.

Fig. 18-ARIES with the Mode-S engineering-model sensor.
Consequently, capacity testing of the en-gineering-model sensors was accomplished with a traffic simulator known as the Aircraft Reply and Interference Environment Simulator (ARIES) [14-16). ARIES (Fig. 18) is interfaced at the analog level with the front end of a sensor and thus exercises the entire sensor. notjustthe computer subsystem. In operation. ARIES lis-tens to interrogations from the engineering model, and then inserts signals into the front endatthe timethatthetransponderreplywould have been received from the real aircraft. ARIES also correctly simulates the monopulse signals according to the off-boresight angle ofthe simu-lated aircraft. This monopulse simulation is accurate enough to permit operation with a mix of simulated and real aircraft.

ARIES Capacity Testing
A principal objective of developing the engi-neering-modelsensorwas toverifYthat Mode-S-sensor algorithms could achieve the reqUired surveillance and communication capacity for Mode-S operation. A traffic model that repre-sented a future worst-case scenario for the Los Angeles Basin enabled capacity testing of ARIES.
Figure 19 shows a typical display of traffic

The Lincoln Laboratory Journal. Volume 2. Number 3 (J 989;
Orlando -The Mode S Beacon Radar System

that the engineering-model sensor processed during capacity testing. In the figure, a square indicates a Mode-S aircraft. and a circle an ATCRBS aircraft. The total traffic load is over 300 aircraft, most of which are contained in a 90°, 60-nmi sector.
Implementation
The FAA is procuring 137 dual-channel Mode-S sensors from ajoint venture comprised of Westinghouse Electric Corp. and Unisys Corp. The first operational implementation at a site is scheduled for 1991. To outfit all of its beacon-radar sites with Mode S. the FAA is currently considering an additional purchase of 259 Mode-S sensors.
The sensors are designed to provide a total communication data rate of 92.5 kbls for a target load of 700 aircraft. Thus the initial sys-tem of 137 sensors will have a total capacity of more than 12 Mb/s. The characteristics of the sensor determine maximum data-link transfer to a specific aircraft. The production version of the Mode-S sensor, which has a rotatingnarrow-beam antenna, can deliver up to 360 bls on the uplink and 313 bls on the downlink for a total eqUivalent simplex rate of 673 b/s. A next-generation sensor eqUipped with an electroni-cally scanned antenna could transfer data to aircraft at a rate as high as 5 kb/s.
Fig. 19-ARIES traffic plot.

Summary
Mode S, an evolutionary improvement ofthe current ATCRBS, provides enhanced surveil-lance performance through monopulse, discrete addressing, and error protection. In addition, Mode S includes an integral data link with unique benefits to ATC because of the link's association with the surveillance function and its resistance to interference.
Extensive field measurements and the devel-opment of an engineering-model sensor have
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validated the Mode-S techniques. The FAA is currently implementing Mode S for operational use in the United States.
Acknowledgments
The development of the Mode-S beacon sys-tem required the efforts of many individuals at Lincoln Laboratory. This article is dedicated to them and to our sponsor, the FederalAviation Administration.
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References
1.  
V.A. Orlando and P.R Drouilhet, "Mode S Beacon System: Func~ional Description," Project Report ATC-42D, Lincoln Laboratory (29 Aug. 1986), FAA/PM-86/19.

2.  
SelectionOrder:U.S.NationalStandardfortheIFFMark X (SIF) Air Traffic Control Radar Beacon System (ATCRBS) Characteristics, Dept. of Transportation/ Federal Aviation Administration Order 1010.51A (8 Mar. 1971).

3.  
D. KarpandM.L. Wood, "DABS Monopulse Summary," Project Report ATC-72, Lincoln Laboratory (4 Feb. 1977), FAA-RD-76-219.
　
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