A portable detachable tracking unit for shipping containers which provides communication via satellite with a central server is provided. The tracking unit has an antenna to communicate both with the GPS satellite and a two-way satellite, a computer, a GPS receiver, a satellite transmitter, a modem and an internal power source. The tracking unit can be attached and detached from the shipping container and is can be attached to the locking bars on the doors of the shipping container. The tracking unit may have sensors for determining when it has been tampered with or detached from the shipping container and transmitting that information to the central server. The tracking unit may have a memory capable of receiving and storing geo-fencing information on the specific route to be taken to the destination with the computer being programmed to determine if the tracking unit is outside of the geo-fence or not and communicating that information to the central server. The tracking unit may have a sensor to determine whether it has been detached from a door of the shipping container and transmitting that information to the central server. A built-in tracking unit is also provided which can communicate whether a door of the shipping container has been opened or not from information supplied by a sensor.