A transponder landing system (TLS) is an all-weather, precision landing system that uses existing airborne transponder and instrument landing system (ILS) equipment to create a precision approach at a location where an ILS would normally not be available. Description Conventional ILS systems broadcast using a number of "single purpose" antennas. The localizer array consists of 8 to 14 or more antenna elements and is typically located 2000+ feet from the runway approach threshold, which is often beyond the departure end of the runway and provides a fan-shaped signal for azimuth direction (side to side). The other antenna array is located beside the runway and provides elevation to indicate a standard glideslope. This complex set of antennas is expensive to install and maintain and is often difficult to site in areas with uneven terrain or obstacles that could interfere with its guidance signals. Function The TLS facility interrogates the transponders of all aircraft within . After receiving a response, TLS determines the aircraft's location using three sets of antenna arrays: one for horizontal position using monopulse techniques, the other for vertical monopulse and a third for trilateration. TLS then calculates the position of all aircraft using the transponder replies and converts that into real-time guidance for an aircraft cleared for approach based on its position relative to the desired approach path. The position tracks for all aircraft in the TLS service volume are displayed on an ATC monitor similar to a secondary surveillance radar but with an increased update rate and higher accuracy. For an aircraft conducting an instrument approach, the TLS will transmit horizontal and vertical guidance signals to the ILS receiver and cockpit avionics that are identical to those of an ILS. The signals will thus appear to emanate from fixed locations where ILS antennas are typically installed. However, TLS guidance is not a function of antenna location - the TLS can provide guidance from "virtual emanation points" that may be anywhere as required by an instrument approach procedure but are configured in the TLS software. This allows the TLS to support multiple approach procedures at a given airport, including steeper glide slope angles for rotary-wing aircraft or increased obstacle clearance, with a single complete system installed adjacent to runway threshold. The TLS will also produce marker beacon-like audio to indicate distance from the runway, but without the requirement for physical equipment. All the pilot has to do is