Instrument Landing System - Limitations and Alternatives

Limitations and Alternatives

Due to the complexity of ILS localizer and glideslope systems, there are some limitations. Localizer systems are sensitive to obstructions in the signal broadcast area like large buildings or hangars. Glideslope systems are also limited by the terrain in front of the glideslope antennas. If terrain is sloping or uneven, reflections can create an uneven glidepath causing unwanted needle deflections. Additionally, since the ILS signals are pointed in one direction by the positioning of the arrays, ILS only supports straight-in approaches. A modified ILS called an Instrument Guidance System (IGS) is also occasionally used, the most famous example being that which was in use at one of the approach direction (13 approach) of Kai Tak Airport, Hong Kong to accommodate a non-straight approach; IGSes are also called Localizer Type Directional Aids in the US. Installation of ILS can also be costly due to the complexity of the antenna system and siting criteria. To avoid hazardous reflections that would affect the radiated signal, ILS critical areas and ILS sensitive areas are established. Positioning of these critical areas can prevent aircraft from using certain taxiways. This can cause additional delays in take offs due to increased hold times and increased spacing between aircraft.

In the 1980s, there was a major US & European effort to establish the Microwave Landing System (MLS), which is not similarly limited and which allows curved approaches. However, a combination of airline reluctance to invest in MLS, and the rise of Global Positioning System (GPS) has resulted in its failure to be adopted in civil aviation. The Transponder Landing System (TLS) is another alternative to an ILS that can be used where a conventional ILS will not work or is not cost-effective.

Localizer Performance with Vertical guidance (LPV) is the latest alternative to the ILS. Based on the Wide Area Augmentation System (WAAS), LPV has similar minima to ILS for appropriately equipped aircraft. As of November 2008, the FAA has published more LPV approaches than Category I ILS procedures.

Another potential alternative to ILS is the Ground-Based Augmentation System (GBAS), a safety-critical system that augments the GPS Standard Positioning Service (SPS) and provides enhanced levels of service. It supports all phases of approach, landing, departure, and surface operations within the VHF coverage volume. (Local Area Augmentation System is the United States' implementation of GBAS). GBAS is expected to play a key role in modernization and in all-weather operations capability at CATI/II and III airports, terminal area navigation, missed approach guidance and surface operations. GBAS provides the capability to service the entire airport with a single frequency (VHF transmission) whereas ILS requires a separate frequency for each runway end. GBAS CAT-I is seen as a necessary step towards the more stringent operations of CAT-II/III precision approach and landing. Until recently, the technical risk of implementing GBAS prevented widespread acceptance of the technology. The FAA, along with industry, have fielded Provably Safe Prototype GBAS stations which mitigate the impact of satellite signal deformation, ionosphere differential error, ephemeris error and multipath.