The Transponder Landing System (TLS) is an all-weather, precision landing system that uses existing airborne transponder and ILS equipment to create a precision approach at a location where an ILS would normally not be available.
Conventional ILS systems broadcast using a number of "single purpose" antennas. One, located just off the end of the runway, provides a fan-shaped signal for azimuth direction (side to side) and another located beside the runway provides elevation to indicate a standard glideslope. ILS installations also include one or more "marker beacons" located off the end of the runway to provide distance indications as the aircraft approaches the runway. This complex set of antennas is expensive to install and maintain, and are often difficult to site in built-up areas.
The TLS facility interrogates the transponder of the aircraft. After receiving a response, it determines the aircraft's location using two sets of direction finding antenna arrays: one for horizontal position, the other for vertical. It then calculates the signal that the aircraft would "see" if they were located at that location and approaching a conventional ILS system, and then broadcasts that signal to the aircraft. The aircraft's ILS receivers receive a signal that is indistinguishable from a normal ILS signal, and displays this information on their glideslope and localizer displays. Certain TLS configurations can also produce marker beacon-like audio to indicate distance at appropriate times during the approach. All the aircraft has to do is tune in the TLS system as if it were an ILS, and tune its transponder to a predetermined channel.
A TLS can be installed in areas where a conventional ILS would not fit or would not function properly, like, for example, an airport that doesn't have a proper reflecting surface for an ILS glideslope because of uneven terrain like steep hills or mountains, or airports that have large buildings like hangars or parking garages that create disruptive reflections that would prevent an ILS localizer from being used. TLS does not even have to be installed at a particular location relative to the runway, but can "offset" its signals from wherever it is installed to appear as if it were at the end of the runway. This makes it much less expensive to install while still providing ILS-class blind-landing approaches. As of 2001, TLS was certified by the FAA for Category I ILS usage.
TLS systems are privately operated and maintained. However, unlike standard ILS equipment, the FAA is not publishing approach procedures for TLS facilities. This leaves the approaches to be determined by the local airport. TLS approach procedures are designated Special Instrument Approach Procedures and require special aircrew training.