Abstract: A cargo sensor system uses reflective patterns on the back door of a cargo trailer that are easily recognized by a camera mounted in the nose of the trailer. Preferred embodiments rely on the assumption that if cargo has been loaded into a trailer, that cargo will be present on the floor of the trailer, no matter whether the trailer was loaded from front to back, or from back to front. Looking across the floor at a relatively low height toward the door having the reflective pattern at a low height, any cargo on the floor will interrupt the camera's vision of the pattern, by which it can be inferred that there is cargo on the floor of the trailer.