Quincy, IL - Oct 9, 2001 – Like many office buildings, the employees in the World Trade Center were issued security cards to permit access into various parts of the building. These access cards, sometimes called smart cards, transmit a data burst when excited by a specific radio frequency. With this in mind, technicians at Motorola began developing a system that may be useful in locating people in the rubble of what was once the World Trade Center.
As previously reported in the BE Radio Currents Online, this highly modified 1kW AM transmitter, operating in the very low frequency (VLF) band, is being used to search for trapped victims at the WTC. All WTC employees received smart cards last August, designed to operate on an exclusive VLF channel. Normally, the wall packs emit only a few mW of power at 125kHz to reach the cards at a distance of one to three feet, and the smart cards emit a return coded signal at 187.5kHz.
The higher-powered, modified Broadcast Electronics AM1A transmitter allows a signal to penetrate the rubble at a distance of 8 to 10 feet, which activates the cards that may be located in the area. An experimental high sensitivity receiver and loop antenna is being used to listen for the card's return signal.
BE Radio has been informed that the first transmitter Broadcast Electronics modified is now on site and operating from a cage suspended over the rubble pile by a crane. Two operators, the transmitter, the receiver and a generator all hang in a basket a short distance above the rubble. The transmit and receive antennas are located ten feet below the basket and the system is moved over one section of the pile at a time. When a card is located by a return ping, the spot is marked and a group digs by hand until the card is located. If there is no return ping, heavy equipment is brought in, and about 6 feet of the rubble is cleared and the process begins again. No official numbers have been released, but local reports indicate that about 30 victims have been located using this system.
A second transmitter has been shipped and modifications to it and the overall system have now been completed. The second system has an improved receiver and a weatherproof housing. As of the date of this release, the second system should be at ground zero. The original system will be returned to Motorola for updating or may be sent directly to Staten Island to search through the relocated rubble for further victim identification. It is expected that these systems will remain in use for the next 9 to 10 months.