an automatic vehicle location system obviates the need for frequent , scheduled position and movement change messages from field vehicles by having each field vehicle send position and movement update messages only when a change occurs in a monitored vehicle characteristic , and only when that change is accompanied by traversing a decision point on a stored electronic map . sensors for monitoring such characteristics include , turn signal indicators , brake indicators , back - up indicators , speedometers , compasses and accelerometers . upon reaching a decision point , and upon one of the signals from a sensor reaching a threshold , the automatic vehicle location box in the mobile unit of the field vehicle causes the mobile data terminal to transmit a position update message to a dispatch center . the dispatch center and the mdt contain identical maps , and extrapolate the vehicle &# 39 ; s position between decision points according to identical algorithms .