Abstract: A communication system transmitting messages includes two-way ranging by communicating between a target vehicle and a traffic controller station through a plurality of satellites. A set of the plurality of satellites may be used simultaneously or sequentially to transmit and receive signals from the ground station to a target vehicle, preferably an aircraft in the preferred embodiment. Each aircraft includes a particular ranging code combined with a message signal to selectively process the ranging data supplied to each vehicle. The ranging determination may be used in conjunction with alternative ranging devices, such as a global positioning system, in order to improve the accuracy of the estimations provided by the ranging processing. Nevertheless, the method and apparatus of the present invention provide relatively accurate state vectors without the need for precisely accurate timing synchronization at each of the stations connected by communication links.