ILS-GLS background information

  • Airports control their landing system.  Today, SFO (like the vast majority of US airports) is using an outdated radar-based Instrument Landing System (ILS) but is planning to implement a GBAS Landing System (GLS), which is GPS-based. GBAS stands for Ground-Based Augmentation System.
  • A GLS system allows precision approaches with much more flexibility. At SFO, GBAS can be used to distribute landing approaches (thus creating dispersion), make better use of the Bay, substantially increase flying altitudes over residential areas, and increase SFO’s overall safety given the level of air traffic. GLS could be implemented relatively quickly (within 2 years or so) and help reduce noise substantially for many Bay Area residents. The current scope of the GBAS project is limited to SFO arrivals only.
  • ILS is a static implementation (e.g. it cannot be reconfigured) that only supports one static low angle approach: planes must descend to a low altitude point far from the airport to capture the radio beams of the ILS system. With ILS, airplanes must approach in a straight direction about 10 to 15 miles from the airport.
  • In contrast, GLS is a dynamic implementation (approaches can be reconfigured) that supports higher approach angles as well as curved approaches. Furthermore, GLS enables approaches to be distributed widely (one GBAS station can support as many as 28 different approaches of any shape).  GLS at SFO could enable the dispersion that residents are demanding and increase the overall safety by providing larger aircraft separations and make better use of the overall airspace.

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