Systems and methods can provide alternatives to a global positioning system (GPS). For example, certain systems can operate on 1090 MHz and provide for methods of estimating location that can be used in place of GPS. Thus, a method can include obtaining an estimate of position of an own aircraft based on time of arrival of signals from a plurality of ground stations. The calculation of the estimate can be performed in the own aircraft. The method can also include using the estimate of position instead of a position from a global positioning system.