Mobile computing devices have changed the way we look at computing. Laptops and personal digital assistants (PDAs) have unchained us from our desktop computers. A group of researchers at AT&T Laboratories Cambridge are preparing to put a new spin on mobile computing. In addition to taking the hardware with you, they are designing a ubiquitous networking system that allows your program applications to follow you wherever you go. The ultrasonic location system is based on the principle of trilateration position finding by measurement of distances (the better-known principle of triangulation refers to position finding by measurement of angles). A short pulse of ultrasound is emitted from a transmitter (a Bat) attached to the object to be located, and we measure the times-of-flight of the pulse to receivers mounted at known points on the ceiling. The speed of sound in air is known, so we can calculate the distances from the Bat to each receiver - given three or more such distances, we have enough information to determine the 3D position of the Bat (and hence that of the object on which it is mounted).