Local Area Augmentation System
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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).
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[edit] History
The Local Area Augmentation System (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.[1]
[edit] 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.
[edit] 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 Category. III system. However, a Category III approach allows aircraft to land with zero visibility utilizing 'autoland' systems and indicates a very high accuracy of better than 1 metre.[2]
[edit] Benefits
One of the primary benefits of LAAS is that a single installation at a major airport can be used for multiple precision approaches within the local area. For example, if Chicago O'Hare has 12 runway ends each with a separate ILS, all 12 ILS facilities can be replaced with a single LAAS system. This represents a significant cost savings in maintenance and upkeep of the existing ILS equipment.
Another benefit is the potential for approaches that are not straight in. A GPS with LAAS capability can guide an aircraft on any approach necessary to avoid obstacles or to decrease noise levels in areas surrounding an airport.
The FAA also contends that only a single set of navigational equipment will be needed on an aircraft for both LAAS and WAAS capability. This lowers initial cost and maintenance per aircraft since only one receiver is required instead of multiple receivers for NDB's, DME, VOR, ILS, MLS and GPS. The FAA hopes this will result in decreased cost to the airlines and passengers as well as general aviation.
[edit] Drawbacks
LAAS covers a 20 to 30 mile area surrounding a single airport in support of precision approach and up to 200 nautical miles for positioning service (line of sight). LAAS mitigates GPS threats in the Local Area to a much greater accuracy than WAAS and therefore provides a higher level of service not attainable by WAAS.
LAAS's VHF uplink signal is currently slated to share the frequency band from 108 MHz to 118 MHz with existing ILS localizer and VOR navigational aids. LAAS utilizes a Time Division Multiplexed data link as well as services the entire airport with a single frequency allocation. With future replacement of ILS, LAAS will reduce the congested VHF NAV band.
Another drawback of LAAS is the potential for a single point of failure if the system is not designed properly. Therefore, the LAAS equipment designers and service providers consider these scenarios and provide the necessary mitigation. LAAS shares in some of the same threats as all RF based landing systems; that being unintentional jamming, signal derogation due to multipath causing loss of accuracy or signal fading. GPS spoofing is difficult since the signal structure and time base are very tightly coordinated. The VHF data link spoofing is more difficult and requires more system coordination than ILS or TLS and requires.
In order to mitigate these problems, the resulting national system will likely have LAAS capability at major airports, WAAS capability for the rest of North America with a limited amount of conventional navaids as a national backup.
[edit] Variations
The Joint Precision Approach and Landing System (JPALS) is a similar system for military usage.
[edit] Future
It is likely that the FAA's goal for LAAS is to replace the existing ILS equipment for all categories of precision approaches. Due to the similarity between JPALS and LAAS, the FAA may choose to adopt JPALS instead of LAAS. The Fault tolerance issues have pushed the program back into R&D and forced the FAA to extend ILS support through at least 2030 for all weather operations down to CAT III minimums.
[edit] See also
- Differential GPS
- Global Positioning System
- GPS and Geo Augmented Navigation
- Instrument Landing System: ILS
- Joint Precision Approach and Landing System is a similar system for military usage.
- Precision approach
- WAAS
- EGNOS
- Autoland
[edit] References
- ^ Federal Aviation Administration (FAA) (February 27, 2004). "Local Area Augmentation System (LAAS) Fact Sheet". http://gps.faa.gov/Library/laas-f-text.htm. Retrieved November 20 2005.
- ^ Department of Transportation and Department of Defense (March 25, 2002). "2001 Federal Radionavigation Plan" (PDF). http://www.navcen.uscg.gov/pubs/frp2001/FRP2001.pdf. Retrieved November 27 2005.
- IHS Aerospace - Honeywell (July 7, 2005). Honeywell to Update Local Area Augmentation System (LAAS) Prototype. Press Release.
[edit] External links
- FAA GPS Programs
- FAA LAAS Fact Sheet
- Stanford LAAS Page
- Honeywell Aerospace - DGPS - Satellite Landing System
- Thales ATM Satellite Navigation Systems
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