The Local Area Augmentation System (LAAS), now more commonly known as the Ground Based Augmentation System (GBAS), is an all-weather aircraft landing system based on real-time differential correction of the GPS signal. Local reference receivers located around the airport send measurements to a nearby processing unit, which use these measurements to formulate differential corrections for the GPS satellites being tracked by the reference receivers. Satellite and receiver measurements are monitored for potential faults, and measurements with detected faults are removed from the differential corrections. In addition, as with SBAS, standard deviations of the errors remaining after differential corrections are applied are included with the differential corrections.
GBAS transmits these differential corrections, error bounds, and other approach guidance information to nearby user aircraft via a VHF Data Broadcast (VDB) that uses the existing ILS Localizer frequency band (108 – 118 MHz). VHF and GPS receivers on aircraft use this information to correct errors in their GPS measurements, remove untrustworthy satellites, and compute protection levels that bound remaining user errors at the safety probabilities required while flying precision approaches.
Stanford’s research in this area started in early 1990’s, when Stanford developed and demonstrated a unique approach to carrier-phase differential GPS and RAIM-based integrity. After many years of research, development, and testing, an operational system based upon carrier-smoothed pseudorange differential GPS with integrity provided by ground monitoring was developed and tested by the FAA (including Stanford and other researchers), Honeywell, and other companies.
Currently, LAAS / GBAS has been certified for Category I precision approaches and has been fielded at airports in the U.S., Europe, Asia, and Australia (see http://flygls.net). The focus of research and development is now on GBAS systems that can support Category II/III precision approaches and landings. The first of these systems is expected to be operational around 2020.
For more information, see the LAAS Wikipedia article.