Coverage
The GPS device sends the trailer's GPS position and any other data collected over a satellite phone/data network. The diagram below shows the network coverage. As you can see it is wide reaching and without regional blackspots.
Transmitting data over the satellite phone/data network also means you are charged the same rate regardless of where you are. Furthermore from a security perspective the satellite network used cannot be interfered with by would-be criminals, who would have recently be known to purchase jamming equipment. Such jamming equipment is used when a trailer's GPS device transmits its data over the mobile phone networks. The criminals use the network-jammer to stop a trailer/HGV being able to send its GPS position back to base while a thief is in progress.
