Local Area Augmentation System

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LAAS Architecture

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).

1 History
2 Operation
3 Accuracy
4 Benefits
5 Drawbacks
6 Variations
7 Future
8 See also
- 8.1 References
- 8.2 External links

[edit] 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.^[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 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.^[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

As of 2005, LAAS has a Category I precision approach capability. This is similar to the WAAS program, but since it covers most of North America, WAAS is far more cost effective. LAAS only covers a 20 to 30 mile area surrounding a single airport.

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. Some existing navaids will need to be turned off because of congestion in the band. Additionally, before LAAS is fully implemented, users may be required to have multiple sets of radio equipment to support all possible situations.

Another drawback of LAAS, is the potential for a single point of failure. If the GPS system is interfered with it could result in serious problems if there is no backup method to land at the airport. Interference could be certain weather conditions, solar activity or malicious jamming of the GPS signal. It is possible that the FAA or local airports will keep existing ILS equipment to provide a backup in the event that the LAAS system should fail or if GPS is jammed. Requiring backup navigational equipment would seem to negate the justification of cost savings since redundant radios on the aircraft would cost users more than the current system. This also makes frequency management difficult because LAAS shares frequency space with its backup.

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 chose 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 minima.