Talk:Local-area augmentation system

Status of LAAS

At this point it would be useful to cover exactly why the LAAS program is not going anywhere.

Content to be integrated

The follow is from the NAS Description of LAAS CAT I

• The Local Area Augmentation System Category I (LAAS CAT I) is a safety-critical precision navigation and landing system that augments Global Positioning System (GPS) range data to provide aircraft position accuracy necessary for CAT I precision approaches; i.e., 200 foot decision height and one-half mile visibility.
• The LAAS signal-in-space will provide: (1) local area differential corrections for GPS satellites and Wide Area Augmentation System (WAAS) Geostationary Earth Orbit (GEO) satellites; (2) the associated integrity parameters; and (3) the path points that describe the final approach segment.
• In FY-04 LAAS reverted from development to research & development (R&D) for resolution of some integrity issues. As of April 2006 the FAA, Honeywell and the LIP (LAAS Integrity Panel) are making progress on LAAS integrity issues. By the fall of 2006 the FAA and FedEx aircraft plan to fly tests to validate the technical and operational performance of the LAAS prototype installed in Memphis, Tennessee. Following those tests, the processing architecture will be upgraded and a complete set of prototype software functions to host all International Civil Aviation Organization (ICAO) SARPs (Standards And Recommended Practices) Category-I functions will be integrated at Memphis and also at a second new LAAS facility at the FAAs William J. Hughes Technical Center in Atlantic City, New Jersey. This is scheduled to be accomplished by December 2007.

Regarding CAT II/III

• LAAS consists of a precisely surveyed ground station with multiple Global Positioning System (GPS) receivers, a very high frequency (VHF) radio data broadcast (VDB), and possibly one or more pseudolites to increase availability. The LAAS ground station will receive, process, and communicate differential correction information, together with an integrity message, to aircraft avionics within a nominal radius of 20 to 30 nautical miles from the airport.
• The CAT II/III Local Area Augmentation System (LAAS) will provide guidance that meets the accuracy, integrity and availabililty requirements for CAT II and III precision approaches. The Wide Area Augmentation System (WAAS) and LAAS together will provide a seamless satellite-based navigation capability for all phases of flight.
• CAT II/III LAAS is an ongoing R&D effort which, if successful, is envisioned to lead to a follow-on development and procurement program. CAT II/III LAAS installations might ultimately complement or replace the CAT II/III Instrument Landing Systems (ILS) that are currently in the NAS.
• Pseudolites are ground-based transmitters that broadcast GPS-like signals. Although not currently envisioned as part of the LAAS architecture, pseudolites may be required to ensure that LAAS meets CAT II/III requirements. Peudolites can be used as a data link to transmit differential corrections and integrity status to aircraft avionics and as a supplementary ranging source. When used as ranging sources, pseudolites can improve system accuracy by improving the local constellation geometry and system availability.

Regarding accuracy, the Stanford LAAS page has:

• LAAS, or the Local Area Augmentation System, is the FAA version of the of the Ground Based Augmentation System, or GBAS, that has been defined by the International Civil Aviation Organization (ICAO).
• LAAS is based on a single GPS reference station facility located on the property of the airport being serviced. This facility has three or more (redundant) reference receivers that independently measure GPS satellite pseudorange and carrier phase and generate differential carrier-smoothed-code corrections that are eventually broadcast to user via a 31.5-kbps VHF data broadcast (in the 108 - 118 MHz band) that also includes safety and approach-geometry information. This information allows users within 45 km of the LAAS ground station to perform GPS-based position fixes with 0.5-meter (95%) accuracy and to perform all civil flight operations up to non-precision approach. Aircraft landing at a LAAS-equipped airport will be able to perform precision approach operations up to at least Category I weather minima.
• ...the largest challenge in designing and fielding LAAS is the need to verify aircraft safety (in terms of not exceeding a safe error bound known as the alert limit) to a probability of two in ten million (2e-7) per approach for Category I and one in one billion (1e-9) per approach for Category III. In addition, the probability that approaches must be aborted due to detected failures or false alarms must be below one in one hundred thousand (1e-5) per 15 seconds. The LAAS Ground Facility, or LGF, meets these requirements by detecting and excluding anomalous reference receiver measurements before differential corrections are broadcast. The corrections that are broadcast come with bounding standard-deviation values ("sigmas") on errors in the corrections that allow users to compute position error bounds (known as "protection levels") in real time and to compare them to the alert limits for their current operation to verify that the operation remains safe to conduct.

Davandron | Talk 22:03, 5 December 2006 (UTC)