Advanced Navigation and Positioning Corporation (ANPC) based in Hood River, OR has developed a landing system that uses transponder signals to track and guide aircraft on final approach.
FAA has qualified ANPC's Transponder Landing System (TLS) to meet or exceed ICAO standards for CAT I precision approaches.
The TLS locates and tracks an aircraft using the transponder's reply to 2 ground-based sensors based on its azimuth, elevation angle and range. The system's guidance is given to the aircraft by emulating standard ILS signals. TLS required no additional equipment to benefit from the system's guidance feature.
Professional Pilot, Nov 2001