The underutilization of highspeed runway turnoffs in low visibility conditions was also due in part to the fact that pilots lacked the necessary guidance to help them find the exits and taxi safely off the runway. In an effort to overcome this obstacle, NASA researchers began investigating the possibility of using a buried magnetic cable to provide rollout, turnoff and taxi guidance for airplanes once they landed.

A magnetic cable system consisted of an electrical cable buried in the center of the runway, turnoffs and taxiways of an airport. An audiofrequency current was sent through the cable, setting up a magnetic field. By measuring the strength of the field, onboard airplane sensors could indicate how far away from the cable the airplane was, and the aircraft autopilot could use that information to follow the cable along the runway, turnoffs, and taxiways.

The idea of using a magnetic cable for aircraft guidance was first tested by the British in the 1950s and 1960s as a way to allow landing operations in extremely low visibility conditions. The British wanted to use the cable to provide guidance during an airplane's final approach to the airport as well as steering guidance on the ground, however, which proved to be beyond the capabilities of the technology. [Ref 7-36] Although the British never implemented the cable guidance system, researchers working with the TCV program in the late 1970s resurrected the idea as a way to improve the efficiency of ground operations at airports in poor weather conditions.

A couple of simulator studies evaluating the feasibility of a cable system for airplane ground navigation and guidance were conducted in the late 1970s and early 1980s at the Langley Research Center. The studies indicated that the idea might be both feasible and practical for commercial aircraft. At the same time, other researchers at Langley were working on possible designs for the cable system itself. [Ref 7-37]

The system was initially tested using a temporary cable and a vanmounted sensor at the Wallops Island Flight Facility in a series of experiments from 19791984. Then in 1988, both static and taxi tests were conducted with the cable system and the TSRV 737 airplane at Wallops Island to gather additional information on the performance of different signal gains and frequencies. Researchers were concerned that the large amounts of metal in an airplane might interfere with the signals, but the tests indicated that although the metal interfered with the system's ability to give accurate heading information, it could still reliably indicate the airplane's distance from the cable. There were some electromagnetic interference (EMI) problems caused by the airplane's power system and VHF radio communications, but it appeared that use of filtering techniques could reduce the EMI to an acceptable level. [Ref 7-38]

Using the data gathered in the 1988 tests, researchers finetuned the cable system and developed the necessary control laws to allow the 737's autopilot to process and use the magnetic cable signals. The entire system was then evaluated in highspeed taxi tests at Wallops Island in 1991. The tests used a combination of the Wallops Island microwave landing system/distance measuring equipment (MLS/DME) and the magnetic leader cable to provide the guidance the TSRV 737's autopilot. The results of the tests indicated that the cable system could work, but it needed further development to smooth its operation to an acceptable level.

Additional research and tests with the magnetic leader cable system were put on hold at that point, however, because another potential solution to the same problem had appeared on the technology horizon. As research progressed with the satellitebased global positioning system (GPS), it appeared that the system might be able to provide guidance on the ground as well as in the air. In the spring of 1991, the TSRV 737 began ground tests using GPS to guide the airplane through its rollout and turnoff from the runway. Obviously, if the same GPS receiver that airliners were using for airborne navigation could also be used for ground guidance in low visibility conditions, it would save the cost of installing and maintaining the extra ground and airborne equipment the magnetic cable system would require. [Ref 7-39]

The research goal remained the same, however, even if the specific technology being evaluated had changed. In order to significantly reduce the amount of spacing between arriving aircraft, ground operations had to be made more efficient, especially in poor weather. The magnetic cable leader system and GPS navigation both held potential for providing better ground navigation guidance to pilots, and the TSRV 737 made it possible for the NASA researchers to evaluate their performance not just in theoretical or simulated conditions, but with a transport class airplane in a realistic airport environment.