Send Out the Bat Signal
In order for a computer program to track its user, researchers had to develop a system that could locate both people and devices. The AT&T researchers came up with the ultrasonic location system. This location tracking system has three basic parts:
- Bats - small ultrasonic transmitters worn by users
- Receivers - ultrasonic signal detectors embedded in ceiling
- Central controller - coordinates the bats and receiver chains
Users within the system will wear a bat, a small device that transmits a 48-bit code to the receivers in the ceiling. Bats also have an imbedded transmitter which allows it to communicate with the central controller using a bidirectional 433-MHz radio link.
Bats are 3 inches long (7.5 cm) by 1.4 inches wide (3.5 cm) by .6 inches thick (1.5 cm), or about the size of a pager. These small devices are powered by a single 3.6-volt lithium thionyl chloride battery, which has a lifetime of six months. The devices also contain two buttons, two light-emitting diodes (LEDs) and a piezoelectric speaker, allowing them to be used as ubiquitous input and output devices, and a voltage monitor to check the battery status.
A bat will transmit an ultrasonic signal, which will be detected by receivers located in the ceiling approximately 4 feet (1.2 m) apart in a square grid. There are about 720 of these receivers in the 10,000-square-foot building (929 m2) at the AT&T Labs in Cambridge. An object’s location is found using trilateration, a position-finding technique that measures the objects distance in relation to three reference points.
If a bat needs to be located, the central controller sends the bat’s ID over a radio link to the bat. The bat will detect its ID and send out an ultrasonic pulse. The central controller measures the time it took for that pulse to reach the receiver. Since the speed of sound through air is known, the position of the bat is calculated by measuring the speed at which the ultrasonic pulse reached three other sensors. This system provides a location accuracy of 1.18 inches (3 cm) throughout the Cambridge building.
By finding the position of two or more bats, the system can determine the orientation of a bat. The central controller can also determine which way a person is facing by analyzing the pattern of receivers that detected the ultrasonic signal and the strength of the signal.