area safety system does not provide controllers with identification of targets.

The aircraft target identification system (ATIDS), under study, would use triangulation and automatic dependent surveillance—rather than airport surface detection equipment radar—to pinpoint the location of aircraft on the ground. The airport target identification system would also use the secondary surveillance radar signal (mode A/C/S) to identify aircraft and to provide to controllers call sign information in association with ASDE-3 and AMASS-like target displays (Castaldo et al., 1996; Smith et al., 1996). An aircraft target identification system prototype has been undergoing tests at the Atlanta airport.

Automated aids under consideration include the capability to present displays to pilots that indicate both the situation information provided to controllers and controller instructions. The runway status lights (RWSL) system is a radar-based safety system intended to improve on-airfield situation awareness by providing pilots a visual advisory of runway status. The system consists of a series of "stop/go" lights on the airport surface that indicate to pilots whether it is safe or unsafe to enter or cross a runway or to begin or hold takeoff. The runway status lights system, whose prototype has been undergoing field tests, controls the lights according to target position, status, and logic information derived from the airport movement area safety system. The runway status lights are intended as an independent backup to controllers' assessments and instructions; procedures dictate that, when pilots identify a discrepancy between the lights and controller instructions, pilots must resolve the discrepancy with controllers before proceeding (Architecture Technology Corporation, 1996; Federal Aviation Administration, 1996d).

The airport surface traffic automation (ASTA) system would extend the mode S surface surveillance system to provide a two-way data link between the tower and the cockpit. The system may introduce into the cockpit an electronic moving map display that provides to pilots the target position, identification, and movement information presented to controllers, as well as the airport movement area safety system and controller instructions and alerts (Architecture Technology Corporation, 1996; Jones and Young, 1996; National Aeronautics and Space Administration, 1995). The airport surface traffic automation system is currently undergoing in-field prototype testing.

Enhancements to cockpit displays, integrated with the airport surface traffic automation system, may also permit improvements in the efficiency of taxiway navigation. The taxi navigation and situation awareness (T-NASA) system, under development for NASA's terminal area productivity (TAP) low-visibility landing and surface operations (LVLAS) program, is intended to support the goals of increased nonvisual ground capacity, runway occupancy time for instrument flight rules equivalent to that for clear weather, and maintenance of safety (National Aeronautics and Space Administration, 1997a). The T-NASA system includes three features intended to provide situation awareness to pilots during



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