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Transponder landing system: next-generation precision approach aid. (Air Traffic Management).

The traditional Instrument Landing System (ILS) was developed back in the 1930s and hasn't changed much since then. Even today, every IFR equipped aircraft has an ILS receiver to display localizer and glide slope information when the pilot flies an approach.

The ILS technology must abide by some difficult constraints to ensure that it functions correctly. In order for an airport to install an ILS, it is often necessary to purchase and grade substantial real estate. And, if there is a hill, river or ditch off the approach end of the runway, an ILS just will not work. There have been many advances in navigation aids other than ILS. The Global Positioning System (GPS) is being adapted to replace all ground-based equipment, however accuracy and availability problems have plagued the program, rendering the need for ground-based augmentation. In addition to the satellite and ground-based equipment costs, the aircraft must be equipped with new avionics to take advantage of this new system. Currently, any GPS approach aid only provides localizer guidance, with no glide slope information available.

A new landing system, called the Transponder Landing System (TLS), provides a cost-effective alternative. The TLS was developed by Advanced Navigation and Positioning Corporation (ANPC) in Hood River, OR and provides Category I approach guidance. ANPC is a privately-held company innovating new technologies with application to commercial, general and military aviation. The TLS technology provides a highly accurate tracking capability. The system interrogates the aircraft's transponder and is able to locate its position from its reply. Figure 1 explains how the TLS functions.


The TLS provides many advantages over any other landing system. First and foremost, no new avionics are necessary for the aircraft. The TLS utilises the installed-base of ILS receivers and displays.

Secondly, terrain that would have prohibited an ILS installation poses no problem for the TLS. Sophisticated site calibration enables the TLS to be tolerant to difficult terrain that may surround an airfield, as seen at Hailey, ID, and Aspen, CO. Thirdly, the system is extremely flexible in siting criteria, which benefits those airfields with limited airport property like St George, UT, and Leesburg, VA. System components are all contained within a 350 square feet (106 sq metres) footprint that can span the runway itself. In some cases, the fact that no new land needs to be acquired amounts to substantial savings in installation costs when compared to an ILS.

Essentially, the Transponder Landing System provides a Category I ILS approach to airfields that previously could not have one due to terrain or land acquisition issues. To the pilot and the airplane, there is no difference between ILS and TLS needles, which also means the autopilot handles TLS guidance without a hitch. Safety, of course, is a major consideration for any approach aid. ANPC has developed the TLS as a [D.sup.2]-Architecture[TM], meaning Dual Dissimilar Architecture. The system software runs in parallel on two different types of processors running two different types of operating systems. The solutions are then compared to ensure fail-safe guidance to the approaching aircraft.

ANPC has also developed a Transportable Transponder Landing System (TTLS), the electronics suite of which is identical to TLS, for military customers. This system is entirely contained within one HMMWV-mounted shelter and generator trailer, both of which can be driven onto one C-130 aircraft for transport. Upon deployment, the system can be set-up, calibrated and flight checked within 6 hours. Operators will be able to capitalise on all of the TLS advantages in a mobile configuration.

Future growth potential is inherent in the TLS technology. Installed system augmentation will easily be accomplished, as new options become available and are approved by the Federal Aviation Administration. Expansion of the TLS capabilities is planned to include:

Non-linear Approaches -- Curved and segmented approaches will be available to address terrain and noise abatement concerns.

Auto-Acquisition -- A pilot will be able to use the TLS system without the assistance of a ground-based operator.

Remote Maintenance Monitor -- ANPC will be able to monitor any installation's status at any time from the company headquarters.

Localizer-Only Guidance -- If the glideslope sensor path degrades to an unacceptable level, the TLS will be able to provide non-precision localizer guidance to the approaching aircraft.

Missed Approach Guidance -- The TLS will be able to deliver localizer guidance to an aircraft executing a missed approach. This will lower approach minimums at those TLS sites where an obstacle in the missed approach sector dictates decision height. This also enables the system to deliver departure guidance. The Federal Aviation Administration has awarded the TLS technology a certification of Category I accuracy. Type Acceptance, and then imminent integration into the National Airspace System, will be granted in the early summer of 2001.

ANPC Talking Points

* The TLS is tolerant to terrain that would otherwise preclude an ILS installation. A powerful calibration procedure enables the system to bear everything from uneven to mountainous topography.

* No new avionics are needed in the aircraft. The TLS adds value to the legacy installed-base of ILS. To the pilot, there is no difference in flying the TLS versus an ILS.

* All of the TLS components are located in a small footprint on the airport property. Nominally, a 350ft diameter area is sufficient for all TLS hardware.

* The system's configuration is extremely flexible and is easily customised to each site's requirements. Components can be placed entirely on one side of a runway or can sit astride the runway. The TLS system software makes this unique configuration variation possible.
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Author:Botts, Renee Harwood
Publication:Airports International
Date:Oct 1, 2001
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