will feature three kinds of alerts: those from the president, those involving imminent threats to safety and Amber Alerts about possible child abductions.

You won't have to sign up to get the free alerts, but a special chip is required to allow your phone to receive them.

Some smartphones already have the chip, and starting next year all new phones will have it.

The alerts can be targeted to a specific geographic area using local cell phone towers. Officials say by overriding other messages, the messages will not be slowed by high network traffic. The alerts will not be held up, even if the network is congested.

"If people need to take action, the system can transmit instructions clearly, accurately, and in a timely manner, which is something that could save countless lives," said the mayor.

“We ask the public, when they see something, say something. Now with this new system, when government knows something, it can say something,” said Police Commissioner Ray Kelly.

"Those experiences taught us all and they certainly gave me a deep appreciation of the necessity of communications technologies in times of crisis and times of disaster," said Federal Communications Commission Chairman Julius Genachowski.

The new alert system aims to prevent communication crises like during the terrorist attacks of September 11, 2001, when cell phone service overloaded, millions of New Yorkers had no way of making or getting calls and official updates were hard to come by.

“There were many lessons learned from 9/11. One of them was that we were under-utilizing communications technologies in emergencies,” said Genachowski. “It would have been great if we had had something like this available as recently as last month for the tornadoes in the South.”

“It isn’t just what happened behind us here [at the World Trade Center], but it’s every tornado, it’s every hurricane, it’s every rainstorm,” said Verizon Chairman and CEO Ivan Seidenberg .

The service will be available in the city and in Washington by the end of the year, and will be available nationwide by April 2012.