|TLS electronics shelter, elevation sensor and uplink antennas|
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. TLS can also produce marker beacon-like audio to indicate distance at appropriate times during the approach. All the pilot has to do is tune in the TLS system as if it were an ILS.
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.
Radio-navigation aids must keep a certain degree of accuracy (given by international standards, FAA, ICAO...); to assure this is the case, Flight Inspection organizations check periodically critical parameters with properly equipped aircraft to calibrate and certify TLS precision.
BenefitsOne of the primary benefits of TLS is the ability to function using equipment that is currently widely used by the aviation industry. TLS uses the existing transponder equipment to determine the aircraft's position. It then transmits the correct signal on the same frequencies used for the current ILS system. All the aircraft is required to do is set the transponder to the assigned code and tune an ILS receiver to the appropriate frequency. As of 2005, modern GPS systems, such as Wide Area Augmentation System (WAAS) and Local Area Augmentation System (LAAS), have not gained wide acceptance and there is not a large user base. TLS uses equipment most airplanes already have.
For military users, TLS also provides a Precision Approach Radar (PAR) graphic display of aircraft position compared to the desired approach course in order for a PAR operator to provide talk-down guidance to the pilot.
DrawbacksSince the system locks on to a single aircraft's transponder then transmits a signal that is specific to that aircraft's location, only one aircraft at a time can fly the landing approach. Any other aircraft in the area will receive the same guidance regardless of their location relative to the approach.
TLS systems are privately operated and maintained. However, unlike standard ILS equipment, the US Federal Aviation Administration (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.