Air traffic control (ATC) systems rely on assembling and managing accurate information on aircraft location and course within specified zones and in communicating data, voice and other information to and from such aircraft. ATC includes at least communications, navigation and surveillance functions. ATC seeks to answer the questions: where is each aircraft in space and time, where is it going and how do you send it voice and data to provide route guidance, weather and safety alerts, landing instructions and, if necessary, instrument landing functions. The required accuracy of aircraft location information varies greatly depending upon the control function being performed. For example, in the case of general aircraft routing control one needs to know the aircraft location only within an accuracy of about 10.sup.3 meters, while for computer controlled instrument landings one needs to know aircraft position to about one meter.
Present day ATC systems use ground based radar to determine aircraft position in three dimensions, transponders to provide aircraft identification and various communication radio systems to exchange voice and other messages with the aircraft. Instrument landings can only be made at airports equipped with local, radar based, instrument landing systems. Various radio beacons are provided along major flyways so that pilots or on-board navigators can tell their general locations and whether or not they are within an assigned flyway. Some problems with present day ATC systems are that it is difficult or impossible to: (i) provide wide area coverage with uniform resolution, i.e., to track all aircraft with uniform high precision over a large geographic area without a substantial increase in the network of ground radar stations, (ii) provide emergency instrument landing capability at emergency (or other) airfields that do not have local instrument landing systems, (iii) have automatic, wide area reporting of accurate aircraft locations to ATC stations, (iv) have uninterrupted communication capability anywhere within the wide area and (v) have a single integrated system which provides, over wide areas, all of the communication, navigation and surveillance functions required by ATC. As used herein, "wide area" singular or plural, is meant to refer to regions encompassing on the order of 10.sup.5 -10.sup.7 km.sup.2 or more.
The advent of the Global Positioning System (GPS) satellite network and the development of inexpensive GPS receivers has opened new possibilities for wide area air traffic control. Unfortunately the existing GPS system was developed primarily for military purposes and has significant limitations when used for ATC purposes. For example, there are times when the angular relationship between satellites in the GPS satellite constellation is such that accuracy of position fix is degraded by geometry, particularly in the event of a satellite outage. Another weakness of the existing GPS system is the great difficulty in and time required for replacing failed satellites. A further weakness is the lack of assurance that the existing GPS system will be operated at its maximum accuracy all of the time. Further, even under the best of conditions, it is very difficult with GPS to obtain the desired one meter accuracy over large geographic areas. Thus, the reliability, availability and accuracy of the present GPS system leaves much to be desired for ATC purposes.