Editing Automatic vehicle location (section)

=== Simple direction finding ===
[[Amateur radio]] and some cellular or [[Personal Communications Service|PCS]] wireless systems use '''direction finding or triangulation''' of transmitter signals radiated by the mobile. This is sometimes called '''radio direction finding''' or '''RDF'''. The simplest forms of these systems calculate the bearing from two fixed sites to the mobile. This creates a triangle with endpoints at the two fixed points and the mobile. Trigonometry tells you roughly where the mobile transmitter is located. In wireless telephone systems, the phones transmit continually when off-hook, making continual tracking and the collection of many location samples possible. This is one type of location system required by [[Federal Communications Commission]] Rules for wireless [[Enhanced 911]].