| 摘要 |
<p>A position tracking system comprises a primary unit that transmits a position signal derived from e.g. GPS, and two or more monitoring units that receive and retransmit the position signal. The primary unit 10 is attached to an object to be tracked and comprises a GPS receiver and a transmitter to broadcast its position to an in-range monitoring unit 20A. The monitoring unit 20A receives and retransmits the position signal to other monitoring units 20B which may not be in range of the primary unit. The monitoring units 20 and primary unit 10 are each assigned a time slot within a TDMA transmission cycle to avoid clashes between transmissions. Within the TDMA cycle each monitoring unit 20 is arranged to transmit in its assigned time slot a position signal of the primary unit 10 in the event that the monitoring unit has received such a signal from the primary unit but not from any other monitoring unit.</p> |
