Abstract: Location of an object to be tracked is determined by measuring, at a receiver situated at the object, the propagation time differences between the signals from a plurality of GPS satellites, each of which is received by the receiver situated at the object. The measured propagation time difference values are transmitted to a central station where the location of the object to be tracked is calculated based upon the propagation time differences of the signals transmitted from the satellites and data derived from a receiver apart from the object but also receiving signals from the satellites.

Abstract: A method of tracking railcars in transit using global positioning techniques involves determining the location of a railcar by using a Global Positioning System (GPS) receiver and a satellite transceiver on board the locomotive, and a local area network of railcar tracking units. Instead of computing the GPS solution independently and transmitting it to a central station, the railcar tracking units transmit their unique identifications (IDs) to one of the tracking units in the local area network acting as an administrator tracking unit. The administrator tracking unit maintains a list of IDs and periodically transmits the list of IDs to the locomotive. The locomotive locates itself using the GPS system and periodically transmits its position and velocity to the central station.