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Low altitude windshear phenomena has been causally linked to numerous civil transport aircraft incidents with fatalities and injuries exceeding 800, covering a period of more than 25 years. The crash of a USAir DC-9 at Charlotte, NC on July 2,1994 has caused an increased focus on windshear and microburst activity. FAA, NASA, the airline industry and equipment manufacturers have been working with the problem since the 1980's, to define, design and test predictive sensor systems that would provide adequate and dependable advance warning to pilots that would allow safely executed evasive actions during take off's and landings. Three sensor systems were investigated: radar, laser radar (lidar), and infrared imaging, with the pulsed Doppler radar consistently detecting both wet and dry microbursts indicative of hazardous windshear conditions at longer ranges than the other two. Numerous documents were reviewed, including the NASA Airborne Doppler Radar together with meetings held with AlliedSignal, Rockwell Collins and Westinghouse to verify radar parameters needed to analyze and present the tester concept described herein. The weather and windshear models defined use the identical criteria established for the Doppler radar in terms of F-factor. The basic concept of the tester is to transmit coherent simulated radar returns in response to the airborne radar's transmission while mounted on a tripod in the far field of the radar when parked on the ramp. The varying amplitude of the received radar pulses are analyzed and put into memory as the tester antenna is illuminated by the scanning main beam and side lobes of the radar's antenna patterns. The tester controls the power of its outputted simulated radar returns in inverse relation to the power of the received radar pulses. These simulated radar returns, outputted into the main beam and/or side lobes of the scanning radar antenna, are interpreted by the radar system as received in the main lobe. The tester transmissi ons, incorporating microburst, storm and gust front models, previously defined, can thereby test the aircraft radar system performance in various hazard environments. The tester is designed to: verify operational performance of the radar; demonstrate installed radar performance; verify crew reports and minimize radar or LRU's removal for maintenance; test before and after a repair; and verify radome effects on radar performance.