In a recent case, we tracked a shipment from its point of origin, bound for a regional distributor. It was suspected that the shipment would be diverted en-route, and some product stolen.
Due to the route being including large sections outside mobile phone coverage, our GPS GSM tracking devices were not suitable for tracking the shipment's location.
Instead, we constructed and installed miniature long-range radio beacons, which we were then able to locate using radio-direction-finding equipment.
This allowed us to follow the shipment at a safe distance, identify when it left the planned route, and follow it to an isolated property where we obtained video evidence confirming the suspected theft.
We have previously used similar devices to follow 4WD vehicles in rough and remote terrain where other tracking devices fail.