The Local Area Augmentation System (LAAS) is an all-weather aircraft landing system based on real-time differential correction of the GPS signal. Local reference receivers send data to a central location at the airport. This data is used to formulate a correction message, which is then transmitted to users via a VHF data link. A receiver on an aircraft uses this information to correct GPS signals, which then provides a standard ILS-style display to use while flying a precision approach. The International Civil Aviation Organization (ICAO) calls this type of system a Ground Based Augmentation System (GBAS).
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The Local Area Augmentation System (LAAS) is an all-weather aircraft landing system based on real-time differential correction of the GPS signal. Local reference receivers send data to a central location at the airport. This data is used to formulate a correction message, which is then transmitted to users via a VHF data link. A receiver on an aircraft uses this information to correct GPS signals, which then provides a standard ILS-style display to use while flying a precision approach. The International Civil Aviation Organization (ICAO) calls this type of system a Ground Based Augmentation System (GBAS).
History
LAAS is designed to correct some of the aviation-related problems of GPS. One problem is the lack of a real-time, rapid-response monitoring system. ILS systems have built-in monitoring equipment that will shut the system down if the monitor detects problems. Category I equipment will normally alert the user of the problem within six to ten seconds of an alarm. GPS has no such rapid-warning system. For example, if a satellite develops a clock problem, there is no way to rapidly warn the user not to use that satellite. WAAS, LAAS and other differential solutions fix this problem and provide GPS system integrity. Another problem is positional accuracy. Sources of error such as satellite clock drift or ionospheric delay can introduce several meters of error in an aircraft's position. These errors must be corrected in real time for a precision approach where there is little or no visibility.
Ten Category I-capable LAAS systems were ordered from Honeywell in 2003. According to the Federal Aviation Administration (FAA) fact sheet, four systems will be for testing and evaluation while the other six will be used at major airports in the US to duplicate existing approaches.ref: FAA_FS
Operation
Local reference receivers are located around an airport at precisely surveyed locations. The signal received from the GPS constellation is compared to the surveyed location and sent to a central location at an airport. This data is used to formulate a correction message which is transmitted to users via a VHF data link. A receiver on the aircraft uses this information to correct the GPS signals it receives. This information is used to create an ILS-type display for landing purposes.
LAAS is designed exclusively for aircraft and is only intended for use within 20 to 30 miles of its installed airfield location. Because LAAS is primarily for aircraft, there are no 'consumer-grade' LAAS-capable GPS receivers.
Accuracy
As of 2001, LAAS was capable of achieving a Category I ILS accuracy of 16 m laterally and 4 m vertically. The goal of the LAAS program is to provide Category III ILS capability. The FAA has not specified the required minimum accuracy for lateral and vertical error of a Cat. III system. However, a Category III approach allows aircraft to land with zero visibility utilizing 'autoland' systems and indicates a very high accuracy of < 1 m.ref: FRP2001
