Abstract: A vehicle monitoring system provides real time monitoring of vehicles by receiving position and operational conditions from mobile units over a wireless communication link, such as a cellular telephony connection. In one embodiment, the mobile unit is equipped with both a Global Positioning System (GPS) receiver and a dead reckoning device, which provides position information when the GPS receiver is unable to receive GPS positions from a GPS satellite (e.g., when the vehicle is in a city area surrounded by tall buildings). A novel position request protocol allows multiple position reports to be included in a single data packet, thereby reducing the air time cost of transmitting data over a cellular telephony link. A novel antenna cable detects and reports tempering of a GPS antenna.