A cargo tracking apparatus for detecting proximity to a cargo structure includes a cargo sensor, a location sensor, a wireless transmitter, and a processor, all disposed within a housing that is configured to be attached to the cargo structure. The cargo sensor, which is adjacent a sensor window in the housing, detects a portion of the cargo structure disposed near the sensor window and generates a cargo sensor signal based on detection or lack of detection of the cargo structure. The location sensor generates location information based on the location of the cargo tracking apparatus. The processor receives the cargo sensor signal and the location information, and based thereon generates alert messages that contain the location information and information indicative of whether the cargo structure is disposed near the cargo tracking apparatus. The wireless transmitter transmits the alert messages.