V.A. Orlando
The Mode S Beacon Radar System

Air traffic controllers rely on primary and secondary radars to locate and identifY aircraft. Secondary, or beacon, radars require aircraft to carry devices called transpon-ders that enhance surveillance echoes and provide data links. Airports currently use a secondary-radar system known as the Air Traffic Control Radar Beacon System (ATCRBS). However, ATCRBS has limitations in dense-traffic conditions, and the system's air-to-ground data link is limited. In response to these shortcomings, Lincoln Laboratoryhas developed the Mode SelectBeaconSystem(referred to as Mode S), a next-generation system that extensive laboratory and field testing has validated. In addition to significantsurveillance improvements, Mode S provides the general-purpose ground-air-ground data link necessary to support the future automation of air traffic control (ATC). The FederalAviationAdministration (FAA) is currentlyinstalling the system with initial operation scheduled for 1991.
Airports around the world currently use a type of secondary radar known in the United States as the Air Traffic Control Radar Beacon System (ATCRBS) [11. Because this radar sys-tem was developed more than 30 years ago, itis beginning to strain under today's increased levels ofair traffic.To replace ATCRBS, Lincoln Laboratory has developed the Mode Select Bea-con System (referred to as Mode S) [21, which is scheduledforinitial operation atU.S. airports in 1991.
Mode S (a combined beacon radar and ground-air-ground data-link system) is de-signed for the dense traffic environments ex-pectedin the future. To supP9rtthe automation
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ofairtrafficcontrol (ATC),ModeS boaststwoat-tributes: accurate surveillance even in dense traffic conditions. and reliable ground-air-ground communications capability.
Because Mode S is capable ofcommon-chan-nelinteroperationwith ATCRBS, the fonner can be installed over an extended transition period dUring which Mode-S systems will eventually replace ATCRBS ones. In fact, a major design reqUirementindevelopingModeS was toensure that the system could be implemented in an evolutionary manner. By the time Mode-S de-ployment begins in 1991. about 200.000 air-craftand 500ground-basedradarswill beusing ATCRBS. Mode S is designed to operate in this ATCRBS environment in a way that would permit a gradual transition to all-Mode-S operation.
Providing a high degree of compatibility be-tween Mode S and ATCRBS has achieved the capabilityforsuchatransition. ModeSusesthe same interrogation and reply frequencies as ATCRBS, and the signal formats have been cho-sen to pennit substantial commonality in hard-ware. Such compatibility will permit a smooth, economictransitioninwhich Mode-S radarswill provide surveillance of ATCRBS-equipped air-craft and Mode-S transponders will reply to ATCRBS radars.
This article begins with a description of ATCRBS. followed by a discourse on the system's limitations in regions of high traffic and sensor densities. Next, details of Mode S are presented with an emphasis on the im-provements provided by the monopulse direc-tion-finding techniques and the specific fea-tures provided by Mode-S surveillance and the Mode-S integral data link. A descrip-tion follows of the field measurements that were made to validate the Mode-S design. Fi-nally, this article comments on the current status ofMode-S implementation in the United States.
'l11e Lincoln Laboratory Journal, Volume 2, Number 3 (1989)
Air Traffic Control Radar Beacon System
Figure 1 schematically illustrates the opera-tion of the current ATCRBS. The ATCRBS an-tenna, which is typically mounted above the primary-radarantenna,hasa fan-beampattern with a horizontal beam width of 2° to 3°. The scan rate is 4.8 s for a sensor used at an airport terminal, and 10 s to 12 s elsewhere. Civil ATCRBS transponders accepttwo types ofinter-rogations. Mode A has an 8-f.Js p)-to-P3 spacing and elicits a 20.3-f.Js reply that contains one of 4,096 pilot-entered identity codes. Mode C has a 21-,us p)-to-P3 spacing and elicits a similar reply that contains the aircraft's barometric altitude referenced to standard atmospheric conditions. The purpose of the Pz pulse is described in the following section.
　
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