When a cargo trailer is coupled to a truck, a trailer tracking device on the trailer acquires an identifier of a truck tracking device attached to the truck. The trailer tracking device then transmits a message to a central server indicating that the trailer is coupled to the truck, which message includes the identifiers of the two tracking devices. The central server associates the identifiers of the two tracking devices in a database. When the trailer is decoupled from the truck, the trailer tracking device transmits a message to the central server indicating that the trailer is decoupled. The trailer tracking device then goes into a sleep mode until the trailer is again coupled to a truck, a predetermined time interval expires, a motion sensor indicates trailer movement, location coordinates indicate a geofence violation, or a low battery voltage condition occurs. Upon any of these events, the trailer tracking device wakes up and transmits to the central server a message indicating which situation caused the transmission and including the location coordinates of the trailer.