Importance

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Abstract

An aircraft local-area augmentation system that employs a differential global positioning system (GPS) to assist aircraft (102) landing is disclosed. One or more GPS ground stations (120), each including at least two GPS receivers (122) and a datalink transmitter (126), calculate and transmit GPS correction data to an aircraft (102). An aircraft (102) employs a GPS receiver (106) for receiving ranging signals (112) from GPS satellites (108), and a datalink receiver (116) for receiving GPS correction data and other information from a GPS ground station (120). The aircraft (102) further includes a data processor (110) for determining a global position of the aircraft (102) as a function of the aircraft GPS pseudorange data and the GPS correction data. The system minimizes the introduction of non-common errors by the use of double-differencing calculations using multiple combinations of satellite (108) and GPS ground station receiver (122). The system further comprises a method of transmitting a digital signature (626) from a GPS ground station (120) to an aircraft (102), in order to authenticate transmitted messages, thereby detecting spoofing. Messages transmitted from a GPS ground station (120) on a datalink (128) include an almanac message (930), providing data useful for locating other GPS ground stations (120), and a NOTAM message (940), providing satellite (108) status information. The airborne datalink receiver (116) includes an autonomous scanning mode that scans available datalink frequencies and time slots, searching for datalink signals (128). One or more optional pseudolite stations (130) employing a spread spectrum code sequence improve the accuracy and reliability of the system. The inclusion of an ILS glideslope frequency (952) in the datalink allows an airborne receiver to operate in a hybrid mode, combining a differential GPS determination of horizontal position with an ILS determination of altitude.

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First Claim