or alternatively the motion detector 54 can initiate operation when motion is detected u2014 i e movement of the freight container to which the remote unit 10 is attached although a low cost mercury switch is used as the motion detector 54 many other types of motion detectors may be used the battery 56 is sized depending on the load imposed a rechargeable 6 volt d cell nickel cadmium rechargeable battery works for most applications the repeater system of fig6 uses gps time to allocate a transmit window to each remote unit thus avoiding the handshake protocol communications overhead associated with conventional communications schemes each repeater 50 has a unique identification which is transmitted along with position data each repeater 50 is allocated for example a 5 second transmit time window to transmit its data because the base station 12 and all of the repeaters 50 have accurate gps time data such a time window allocation is possible the timer 52 initiates operation of the remote unit and during operation the timer is reset to gps time to ensure accurate time in the timer 52 a repeater 50 receives timing signals from 4 satellites and stores the signals in a temporary memory buffer compressing if desired for transmission in its allocated time window these raw data timing signals include an identification of the satellite different signal processing techniques may be employed if desired to obtain an accurate position estimate of these raw data timing signals such as filtering and compressing the base station collects each timing signal from the repeaters 50 and processes the timing signals to determine a location of the repeater the base station 12 can employ the amount of processing desired to the timing signals to improve the.