Publications

Many airports across the United States will soon be equipped with Mode S, a next generation beacon (or secondary) radar system. One feature of Mode S is that it provides a data link between airborne aircraft and air traffic controllers. If Mode S could be used to communicate with aircraft on the airport surface, the radar system would improve airport safety and efficiency on runways and taxiways. The airport surface, however, is a hostile propagation environment. This article outlines a candidate design for the propagation of Mode-S beacon signals on the airport surface. Data that support the feasibility of Mode S for surveilling runways and taxiways are presented.

The Traffic Alert and Collision Avoidance System (TCAS) has been developed to reduce mid air collisions between transponder equipped aircraft. The TCAS concept encompasses a range of capabilities. TCAS I is a low-cost version which provides traffic advisories only. TCAS II adds vertical resolution advisories and is intended to provide a comprehensive level of separation assurance in all current and predicted airspace environments through the end of this century. Enhanced TCAS II uses more accurate intruder bearing data to allow it to generate horizontal resolution advisories. All three forms of TCAS equipment track aircraft equipped with both the existing Air Traffic Control Radar Beacon System (ATCRBS) transponders and with the new Mode S transponders. A TCAS equipped aircraft makes ATCRBS or Mode S range measurements on nearby aircraft. The ATCRBS or Mode S replies contain the altitude of the aircraft if it has an encoding altimeter. TCAS II uses range rate and altitude rate to decide if a collision is imminent. Therefore the replies from a given aircraft must be tracked and correlated in range and altitude. This report documents surveillance techniques developed by Lincoln Laboratory for use by TCAS II equipment in tracking aircraft equipped with ATCRBS transponders. Specifically, it describes the two tracking algorithms used for ATCRBS replies. One algorithm is for aircraft that report altitude, and the other is for those that do not.

Lincoln Laboratory, under FAA sponsorship, is developing an Active Beacon Collision Avoidance System (BCAS), concentrating primarily on the air-to-air surveillance subsystem. The surveillance functions required are to detect the presence of nearby aircraft (whether they are equipped with ATCRBS transponders or DABS transponders), and then generate a surveillance track on each aircraft, issuing range and altitude reports once per second. The development effort consisted of airborne measurements complemented by simulation studies and analyses. The basic effects of ground-bounce multipath, interference, and power fading were assessed by air-to-air measurements. In other measurements, the BCAS interrogation and reply signal formats were transmitted between aircraft, and the results recorded for later playback and computer processing using the BCAS surveillance algorithms. This is a flexible means of experimentation which allows many of the design parameters to be changed as the effects are noted. In the most recent phase of the program, Lincoln designed and built realtime BCAS Experimental Units (BE Us), flight tested them, and then delivered them to the FAA for more extensive flight testing. In one of these flight tests, a BEU-equipped Boeing 727 flew to New York, Atlanta, and other major terminal areas in the eastern U.S. An analysis of BEU performance during this "Eastern Tour" is given in this report.

Improved azimuthal resolution of proximate aircraft necessary to support ATC automation can be achieved by beacon surveillance systems employing monopulse angle estimation techniques described in this report. Included in the report are the results of beacon surveillance monopulse system analyses relating to off-boresight angle estimation using short (1/2 micro sec) pulses: the effects of specular and diffuse multipath signal return; the effects of overlapping ATCRBS fruit replies, and the problems of antenna pattern design. These topics have been studied in detail as part of the Lincoln Laboratory disign of the Discrete Address Beacon System (DABS). This report summarizes analytical results obtained. In general, it has been concluded that the ATC environment does not pose a serious problem to the use of the monopulse concept for beacon system direction finding and that sufficient direction finding accuracy can be obtained using a small number of narrow pulses for each scan.