Stone et al. -Advances in Primary-Radar Technology Fig. I-Detection of aircraft flying in rain. (a) Output of a conventional MTI radar taken with a 5-min exposure. The aircraft flying within the rain echoes is obscured. (b) Output of moving target detector (MTD). The screen is free of rain clutter and the overall track of the aircraft is clearly delineated. Because only one display was available, the photographs were made sequentially. [9, 101. Figure 1 gives a qualitative illustration of the

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... periority of MTD over MTI in detecting aircraft flying in rain. MTD and the weather channel used in ASR-9 represent a significant improvement in primary-radar technology. The FAA is installing ASR-9 systems at more than 100 major airports in the United States (Fig. 2 ). This article first delineates the primary-radar requirements for aircraft and weather surveillance. Next, MTD is described and the system's test results are discussed. Finally, we comment on possible future enhancements to the MTD concept. Performance Requirements for Airport Primary Radars The Lincoln Laboratory Journal. Volum 2, Number 3 (1 989) Stone et al. -Advances in Primary-Radar Technology Fig. 2-Planned ASR-9 installations in the United States. rected toward the horizon. to circular polarization. By doing so, the sensor Although the antenna normally both radiates achieves an additional 12 to 20 dB of precipitaand receives vertical polarization, whenever tion-echo rejection. During the time that circuthere is heavy precipitation over a significant lar polarization is used, weather signals are portion of the coverage area, the radar switches derived from the orthogonal-polarization ports -'2. . !:-'2-,-',? Maximum range ='60 nmi. Altitude coverage = 0 ft to 25,000 ft. Update rate = 4.8 s. Probability of detecting a small aircraft = 0.9. False-alarm rate at the output of a tracking filter = 1 per scan. False-alarm rate at the output of the correlation and interpolation filter = 20 per scan. Range accuracy = 1/32 of a nautical mile with a 2004 rms. Azimuth accuracy = 0.1 6" rms. Capacity = 400 aircraft distributed nonuniformly in azimuth. Weather reporting: six National Weather Service (NWS) levels are available for pairwise selection by the air traffic controller. .The Lincoln Laborato y Journal, Volume 2. Number 3 (1 989) Stone et al. -Advances in Primary-Radar Technology