A completely automatic system for monitoring auto racers or the like. Each vehicle carries a transponder which senses its own passage of a position event such as crossing the start-finish line, entering or leaving the pit area, etc., and transmits a signal indicating same to a track side receiver adjacent that event position on a time shared basis with the transponders of all other vehicles. A master transmitter sends a time divisible sync signal to all of the vehicle transponders and each transponder is adapted to transmit a position event signal during only a predetermined time division of the sync signal. Thr track side receivers feed into a central control unit which is also synchronized with the master transmitter and logs the appearance of its positional event of each vehicle in real time. A clock and a computer are provided so that racing rank, lap speeds, etc., may be determined from the log data. A unique position event occurrence sensing system is also disclosed in which an inductance carried by the vehicle passes through a fluctuating magnetic field, the polarity orientation of which defines the position of the particular event.