Passive landing aids for precision EVS approach and landing

ICAO forecasts a growth in world air travel of 5% per annum until 2005. Based on recent experience in Europe, this appears likely to be a conservative estimate. As traffic grows steadily, airport congestion is evident and becomes the limiting factor at some airports. The problem becomes even more serious under low visibility weather conditions, because the nominal airport capacity has to be reduced further due to safety concerns. Consequently, there is a need to close the gap between capacity under low visibility conditions and good visibility conditions. Enhanced Vision Systems can help to close this gap. The use of weather penetrating forward looking sensors allows the pilot of an aircraft to obtain the necessary external references to continue approach procedures maintaining VMC separation minima even under low visibility conditions. Consequently, the decrease of capacity under low visibility due to safety reasons can be reduced or even be avoided. Up to now, radar systems seem to be the most promising sensors to be applied for Enhanced Vision Systems. But radar data are often difficult to interpret, so it\´s not always possible to obtain a reliable detection of runway structures within the radar images. Therefore, passive radar retro-reflectors can be installed along the runway to give unique "visual" cues similar to the runway and approach lights within the visual spectral range. This article deals with the use of retro-reflectors to mark the runway. Further more, configurations of different so-called "diplane-reflectors" are described which act as landing aids similar to the optical PAPI or VASI lights. Both, analytical accuracy assessments and results from flight trials with the MMW-HiVision radar of EADS Germany are presented.