The present invention relates to satellite navigation systems. More particularly, the present invention relates to satellite navigation receivers and systems adapted for use in automatic landing of aircraft.
Global navigational satellite systems (GNSS) are known and include the global positioning system (GPS) and the Russian global orbiting navigational satellite system (GLONASS). GNSS-based navigational systems are used for navigation and positioning applications. In the GPS navigational system, GPS receivers receive satellite positioning signals from a set of up to 32 satellites deployed in 12-hour orbits about earth and dispersed in six orbital planes at an altitude of 10,900 nautical miles. Each GPS satellite continuously transmits two spread spectrum, L-band signals: an L1 signal having a frequency fL1 of 1575.42 MHz, and an L2 signal having a frequency fL2 of 1227.6 MHz. The L1 signal from each satellite is modulated by two pseudo-random codes, the coarse acquisition (C/A) code and the P-code. The P-code is normally encrypted, with the encrypted version of the P-code referred to as the Y-code. The L2 signal from each satellite is modulated by the Y-code. The C/A code is available for non-military uses, while the P-code (Y-code) is reserved for military uses.
GPS navigational systems determine positions by timing how long it takes the coded radio GPS signal to reach the receiver from a particular satellite (e.g., the travel time). The receiver generates a set of codes identical to those codes (e.g., the Y-code or the C/A-code) transmitted by the satellites. To calculate the travel time, the receiver determines how far it has to shift its own codes to match the codes transmitted by the satellites. The determined travel times for each satellite are multiplied by the speed of light to determine the distances from the satellites to the receiver. By receiving GPS signals from four or more satellites, a receiver unit can accurately determine its position in three dimensions (e.g., longitude, latitude, and altitude). A conventional GPS receiver typically utilizes the fourth satellite to accommodate a timing offset between the clocks in the receiver and the clocks in the satellites. Additional satellite measurements can be used to improve the position solution.
Modern aircraft are equipped with guidance equipment to enable automatic landings in low-visibility conditions. The equipment, which generates deviations from a defined path based on signals received from terrestrial radio-navigation aids, must be designed to ensure that it is extremely improbable that it provides misleading guidance data (i.e., the equipment must have high integrity). Stringent integrity requirements can be met with monitoring of receiver performance. These monitoring functions must detect faults that could cause misleading information while providing a low false alarm rate (i.e., high continuity).
The typical accuracy of satellite navigation systems is comparable to terrestrial radio-navigation systems. However, current satellite navigation receivers do not meet the stringent integrity and continuity requirements necessary for incorporation into aircraft automatic landing systems.
A satellite navigational receiver landing system having retrofit compatibility with integrated landing system (ILS) receivers, in accordance with the invention, includes radio frequency circuitry which converts satellite signals into intermediate signals. A first processing channel coupled to the radio frequency circuitry generates a first position, velocity and time (PVT) solution as a function of the intermediate signals, and provides a PVT and guidance data output. A second processing channel coupled to the radio frequency circuitry generates a second PVT and guidance solution as a function of the intermediate signals. Monitor circuitry provides monitor output signals as a function of the first and second PVT and guidance solutions. Shutdown circuitry provides the first PVT and guidance solution to the PVT and guidance data output under the control of a shutdown signal from the monitor circuitry. Monitoring functions ensure PVT and guidance accuracy is maintained within specified limits.